SSC CGL Physics · Electricity and Magnetism

Circuits, Power and Magnetism

Ohm's law and resistors in parallel, resistivity and its unit, electrical energy in units and fuse ratings, and magnets, the right-hand thumb rule and electromagnetic devices.

All Electricity and Magnetism notes

Why this matters

Fifteen questions in the bank, five from each year's shifts, 13 of them EASY. The four sums use two formulas, and the rest are single facts about magnets and devices.

Concept 1 of 3: Ohm's law and resistance

Current is pushed by voltage and held back by resistance. In parallel, each resistor gets the full voltage, so the currents simply add.

Definition

The rules:

Ohm's law: V = IR, so with V fixed, doubling R halves I.
Parallel: 1/R = 1/R1 + 1/R2; for two resistors, R = R1R2 / (R1 + R2). Series: R = R1 + R2.
Resistivity (specific resistance) has the unit ohm metre.
Alloys such as nichrome have far higher resistivity than pure metals (silver lowest).

Ohm's law and parallel resistors

I = \frac{V}{R}, \qquad \frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2}

$I$ Current
$V$ Potential difference
$R_p$ Resistance of the parallel combination

Worked example

Resistors of 3 ohm and 6 ohm are in parallel across a 12 V battery. Find the total current.

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2025 · Tier I, 26 Sep 2025, Shift 1 · Q30Easy

Example 1 · Electricity and Magnetism · Circuits, Power and Magnetism

Two resistors, one with a resistance of 2 ohms and the other with a resistance of 4 ohms, are connected in parallel to a 6-volt battery. What is the total current flowing through the circuit?

Parallel is less than either

Two resistors in parallel give less resistance than the smaller one. If your answer is bigger, you added them as in series.

Drill 3 more on ohm's law and resistance

Concept 2 of 3: Electrical energy and heating

Energy is power times time. The electricity board's unit is the kilowatt-hour, so turn watts into kilowatts and minutes into hours first.

Definition

The rules:

Energy = power x time; 1 unit = 1 kilowatt-hour.
Current = power / voltage. A 1 kW iron at 220 V draws about 4.5 A, so it needs a 5 A fuse.
A heating element (nichrome) needs high resistivity and a high melting point.

Electrical energy in units

E\ (\text{kWh}) = P\ (\text{kW}) \times t\ (\text{h})

$P$ Power in kilowatts
$t$ Time in hours

Worked example

A 1,500 W heater runs 2 hours a day for 10 days. How many units does it use?

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2025 · Tier I, 20 Sep 2025, Shift 1 · Q40Easy

Example 2 · Electricity and Magnetism · Circuits, Power and Magnetism

A 60 W electric lamp is used for 5 hours daily. Calculate the energy consumed in 'units' (kilowatt-hours) in 30 days.

Convert watts first

60 W is 0.06 kW. Multiplying 60 by hours gives watt-hours, a thousand times too big for units.

Drill 2 more on electrical energy and heating

Concept 3 of 3: Magnets and devices

A current makes a magnetic field, and a changing field makes a current. Most devices here use one or the other.

Definition

Magnets:

A hanging bar magnet's north pole points north.
Steel makes permanent magnets; an electromagnet is temporary.
The tesla is the field giving 1 N on 1 m of wire carrying 1 A.
Right-hand thumb rule: thumb along the current, fingers curl along the field.
Faraday discovered electromagnetic induction in 1831.

Devices:

Electric bell: the magnetic effect of current. Generator: mechanical to electrical energy. Transformer: AC only.
Cathode ray tube: an electron beam on a phosphor screen.
Gauss and Weber built the first electromagnetic telegraph in 1833.

Item	Fact
Electric bell	Magnetic effect of currentQ
Thumb in the rule	Direction of currentQ
Permanent magnet	SteelQ
First electromagnetic telegraph	Gauss and Weber, 1833Q
Electron beam on phosphor	Cathode ray tubeQ

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2024 · Tier I, 23 Sep 2024, Shift 3 · Q30Easy

Example 3 · Electricity and Magnetism · Circuits, Power and Magnetism

Which famous experiment was done by Michael Faraday in 1831?

Generator and motor run opposite ways

A generator turns mechanical energy into electrical; a motor does the reverse. A pair giving the generator the motor's job is false.

Drill 7 more on magnets and devices

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Ohm's law and resistance
Ohm's law and parallel resistors
$I = \frac{V}{R}, \qquad \frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2}$
Electrical energy and heating
Electrical energy in units
$E\ (\text{kWh}) = P\ (\text{kW}) \times t\ (\text{h})$

Reference tables (1)

Magnets and devices5 rows

Item	Fact
Electric bell	Magnetic effect of currentQ
Thumb in the rule	Direction of currentQ
Permanent magnet	SteelQ
First electromagnetic telegraph	Gauss and Weber, 1833Q
Electron beam on phosphor	Cathode ray tubeQ

Watch out for (3)

Parallel is less than either
→ Ohm's law and resistance
Convert watts first
→ Electrical energy and heating
Generator and motor run opposite ways
→ Magnets and devices

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