Class 10 Science Chapter 12 Electricity Notes

Class 10 Science

Chapter 12 Electricity

Topics in the Chapter

Introduction

• Current

• Potential Difference

→ Volt definition

→ Voltmeter

•  Symbols of some commonly used components in Circuit

 • Ohm's Law

→ Mathematical expression for Ohm's law

→ V-I gragh Ohm's law

→ Resistance

→ Ohm

→ Rheostat

• Factors on which the Resistance of a conductor depends Resistivity

→ Resistivity

• Resistors in Series

→ Total/resultant/overall/effective resistance in series

→ Voltage across each resistor

• Resistors in parallel

• Advantage of parallel combination over series combination

• Heating effect of electric circuit

 → Joule's law of heating effect of electric current

→ Filament of electric bulb is made up of tungsten

→ Electric Fuse

→ Electric Power

 Introduction

→ Charge is a fundamental particle in an atom. It may be positive(Protons) or negative(Electrons).

→ Like charges repel each other.

→ Unlike charges attract each other.

→ S. I. unit of charge is Coulomb (C) 

→ 1 Coulomb charge = Charge present on approx. 6 × 10^18 electrons

→ Charge on 1 electron = Negative charge of 1.6 × 10^-19 C

i.e. Q = ne

Where,

 Q = Charge (total)

n = No. of electrons

e = Charge on 1 electron

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.

→ Direction of current is taken opposite to flow of electrons as electrons were not known at the time when the phenomenon of electricity was discovered first and current was considered to be flow of positive charge.

Current = Charge/Time

⇒ I = Q/T

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

if 1 coulomb of electric charge flows through a cross section for 1 second, it would be equal to 1 ampere.

Therefore,  1 A = 1 Cs^-1

⇒ 1 mA = 10^-3 A

⇒ 1 µA = 10^-6 A

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

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

→ Ammeter has low resistance and always connected in series.

Potential Difference

The difference in the amount of electric potential energy between two points in an electric circuit is called electric potential difference.

Electric potential difference is known as voltage, which is equal to the amount of work done to move the unit charge between two points against static electric field.

Therefore, Voltage = Workdone/Charge

Voltage or electric potential difference is denoted by V’.

Therefore, V = W/Q

Where, W = Work done and Q = Charge

• 1 Volt: When 1 joule work is done in carrying one Coulomb charge then potential difference is called 1 volt.

→ S. I. unit of Potential difference is Volt (V)

1 V = 1 JC^-1

• Voltmeter: It is an instrument to measure the potential difference.

→ It is always connected in parallel.

 Symbol is

→ Cell is the simplest device to maintain potential difference.

→ Current always flow from higher potential to lower potential.

Galvanometer: It is a device to detect current in an electric circuit.

Ohm’s Law:

Ohm’s Law states that the potential difference between two points is directly proportional to the electric current, at a constant temperature.

Mathematical expression for Ohm’s law

V ∝ I

⇒ V = IR

→ R is a constant called resistance for a given metal.

• V-I graph for Ohm’s law

Resistance (R): It is the property of a conductor to resist the flow of charges through it.

S. I. unit of resistance is Ohm (Ω)

 1 ohm = 1 volt/1ampere

→ When potential difference is 1 V and current through the circuit is 1 A, then resistance is 1 ohm.R

Rheostat: Variable resistance is a component used to regulate current without changing the source of voltage.

Factors on which the Resistance of a Conductor depends

(i) length of conductor

(ii) area of cross-section

(iii) temperature

(iv) depend on nature of material.

• Resistivity (P): It is defined as the resistance offered by a cube of a material of side 1m when current flows perpendicular to its opposite faces.

• Its S.I. unit is ohm-metre (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 10^12 to 10^17 Ωm.

→ Resistivity of alloy is generally higher than that of its constituent metals.

→ Alloys 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 lowresistivity.

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

Rs = 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 reistances.

• 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/Rp = 1/R1 + 1/R2 + 1/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.

As E = P×T ∝  VIt  {E = H}

Heat produced, H = VIt  {V = IR}

Or, Heat produced, H = I^2Rt

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 ∝ Rt^2

→ It is directly proportional to resistance for a given current, H ∝ R

→ It is directly proportional to time for which current flows through the conductor, H ∝ t.So,

H =  I^2Rt

→ 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 because:

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

(ii) it has high melting point (3380º 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 = I^2R = V^2/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

Click here 👉Class 10 Science Notes