Maharashtra Board Class 9 Science Chapter 3 Current Electricity PDF Download

3. Current Electricity

Electricity is of utmost importance in the modern world. We depend on it for almost everything in our day to day life. In order to avoid the inconvenience faced due to failure of power supply, hospitals, banks, offices and private institutions make alternative arrangements with the help of generators. Electricity is used for running electric furnaces, electric motors and several other instruments used in industries.

Domestic appliances like the fridge, electric oven, mixer, fans, washing machines, vacuum cleaner, rotimaker, etc. have helped us by saving time and labour. All these devices cannot be run without electricity.

Not only human beings but some animals also use electricity. For example, fishes such as eels use electricity to catch their prey and also for self-defence. The lightning that strikes the earth is an excellent example of natural flow of electricity. What if we could collect and store this electricity!

You must have seen a waterfall. Which way does the water flow?

For the generation of electricity, water is released from a dam at a higher level and because of gravity, it falls to a lower level. Thus, as we know, the direction of flow of water between two points depends on the level of the two points.

Potential And Potential Difference

Equipment: Two plastic bottles, rubber tube, clamp, water.

Procedure: Set up the experiment as shown in figure 3.1. Then remove the clamp from the rubber tube. Note your observations. Answer the following questions.

1. What happens when the clamp is removed?

2. Does the water stop flowing? Why?

3. What will you do to keep the water flowing for a longer duration?

Just like water, the flow of electric charge between two points depends on a kind of electric level at those points. This level is called electric potential.

Teacher's Note

Electricity works like water from a tap. Water flows from high to low. Current also flows from high to low. You can see this when you use your phone charger at home.

Exam Trick

Remember: Water flows downhill. Current flows from high potential to low potential. Just like water always flows down, current always flows from positive to negative.

Points To Remember

Electric potential is like the height of water in a tank.
Water flows from high level to low level.
Electric current flows from high potential to low potential.
Potential difference is the difference between two levels.
A battery creates this difference to make current flow.

Work has to be done against the electric field to take a positive charge from a point of lower potential to a point of higher potential.

Potential Difference Of A Cell

The difference in potential between the positive and negative terminals of a cell is the potential difference of that cell. This potential difference is caused by chemical reactions occurring inside the cell. The potential difference sets the electrons in motion and results in the flow of electricity through a conducting wire connected to the two ends of the cell.

The amount of work done to carry a unit positive charge from point A to point B is called the electric potential difference between the two points.

Potential difference between two points = \[\frac{W}{Q}\], \[V = \frac{W}{Q}\]

A positive charge flows from a point of higher potential to a point of lower potential. We have seen earlier that electricity flows due to the conduction of negatively charged electrons. Electrons flow from the point of lower potential to a point of higher potential. A lightning strike is the flow of electrons from point of lower (negative) potential on the clouds to the point of higher (zero) potential on the earth. We shall study the definition of electric potential in higher standards.

The difference between the values of potentials at two points A and B is called the potential difference between them.

In the figure 3.2, conductor A is at a higher potential than conductor B. When these two are connected by a conducting wire, a potential difference is created between its two ends and electrons will flow from B to A through the wire. This flow will continue until the two conductors, A and B have the same potential, i.e. until their potential difference becomes zero. Only then will the flow of electrons stop.

The unit of potential difference in SI system is volt.

\[1 V = \frac{1J}{1C}\]

Teacher's Note

Potential difference is like the pressure difference that makes water flow. In your home, the electrical pressure is 220 volts. This makes the current flow to your appliances.

Exam Trick

Remember: 1 Volt = 1 Joule per 1 Coulomb. Think of volt as the push that makes charges move. More volts = more push = more current flows.

Points To Remember

Potential difference is the work done on one coulomb of charge.
The unit of potential difference is volt (V).
Higher potential difference means more force on charges.
Electrons move from low potential to high potential.
A battery creates the potential difference in a circuit.

Very small values of potential difference are expressed in the following units.

1. 1 mV (millivolt) = \[10^{-3}\] V

2. 1 μV (microvolt) = \[10^{-6}\] V

Large values of potential difference are expressed in the following units.

1. 1 kV (kilovolt) = \[10^{3}\] V

2. 1 MV (megavolt) = \[10^{6}\] V

Free Electrons

Every atom of a metallic conductor has one or more outermost electrons which are very weakly bound to the nucleus. These are called free electrons.

As shown in figure 3.3, these electrons can easily move from one part of a conductor to its other parts. The negative charge of the electrons also gets transferred as a result of this motion. The free electrons in a conductor are the carriers of negative charge.

Volta's Simple Electric Cell

The Italian scientist Alessandro Volta constructed the first electric cell. The unit of potential difference is named 'volt' in his honour.

Teacher's Note

Free electrons are like tiny beads that can roll freely inside a metal wire. When you switch on a light, these electrons start moving and create current. Think of them like cars on a highway moving together.

Exam Trick

Remember: Free electrons = electrons that can move freely in metals. Metals have many free electrons. Non-metals have few free electrons. This is why metals are good conductors.

Points To Remember

Free electrons are weakly bound to the nucleus.
They can move easily from atom to atom in a metal.
These electrons carry the negative charge in a conductor.
More free electrons = better conductor of electricity.
Copper has more free electrons than iron.

Current Flowing Through A Wire

As shown in the figure 3.4 A, if a conducting wire is not connected to a cell, its free electrons move randomly in all directions in the space between the atoms. When we connect the ends of the wire to the two terminals of a cell, electric force acts on the electrons. Being negatively charged, they start moving from the negative (lower potential) to the positive (higher potential) terminal of the cell, as shown in figure 3.4 B. Due to the flow of these electrons, current starts to flow through the wire. This motion of electrons is irregular but there is a definite, non-zero value to their average velocity.

Even though, electrons move from negative end to positive end, conventionally, the direction of current flow is taken to be opposite of the direction of flow of electrons i.e. from the +ve end to the –ve end of a cell.

Electric Current

An electric current is the flow of electrons through a conductor. Quantitatively, current (I) is defined as the charge passing through a conductor in unit time.

If charge Q is flowing through cross-section of a conductor in time t then the

\[current = I = \frac{Q}{t}\]

\[1A = \frac{1C}{1s}\]

The unit of charge in SI units is Coulomb (C). Current is expressed in Ampere (A). The charge of one electron is \[1.6 \times 10^{-19}\] C.

Ampere: One ampere current is said to flow in a conductor if one Coulomb charge flows through it every second.

Very small values of current are expressed in the following units.

1. 1 mA (mili ampere) = \[10^{-3}\] A

2. 1 μA (micro ampere) = \[10^{-6}\] A

The French mathematician and scientist, Ampere, conducted a number of experiments on electricity. Today, we can measure the current flowing in a conductor only because of his extraordinary work. The unit of current is called ampere in recognition of his great work.

Example: A current of 0.4 A flows through a conductor for 5 minutes. How much charge would have passed through the conductor?

Given: I = 0.4 A

t = 5 min = 5 × 60 s = 300 s

Formula: Q = I × t

Q = 0.4 A × 300 s

Q = 120 C.

Therefore, Charge passing through the conductor = 120 C

Teacher's Note

Current is like the flow of water in a pipe. More water flowing = more current. If you turn on more taps, more water flows. Similarly, if you connect more appliances, more current flows.

Exam Trick

Remember: Current = Charge ÷ Time. If more charge passes in less time, current is high. Think of it like counting how many students pass through a door per minute.

Points To Remember

Current is the flow of electrons in a conductor.
Current is measured in Amperes (A).
One Ampere = 1 Coulomb per 1 second.
The formula is I = Q/t.
Electrons actually move from negative to positive inside a wire.

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