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What is an Charge?

Charge is a fundamental particle in an atom. It may be positive or negative. Like charges repel each other and unlike charges attract each other.

Charge on 1 electron (e) = - 1.6 × 10⁻¹⁹ C

Charge on 1 proton (p) = 1.6 x 10⁻¹⁹ C

S.I. unit of charge = Coulomb (C)

1 Coulomb charge = Charge present on approx. 6 × 10¹⁹ electrons

Or We can say that, Charge on 1 electron (e) = - 1.6 × 10⁻¹⁹ C

Q = ne ; n = number of electrons

e = charge of electrons

What is electric current?

Just as the flow of water is called as water current, the flow of electrons is called as electric current.

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.

In this diagram, the direction of flow of positive charges is taken as the direct of current which flows from the positive terminal of the battery.

A continuous and closed path of an electric current is called an electric circuit. The flow of charges through the circuit is controlled by means of a switch which can be used to turn on or off the current.

Electric current thus can be expressed as the amount of charge flowing through a particular area in unit time.

Thus, 1 Ampere can be defined as flow of 1 Coulomb of charge per second.

How can we measure electric current?

Electric current can be measured by using an instrument called Ammeter. An ammeter must always be connected in series and should have low resistance to allow for easy flow of electrons.

What is Potential Difference?

The flow of electrons requires a difference of electric pressure – called the potential difference. It can be understood as difference in concentration of positive and negative charges.

This difference of potential may be produced by a battery, consisting of one or more electric cells or through friction as in the case if static electricity.

Inside a battery, the chemical action generates the potential difference across the terminals of the cell, even when no current is drawn from it. When this cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current.

So, it can be said that the potential difference is the driving force behind the flow of electrons from one place to another. In order to continue the flow of electrons, the battery has to use the chemical energy stored in it.

Potential difference acts as source of energy and thus can be defined as the work done to move a unit charge from one point to another.

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

1 Volt is thus 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.

How can we measure potential difference?

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. It should have high resistance so that current flow through it is minimum allowing it for better accuracy.


The potential difference, V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same.

This constant is Resistance – the property of any material to resist the flow of charges through it and is constant for a given wire of constant length and cross section made up of a given material at a given temperature.

If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω.

That is, 1 ohm (Ω) = 1 Volt

1 Ampere

From the equation above we can see that the current through a resistor is inversely proportional to its resistance. The greater the resistance of a material, the smaller the current flowing through it for a given potential difference.

What is a resistor, conductor and insulator?

A conductor having some appreciable resistance is called a resistor.

A component of identical size that offers a higher resistance is a poor conductor.

An insulator of the same size offers even higher resistance.

What is a Rheostat or Variable Resistance?

A component used to regulate current without changing the voltage source is called variable

resistance. In an electric circuit, a device called rheostat is often used to change the resistance in the circuit. The change in the length of the conductor is used to vary the resistance.

Factors affecting the resistance of a material

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.

Resistivity (ρ) : 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 does not change with change in length or area of cross-section but it changes with

change in temperature.

Range of resistivity of metals and alloys is 10−8 to 10−6 Ωm.

Range of resistivity of insulators is 1012 to 1017 Ωm.

Why are alloys used in electrical heating devices?

Resistivity of alloy is generally higher than that of its constituent metals and they do not oxidize (burn) readily at high temperature, so they are commonly used in electrical heating devices.

Copper and Aluminium are used for electrical transmission lines as they have low resistivity.

Resistors in Series

When two or more resistors are connected end to end, the arrangement is called series combination. Current through each resistor is same in a series circuit.

Total/resultant/overall/effective resistance in series; Rs = R1 + R2 + R3


Total voltage = Sum of voltage drops

V = V1 + V2 + V3

IRs = IR1 + IR2 + IR3

IRs = I(R1 + R2 + R3)

Rs = R1 + R2 + R3

Equivalent resistance is larger than the largest individual resistance.

Resistors in Parallel

When two or more resistors are connected to the corresponding ends of the other resistors having the same potential difference, then arrangement is called parallel combination. Voltage through each resistor is same in a parallel circuit.

Total/resultant/overall/effective resistance in parallel; 1 = 1 + 1 + 1

Rp R1 R2 R3


I = I1 + I2 + I3

V = V + V + V

Rp R1 R2 R3

V = V ( 1 + 1 + 1 )

Rp R1 R2 R3

1 = 1 + 1 + 1

Rp R1 R2 R3

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

Advantages of Parallel Combination over Series Combination

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

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

(iii) The total resistance in a parallel circuit is decreased.

Heating Effect of Electric Circuit

If an electric circuit is purely resistive, the source of energy continually get dissipated entirely in form of heat. This is known as heating effect of electric current.

W = H = E = V x Q = VIt (Since, Q = It)

Heat produced, H = VIt = I2Rt = V2t


Joule’s Law of Heating Effect of Electric Current It states that the heat produced in a resistor is

(i) directly proportional to square of current, H ∝ I2

(ii) directly proportional to resistance for a given current, H ∝ R

(iii) directly proportional to time for which current flows through the conductor, H ∝ t.

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 and a small part of it is radiated in form of light.

Filament of electric bulb is made up of tungsten as

(i) it does not oxidize readily at high temperature.

(ii) it has high melting point (3380º C).

(iii) 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 and is always connected in series with live wire.

Fuse has low melting point and current capacity of fuse is slightly higher than that of the


Electric Power

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

P = W = H = E = VI = I2R = V2

t t t R

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

Commercial unit of electric energy = Kilo Watt hour (KWh)

KWh = 1 unit of electric energy

1 KWh = 3.6 × 106 J


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