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**Electric Current: **Electric current is the rate of flow of electric charge from a body at higher potential to a body at lower potential.

- $$\text{I}=\frac{\text{Q}}{\text{t}}$$
where

I is the current, Q is the charge and t is the time.

- Q=n x e

- $$\text{I}=\frac{\text{Q}}{\text{t}}$$

where n is the number of electrons

and e is the charge on one electron i.e 1.6 x 10 ^{-19} C

- $$\text{I}=\frac{(\text{n×e})}{\text{t}}$$

The S.I unit of current is Ampere(A).

- $$\text{I}=\frac{(\text{n×e})}{\text{t}}$$

**1 Ampere Current: **When one Coulomb charge flows through any cross section of a conductor in one second, the current passing through it is said to be one Ampere.

**Measurement of Current: **Ammeter is a device used to measure electric current.

The number of electrons required to carry 1 Coulomb charge are 6.25 x 10 ^{ 18 .}

**Potential difference:** Potential difference is the amount of work done ( energy spent ) in moving a unit positive charge from one point to another point in an electric field .

$$\text{V}=\frac{\text{W}}{\text{Q}}$$

where V is the potential difference,W is the work done and Q is the charge.

The S.I unit of potential difference is volt or J/C.

**One volt: **is the potential difference when one joule of work is done in moving one coulomb electric charge, from infinity to a given point in an electrical field.

Measurement of Potential difference is done by voltmeter.

**Conductors: **The substances which allow the charges to pass through them freely, are known as Conductors.

**Insulator: **The substances which do not allow the charges to pass through them are known as Insulators.

## Ohm’s Law:

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 is constant.This is called Ohm’s law.

V ∝ I

V=IR

Where V is the voltage, I is the current and R is the resistance.

**Resistance: **Resistance is defined as the property of the conductor which opposes the flow of electric current.

The S.I unit of resistance is ohm or volt/Ampere.

The resistance of the conductor is one ohm if a potential difference of one volt applied between its ends, causes a current of one ampere to flow through it.

## Factors Affecting Resistance Of The Conductor

Resistance is directly proportional to the length of the conductor.

Resistance is inversely proportional to the area of cross section.

Resistance depends on the nature of the material.

Resistance is directly proportional to the temperature.

So from the above points we can conclude that

$$\text{R}∝\frac{\text{I}}{\text{A}}$$

where R is the resistance,l is the length of the conductor and A is the area of the cross section.

$$\text{R}=\frac{\rho \text{l}}{\text{A}}$$

ρ=resistivity

**Resistivity:** Resistivity of a material is defined as the resistance offered by a cylindrical conductor of the material of cross sectional area one metre square and length one metre when the Current flows perpendicular to the opposite ends of the cylinder.

The S.I unit of resistivity is ohm-meter.Conductors have very low resistivity while insulators have very high resistivity.

Heating elements of electrical heating appliances are made up of alloys rather than pure metals because resistivity of alloys is much more than metals.

### Resistance Can Be Connected In Two Ways

Series

Parallel

**Series:**If we connect a number of resistances in such a way that the same current flows through each resistance, then the arrangement is called resistance in series.

R_{s} = R_{1}+ R_{2}+ R_{3} + R_{4}

**Parallel:**If we connect a number of resistances between two common points in such a way that the potential difference across each resistance is the same, then the arrangement is called as resistance in parallel.

$$\frac{1}{\text{R}\rho}=\frac{1}{\text{R}_1}+\frac{1}{\text{R}_2}+\frac{1}{\text{R}_3}$$

Parallel circuits are used in domestic wiring because:

(1) If any of the appliances in the parallel circuit stops working then all the other appliances in the circuit keep on working.

(2) All the appliances in the parallel circuit have their separate switch.

Hence, all the appliances can not be turned ON or OFF, separately.

(3) All the appliances in the parallel circuit will get the same voltage (220 V)

## Heating Effect Of Electric Current :

When an electric current passes through a conductor (like a high resistance wire) the conductor becomes hot after some time and produces heat. This is called the heating effect of Electric Current.

### Heat Produced In The Conductor Depends On The Following:

Electric current (I)

Resistance (R)

Time (t)

Potential difference (V)

**Joule’s Law of Heating: **Quantity of heat generated (H) in a conductor of resistance (R) when a current (I) flows through it for time (t) is directly proportional to square of the current, resistance of conductor, time for which the current flows

H = I^{2} Rt

OR

H=VIt

Or

$$\text{H}=\frac{\text{V}^{2}\text{t}}{\text{R}}$$

**1 Calorie = 4.18 joule**

Since H ∝ I^{2}, If the current is Doubled, the Heat produced will become Four times.

Since H ∝ R, If the Resistance is Halved, the Heat produced will become Halved.

Since H ∝ t, If the time for which the Current is passed for Halved, than the Heat produced will also be halved.

### Application Of Heating Effect

Electric bulb

Electric heater

**Electric Power: **Electric power is the rate at which electric energy is consumed in an electric circuit.

Electric energy = Power x Time

$$\text{Electric power (P)}=\frac{\text{electric energy(H) }}{\text{time(t)}}$$

$$\text{P}=\frac{\text{I}^{2}\text{Rt}}{\text{t}} \qquad\text{ (we know H}=\text{I}^2\text{Rt})$$

P=I^{2}R

Or

P =VI

Or

$$\text{P}=\frac{\text{V}^{2}}{\text{R}}$$

The S.I unit of power is Watt or J/s.

1 Kilowatt = 1000 W = = 10^{3} W

1 Megawatt = 1000000 W = = 10^{6} W

1 H.P. = 746 W

## What are Electricity and Charge?

## Charge

- A physical property of a substance which causes it to experience a force when placed near other matter or substances.
- Unit of electric charge is Coulomb.
- Like charges repel each other.
- Unlike charges attract each other.

## Electricity

- It is the term used to represent a set of physical phenomenon associated with the presence of electric charge.
- It is used in our homes for lightning operating fans, heating purpose etc.