# CBSE Class 10 Science Chapter 12 Electricity  Written by

# Electricity Electricity

## Electricity and a few Terminologies

Electricity is a controllable form of energy used to run a variety of electrical and electronic equipment like fan, bulb, tv, radio etc.
• Electrical appliances convert the electricity directly into heat, light or motion whereas electronic appliances have micro-chips that take electricity and convert it into endless useful things (display picture on TV, play songs on radio etc.) and not just simple heat, light or motion.
• When electric charge flows through a conductor (like a metallic wire), it is said that the conductor has electric current.
• Electric current, generated by cells or batteries inside a torch, makes the torch glow.
• switch is a conducting path between cells and the bulb. An ON switch makes the bulb glow whereas an OFF switch puts it in non-glowing state.
• An electric circuit is a complete and continuous path for electric current. Current does not flow when the path is broken. A switch makes and breaks the circuit inside a torch.

## Electric Current and Electric Circuit

Electric current is rate of flow of electric charge or the amount of charge flowing through a particular area in unit time.
• Until the discovery of electrons, the electric current was considered to be a flow of positive charge and the direction was considered from positive terminal of the battery to the negative terminal.
• In reality, electric current is flow of negatively charges particles called electrons. These flow from negative terminal of the battery to the positive terminal.
• As a convention, the direction of current is taken opposite to the direction of flow of electrons. ## Measurement of Electric Current

Ammeter is an instrument used to measure electric current. It is always connected in series in a circuit.
• SI Unit of electric charge is Coulomb (C).Charge is usually denoted by letter Q.
• SI Unit of electric current is Ampere (A).Current is usually denoted by letter I.
I = Q/t, which means current is net charge flowing through a cross-section in time t.
Hence,  1 A = 1 C/ 1s.
• 1 Coulomb is equivalent to 6 X 1018 So an electron possess negative charge of 1.6 X 10-19 C. ## Electric Potential and Potential Difference

Electric potential is the work done per unit charge in bringing the charge from infinity to that point against electrostatic force.
In a conductor, electrons flow only when there is a difference in electric pressure at its ends. This is also called potential difference. In a circuit, this potential difference is created by a battery.
• A battery or a cell has a potential difference due to reaction of chemicals inside it even when it is not connected to any circuit. Once connected, the potential difference makes the electrons flow and the current is generated in the circuit/conductor. A battery generates this potential difference till the chemicals inside it are not completely spent.
• Potential difference is also defined as work done to move a unit charge from one point to another.
V = W/QOrPotential difference =Work done/Charge

## Measurement of Potential Difference

Potential difference is nothing but voltage and is measured by a device called Voltmeter. It is always connected in parallel across the points whose potential difference is to be measured.
• SI unit of electric potential difference is Volt (V).
• SI unit of Work done is Joule (J).
1 V = 1 J/ 1 C, potential difference is 1 when 1 joule work is done to move 1 coulomb of charge from one point to another.

## Electric Circuit Symbols   ## OHM’s Law

OHM’s law states that under no same temperature, electric current flowing through an ideal conductor is directly proportional to the potential difference across its ends.
V ∝ I
V = IR, R – Resistance
• Resistance is a property of conductor to resist flow of charge through it. Its SI unit is Ohm (Ω).
R = V/I, 1 Ohm = 1 Volt/ 1 Ampere
• I = V/R denotes that Current is inversely proportional to Resistance. Hence, greater the resistance lesser will be the flow of current and vice-versa.
• rheostat or a variable resistor is a device or a component which allows changing of resistance in a circuit keeping the voltage same.
• In a conductor, electrons are attracted by the atoms. This is a resistive force which is lower for a good conductor but very high for an insulator. A conductor having some resistance is called a Resistor.
Ohm’s Law Experiment

## Factors on which Resistance Depends

Resistance of a conductor depends:
1. On its length
2. On its area of cross-section
3. On the nature of its material
• Resistance of uniform metallic conductor is directly proportional to its length and inversely proportional to area of cross-section.
R∝l
R∝ 1/A
Therefore, R = ρ(l  / A),ρ(rho) is constant of proportionality.
• ρ is called electrical resistivity of the material of the conductor. SI unit is Ωm.
• Resistivity of conductors is very low whereas the insulators have a very high resistivity.
• Resistivity varies with Temperature.
• Alloys having higher resistivity than metals are used in electrical heating devices, like iron and toasters, tungsten is used in filament of electric bulbs and copper and aluminium are used for electrical transmission lines.
Resistivity of Conductors < Resistivity of Alloys < Resistivity of Insulators

## Resistors in Series

When two or more resistors are connected in series:
• The current through the circuit remains the same.
• The potential difference becomes sum of the individual potential difference across each resistor.
• Equivalent resistance of the circuit is the sum of individual resistances.
• Disadvantages of Series Arrangement of Resistors
• Two different electrical appliances, having different current requirements, cannot be connected in series as the current is constant in a series circuit.
• If one of the components fails in a series circuit, the circuit gets broken and none of the other components get the current.

## Resistors in Parallel

When two or more resistors are connected in parallel:
• The current through the circuit is the sum of currents through each branch of the circuit.
• The potential difference across the two points of the circuit remains the same.
• The reciprocal of equivalent resistance of the circuit is the sum of reciprocal of the individual resistances.
Advantages of parallel arrangement of resistors
• Parallel arrangement divides current in different branches and hence each component receives the required amount of current.
• If one of the components fails in the parallel circuit, the rest work as usual.

## Heating effect of Current

When a voltage source like a battery generates current for a circuit, most of the energy is used up in maintaining the current inside the circuit. However, a portion of the energy is dissipated in the form of heat. In a circuit containing only battery and resistance, the entire energy is dissipated in the form of heat. This is heating effect of current.
Power = Work Done / Time TakenorP = W / t
Since, Potential Difference = Work Done / ChargeorV = W / Q
P = VQ / tor P = VI, where I (current) = Charge / Time Taken
Energy supplied by power source to a circuit in time t is, H = P x t = VIt
Or from Ohm’s law, H = I2Rt
This is known as Joule’s law of heating. It states that heat produces in a circuit is directly proportional to the square of current flowing, resistance for current and time for which current flows.
Disadvantages of heating effect
• Loss of energy in the unwanted heat.
• Wear and tear of components.

## Applications of Heating Effect of Current

• Appliances based on Joule’s heating are electric iron, electric toaster, electric heater, oven etc.
• Heating effect produces light in a bulb. Tungsten wire inside the bulb becomes hot and emits light when current passes through it. Since it has a high melting point of 3380°C, it does not break. Also, due to the presence of gases like chemically inactive nitrogen and argon inside the bulb, the filament remains thermally isolated and its life increases.
• Joule’s heating is also used in fuses where a thin wire protects the household appliances from power surge. Fuse wires are made of thin aluminium, copper etc. with an appropriate melting point. These wires melt and break the circuit due to heating during excessive current.

## Electric Power

Rate at which electric energy is dissipated or consumed in a circuit is called Electric Power.
P = VI = I2R = V2/R
• SI unit of Power is Watt (W).
• 1 Watt power is consumed when 1 Ampere of current flows through a device at a potential difference of 1 Volt.
• Electrical energy is the amount of work done or energy consumed in a given amount of time. So, it is measured in Joules or Wh (watt hour) or most commonly as kWh (Kilowatt hour).
• 1 kWh = 3.6 x 106 Joule (J).

## SCIENCE Revision Notes

Chapter:01  Chemical Reaction & Equation
Chapter:02  Acid Base & Salt
Chapter:03  Metals & Non Metals
Chapter:04  Carbon & its Components
Chapter:05  Periodic Classification of Elements
Chapter:06  Life Processes
Chapter:07  Control & Coordinates
Chapter:08  How do Organisms Reproduce
Chapter:09  Heridity & Evolution
Chapter:10  Light Reflection Refraction
Chapter:11  The Human Eye & the Colourful World
Chapter:12  Electricity
Chapter:13  Magnetic Effect of Electric Current
Chapter:14  Source Of Energy
Chapter:15  Our Environment
Chapter:16  Management of Natural Resource

Maths Revision Notes

English Revision Notes

Economics Revision Notes

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