Current Electricity and Magnetism: Ohm's Law, Resistance, Power, Electric Motor, Generator and Transformer for SSC

Electricity and magnetism carry 2 to 3 questions in SSC general science, and most of them are based on formulas, units and applications. This guide on current electricity and magnetism for SSC explains Ohm’s law, resistance, series and parallel circuits, electric power and energy, the heating effect, the magnetic effects of current, Fleming’s rules and the motor, generator and transformer, with solved examples and practice questions.

Basic Quantities and Units

Ohm’s Law

At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = I R. The resistance of a wire is directly proportional to its length and inversely proportional to its area of cross-section: R = ρ L/A. Resistance increases with temperature for metals.

Combination of Resistors

Household appliances are connected in parallel so that each gets the same voltage and can work independently.

Electric Power and Energy

• P = V I = I² R = V² / R.
• 1 kilowatt hour (kWh) = 1 unit of electricity = 3.6 × 10⁶ J.
• 1 horsepower = 746 watts.
• Heating effect (Joule’s law): H = I² R t. It is used in heaters, irons and bulbs.
• A fuse is a thin wire of a low-melting alloy connected in series; it melts if the current is too high.

Domestic Electric Circuits in India

• The supply voltage is 220 V (about 230 V) and the frequency is 50 Hz.
• Live wire (red, positive), neutral wire (black) and earth wire (green).
• The earth wire protects from electric shock by passing the current into the ground.
• AC changes direction periodically; DC flows in one direction.

Magnetic Effect of Current

• Oersted discovered in 1820 that a current-carrying wire produces a magnetic field.
• Magnetic field lines go from the north pole to the south pole outside a magnet and never cross each other.
• A solenoid (a coil of wire) behaves like a bar magnet; an electromagnet is made by placing a soft iron core in it.
• The force on a current-carrying conductor in a magnetic field is given by Fleming’s left-hand rule (thumb: force; forefinger: field; middle finger: current).

Electromagnetic Induction

Michael Faraday discovered in 1831 that a changing magnetic field produces an electric current in a coil. Fleming’s right-hand rule gives the direction of the induced current.

Transformer

V_s / V_p = N_s / N_p. Power is transmitted at a high voltage to reduce the loss of energy.

Solved Examples

Example 1. A current of 2 A flows through a resistance of 10 Ω. Find the voltage.
V = IR = 20 V.

Example 2. Three resistors of 2 Ω, 3 Ω and 5 Ω are in series. Find the total resistance.
10 Ω.

Example 3. A 100 W bulb is used for 10 hours. How many units are consumed?
100 × 10 = 1000 Wh = 1 unit.

Example 4. Two resistors of 6 Ω each are connected in parallel. Find the equivalent resistance.
1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω.

Quick Tips

• Memorise the units and formulas.
• Remember that appliances are connected in parallel.
• Learn Fleming’s left-hand rule for motors and right-hand rule for generators.
• Remember that a transformer works only on AC.

Quick Revision Points

• V = IR; P = VI; E = Pt.
• Series: resistances add; parallel: reciprocals add.
• Motor: left-hand rule; generator: right-hand rule.
• 1 kWh = 3.6 × 10⁶ J.
• The fuse is connected in series with the live wire.

Practice Questions With Answers

1. What is the SI unit of resistance?
   Answer: Ohm
2. A current of 3 A flows through 5 Ω. What is the voltage?
   Answer: 15 V
3. What is the commercial unit of electrical energy?
   Answer: Kilowatt hour (kWh)
4. Which device converts electrical energy into mechanical energy?
   Answer: Electric motor
5. Which device works on electromagnetic induction?
   Answer: Generator and transformer
6. Who discovered the magnetic effect of current?
   Answer: Oersted
7. Which wire is connected to the metal body of an appliance for safety?
   Answer: Earth wire
8. Two 4 Ω resistors are in series. What is the total?
   Answer: 8 Ω
9. What is the frequency of AC supply in India?
   Answer: 50 Hz
10. Which transformer increases the voltage?
    Answer: Step-up transformer

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the General Science notes page, read the previous topic Light and the Human Eye and the next topic Cell Biology and Genetics.

Frequently Asked Questions

What is Ohm’s law?

The current through a conductor is proportional to the voltage across it at constant temperature: V = IR.

What is the SI unit of current?

Ampere.

What is the SI unit of power?

Watt.

How are appliances connected in a house?

In parallel.

What is a fuse?

A safety device with a thin wire that melts if the current is too high.

What is the commercial unit of electricity?

The kilowatt hour (unit).

What is an electric motor?

A device that converts electrical energy into mechanical energy.

What is a generator?

A device that converts mechanical energy into electrical energy.

What is a transformer?

A device that changes the voltage of AC supply by mutual induction.

What is electromagnetic induction?

The production of current in a coil by a changing magnetic field.

What is Fleming’s left-hand rule?

A rule that gives the direction of force on a current-carrying conductor in a magnetic field.

Is this topic asked in SSC?

Yes. Electricity and magnetism are asked in SSC general science.