NCERT Solutions Class 8 Science Chapter 14 Chemical Effects of Electric Current

NCERT Solutions for Class 8 Science for chapter 14 Chemical Effects of Electric Current

Question. The light bulbs shown in the diagram below are connected in a series circuit. The bulbs do not light up, apparently because one of the bulbs is fused. How can it be determined which bulb(s)  is/are  fused?

(a) Put the switch on and substitute with a new bulb, one by one, until all three bulbs light up.
(b) Put the switch on and notice which bulb does not light up.
(c) Put the switch on and remove one bulb at a time until the other two light up.
(d) There is no way to determine which bulb is burned out when they are connected in series.

Answer : D

Question. Some electric devices like air conditioners (AC) use plugs which are bigger in size than those used by radios, televisions (TV), etc. Why is this?
(a) Devices like AC's require higher voltage.
(b) Devices like AC's draw higher current.
(c) Devices like AC's require greater protection from electric shock.
(d) There is no reason for this - the difference is only in appearance.

Answer : B

Question. If you reverse the direction of rotation of an air cooler's fan, what will happen?
(a) Nothing. It will continue to work like before.
(b) It will stop working
(c) It will work like an exhaust and pull out air.
(d) Its motor will burn out.

Answer : C

Question. Which switch A, B, C or D MUST be open to ensure that no bulb will glow

(a) A
(b) B
(c) C
(d) D

Answer : A

Question. See the diagram of an electric bell given below. Now arrange the steps that describe its functioning in the correct order.
1.    The switch is closed and the current flows in the electromagnetic coil
2.    The spring returns the iron shaft to its original position
3.    3. The iron shaft is attracted to the electromagnetic coil and strikes the bell 
4.    The contacts close the circuit causing electricity to flow again
5.    As the iron shaft moves towards the bell, contact is broken

(a) 3, 4, 5, 2, 1
(b) 1, 2, 3, 4, 5
(c) 2, 4, 5, 3, 1
(d) 1, 3, 5, 2, 4

Answer : D

Question. Study this circuit which uses two 2-way switches. Unlike a regular switch, a 2-way switch is connected to a circuit even when it is in the 'off' position.
The special feature of this circuit is that:

(a) The bulb can be switched on only if both the switches are on.
(b) The bulb can be switched on by any one of the two switches.
(c) One switch can only turn the bulb on, and the other can only turn it off.
(d) Multiple bulbs can be controlled through multiple switches.

Answer : B

Question. Study the simple circuit shown here. A compass is placed near the circuit as shown. Which of these statements is TRUE?

(a) If the battery terminals are connected the other way, the compass needle will deflect in the other direction.
(b) The bulb will light up only when the battery terminals are connected in one particular way, not the other.
(c) There can be situations when the compass needle may deflect, but the bulb does not light up or vice versa.
(d) The extent of deflection of the compass needle will change if it moved along the wire from battery to bulb

Answer : A

Question. Raju has two batteries. One is a 9V battery and the one is a regular 1.5V pencil cell. Each battery has a + and - noted on it as shown. How can Raju connect the two batteries together in a simple circuit?

Answer : C

Question. While repairing a fan, a trainee electrician connects the wires wrongly. When the fan is switched on, it rotates at the proper speed, but in the OPPOSITE direction. What will happen as a result of this?

(a) The fan will work for sometime and then stop.
(b) The fan will consume more power.
(c) The fan will not throw down any air.
(d) The fan will work perfectly.

Answer : C

Question. A switch controls whether electricity flows in a circuit or not. In the figure given below, S1, S2, S3 and S4 are switches. Under what case will the bulb glow?

(a) S1, S2, S3 are closed and S4 is open
(b) S1, S2, are closed and S3,  S4 are open
(c) S1,  S3 are closed and S2, S4 is open
(d) S1, S4 are closed; S2, S3 are open

Answer : D

Question. A train with a diesel engine is not connected to any external source of electricity while it is moving. What is the MAIN source of the electricity on which the fans and lights in the coaches run?
(a) The train draws power from electrical wires overhead.
(b) The train draws power through the tracks.
(c) The train generates power from its motion using a generator.
(d) The train carries a large battery that provides electricity during its journey.

Answer : C

Question. Electromagnets are created by coiling a wire that conducts electricity around a piece of iron and then passing a current through the wire. Below are the results of an investigation to determine the relationship between the number of coils and the strength of an electromagnet. What do the results of this experiment suggest?

Number of coils	Number of metal screws picked up by the magnet
5 coils	1.0
10 coils	3.0
20 coils	5.7

(a) The strength of the electromagnet increases when the current increases.
(b) Increasing the number of coils of an electromagnet increases its current.
(c) Increasing the number of coils of an electromagnet increases its strength.
(d) The strength of an electromagnet does not depend on the number of its coils.

Answer : C

Question. Which of the following shows a magnet being attracted to another magnet?

Answer : B

Question. A bar magnet is shown along with 4 compasses placed near it. If there are no magnetic substances near it, a compass needle will point north. Which of the arrows in the options below is pointing towards North?

Answer : A

Question. In the solar panel shown here, one form of energy is converted into another.
An example of a device in which the energy conversion is OPPOSITE to that of the solar panel is -

(a) a candle
(b) a radio
(c) a tube light
(d) a steam engine

Answer : C

Question. A regular telephone neither uses batteries, nor is it connected to the electric supply. From where does it get the energy (electricity) required for its working?
(a) It does not require any energy for its working.
(b) It gets energy through the regular telephone wires.
(c) It gets energy every time someone speaks into it.
(d) It gets energy every time someone dials a number.

Answer : B

Question. In a car battery, plates of lead and lead dioxide are immersed in a liquid, L. A chemical reaction occurs leading to the production of lead sulphate and water. What kind of a liquid is L likely to be?
(a) An acid
(b) A base
(c) Salt solution
(d) Distilled water

Answer : A

Question. A car headlight bulb is marked 48 W 12 V. Can this bulb be used at home in place of a regular electric bulb?
(a) Yes, it can. The bulb will glow much brighter.
(b) Yes, it can. But the bulb will glow less brightly.
(c) No, it cannot. The bulb will burn out.
(d) No, it cannot. The bulb will not fit into the holder.

Answer : C

Question. Given below is a graph showing the Current (I) versus the Voltage (V) in two different electrical circuits. If the Resistance (R) of a circuit is given by:  
R = V/I.  What VALID conclusion can be drawn from this graph?

(a) For both the circuits the resistance is the same.
(b) The resistance of 'X' is more than that of 'Y'.
(c) The resistance of 'Y' is more than that of 'X'.
(d) Resistance increases as current is increased.

Answer : C

Question. A circuit has five bulbs connected to a battery. Initially they all glow. After some time one of them fuses, but the other four continue to glow. Which of the following circuits could it be?

Answer : B

Question. If a non-magnetic object is inserted, what will be the effect of the electromagnet on it?

(a) It will slow it down more than the 'correct' coin.
(b) It will slow it down, but less than the 'correct' coin.
(c) It will make it move faster than its original speed.
(d) It will neither slow it down nor make it move faster.

Answer : D

Question. In a vending machine based on this design, the strength of the electromagnet can be varied. What benefit does this design possibly offer?

(a) The machine can be used with non-metallic coins, if required.
(b) The machine can be used with different coins, if required.
(c) The machine can be used on battery, rather than electricity.
(d) The machine can be easily repaired if something goes wrong.

Answer : B

Question. Srihari has a torch with him and 2 new batteries. However, the torch requires 3 batteries to work. Is there any way Srihari can get the torch to glow?

Answer : A

Question. Consider the comparison shown here between a regular 60 W bulb and a new type of bulb that has been introduced What would be the advantage of using the new type of bulb?

(a) The new bulb will be brighter.
(b) The power consumed will be less.
(c) The new bulb is cheaper to buy.
(d) All the above are true.

Answer : B

Question. Five friends, Lata, Mala, Shaila, Rohit and Ali each tried out two of the five different brands of batteries to check which brand lasted the longest. The data collected is shown below.
If they had to choose the brand with the longest expected life based on the above experiments, they should choose

(a) Brand P
(b) Brand Q
(c) Brand R
(d) Brand T

Answer : B

Question. Which BEST describes the surface of the Earth over billions of years?
(a) A flat surface is gradually pushed up into higher and higher mountains until it is covered with mountains.
(b) High mountains gradually wear down until most of the Earth is at sea level.
(c) High mountains gradually wear down as new mountains are continuously being formed, over and over again.
(d) High mountains and flat plains stay side by side for billions of years with little change.

Answer : C

Question. This pie-chart shows the percentage composition by weight of the elements of the earth's crust. The two elements that are  present the most on the earth's crust are-

(a) Silicon and Oxygen
(b) Oxygen and Iron
(c) Aluminium and Iron
(d) Silicon and Iron

Answer : A

Question. The diagram here shows a mountain of height 2000 metres. The direction of the wind and some air temperatures are indicated. From this information, it is very likely that the region at X will be a

(a) dry region
(b) Jungle
(c) large lake
(d) Plateau

Answer : A

Question. Contour lines on a map connect points on the surface that have the same elevation (height) above sea level.
What can be said about the flow of the river in this map?

(a) It flows in a south-east direction
(b) It flows in a north-west direction
(c) It flows in a south-west direction
(d) We cannot be sure of the flow direction.

Answer : A

Question. See the map showing present day South America and Africa. Which of the following provides the most compelling scientific evidence that the two continents were probably joined in the past?

(a) The shapes of the continents fit quite well together like a jigsaw puzzle.
(b) It is known that other continents like Asia and Australia were joined in the past.
(c)  Fossils of the same age of extinct animals have been found in both continents.
(d) The present day climatic conditions in both continents are very similar.

Answer : C
 

1. Fill in the blanks.

(a) Most liquids that conduct electricity are solutions of ______,  _____and ____.

(b) The passage of an electric current through a solution causes   ______effects.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ____ terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called _____.

Answer:

(a) Most liquids that conduct electricity are solutions of acids, bases and salts.

Explanation: The solutions of acids, bases or salts can conduct electricity.

(b) The passage of an electric current through a solution causes chemical effects.

Explanation: The solution decomposes into positive and negative ions when electric current passes through it.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the negative terminal of the battery.

Explanation: On passing an electric current through a copper sulphate solution, the solution decomposes into positively charged copper ions and negatively charged sulphate ions. The positively charged copper ions get attracted towards the plate which is connected to the negative terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.

2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Answer: The deflection in the compass needle proves that current is flowing through the circuit. It shows that the solution is a conducting solution.

3. Name three liquids, which when tested in the manner shown in Fig. 14.9, may cause the magnetic needle to deflect.

Answer: The liquids such as lemon juice, salt water and vegetable oil allow electricity to pass through them. Therefore, these liquids can be used in the beaker shown here.

4. The bulb does not glow in the setup shown in figur List the possible reasons. Explain your answer.

Answer: There may be two reasons for this.
(i) The battery connected in the circuit has insufficient energy to generate electricity.

(ii) Liquid kept in the beaker is non-conducting. In this case, current would not be able to pass through the liquid.

5. A tester is used to check the conduction of electricity through two liquids, labelled A and It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that

(i) liquid A is a better conductor than liquid B.
(ii) liquid B is a better conductor than liquid A.
(iii) both liquids are equally conducting.
(iv) conducting properties of liquid cannot be compared in this manner.

Answer: The correct option is (i).

Explanation: The magnitude of current flowing through a conducting solution depends on the conductivity of the solution. The solution having more conductivity will allow more current to pass through it.

6. Does pure water conduct electricity? If not, what can we do to make it conducting?

Answer: No. Pure water does not conduct electricity because it does not contain salt. When some common salt is added to the pure water, it can conduct electricity.

7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Answer: Water can act as a conductor of electricity. In case of a fire, if the electrical supply for the area is not shut off and water is poured over electrical appliances, then the fireman can get electric shocks.

8. A child staying in a coastal region tests the drinking water and also the seawater with his test He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Answer: Sea water is more salty than the drinking water. Therefore, it is more conducting than the drinking water. Due to this, the compass needle deflects more in the case of seawater than in the drinking water.

9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Answer: No. During heavy downpour, it is not safe to repair electrical appliances outdoors. Rain water can conduct electricity as it contains dissolved salts. Hence, the electrician may get electrical shocks while working outdoors during rain.

10. Paheli had heard that rain water is as good as distilled wate So, she collected some rain water in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the reasons?

Answer: Rain water acts as conducting solution as it contains dissolved salts. On the other hand, there are no dissolved salts present in the distilled water. Therefore, rain water can allow electricity to pass through it.

11. Prepare a list of objects around you that are electroplated.

Answer:

(i) In gold-plated ornaments, a fine layer of gold is deposited on the silver ornaments.

(ii) Chromium plating is done on different parts of vehicles to make them shiny.

(iii) To protect iron from corrosion, iron is coated with zinc layer.

12. The process that you saw in Activity 7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?

Answer: Copper ions have positive charges. Hence, they are attracted towards the plate which is connected to the negative terminal of the battery. As copper ions are transferred to the thin copper plate, this thin pure copper plate should be connected to the negative terminal of the battery. As a result, impure copper rod should be connected to the positive terminal of the battery.