CBSE Class 10 Physics HOTs Electricity

MCQ Questions for NCERT Class 10 Science Electricity

Question. The accumulator which is used for the domestic purpose has the electromotive force of 10 V and with an internal resistance of 0.8 Ω is externally charged by 150 V of the direct current power supply using a series resistor 18 Ω. Calculate the terminal voltage of the accumulator during using.
(a) 16.8 V
(b) 17.1 V
(c) 11.3 V
(d) 15.9 V
Answer: D

Question. The maximum resistance which can be made using four resistors 1/2 each of resistance W is :
(a) 2 W
(b) 1 W
(c) 2.5 W
(d) 8 W
Answer: A

Question. An electric fuse is connected with :
(a) live wire
(b) earthing
(c) neutral wire
(d) parallel to the line wire
Answer: A

Question. Electrical resistivity of an alloy of copper and nickel is ________ when compared with the electrical resistivity of an alloy of copper, manganese and nickel.
(a) same
(b) double
(c) more
(d) less
Answer: C

Question. A cylindrical conductor of length ‘l’ and uniform area of crosssection ‘A’ has resistance ‘R’. The area of cross-section of another conductor of same material and same resistance but of length ‘2l’ is :
(a) A/2
(b) 3A/2
(c) 2 A
(d) 3 A
Answer: C

Question. Let us consider that because of the flow of the current flowing through a metallic wire if the temperature of the entire system increases. What will happen from the following options ?
(a) Potential difference (V) increases
(b) Resistance (R) decreases
(b) Potential difference (V) decreases
(d) V and R remains the same
Answer: C

Question. There is a dual of 8 ohm resistance on the aerial. Determine the aerial’s new resistance.
(a) 2 W
(b) 4 W
(c) 7 W
(d) 10 W
Answer: A

Question. On which of the given factor, resistance does not depend:
(a) length of conductor
(b) area of cross-section
(c) temperature
(d) density
Answer: D

Question. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be:
(a) 1 : 2
(b) 2 : 1
(c) 1 : 4
(d) 4 : 1
Answer: C

Question. An ammeter has 10 divisions between 0 and 0.5 A marks on its scale. The least count of ammeter is :
(a) 0.01 A
(b) 0.5 A
(b) 0.05 A
(d) 0.1 A
Answer: C

Question. For which of the following substances, resistance decreases with increases in temperature?
(a) Mercury
(b) Silver
(b) Copper
(d) Carbon
Answer: D

Question. Commercial unit of electrical energy is :
(a) joule
(b) ampere
(b) volt
(d) kilowatt-hour
Answer: D

Question. When a current I flows through a resistance R for time t, the electrical energy spent is given by :
(a) I2R/t (b) IRt
(c) I2Rt
(d) IR2t
Answer: C

Question. The electric meter in a house records :
(a) current
(b) energy
(c) power
(d) voltage
Answer: B

True/Fa;se

Question. When a metallic conductor is heated the atoms in the metal vibrate with greater amplitude and frequency.
Answer: True

Question. A dentist uses a convex mirror to view the inner parts of a patient’s mouth.
Answer: True

Question. In the circuit to verify Ohm’s law, ammeter and voltmeters both are connected in series with resistance and cell in the circuit.
Answer: False

Question. The reciprocal of resistance is called specific
Answer: False

Question. Pure tungsten has high resistivity and a high melting point (nearly 3000cC).
Answer: True

Question. The resistivity of a wire is directly proportional to cross-sectional area.
Answer: True

Question. Two wires of resistances 2 W and 4 W are connected in series. The combination is connected to a 220 V supply. The power dissipated in 2 W resistor is more.
Answer: False

Question. A copper wire of length L and cross-sectional area A carries a currentI . If the specific resistance of copper is S , then electric field in the wire is Is/A.
Answer: True

Question. The equivalent resistance of several resistors in series is equal to the sum of their individual resistances.

Answer: True

Question. In parallel combination, the reciprocal of equivalent resistance is the sum of the reciprocal of individual resistance.

Answer: True

Important Questions for NCERT Class 10 Science Electricity 

Very Short Answer Type Questions :

Question. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.
Answer: 4800 W

Question. A nichrome wire has a resistance of 10 W. Find the resistance of another nichrome wire, whose length is three times and area of cross-section is four times the first wire.
Answer: 7.5 W

Question. The equivalent resistance of the combination of resistors given in figure is 4 W. Calculate the value of x.
Answer: 5 W

Question. Write three points of difference between Ohmic resistor and non- Ohmic resistor.
Answer: 

Question. Write two points of difference between resistance and resistivity (or specific resistance). 
Answer: 

Question. Write three points of difference between series combination and parallel combination of resistors.
Answer: 

Question. There are three 2 V cells connected in series. How many joules of energy does 1 C gain on passing through all the three cells?
Answer: Here, the potential difference,
V = 2 + 2 + 2 = 6 V and
charge, Q = 1 C
We know that,
Work done, W = VQ
Substituting the values, we get
W = 6 × 1 = 6 J.

Question. The following table gives the value of electrical resistivity of some materials :

Which one is the best conduct or of electricity out of them?
Answer: Silver, because its electrical resistivity is least out of the given materials.

Question. What is the resistance of an (a) ideal ammeter and (b) ideal voltmeter ?
Answer: (a) Zero
(b) Infinite

Question. In series combination which remains constant—current or voltage ?
Answer: Current.

Question. Name two devices in which electricity is converted into heat.
Answer: Electric heater and electric iron.

Question. Name the alloy which is used for making the filament of bulbs.
Answer: Tungsten is used for making the filament of bulbs.

Question. Should the resistance of a voltmeter be low or high ? Give reason.
Answer: The resistance of a voltmeter should be high, because voltmeter is connected parallel to the component of a circuit and it also takes negligible current from the circuit in order to measure the potential difference accurately.

Question. Name a material whose resistivity becomes zero at a particular temperature.
Answer: Mercury.

Question. Electric current flows through a metallic conductor from its one end A to other end B. Which end of the conductor is at higher potential ? Why ?
Answer: Since, current flows from a region higher potential to a lower potential. So, it flows from A to B where A is the end with higher potential.

Question. Which material is the best conductor of electricity ?
Answer: Silver.

Short Answer Type Questions :

Question. Though same current flows through the electric line wires an the bulb filament, yet only the filament glows. Give the reason behind it?
Answer: The resistance of electric line wire is very very less than that of the filament of the bulb. Therefore, the current through the high resistance filament produces more heat which makes it glow.

Question. Which is analogous to temperature: a current or electric potential?
Answer: Electric potential is analogous to temperature. As heat flows from higher temperature to lower temperature, so charges flow from higher potential to lower potential.

Question. Ammeter burns out when connected in parallel. Give reasons.
Answer: Ammeter consists of a wire of low resistance when connected in parallel; a large amount of current passes through it hence gets burnt i.e. short circuited.

Question. For domestic purpose, we connect the electrical devices in parallel instead of connecting them in series. What are the advantages of connecting electrical devices in parallel?
Answer: By connecting the electrical devices in parallel:- 1 each device gets the full or same voltage 2. each device gets proper current depending on its resistance. 3. if one device is switched off/on , other electrical appliances remain unaffected That is why for domestic purpose we connect the electrical devices in parallel instead of connecting them in series.

Question. The heating elements of electric toasters and electric iron are made of an alloy rather than a pure metal. Why?
Answer: The heating elements of electric toasters and electric irons are made of an alloy rather than a pure metal due to the following reasons:- 1. Alloys don’t oxidize readily at higher temperature. 2. Alloys have higher resistivity than their constituent elements

Question. An ammeter is always connected in series across a circuit element. What happens when it is connected in parallel with the circuit element?
Answer: The ammeter is alow resistance device. When it is connected in parallel, the resistance of the circuit reduces considerably. Therefore, a large current flows through the circuit, by virtue of which it may be damaged.

Long Answer Type Questions :

Question. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below : 

Plot a graph between V and I and calculate the resistance of that resistor.
Answer: The graph between V and I is given below :
Let us consider two points A and B on the slope.
Draw two lines, one from point B along X-axis and another from point A along Y-axis, which meet at point C.
The slope of the graph will give the value of resistance, thus
Slope = R = AC/BC
Now, BC = 3 – 1 = 2 A
AC = 10.2 – 3.4 = 6.8 V
Slope = 6.8/2 = 3.4 W

Thus, resistance (R) = 3.4 Ω.

Question. An electric lamp of resistance 20 W and a conductor of resistance 4 W are connected to a 6 V battery as shown in the circuit. Calculate : 
(a) the total resistance of the circuit,
(b) the current through the circuit,
(c) the potential difference across the (i) electric lamp and (ii) conductor,
(d) power of the lamp.
Answer: (a) Given, R1 = 20 W, R2 = 4 W
Since, in Series R = R1 + R2
∴ Total resistance of circuit : R = 20 + 4
= 24 W
(b) Current through circuit V = 6 V, R = 24 W
According to Ohm’s law V = IR
So, I = V/R
I = 6/24
= 1/4 = 0·25 ampere
(c) (i) Potential difference across electric lamp :
I = 1/4 A, R1 = 20 W
V1 = IR1
V1 = 1/4 × 20
= 5 V
(ii) Potential difference across conductor
V2 = IR2
= 1/4 × 4
V2 = 1 V
(d) Power of lamp : P = I2R
= (1/4)2 x 20 
= 1/4 x 1/4 x 20
= 5/4 W
or P = 1.25 W.

Question. The current flowing through a resistor connected in a circuit and the potential difference developed across its ends are as shown in the diagram by milliammeter and voltmeter readings respectively:

(a) What are the least counts of these meters?
(b) What is the resistance of the resistor?
Answer: (a) 10 mA and 0.1 V
(b) V = 2.4 volt, I = 250 mA = 0.25 A
From Ohm’s law. R = V/I = 2.4/0.25 = 9.6 W

Question. Show how would you join resistors each resistance 9 W so that the equivalent resistance of the combination of combination is (a) 13.5 W (b) W ?
Answer: (a) To get an equivalent resistance of 13.5 W, the resistance
should be connected as sown in the figure given below:
So, 1/Rp = 1/R1 + 1/R2
= 1/9 + 1/9
= 1 + 1/9 = 2/9
1/Rp = 2/9
RP = 9/2 = 4.5 W
Now, RS = R3 + 4.5 W
= 9 W + 4.5 W
= 13.5 W
(b) To get equivalent resistance of 6 W, the resistance should be connected as shown in the given below
RS = R1 + R2
= 9 + 9
= 18 W
Now both the resistance are in parallel with each other so,
Rp = 1/18 + 1/9
= 1 + 2/18 = 3/18
= 1/6 w
So, RP = 6 W

Question. Two bulbs rated (60 W – 220 V) and (60 W – 110 V) respectively.
Calculate the ratio of their resistance.
Answer: First bulb:
Power rating P1 = 60 W
Voltage rating V1 = 220 V
∴ Resistance, R1 = V12/P1
= 220 X 220 /60 Ω
Second bulb:
Power rating P2 = 60 W
Voltage rating, V2 = 110 V
∴ Resistance, R2 = V22/P2
= 110 x 110/60 Ω
 220 x 220 / 60
R1/R2 = 110 x 110 /60
= 220 x 220 /60 x 60/110 x 110
= 4/1 
R1 : R2 = 4 : 1

Question. An electric kettle is rated at (100 W – 220 V).
(a) What is the resistance of its element when in use?
(b) What is the safe value of current that can pass through its element?
Answer: Here, Power rating, P = 1000 W
Voltage rating, V = 220 V
(a) Using the relation P = V2/R
Resistance of element when in use,
R = V2/P = 220 x 220/1000
= 48.4 W
(b) Using the relation P = VI
Safe current, I = P/V = 1000/20
= 4.55 A

Question. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?
Answer: Given: Power of one lamp, P1 = 100 W
Power of second lamp, P2 = 60 W
Since, both the lamps are connected in parallel, thus, potential difference will be equal.
Thus, Potential difference = 220 V
We know, that Power (P) = VI
Thus, the total current through the circuit
I = P1/V + P2/V
I = 100/200 + 60/200
= 100 + 60/220
= 160/220 = 0.727 A

Question. If a student wants to connect four cells of 1.5 V each to form a battery of voltage 6 V, then how would he draw the symbol of the battery?
Answer: 

Question. Arrange 1 W, 10 W and 100 W such that the equivalent resistance is greater than 10 W but less than 11 W.
Answer: (a) The resistor have to combined as shown in the diagram
The equivalent resistance of 1 W and 100 W connected in parallel is:
1/R1 = 1/1 + 1/100
= 100 + 1/100
= 101/100
or R1 = 100/101 W
= 0.99 W
Now, 10 W and R1 are in series,
Therefore, equivalent resistance R is
R = (10 + 0.99) W
= 10.99 W
The above value of resistance is greater than 10 W but less than 11 W.

Question. What is (a) the highest, (b) the lowest resistance that can be secured by combination other resistors of 1 W, 10 W, 100 W and 1000 W?
Answer: (a) To obtain the highest resistance, the resistors must be connected in series.
∴ Highest resistance, RS = (1 + 10 + 100 + 1000) W
= 1111 W
(b) To obtain the lowest resistance, the resistors must be connected in parallel. the lowest resistance is given by
1/Rp = 1/1 + 1/10 + 1/100 + 1/1000
= 1000+ 100 + 10 + 1/1000
= 1111/1000
∴ Lowest resistance RP = 1000/1111 W
= 0.9 W.

Question. An electric iron is rated ‘1 kW – 220 V’. Calculate the following :
(a) The resistance of its heating element.
(b) The amount of current that will flow through the element.
(c) The amount of heat that will be produced in 2 minutes.
(d) The power consumed if the line voltage falls to 200 V.
Answer: Here, V = 220 volt, P = 1 kW = 1000 W, t = 2 minute = 2 × 60 = 120 s
(a) Resistance of the heating element, 
R = V2/P = (220)2/1000 = 48.4 W.
(b) Current through the element,
I = P/V = 1000/220 = 4.54 A
(c) Heat produced in 2 minutes = P × t
= 1000 × 120 = 1·2 × 105 J
(d) If line voltage falls to 200 V, the power consumed is,
P = V2/R = (200)2/48.4 = 826.44 W.

Reasoning Based Questions :

Question. Answer the following questions:
(a) List the factors on which the resistance of a conductor in the shape of wire depends.
(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.
Answer: (a) Resistance of a conductor depends directly on its length and is inversely proportional to the area of cross-section.
(b) Metals have free electrons and they move and conduct electricity, whereas glass does not have free electrons and charges to flow as it is an insulator.

Question. Why are coils of electric heaters and electric irons made of an alloy rather than a pure Metal?
Answer: The resistivity of alloys are generally higher than that of its constituent metals and alloys do not oxidize (burn) readily at high temperatures, hence they are commonly used in electrical heating devices, like electric heaters, electric irons etc.

Question. Does Ohm’s law hold good for electrolytic solutions and semiconductors?
Answer: No.

Question. Define the electric circuit. Draw a labelled, schematic diagram of an electric circuit comprising of a cell, a resistor, an ammeter, a voltmeter and a closed switch 175
Answer: A continuous conduction path consisting of wires and other resistance (like bulb, fan, etc) and a switch between the two terminals of a cell or a battery along which an electric current flows, is called a circuit.

Question. Which of the cables, one rated 5 A and the other 10 A will be of thicker wire? Give a reason for your answer.
Answer: The cable carrying 10 A current will be of thicker wire because to carry a heavy current, the resistance of wire should be low, hence its area of cross-section should be large.

Question. Why are copper and aluminium wires used as connecting wires?
Answer: Copper and aluminium wires are used as connecting wires because they have low resistivity and are good conductors of electricity.

Question. Name a material which is used for making standard resistors. Give a reason for your answer.
Answer: Standard resistors are made from alloys such as constantan, manganin etc., because they have high specific resistance and the effect of change in temperature on their resistance is negligible.

Question. Why is a series arrangement not used for domestic circuits?
Answer: Series arrangement is not used for domestic circuits for the following reasons :
1. The voltage of the source gets divided in all the appliances connected in series, in the ratio of their resistances, so each appliance does not operate at its rated voltage.
2. The resistance of the circuit increases and it reduces the current in the circuit, so each appliance gets less power.
3. If any one appliance in series arrangement is switched off (or gets spoilt), no other appliance connected with it in series will then operate.

Question. Why is tungsten used for filaments of electric lamps?
Answer: Tungsten has high melting point and great tensile strength that’s why it is used as light bulb filament in electric lamps.

Question. The given figure shows three lamps and three switches 1, 2 and 3 connected with two cells.
(a) Name the switch/switches to be closed so as to light all the three lamps.
(b) How are the lamps connected : in series or in parallel?
Answer: (a) 2 and 3, (b) in series.

Question. Why is lead-tin alloy used for fuse wires?
Answer: Lead-tin alloy is used for fuse wires because it has low melting point. It will melt when high supply come to prevent the electric circuit from fire.

Question. Two wires P and Q are made of copper. The wire P is long and thin, while the wire Q is short and thick. Which will have more specific resistance? Give a reason for your answer.
Answer: Both the wires will have same specific resistance, since they are both made of same material (i.e., copper) and there is no change in temperature.

Question. Why are the heating elements of electric toasters and electric irons made of an alloy rather than a pure metal?
Answer: The resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at higher temperatures. Therefore, conductors of electric heating devices, such as toasters and electric irons, are made up of an alloy rather than pure metal.

Question. Why should a connection wire be thick?
Answer: Resistance of a wire is inversely proportional to its area of cross-section (or thickness). Hence, a connection wire should be thick to reduce its resistance.

Creating Based Questions :

Question. Brisilia designed a prototype in which she used a very sensitive electrical device. But she does not know how to protect the sensitive electrical device from high current. Suggest her with one idea to protect the sensitive device from high current.
Answer: The suggestion to protect the sensitive electrical device from high current is by using a parallel low resistor known as shunt resistor. The resistance value of shunt resistor is very low. It is made up of the material having low temperature coefficient of resistance. It measures the electric current, alternating current or direct current.

Question. Rita designed a circuit in which resistors are connected in series. Yet she is not satisfied with the series resistors because if there is a fault in some component of the circuit, the whole circuit stops working. What would be your suggestion in alternative to the resistance in series ?
Answer: To overcome the problem faced by Rita, I would suggest to connect the resistors in parallel because if the resistors are connected parallel, the whole circuit does not stop working. If the fault is with one component of the circuit, the current continues to flow through the other components of the circuit which makes the device work further.

Question. Electrical resistivities of some substance at 20°C are given below. 

Answer the following relations in relation to them.
(a) Among silver and copper, which one is a better conductor ?
(b) Which material would you advise to be used in electrical heating devices ?
(c) Define resistivity.
(d) What is the effect of temperature on resistivity of a substance ?
Answer: (a) Silver
(b) Nichrome
(c) Resistivity of a conductor is defined as the resistance of the conductor of unit length and unit area of cross-section.
(d) Resistivity of a material changes if its temperature changes.

Question. Rhea noted the readings of her home’s electricity meter on Monday at 9 a.m. and again on Tuesday at 9 a.m. (as shown in figure given below). 

(a) What was the meter reading on Monday ?
(b) What was the meter reading on Tuesday ?
(c) How many units of electrical energy have been used ?
(d) How much time (in hours) have these units been used ?
(e) Calculate the cost of electrical energy used during this time, if the rate is ` 8 per unit.
Answer: (a) Meter reading on Monday = 53712
(b) Meter reading on Tuesday = 53725
(c) Number of units of electrical energy used = 53725–53712
= 13 units
(d) Time = 24 hours
(e) Electrical energy consumed = ₹ 8 × 13
= ₹ 104

Question. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.
Answer:


Question. How will you infer with the help of an experiment that the same current flows through every part of a circuit containing three resistors in series connected to a battery ? 
Answer: Let three resistors R1, R2 and R3 are connected in series which are also connected with a battery, an ammeter and a key as shown in figure. When key is closed, the current starts flowing through the circuit. Take the reading of ammeter. Now change the position of ammeter to anywhere in between the resistors and take its reading. We will observe that in both the cases reading of ammeter will be same showing same current flows through every part of the circuit above.

Question. Answer the following question:
(a) Write Joule’s law of heating.
(b) Two lamps, one rated 100 W; 220 V, and the other 60 W; 220 V, are connected in parallel to electric mains supply. Find the current drawn by two bulbs from the line, if the supply voltage is 200 V.
Answer: (a) According to Joule’s law of heating, the heat produced in a
wire is directly proportional to :
(i) square of current (I2),
(ii) resistance of wire (R),
(iii) time (t) for which current is passed.
Thus, the heat produced in the wire by current in time ‘t’ is
H ∝ I2Rt
or H = KI2 Rt
But K = 1, H = I2Rt
(b) We know that, P = VI
⇒ I = P/V First lamp : P1 = 100 W, V = 220 volt
I1 = P1/V = 100/220 = 0·45 A
Second lamp : P2 = 60 W, V = 220 volt
I2 = P2/V = 60/220 = 0·27 A
So, Total current = I1 + I2
= 0·45 + 0·27
= 0·72 A

Question. For the series combination of three resistors establish the relation:
R = R1 + R2 + R3
where the symbols have usual meaning. Calculate the equivalent resistance of the combination of three resistor of 6 W, 9 W and 18 W joined in parallel.
Answer: Same current (I) flows through different resistance, when these are joined in series, as shown in the figure. 

Let R be the combined resistance, then
V = IR
V1 = IR1, V2 = IR2, V3 = IR3
IR = IR1 + IR2 + IR3
∴ IR = IR1 + IR2 + IR3
⇒ IR = I(R1 + R2 + R3)
∴ R = R1 + R2 + R3
Now, R1 = 6 W, R2 = 9 W,
R3 = 18 W
In parallel combination
1/R = 1/R1 + 1/R2 + 1/R3
⇒ 1/R = 1/6 + 1/9 + 1/18 = 3 + 2 + 1/18
= 6/18 = 1/3
⇒ 1/R = 1/3
⇒ R = 3 W.

Question. The following apparatus is available in a laboratory which is summarised in tabular form: 
(a) For what purpose an ammeter is used ?
(b) If we require the maximum resistance from a number of given resistors we connect :
(i) all in series
(ii) all in parallel
(iii) less resistors in series and more in parallel
(c) The best combination of voltmeter and ammeter for finding the equivalent resistance of the resistors in series would be :
(i) ammeter A1 and voltmeter V1
(ii) ammeter A1 and voltmeter V2
(iii) ammeter A2 and voltmeter V1
(iv) ammeter A2 and voltmeter V2
(d) For the experiment to find the equivalent resistance of the parallel combination of the two given resistors, the best choice would be :
(i) ammeter A1 and voltmeter V1
(ii) ammeter A1 and voltmeter V2
(iii) ammeter A2 and voltmeter V1
(iv) ammeter A2 and voltmeter V2
Answer: (a) To measure current in the circuit.
(b) (i) All in series
(c) (iv) Ammeter A2 and voltmeter V2
(d) (iii) Ammeter A2 and voltmeter V1

Question. What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
Answer: Advantages of connecting electrical appliances in parallel instead of connecting in series:
(a) Voltage remains same in all the appliances.
(b) The total effective resistance is less.
(c) Switching ON/OFF of one device does not affect others.

Question. State the energy conversion taking place in the following electric appliances :
(a) Electric heater, (b) Electric -motor, (c) Loud- speaker, (d) Electrolysis.
Answer: (a) Electrical energy gets converted into heat energy in an electric heater.
(b) Electrical energy changes into mechanical energy in an electric motor.
(c) Electrical energy gets converted into sound energy in a loudspeaker.
(d) Electrical energy changes into chemical energy during electrolysis.

Question. Shyam designed a burglar alarm circuit in which the resistors are connected in series. The circuit breaks and the current did not flow through the circuit. What is the alternate method he should opt to prevent the circuit break when the resistors are connected in series ?
Answer: There is only a single path which connects from the electric source to the output devices. The electrical appliance damage can be prevented by connecting the fuse in series with the mains as well as the electrical appliance. To maintain the current level efficiently series of resistors can be used.

Question. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
Answer: (a) Since Resistance (R) = Potential difference(V) / Electric (I)
Therefore, if potential between to ends of the component will be halved, and resistance remains constant then electric current would also be halved.

Please refer to link below for CBSE Class 10 Biology HOTs-Electricity.

Question. In the given figure what is the ratio of current in A₁,and A₂

Answer: V=IR V=const.
I ∞ 1/R I₁/I₂ = R/2R I₁/I₂ = 1/ 2

Question. A wire of resistance R is bent in form of a closed circle, what is the resistance across a diameter of the circle?
Answer: 1/R’ =1/(R/2)+1/(R/2) R’=R/4.

Question. A charge of 6 C is moved between two points P and Q having , potential 10V and 5V respectively. Find the amount of work done.
Answer: W=q(V₂-V₁)=6(10-5)=30 joule

Question. Name the physical quantity whose SI unit is JC-1.
Answer: Potential

(2 Marks Questions)

Question. Two wires of equal cross sectional area , one of copper and other of manganin have same resistance. Which one will be longer?
Answer: R=ρL/A (R,A=const .L=1/ρ)
ρ _manganin > ρ _copper
L _copper>L _manganin

Question. A Rectangular block of iron has dimensions L X L X b. What is the resistance of the block measured between the two square ends ? Given ρ= resistivity.
Answer: R=ρ b/L²

Question. Three equal resistances are connected in series then in parallel. What will be the ratio of their Resistances?
Answer: R_series =3R.
R_parallel =R/3
R_series / R_parallel =3R/(R/3)=9

Question. Jusitfy for any pair of resistance the equivalent resistance in series is greater equivalent resistance in parallel .

Answer: Since, R=V/I
R_A>R_B
A=Series,B=Parallel

Question. How many bulbs of 8Ώ should be joined in parallel to draw a current of 2A from a battery of 4 V?
Answer: R=V/I=4/2=2 Ώ, let ‘n’ be the no of bulbs.
1/R=1/R₁ + 1/R₂ +……..1/R_n =n/8
½=n/8, n=4.

Question. Two cubes A and B are of the same material. The side of B is thrice as that of A. Find the ratio R_A/R_B.
Answer: R_A = ρL/A R_B=ρ3L/9A
R_A : R_B =3:1

Question. 3 X 10¹¹ electrons are flowing through the filament of bulb for two minutes. Find the current flowing through the circuit. Charge on one electron=1.6X10^-19 C.
Answer:  q=ne=3x10¹¹x1.6x10^-19=4.8x10⁸C
I=q/t=4.8x10⁸/(2x60)=4x107A

Question. A nichrome wire of resistivity 100X10-6ohm- m and copper wire of resistivity 1.62X10-8 ohm-m of same length and same area of cross section are connected in series , current is passed through them, why does the nichrome wire gets heated first?
Answer: Q=I² Rt
Q= I² { ρ L/A}t
Nichrome wire has higher resistivity than copper wire . Therefore, it is heated first

(3 Marks Questions)

Question. In the given figure what is ratio of ammeter reading when J is connected to A and then to B.

Answer: when J is connected to A
I=V/R=3/5A=O.6A
When J is connected to B
V=1+2+3+4=10V
I=10/5=2A

MCQ Questions for NCERT Class 10 Science Electricity 

Question. Electrical resistivity of a given metallic wire depends upon:
(a) its length
(b) its thickness
(c) its shape
(d) nature of the material
Answer: D

Question. If a person has five resistors each of value 1/5 W, then the maximum resistance he can obtain by connecting them is:
(a) 1 W
(b) 5 W
(c) 10 W
(d) 25 W
Answer: A

Question. The proper representation of series combination of cells obtaining maximum potential is: 


Answer: A

Question. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R', then the ratio R/R' is :
(a) 1/25
(b) 1/5
(c) 5
(d) 25
Answer: D

Question. An electric heater is rated at 2 kW. Electrical energy costs ₹4 per kWh. What is the cost of using the heater for 3 hours?
(a) ₹ 12
(b) ₹ 24
(c) ₹ 36
(d) ₹ 48
Answer: B

Question. There are three resistors connected in parallel, the resistance of each resistor is 3 ohm. What is the total resistance of all the three resistors ?
(a) 1 Ω
(b) 6 Ω
(c) 15 Ω
(d) 3 Ω
Answer: A

Question. Consider the room temperature is 24°C in summer, the electrical resistance of thermocoil which is used in the AC unit is 150 Ω.
Then calculate the temperature of the thermocoil if the electrical resistance is 175 Ω. Given the temperature coefficient of the thermocoil is 2.98 × 10– 4 °C– 1.
(a) 597°C
(b) 583°C
(c) 546°C
(d) 512°C
Answer: B

Question. Which of the following terms does not represent electrical power in a circuit ?
(a) I2R
(b) IR2
(c) VI
(d) V2/R
Answer: B

Question. Which of the following obeys Ohm’s law ?
(a) Filament of a bulb
(b) LED
(c) Nichrome
(d) Transistor
Answer: C

Question. The values of mA and μA are :
(a) 10–6 and 10–9 A respectively
(b) 10–3 and 10–6 A respectively
(c) 10–3 and 10–9 A respectively
(d) 10–6 and 10–3 A respectively
Answer: B

Question. Which of the following does not apply to silver ?
(a) The resistance provided is directly proportional to its length.
(b) The resistance provided is inversely proportional to the area of cross section.
(b) Their resistivity is in the range 10–8 Ω to 10–6 Ω.
(d) The movement of electrons on their outer most orbital is tightly held together.
Answer: D

Question. The resistance of a resistor is reduced to half of its initial value.
In doing so, if other parameters of the circuit remain unchanged the heating effects on the resistor will become:
(a) two times
(b) half
(c) one - fourth
(d) four times
Answer: A

Question. The resistance of the wire when the length of the wire increases two times :
(a) becomes 2 times
(b) becomes 3 times
(c) becomes 6 times
(d) becomes 4 times
Answer: A

Question. Which of the following terms does not represent electrical energy in a circuit?
(a) I 2Rt
(b) IR2t
(c) VIt
(d) V2t/R
Answer: B

Question. Which among the following is the correct way of connecting ammeter and voltmeter in the circuit to determine the equivalent resistance of two resistors in series ?
(a) Both ammeter and voltmeter in series
(b) Both ammeter and voltmeter in parallel
(b) Ammeter in parallel and voltmeter in series
(d) Ammeter in series and voltmeter in parallel
Answer: D

Question. The instrument used for measuring electric current is:
(a) galvanometer
(b) ammeter
(c) voltmeter
(d) potentiometer
Answer: B

Question. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be:
(a) 100 W
(b) 75 W
(c) 50 W
(d) 25 W
Answer: D

Question. The equivalent resistance of a series combination of two resistances is X ohm. It the resistance are of 10 W and 40 W respectively, the value of X will be:
(a) 10 W
(b) 20 W
(c) 50 W
(d) 40 W
Answer: C

Question. A student plots V-I graphs for three samples of nichrome wire with resistances R1, R2 and R3. Choose from the following statement that holds true of this graph.
(a) R₁ = R₂ = R₃
(b) R₁ > R₂ > R₃
(c) R₃ > R₂ > R₁<
(d) R2 > R1 > R3
Answer: D

Question. R1 and R2 are two resistors and r1 and r2 are equivalent resistances in series and parallel respectively, then R1/R2
(a) r₁ r₂/r₁ + r₂
(b) r₁ + r₂/r₁ r₂
(c) r₁+√r₁ - 4r₁r₂/r₁ +√r₁ +4r₁r₂
(d) r₁ + √r₂¹ - 4r₁r₂/r - √4r₁r₂
Answer: D

Assertion and Reasoning Based Question :

Question. Assertion :When more current flows to an electrical equipment it shows more heating of the same.
Reason : Heat flow is directly proportional to the square of current only.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: C

Question. Assertion : In a circuit which is having 3 series resistors of R W each, the total resistance of the circuit will be 3 R.
Reason : As in parallel circuit the resultant resistance will be 1/R = 1/R1 + 1/R2 + 1/R3 .
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If the assertion is false, but reason is true.
Answer: B

Question. Assertion: The connecting wires are made of copper.
Reason: The electrical conductivity of copper is high.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: A

Question. Assertion : Voltmeter is always connected in parallel across the circuit while measuring the potential difference.
Reason : As the voltage in parallel circuits is measured to be the same.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: A

Question. Assertion : Electric current flowing through a metallic wire is directly proportional to the potential difference across its ends.
Reason : Ohms law expression V = IR, where R (resistance) of the wire is always varying.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: C

Question. Assertion : Bulbs are filled with inactive nitrogen and argon gases.
Reason : As there is a requirement of thermal isolation of the filament.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: A

Question. Assertion : Alloys are commonly used in electrical heating devices like electric iron and heater.*
Reason : Resistivity of an alloy is generally higher than that of its constituent metals but the alloys have low melting points than their constituent metals.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: B

Question. Assertion : In a series circuit, the current is constant throughout the electric circuit.
Reason : All electric devices does not need equal currents to operate properly.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: B

Question. Assertion : When area of the conductor is halved then the resistance of the material gets doubled when length is kept constant.
Reason : Because resistance is inversely proportional to the area of a cross-section of the material.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: A

Question. Assertion : An ammeter is always connected in parallel with the circuit for which current has to be measured.
Reason : As the current in a series circuit is same.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false, but reason is true.
Answer: D

HOTS Questions and Answers

Question. What is represented by joule/coulomb?
Answer: It represents potential difference. 

Question. A charge of 2C moves between two plates, maintained at a p.d of 1V. What is the energy acquired by the charge?
Answer: W=QV=2×1=2J 

Question. Why are copper wires used as connecting wires?
Answer: The electrical resistivity of copper is low.

Question. A wire of resistivity ρ is stretched to double its length. What is its new resistivity?
Answer: It remains same because resistivity depends on nature of material.

Question. What is the resistance of connecting wire?
Answer: The resistance of a connecting wire, which is made of good conductor, is negligible.

Question. What is the resistance of an ammeter?
Answer: The resistance of an ammeter is very small and for an ideal ammeter, its value is zero.

Question. What is the resistance of a Voltmeter?
Answer: The resistance of a voltmeter is very high and for an ideal voltmeter, its value is infinity.

Question. Which has more resistance: 100W bulb or 60W bulb?
Answer: As Rα 1/P.Thus, the resistance of 60W bulb is more.

Question. How will you join three resistances, each of 2Ω so that the effective resistance is 3Ω?
Answer: A parallel combination of two resistances (which will be 1Ω) joined in series with the third resistance (2Ω)

Question. What happens to the current in a circuit if its resistance is doubled?
Answer: As I α 1/R, the current is reduced to half of its previous value.

Question. What happens to the resistance of a circuit if the current through it is doubled?
Answer: The resistance of the circuit does not depend on the current through it.

Question. How does the resistance of a wire depend upon its radius?
Answer: As R α 1/A, R α 1/πr² i.e. R α 1/r².

Question. Two wires are of the same length, same radius, but one of them is of copper and the other is of iron. Which will have more resistance?
Answer: As R = ρl/A, but A and l are same it depends only on resistivity and it is more for iron so iron has more resistance.

Question. Two wires of same material and same length have radii r₁ and r₂. Compare their resistances.
Answer: If R₁ and R₂ are resistances, then R1/R2 = r₂²/r₁² because ρ and l are same.

Question. Given a resistors each of resistors R. How will you combine them to get the (i) maximum and (ii) minimum effective resistance? What is the ratio of the maximum to minimum resistance?
Answer: for maximum resistance Rs= nr (Equivalent of series combination) for minimum resistance Rp= r/n (Equivalent of parallel combination) Rs/Rp= n²

Question. A wire of length L and resistance R is stretched so that its length its doubled.
How will its (a) Resistance change (b) Resistively change ?
Answer: (a) If the original length of the wire is l and its cross-sectional area is A, then
R = ρl/A.When length becomes 2l, cross-sectional area reduces to A/2 because
volume does not change. The new resistance =ρ (2l)/A/2=4 ρl/A = 4R
(b) Resistively does not change.

Question. Two students perform the experiments on series and parallel combinations of two given resistors R1 and R2 and plot the following V-I graphs.  

Which of the graphs is (are) correctly labelled in terms of the words ‘series’ and parallel’ Justify your answer.
Answer: Both are correct because ΔV/ΔI= resistance(R) and ΔI/ΔV=1/R Series means high resistance and parallel means low resistance.

Question. A household uses the following electric appliances :
(i) Refrigerator of rating 400W for ten hours each day.
(ii) Two electric fans of rating 80W each for twelve hours each day.
(iii) Six electric tubes of rating 18W each for 6 hours each day.
Calculate the electricity bill of the household for the month of June if the cost per unit of electric energy is Rs. 3.00.
Answer: Electrical energy consumed per day = 400×10+2×80×12+6×18×6
=4000+1920+648
= 6568 Wh
= 6.568 kWh
Electrical energy consumed in 30 days= 6.568×30
= 197 kWh (units)
E lectric Bill = 197×3
= Rs 591.

Question. Ammeter burns out when connected in parallel. Give reasons.
Answer: Ammeter consists of a wire of low resistance when connected in parallel, a large amount of current passes through it hence gets burnt i.e. short circuited.

More Questions for Practice

Q.1 Name a substance whose resistance almost remains unchanged by increase of temperature.

Q.2 Name two special characteristics of heater coil.

Q.3 A wire of resistance 4Ω is bent to form a circle. What is the resistance between
two diametrically opposite ends ?

Q.4 How does the resistance of a conductor change if its temperature is increased?

Q.5 A current of 4A flows in a wire of resistance 60Ω.Calculate electrical energy consumed in 2 minutes.

Q.6 V-I graph for two resistors is given. Which of the two has minimum resistance? 

Q.7 Alloys are used in electrical heating devices rather than pure metals. Give one reason.

Q.8 An electric geyser has the ratings 2000W, 220V marked on it. What should be the m inimum rating, in whole number of a fuse wire that may be required for safe use with this geyser?

Q.9 The electrical resistivity of few materials is given below in ohm-meter. Which of these materials can be used for making element of a heating device?
A 6.84 x 10^-8
B 1.60 x 10^-8
C 1.00 x 10^-4
D 2.50 x 10¹²
E 4.40 x 10^-5
F 2.30 x 10¹⁷

Q.10 Where do we connect a fuse: with live wire or with neutral wire?

Q.11 What is the resistance of an air gap?

Q.12 Name two safety measures commonly used in electric circuits and appliances.

Q.13 Two metallic wires A and B are connected in parallel. Wire A has length l and radius r, wire B has a length 2l and radius 2r. Compute the ratio of the total resistance
of parallel combination and the resistance of wire A.

Q.14 What is the meaning of the term ‘frequency’ of an alternating current? What is its value in India? Why is an alternating current considered to be advantageous over
direct current for long-range transmission of electric energy?

Q.15 A TV set picture tube shoots out a beam of electrons. The current due to this beam is 10 mA. How many electrons will strike the TV screen every second?

Q. 16 An electric wire is stretched to increase its length by 25%.By what % will the resistance be increased and what will be increase in its resistivity?

Q.17 An electric iron of resistance 20Ω takes a current of 5 A. Calculate the heat developed in 30 sec.

Q.18 A 60 W electric lamp gives off energy in the form of light at the rate of 7.5 J/s.
What percentage of energy does the lamp transform into light?

Q.19 The voltage-current variation of two metallic wires A and B at constant temperature are shown in fig. Assuming that the wires have the same length and same diameter, explain which of the two wires will have larger resistivity. 

Q.20 You are given following current-time graphs from two different sources: 

(i) Name the type of current in two cases.
(ii) Identify any one source for each type of these currents.
(iii) What is the frequency of current in case II in India?

Q.21 The electric power consumed by a device may be calculated by using either of the two expressions P = I²R or P = V²/R . The first expression indicates that it is
directly proportional to R whereas the second expression indicates inverse proportionality. How can the seemingly different dependence of P on R in these expressions be explained.

Q.22. Draw a schematic diagram of a circuit containing the following electrical components:(a) a resistance (b) a voltmeter (c) an electric bulb (d) a cell (e) an ammeter and (f) plug key

1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.

2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 10⁹ N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10^-19 coulomb.

3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.

4. Conductor: A substance which allows passage of electric charges through it easily is called a  'conductor' . A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.

5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an  'insulator‘. For example rubber, glass, plastic, ebonite etc.

6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.

Electric current = Charge/Time or I = Q/t Electric current is a scalar quantity.

7. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere. 1 ampere = 1 coulomb/1 second or 1 A = 1C/1s = 1Cs^-1 1 milliampere = 1 mA =10^-3 A 1 microampere = 1μA = 10^-6 A

8. Electric circuit: The closed path along which electric current flows is called an 'electric circuit‘.

9. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.

10. Electric field: It is the region around a charged body within which its influence can be experienced.

11. Electrostatic potential: Electrostatic potential at any point in an electric field is defined as the amount of work done in bringing a unit positive charge from infinity to that point. Its unit is volt. Positive charges move from higher to lower potential regions. Electrons, being negatively charged, move from lower to higher potential regions.

12. Potential difference between two points: The Potential difference between two points in an electric field is the amount of work done in bringing a unit positive charge from one to another. Potential difference = Work done/Charge or V = W/Q

13. One volt potential difference: The Potential difference between two points in an electric field is said to one volt if one joule of work has to be done in bringing a positive charge of one coulomb from one point to another. 1 volt = 1 joule/1 coulomb or 1 V = 1J/1C

14. Galvanometer: It is device to detect current in an electric circuit.

15. Ammeter: It is device to measure current in a circuit. It is always connected in series in a circuit.

16. Voltmeter: It is a device to measure potential difference. It is always connected in parallel to the component across which the potential difference is to be measured.

17. Ohm’s law: This law states that the current passing through a conductor is directly proportional to the potential difference cross its ends, provided the physical conditions like temperature, density etc. remains unchanged.

V α I or V = RI

The proportionality constant R is called resistance of conductor.

18. Resistance: It is a property of a conductor by virtue of which it opposes the flow of current through it. It is equal to the ratio of the potential difference applied across its ends and the current flowing through it. Resistance = Potential difference/Current or R = V/I

19. Ohm: It is the S.I. unit of resistance. A conductor has a resistance of one ohm if a current of one ampere flows through it on applying a potential difference of one volt across its ends. 1 ohm = 1 volt/1 ampere or 1Ω = 1V/1A

20. Factors on which resistance of a conductor depends: The resistance R of a conductor depends

i) Directly on its length L i.e. R α L.

ii) inversely on its area of cross-section A i.e. R α 1/A

iii) on the nature of material of the conductor on. On combining the above factors, we get R α L/A

R = ρ * L/A The proportionality constant ρ is called resistivity of conductor.

21. Resistivity: It is defined as the resistance offered by a cube of a material of side 1 m when current flows perpendicular to its opposite faces. Its S.I. unit is ohm-meter (Ωm).

Resistivity, ρ = RA/L

22. Equivalent resistance: If a single resistance can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, then that single resistance is called the equivalent resistance.

23. Laws of resistances in series: i) Current through each resistance is same.

ii) Total voltage across the combination = Sum of the voltage drops.

V= V₁ + V₂ + V₃

iii) Voltage drops across any resistor is proportional to its resistance.

V1 = IR₁, V₂ = IR₂, V₃ = IR₃

iv) Equivalent resistance = Sum of the individual resistances.

Rs = R₁ + R₂ + R₃

v) Equivalent resistance is larger than the largest individual resistance.

Laws of resistances in parallel:

i) Voltage across each resistance is same and is equal to the applied voltage.

ii) Total current = Sum of the currents through the individual resistances.

I = I₁ + I₂ + I₃

iii) Currents through various resistances are inversely proportional to the individual

resistances. I1 = V/R₁, I₂ = V/R₂, I₃ = V/R₃

iv) Reciprocal of equivalent resistance = Sum of reciprocals of individual resistances.

1/Rp = 1/R₁ + 1/R₂ + 1/R₃

v) Equivalent resistance is less than the smallest individual resistance.

24. Joule’s law of heating: It states that the heat produced in a conductor is directly proportional to

(i) the square of the current I through it

(ii) proportional to its resistances R and

(iii) the time t for which current is passed. Mathematically, it can be expressed as

H = I²Rt joule = I²Rt/4.18 cal

           or

H = VIt joule = VIt/4.18cal

25. Electric energy: It is the total work done in maintaining an electric current in an electric circuit for given time. Electric energy, W = VIt = I²Rt joule

26. Electrical power: Electrical power is the rate at which electric energy is consumed by an appliance.

P = W/t = VI = I²R = V²/R

27. Watt: It is the S.I. unit of power. The power of an appliance is 1 watt if one ampere of current flows through it on applying a potential differences of 1 volt across its ends.

1 watt = 1 joule/1 second =1 volt x 1 ampere or 1 W = 1 Js^-1 = 1 VA

1 kilowatt = 1000 W            1Mega watt= 10⁶ watt             1Gigawatt=10⁹ watt

28. Kilowatt hour: It is the commercial unit of electrical energy. One kilowatt hour is the electric energy consumed by an appliance of 1000 watts when used for one hour.

1 kilowatt hour (kWh) = 3.6 x 10⁶ J

SUGGESTED ACTIVITIES

1. To verify ohm‘s law.

2. To observe the flow of current is different for different components.

3. To study the factors on which the resistance of conducting wire depends.

4. Resistors in series and resistors in parallel

HOTS QUESTIONS (SOLVED / UNSOLVED)

Question. Why is the tungsten metal more coiled in the bulb and not installed in straight parallel wire form?
Answer: The coiled wire of tungsten increases the surface area of the wire in very less space so as to emit more light and helps in glowing with more intensity.

Question. Why are fairy decorative lights always connected in parallel?
Answer: When the fairy lights are connected in series the resistance offered will be greater and brightness of the bulbs will be affected. But in parallel connection all the bulbs will glow with same intensity and if any more bulbs gets fused the other bulbs will continue to glow.

Question. What will happen when -
a) Voltmeter is connected in series?
b) Ammeter is connected in parallel?
Answer: a) Negligible current will pass through the circuit because the voltmeter has a very high resistance.
b) Ammeter will get damaged due to flow of large amount of current through it, because it has low resistance.

1. A hot plate of an electric oven connected to a 220 volt line has two resistance coil A and B each of 24 ohm resistance which may be used separately in parallel or series (I)What will be the power consumed in each case?
(ii) In each of two cases, find the rating of the fuse required?

2. A wire of length l, area A is made in to a wire of double its length by -
(I) attaching a similar wire.
(ii) Melting the existing wire
If the original resistivity is p, what will be the new resistivity and resistance?

3. Two metallic wires A and B of same material are connected in parallel. Wire A has length (l) and radius (r) and wire B has length (2l) and radius 2r. Compute the ratio of the total resistance of parallel combination and the resistance of wire A.

4.. Two wires are of the same length, same radius, but one of them is of copper and the other is of iron. Which will have more resistance?

5. Two wires of same material and same length have radii r1 and r2. Compare their resistances.

6. A wire of length L and resistance R is stretched so that its length is doubled. How will its (a) Resistance change (b) Resistivity change?

7. You are given following current-time graphs from two different sources 

a.. Name the type of current in two cases.
b.. Identify any one source for each type of these currents.
c.. What is the frequency of current in case (ii) in India?
Use above graphs to write two points of difference between the current in two cases.

8. The electric power consumed by a device may be calculated using either of the two expressions P = I^2 * R or P = V^2/R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality.
How can the seemingly different dependence of P on R in these expressions be explained?

9. A battery of 9 v is connected in series with resistance of0.2Ω, 0.3Ω, 0.4Ω, 0.5Ωand 12Ω respectively. How much current would flow through the 12Ω resistor?

Current in a wire is due to the flow of electrons in the wire. Although the drift speeds of electrons in the wire are very small, yet an electric bulb lights up as soon as the switch is on. Explain. Why?

11. Electrical resistivity of some substances at 20°C are given below:
Silver 1· 60 x 10^-8 Q m
Tungsten 5·2 X 10^-8 Q m
Iron 10·0 x 10^-8 Q m
Mercury 94·0 x 10^-8 Q m
Nichrome 100 x 10^-6 Q m
Answer the following questions in relation to them:
(i) Among silver and copper, which one is a better conductor? Why?
(ii)Which material would you advise to be used in electrical heating devices? Why?

12. If the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half its former value,
what change will occur in the current through it?

13. Figure a, b and c show three cylindrical conductors along with their face areas and length. Which geometric figure will have highest resistance?