DETAILED NOTE | ELECTRICITY | CHAPTER 12 | CLASS 10 | PHYSICS

     

ELECTRICITY

Introduction

- Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.  

- There are two types of electric charge: positive and negative.

- Opposite charges (or Unlike charges) attract each other. 

- Similar charges (or Like charges) repel each other. 

- The SI unit of electric charge is coulomb which is denoted by the letter C. 

ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

Electric Potential

- The electric potential  at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point. 

- It is denoted by the symbol V and its unit is volt. 

- A potential of 1 volt at a point means that 1 joule of work is done in moving 1 unit positive charge from infinity to that point. 

Potential Difference

- The potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit charge from one point to the other point. 

- The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to the other.

- The SI unit of potential difference is volt which is denoted by the letter V. 

- The potential difference is measured by

means of an instrument called voltmeter.

(Voltmeter)

- Voltage is the other name for potential

difference. 

ELECTRIC CURRENT

- When two charged bodies at different electric potentials are connected by a metal wire, then electric charges will flow from the body at higher potential to the one at lower potential (till they both acquire the same potential). This flow of charges in the metal wire constitutes an electric current.

- Electric current is expressed by the amount of charge flowing through a particular area in unit time. 

- If a charge of Q coulombs flows through a conductor in time t seconds, then the magnitude I of the electric current flowing through it is given by : 

- The SI unit of electric current is ampere which is denoted by the letter A. 

- When 1 coulomb of charge flows through any cross-section of a conductor in 1 second, the electric current flowing through it is said to be 1 ampere.

- Current is measured by an instrument called

ammeter.

-  An ammeter should have very

low resistance.

(Ammeter)

1 mA = 10–3 A

1 µA = 10–6 A

ELECTRIC CIRCUITS

- A continuous conducting path consisting of wires and other resistances (like electric bulb, etc.) and a switch, between the two terminals of a cell or a battery along which an electric

current flows, is called a circuit.

CIRCUIT DIAGRAM

- Conventional symbols used to represent some of the most commonly used electrical components are 

(Electrical Symbols)

- In this circuit, a resistor R has been connected to the two terminals of a cell through a switch. An ammeter A has been put in series with the resistor R. This is to measure current in the

circuit. A voltmeter V has been connected across the ends of the resistor R, that is, voltmeter is connected in parallel with the resistor. This voltmeter is to measure potential difference (or voltage) across the ends of

the resistor R. 

Electric circuit

OHM’S LAW

- Ohm’s law gives a relationship between current and potential difference. 

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

- If I is the current flowing through a conductor and V is the potential difference (or voltage) across its ends, then according to Ohm’s law : 

Where R is resistance of conductor (Constant)

The above equation can also be written as :

This is the mathematical expression of Ohm’s law.

The ratio of potential difference applied between the ends of a conductor and the

current flowing through it is a constant quantity called resistance.

Current, I = V/R

- It is obvious from this relation that :

(i) the current is directly proportional to potential difference. 

(ii) the current is inversely proportional to resistance.

- electric current in a given conductor depends on two factors :

(i) potential difference across the ends of the conductor, 

(ii) resistance of the conductor.

RESISTANCE OF A CONDUCTOR

The property of a conductor due to which it opposes the flow of current through it is called resistance.



-  1 ohm is the resistance of a conductor such that when

a potential difference of 1 volt is applied to its ends, a current of 1 ampere flows through it. 

- The resistance of a conductor depends on

 (i) Length 

(ii) Thickness 

(iii) Nature of material 

(iv) Temperature, of

the conductor. 

- The SI unit of resistance is ohm which is denoted by the symbol omega.

RESISTANCE OF A SYSTEM OF RESISTORS

- In the electrical circuits of radio,

television and other similar things, it is usually necessary to

combine two or more resistances to get the required current in the

circuit. 

- The

resistances can be combined in two ways : (i) in series, and (ii) in

parallel.

Two resistance in series and parallel

1. Resistors in series

- If we want to increase the total resistance, then the individual resistances are connected in series.

- According to the law of combination of resistances in series : The combined resistance of any number of resistances connected in

series is equal to the sum of the individual resistances. 

Ex- If a number of resistances R1, R2, R3

...... etc., are connected in series, then their combined resistance R is given by : R = R1 + R2 + R3 +.........

- When a number of resistances connected in series are joined to the terminals of a battery, then each resistance has a different potential difference across its ends . But the total potential difference across the ends of all the resistances in series is equal to the voltage of the battery. 

- When a number of resistances are connected in series, then the same current flows through each resistance (which is equal to the current flowing in the whole circuit).

Resultant Resistance of  Resistances Connected in Series

 Three resistances R1, R2 and R3 connected in series.

A battery of V volts has been applied to the ends of this series combination of resistances.

  Now, suppose the potential difference

across the resistance R1 is V1, R2 is V2 and R3 is V3.

 The total potential difference across the three

resistances should be equal to the voltage of the battery applied. That

is,    V = V1 + V2 + V3        ... (1)

 The total potential difference due to battery

is V. 

 The total resistance of the combination be R.

 The current flowing through the whole circuit is I.

So, applying Ohm’s law to the whole circuit, we get : 

                      or             V = I × R

Since the same current I flows through all the resistances R1, R2 and R3 in series, so by applying Ohm’s

law to each resistance separately, we will get :

       V1 = I × R1 ... (3)

       V2 = I × R2 ... (4)

and V3 = I × R3 ... (5)

Putting these values of V, V1, V2 and V3 in equation (1), we get :

                I × R = I × R1 + I × R2 + I × R3

or             I × R = I × (R1 + R2 + R3)

Cancelling I from both sides, we get :

                R = R1 + R2 + R3

If three resistors R1, R2, and R3 are connected in series then their total resistance R is given by the formula :

                R = R1 + R2 + R3

If there are 'n' resistors R1, R2, R3,...... Rn connected in series, then their resultant resistance

R is given by the formula : 

        R = R1 + R2 + R3 + ........... + Rn

2. Resistors in Parallel

- If we want to decrease the resistance, then the individual resistances are connected

in parallel. 



- According to the law of combination of resistances in parallel : The reciprocal of the combined resistance of a number of resistances connected in parallel is equal to the sum of the reciprocals of all the individual resistances.

Ex- if a number of resistances, R1, R2, R3 ...... etc., are connected in parallel, then their combined resistance R is given by the formula : 

- When a number of resistances are connected in parallel then their combined resistance is

less than the smallest individual resistance. 

- When a number of resistances are connected in parallel, then the potential difference across each

resistance is the same which is equal to the voltage of the battery applied.

- When a number of resistances connected in parallel are joined to the two terminals of a battery, then different amounts of current flow through each resistance. But the current flowing through all the individual parallel resistances, taken together, is equal to the current flowing in the circuit as a whole. 

Resultant Resistance of  Resistances Connected in Parallel

 Three resistances R1, R2 and R3 are connected parallel.

  A battery of V volts has been applied across the ends of this combination. In this case the

potential difference across the ends of all the three resistances will be the same. And it will be equal to the voltage of the battery used. 

 Suppose the total current flowing through the circuit is I.

 The current passing through resistance R1 will be I1, R2 will be I2, R3 will be I3.

DISADVANTAGES OF SERIES CIRCUITS IN DOMESTIC WIRING

1. In series circuit, if one electrical appliance stops working due to some defect, then all other appliances also stop working.

2. In series circuit, all the electrical appliances have only one switch due to which they cannot be turned on or off separately. 

3. In series circuit, the appliances do not get the same voltage (220 V) as that of the power supply line. 

4. In the series connection of electrical appliances, the overall resistance of the circuit increases too much due to which the current from the power supply is low. 

ADVANTAGES OF PARALLEL CIRCUITS IN DOMESTIC WIRING

1. In parallel circuits, if one electrical appliance stops working due to some defect, then all other appliances keep working normally.

2. In parallel circuits, each electrical appliance has its own switch due to which it can be turned on or turned off independently, without affecting other appliances.

3. In parallel circuits, each electrical appliance gets the same voltage (220 V) as that of the power supply line.

4. In the parallel connection of electrical appliances, the overall resistance of the household circuit is reduced due to which the current from the power supply is high. 

HEATING EFFECT OF ELECTRIC CURRENT

 When an electric current is passed through a high resistance wire, like nichrome wire, the resistance wire becomes very hot  and produces heat. This is called the heating effect of current.

  The heating effect of current is obtained by the transformation of electrical energy into heat energy. 

  Heat produced, H = I2 × R × t joules

This formula gives us the heat produced in joules when a current of I amperes flows in a wire of resistance R ohms for time t seconds. 

  This is known as Joule’s law of heating. 

 The heat produced in a wire is directly proportional to :

(i) square of current (I2)

(ii) resistance of wire (R)

(iii) time (t), for which current is passed.

- A given current will produce more heat in a high resistance wire than in a low resistance wire.

- A given current will produce more heat per unit time if the two resistances are connected in series than in parallel.

- All the appliances which run on electricity

do not convert all the electric energy into heat energy.

Applications of the Heating Effect of Current

1. The heating effect of current is utilised in the working of electrical heating appliances such as

electric iron, electric kettle, electric toaster, electric oven, room heaters, water heaters (geysers), etc. 

2. The heating effect of electric current is utilised in electric bulbs (electric lamps) for producing light.

3. The heating effect of electric current is utilised in electric fuse for protecting

household wiring and electrical appliances.

ELECTRIC POWER

- Electric power is the electrical work done per unit time.

- SI unit of electric power is watt.

- The power of 1 watt is a rate of working of 1 joule per second. 

Formula for Calculating Electric Power

Electric power = Potential difference × Current

Electric power = Voltage × Current