CBSE Class 10 Science — Magnetic Effects of Electric Current

Chapter Overview & Weightage

Magnetic Effects of Electric Current is one of the highest-weightage chapters in Class 10 Science. Expect 4–6 marks in board exams — usually a mix of short-answer and one long-answer question.

Year	Marks	Question Types
2024	5	1 SA (2m) + 1 LA (3m)
2023	4	2 SA (2m each)
2022	5	1 SA + 1 LA
2021	4	MCQ + SA

This chapter consistently gives diagram-based questions — especially the solenoid field, motor, and generator diagrams. Practice drawing and labelling all three from memory.

Key Concepts You Must Know

These are ordered by exam frequency — the top ones appear almost every year.

1. Oersted’s Experiment Hans Christian Oersted discovered in 1820 that a current-carrying wire deflects a compass needle placed near it. This proved that electric current produces a magnetic field.

2. Magnetic Field Lines

They are closed continuous curves
Outside a magnet: north pole → south pole
They never intersect (if they did, it would mean two directions of field at one point — impossible)
Denser field lines = stronger field

3. Magnetic Field due to a Straight Current-Carrying Conductor The field forms concentric circles around the wire. The direction is given by the Right-Hand Thumb Rule: thumb points in the direction of current, curled fingers show the direction of field.

4. Magnetic Field due to a Circular Loop Each point on the loop contributes a straight-wire field. At the centre, all contributions add up — the field is perpendicular to the plane of the loop. More turns = stronger field.

5. Solenoid A solenoid is a tightly wound coil of wire. The field inside is uniform and parallel to the axis — just like a bar magnet. One end acts as north (current flows anticlockwise), other as south (clockwise).

6. Force on a Current-Carrying Conductor in a Magnetic Field

F = BIL\sin\theta

Where $B$ = magnetic field, $I$ = current, $L$ = length of conductor, $\theta$ = angle between field and current. Maximum force when $\theta = 90°$ , zero when $\theta = 0°$ .

7. Fleming’s Left-Hand Rule (Motor Rule) Point the forefinger in the direction of the field ( $B$ ), the middle finger in the direction of current ( $I$ ) — the thumb shows the direction of force (motion). Used for motors.

8. Electric Motor Converts electrical energy → mechanical energy. Key parts: coil (ABCD), magnet, split-ring commutator, brushes. The commutator reverses current direction every half rotation to maintain continuous rotation.

9. Electromagnetic Induction A changing magnetic field (or relative motion between a conductor and magnet) induces an EMF. Faraday discovered this. The induced current direction follows Fleming’s Right-Hand Rule.

10. Electric Generator Converts mechanical energy → electrical energy. AC generator: uses slip rings. DC generator: uses split-ring commutator.

11. Domestic Electric Circuits Live wire (red/brown): 220V. Neutral wire (black/blue): 0V. Earth wire (green/yellow): safety. Household appliances are connected in parallel so each gets full 220V.

Important Formulas

F = BIL\sin\theta

Use when: a wire of length $L$ carries current $I$ in a field $B$ , at angle $\theta$ to the field.

B = \frac{\mu_0 I}{2r}

(Class 10 — you need to know the formula conceptually; this level detail is for Class 12)

Solved Previous Year Questions

PYQ 1 — CBSE 2023 (2 marks)

Q: State Fleming’s Left-Hand Rule. Name one device based on this principle.

Solution: Hold your left hand flat. If the forefinger points in the direction of the magnetic field ( $B$ ) and the middle finger in the direction of current ( $I$ ), then the thumb points in the direction of force on the conductor.

Device based on this: Electric Motor.

PYQ 2 — CBSE 2024 (3 marks)

Q: With a labelled diagram, explain the working of an AC generator.

Solution: An AC generator converts mechanical energy into electrical energy using electromagnetic induction.

Key components:

Rectangular coil ABCD rotating in a magnetic field
Two slip rings (one attached to each end of the coil)
Two brushes (one on each slip ring) connected to external circuit
Strong permanent magnets (N and S poles)

Working: When the coil rotates, different parts cut through field lines at different rates. When AB and CD are perpendicular to field lines, the rate of cutting is maximum — EMF is maximum. When parallel, rate is zero — EMF is zero. This produces an alternating (AC) output.

The difference from DC generator: AC uses slip rings (continuous rings), while DC generator uses a split-ring commutator.

PYQ 3 — CBSE 2022 (2 marks)

Q: What happens to the force on a current-carrying conductor if (a) current is doubled, (b) conductor is placed parallel to the field?

Solution: (a) From $F = BIL\sin\theta$ : if $I$ is doubled, $F$ doubles.

(b) When conductor is parallel to field, $\theta = 0°$ , so $\sin 0° = 0$ . Therefore, $F = 0$ .

Difficulty Distribution

Difficulty	% of Questions	Topics
Easy	40%	Definitions, Right-Hand Rule, Fleming’s Rules
Medium	40%	Diagrams (motor, generator, solenoid), Explain working
Hard	20%	Numerical (force), Comparison questions, Application

For the “Hard” 20% — the numericals on force ( $F = BIL\sin\theta$ ) are actually straightforward once you remember the formula. Practice 3–4 variations and you’ve covered this entire difficulty tier.

Expert Strategy

Week 1: Master the diagrams. Draw the motor and generator five times each from memory. Label every part. This alone secures 3–4 marks in most years.

Week 2: Understand the rules (Right-Hand Thumb, Fleming’s Left and Right). Make a simple table: which rule for which situation? Don’t confuse motor (left hand) with generator (right hand).

Week 3: Practice numericals on $F = BIL$ and do the last 5 years’ board questions. Notice the patterns.

Common board exam trap: Examiners ask “why are field lines inside a solenoid uniform?” — Answer: because the contributions from all parts of the coil add up uniformly inside. Students who just memorise the fact without understanding cannot answer follow-up marks.

Common Traps

Trap 1: Confusing Left-Hand and Right-Hand rules. Left hand = motor (force on conductor, current flows in). Right hand = generator (motion given, find current direction). A memory trick: Motor = Left (both have L).

Trap 2: Solenoid vs. electromagnet. A solenoid with an iron core becomes an electromagnet — the core concentrates the field. A solenoid without a core is just a coil. In diagram questions, mark the core clearly.

Trap 3: Applying Fleming’s Left-Hand Rule to generators. Fleming’s Left-Hand Rule is ONLY for motors (external force needed, current fed in). For generators, use Fleming’s Right-Hand Rule (mechanical energy in, current comes out).

Trap 4: Forgetting that household appliances are in PARALLEL, not series. In a parallel circuit, switching off one appliance doesn’t affect others — which is why your TV turning off doesn’t switch off your fan.

Trap 5: Saying a solenoid “creates” a magnetic field. The solenoid creates a magnetic field only when current flows. The iron core doesn’t create the field — it just strengthens it by becoming a temporary magnet.