Class 10 Science Chapter 11 Electricity NCERT Notes

Chapter 11 Electricity Class 10 Science CBSE NCERT Notes

Electric current, electric circuit, voltage or electric potential, resistance and (Ohm’s law).

Electric Current

The flow of electric charge is known as Electric Current, Electric current is carried by moving electrons through a conductor. By convention, electric current flows in the opposite direction to the movement of electrons.

Electric Circuit

Electric circuit is a continuous and closed path of electric current.

Expression of Electric Current

Electric current is denoted by the letter ‘I’. Electric current is expressed by the rate of flow of electric charges. Rate of flow means, the amount of charge flowing through a particular area in unit time.

If a net electric charge (Q) flows through a cross-section of a conductor in time t, then,
Electric Current (I) = Net Charge (Q)/Time (t) or,
I = Q/t

Where I is electric current, Q is a net charge and t is a time in second.

S.I. Unit of Electric Charge and Current: S.I. unit of electric charge is coulomb (C).

One coulomb is nearly equal to 6 x 1018 electrons. S.I. unit of electric current is ampere (A). Ampere is the flow of electric charge through a surface at the rate of one coulomb per second. This means, if 1 coulomb of electric charge flows through a cross section for 1 second, it would be equal to 1 ampere.
Therefore, 1 A = 1C/1sec

Small Quantity of Electric Current

Small quantity of electric current is expressed in milliampere and microampere. Milliampere is written as mA and microampere as pA.
1 mA (milliampere) = 10^-3 A
1 pA (microampere) = 10^-6 A

Ammeter: An apparatus to measure electric current in a circuit.

Charge: Like mass, the charge is the fundamental property of matter. There are two types of charge:

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

Direction of Electric Current

In circuits using metallic wires, electrons constitute the flow of electric charges. However, electrons were not known at the time when the phenomenon of electricity was first observed. So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current.

Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.

It should be remembered that in external circuit, current always flows from positive terminal of the cell to the negative terminal of that energy source.

Ammeter

Current is measured by an instrument called ammeter. An ammeter connected in series. Since the entire current passes through the ammeter, therefore, an ammeter should have very low resistance so that it may not change the value of the current flowing in the circuit.

Electric Field

The region or space surrounding the charge where another charge experiences a force of attraction or repulsion depending upon the nature of charge is called electric field or electrostatic field around that change.

Electric Potential

Electric potential at a point in an electric field is equal to the work required to transfer a unit positive charge from an infinite distance to a given point against an electric field. It is commonly known as voltage. Electric potential (V) at any point in the electric field is given by

V = Work done (W)/Charge (Q)

SI unit of electric potential is volt (V), named in honour of the Italian physicist, Alessandro Volta (1745-1827).

1V = 1 JC-1

Electric potential is a scalar quantity.

Potential Difference

Electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other.

Potential difference (V) between two points = Work done (W)/Charge (Q)
V = W/Q

The SI unit of electric potential difference is volt (V).

One volt is the potential difference between two points in a current carrying conductor when 1 Joule of work is done to move a charge of 1 coulomb from one point to the other.
Therefore,
1 volt = 1 joule/1 coulomb

The potential difference is measured by means of an instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

Circuit Diagram

Ohm’s Law

According to Ohm’s law: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

Current, I = V/R

The current is directly proportional to potential difference. The current is inversely proportional to resistance.

Resistance of a Conductor

The property of a conductor due to which it opposes the flow of current through it is called resistance. The resistance of a conductor depends on length, thickness, nature of material and temperature, of the conductor.

The SI unit of resistance is ohm.

1 Ohm: 1 ohm (Ω) of resistance (R) is equal to the flow 1A of current through a conductor between two points having a potential difference equal to 1V.
This means; 1Ω = 1V1AΩ1 Ohm: 1 ohm (Ω) of resistance (R) is equal to the flow 1A of current through a conductor between two points having a potential difference equal to 1V.
This means; 1Ω = 1V1A

From the expression of Ohm’s Law, it is obvious that electric current through a resistor is inversely proportional to resistance. This means electric current will decrease with an increase in resistance and vice versa. The graph of V (potential difference) versus I (electric current) is always a straight line.

Resistance

Resistance is a property of conductor due to which it resists the flow of electric current through it. A component that is used to resist the flow of electric current in a circuit is called a resistor. In practical application, resistors are used to increase or decrease the electric current.

Variable Resistance

The component of an electric circuit which is used to regulate the current, without changing the voltage from the source, is called variable resistance.

Rheostat

This is a device which is used in a circuit to provide variable resistance.

Cause of Resistance in a Conductor

Flow of electrons in a conductor is electric current. The positive particles of conductor create hindrance to flow of electrons, because of attraction between them, this hindrance is the cause of resistance in the flow of electricity.

Factors on which the Resistance of a Conductor depends:

Resistance of a uniform metallic conductor is
(i) directly proportional to the length of conductor
(ii) inversely proportional to the area of cross-section
(iii) directly proportional to the temperature
(iv) depend on nature of material.

Effect of Length of the Conductor

The resistance of a conductor is directly proportional to its length. A long wire (or long conductor) has more resistance, and a short wire has a less resistance.

Effect of Area of Cross-Section of the Conductor

The resistance of a conductor is inversely proportional to its area of cross-section.
Resistance, R ∝ 1/A (where A is area of cross-section of conductor)

Short length of a thick wire is used for getting low resistance.
Long length of a thin wire is used for getting high resistance.

Effect of the Nature of Material of the Conductor

The electrical resistance of a conductor (say, a wire) depends on the nature of the material of which it is made. Some materials have low resistance whereas others have high resistance.

Effect of Temperature

The resistance of all pure metals increases on raising the temperature; and decreases on lowering the temperature. But the resistance of alloys like manganin, constantan and nichrome is almost unaffected by temperature.

Resistivity

The resistivity of a substance is numerically equal to the resistance of a rod of that substance which is 1 metre long and 1 square metre in cross-section.

The SI unit of resistivity is ohm-metre which is written in symbols as Ω m.

The resistivity of a substance does not depend on its length or thickness. It depends on the nature of the substance and temperature.

The resistivities of alloys are much more higher than those of the pure metals (from which they are made).

The heating elements (or heating coils) of electrical heating appliances such as electric iron and toaster, etc., are made of an alloy rather than a pure metal because,

the resistivity of an alloy is much higher than that of pure metal, and
an alloy does not undergo oxidation (or burn) easily even at high temperature, when it is red hot.

Example: nichrome alloy is used for making the heating elements of electrical appliances such as electric iron, toaster, electric kettle, room heaters, water heaters (geysers), and hair dryers, etc.

Resistors in Series

When two or more resistors are connected end to end, the arrangement is called series combination.
Total/resultant/overall/effective resistance in series

R_S=R₁ +R₂ + R₃

Current through each resistor is same.
Equivalent resistance is larger than the largest individual resistance.
Total voltage = Sum of voltage drops
V = V₁ + V₂ + V₃

Voltage across each resistor:

V₁ = IR₁
V₂ = IR₂ (V₁ + V₂ + V₃ = V)
V₃ = IR₃ (V = IR)
V = IR₁ + IR₂ + IR₃
⇒ IR = I(R₁ + R₂ + R₃)
⇒ R = R₁ + R₂ + R₃

Resistors in Parallel

Voltage across each resistor is same and equal to the applied voltage.

Total current is equal to sum of currents through the individual resistances.

I = I₁ + I₂ + I₃
V/R = V/R₁ + V/R₂ + V/R₃

Reciprocal of equivalent resistance is equal to sum of reciprocals of individual resistances.

1/R_P = 1/R₁ + 1/R₂ + 1/R₃

Equivalent resistance is less than the value of the smallest individual resistance in the combination.

Practical Applications of parallel circuit

Resistance in the circuit is to be decreased.
Current in the circuit is to be increased.

Therefore, all the electrical appliances for a household purpose are connected in parallel combination.

Practical applications of series circuit: Series circuits are used for dependent operations such as

Decorative light string on festivals.
Thermostats in heating devices to control the temperature.
Light switches, fuse with live wire in household wiring.
Batteries to get higher voltage.
Ammeter to measure the current.
Light emitting diodes (LEDs) are usually connected in series in the electronic devices.

Advantages of Parallel Combination over Series Combination

In series circuit, when one component fails, the circuit is broken and none of the component works.

Different appliances have different requirement of current. This cannot be satisfied in series as current remains same.

The total resistance in a parallel circuit is decreased.

Commercial Unit of Electrical Energy

Kilowatt-Hour: One kilowatt hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour.

Relation between kilowatt-hour and joule

1 Watt = 1 Joule/1Second
1 Kilowatt-hour = 1000 joules/seconds for 1 hour
1 hour = 60 ✕ 60 seconds
1 Kilowatt-hour = 1000 joules/seconds ✕ 60 ✕ 60 seconds
1 Kilowatt-hour = 36,00,000 joules or, 3.6 ✕ 10⁶ J

Effects of Electric Current

The presence of electric current can be detected by it effect. The electrical current can be exhibit three basic effects namely:

Heating effect: When an electric current passes through a wire, the wire gets heated and its temperature rises. This is known as heating effect of current. For example, Light bulb, electric iron, electric welding, etc.

Some of the devices in which heating effect is highly undesirable are electric motor, generator or transformer, TV set, monitor, CPU, etc.

Magnetic effect: When an electric current flows through a wire, it produces a magnetic field around it. This effect is known as magnetic effect of current. For example, Electromagnet, electric bell, electric fan etc.

Chemical effect of current: When the current passes through the liquid, it decomposes it into its components. This effect of electric current is called chemical effect of current. For example, Hydrolysis of water, electroplating process.

Joule’s Law of Heating Effect of Electric Current

It states that the heat produced in a resistor is

directly proportional to square of current, H ∝ I²
directly proportional to resistance for a given current, H ∝ R
directly proportional to time for which current flows through the conductor, H ∝ t.

So, H = I²Rt

Heating effect is desirable in devices like electric heater, electric iron, electric bulb, electric fuse, etc.

Heating effect is undesirable in devices like computers, computer monitors (CRT), TV, refrigerators etc.

In electric bulb, most of the power consumed by the filament appears a heat anda small part of it is radiated in form of light.

Filament of electric bulb is made up of tungsten as

it does not oxidise readily at high temperature.
it has high melting point (3380° C).

The bulbs are filled with chemically inactive gases like nitrogen and argon to prolong the life of filament.

Electric Fuse

It is a safety device that protects our electrical appliances in case of short circuit or overloading.

Fuse is made up of pure tin or alloy of copper and tin.
Fuse is always connected in series with live wire.
Fuse has low melting point.
Current capacity of fuse is slightly higher than that of the appliance.

Electric Power

The rate at which electric energy is consumed or dissipated in an electric circuit.

P = VI
P = I²R = V²/R

S.I. unit of power = Watt (W)
1 Watt = 1 volt ✕ 1 ampere

Commercial unit of electric energy = Kilo watt hour (KWh)
1 KWh = 3.6 × 10⁶ J
1 KWh = 1 unit of electric energy