Electric Current


Electric Current (I) – is measured in amperes (A).

  • Current is the rate of flow of charge. A current of 1 A means that 1 coulomb of charge flows past a point in a circuit every second. ( 1 A = 1 C s-1) Current is measured in a circuit using an ammeter which is placed in series with the component of interest in the circuit.
  • I = current in amperes, A
  • Delta Q =  charge in coulombs, C
  • Delta t = time in seconds, s

Charge (Q) – is measured in coulombs (C).

  • A single electron carries a charge of 1.6 x 10-19

Potential difference

Potential difference (V) – is measured in volts (V).

It is the work done per unit charge. A potential difference of 1 V means that 1 joule of work is done per coulomb of charge. ( 1 V = 1 J C-1) Potential difference in a circuit is measured using a voltmeter which is placed in parallel with the component of interest in the circuit.

  • V = potential difference in volts, V
  • W = work done or energy transferred in joules, J
  • Q = charge in coulombs, C


Resistance (R) – is the ratio of the potential difference across a component to the current flowing through it, it is measured in ohms.

  • R = resistance in ohm’s, W
  • V = potential difference in volts, V
  • I = current in amperes, A


Resistivity is the Resistance offered by per meter of wire is called its resistivity

  • The S.I unit of resistivity is ohm-meter.
  • Resistivity varies with temperature.
  • It is the characteristic property of the material.
  • The metal and alloy have the resistivity in the range of 10-8 OHM m.
  • The insulator has the resistivity in the range of 1012 OHM m to 10 17 OHM m

Cause of Resistance in a Conductor:

The flow of electrons in a conductor is electric current. The 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.

Resistance in a conductor depends on nature, length and area of cross section of the conductor.

Nature of material: Some materials create the least hindrance and hence are called good conductors. Silver is the best conductor of electricity. While some other materials create more hindrance in the flow of electric current, i.e. flow of electrons through them. Such materials are called bad conductors. Bad conductors are also known as insulators. Hard plastic is one of the best insulators of electricity.

Length of conductor: Resistance R is directly proportional to the length of the conductor. This means resistance increases with increase in the length of the conductor. This is the cause that long electric wires create more resistance to the electric current.

Thus, Resistance (R) ∝ length of conductor (l)

or R ∝ l --------(i)

Area of cross section: Resistance R is inversely proportional to the area of cross section ( A) of the conductor. This means R will decrease with increase in the area of conductor and vice versa. More area of conductor facilitates the flow of electric current through more area and thus decreases the resistance. This is the cause that thick copper wire creates less resistance to the electric current.

Thus, resistance ∝ 1/Area of cross section of the conductor (A)

Where ρ (rho) is the proportionality constant. It is called the electrical resistivity of the material of conductors.

From equation (iii)

The SI unit of resistivity: Since, the SI unit of R is Ω, SI unit of Area is m2 and SI unit of length is m. Hence

Thus, SI unit of resistivity (ρ) is Ω m

Materials having resistivity in the range of 10−8 Ω m to 10−6 Ω m are considered as very good conductors. Silver has resistivity equal to 1.60 X 10−8 Ω m and copper has resistivity equal to 1.62 X 10−8 Ω m.

Rubber and glass are very good insulators. They have resistivity in the order of 1012 Ω m to 1017 Ω m.

Resistivity of materials varies with temperature.

Resistance of A System of Resistors

Resistors are joined in two ways, i.e. in series and in parallel.

Resistors in Series: When resistors are joined from end to end, it is called in series. In this case, the total resistance of the system is equal to the sum of the resistance of all the resistors in the system.

Let total resistance = R

Resistance of resistors are R1, R2, R3, …Rn

Therefore, R = R1 + R2 + R3 + …………+ Rn


Resistors in parallel: When resistors are joined in parallel, the reciprocal of the total resistance of the system is equal to the sum of reciprocal of the resistance of resistors.



Let the total resistance = R

Resistance of resistors are R1, R2, R3, …Rn

Applications of series and parallel circuits

Series circuits: All mains operated appliances have switches that are connected to the live wire (the wire that carries current into the appliance). When a switch is in series with a device, it controls the device, allowing us to switch it on and off.

Parallel circuits:

Lighting circuits

In the lighting circuit, all the lamps are connected in parallel.

This means that the lights in each room can be switched on and off independently.

Power ring

The power ring circuit is also a parallel circuit.

In the lighting circuit there is only one path for the current to each lamp but in the power ring circuit, there are two paths for the current to each appliance. Because there are two paths, the current in the cables of a power ring circuit is less than those of a usual parallel circuit with only one path.

Heating Effect of Electric Current-Practical Application

Practical Application of Heating Effect of Electric Current & Electric Power

For exploiting the heating effect of electric current, the element of appliances must have a high melting point to retain more heat. The heating effect of electric current is used in the following applications:

Electric Bulb: In an electric bulb, the filament of bulb gives light because of the heating effect of electricity. The filament of bulb is generally made of tungsten metal; having melting point equal to 3380°C.

Electric iron: The element of electric iron is made of alloys having high melting point. Electric heater and geyser work on the same mechanism.

Electric fuse: Electric fuse is used to protect the electric appliances from high voltage; if any. An electric fuse is made of metal or alloy of metals, such as aluminium, copper, iron, lead, etc. In the case of the flow of higher voltage than specified, fuse wire melts and protects the electric appliances.

Fuse of 1A, 2A, 3A, 5A, 10A, etc. are used for domestic purpose.

Suppose, if a refrigerator at home consumes 520W at 220V.

Then electric current in circuit I = P/V

Or, I = 2.4 A

Thus, in this case, a fuse of 3A should be used to protect the electric heater in the case of flow of higher voltage.

Electric Power:

SI unit of electric power is watt (W).

1W = 1 volt x 1 ampere = 1V x 1A

1 kilo watt or 1kW = 1000 W

Consumption of electricity (electric energy) is generally measured in kilo watt.

Unit of electric energy is kilo watt hour (kWh)

1 kWh = 1000 watt X 1 hour = 1000 W x 3600 s

Or, 1kWh = 3.6 x 106 watt second = 3.6 x 106 J