Magnetic Effects of Electric Current — Practice

Question 1

Why do two magnetic field lines never intersect each other?

Accepted Answer

If two lines crossed, the compass at that point would have to point in two directions at once — impossible. So field lines never intersect.

✦ Answer: Because the field can have only one direction at a point.

Question 2

Whose experiment first showed that an electric current produces a magnetic effect?

Accepted Answer

✦ Answer: Hans Christian Oersted (1820) — a current deflected a nearby compass needle.

Question 3

The magnetic field pattern of a current-carrying solenoid resembles that of which familiar magnet?

Accepted Answer

✦ Answer: A bar magnet (with a north and a south end and a uniform field inside).

Question 4

State Fleming's left-hand rule and name the device based on it.

Accepted Answer

Step 1 — Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular.

Step 2 — Forefinger → magnetic Field, Middle finger → Current, Thumb → Force (motion).

Step 3 — Device: the electric motor.

✦ Answer: rule as above; based on it → electric motor.

Question 5

Using the right-hand thumb rule, describe the magnetic field produced around a straight current-carrying wire.

Accepted Answer

Step 1 — Grip the wire in the right hand with the thumb along the current direction.

Step 2 — The curled fingers show the field, which forms concentric circles around the wire.

Step 3 — The field is stronger near the wire and for larger current.

✦ Answer: concentric circular field lines around the wire; direction by the curl of the right-hand fingers.

Question 6

What is an electromagnet? Give one advantage over a permanent magnet.

Accepted Answer

Step 1 — An electromagnet is a solenoid with a soft-iron core that becomes magnetic only when current flows through it.

Step 2 — Advantage: it can be switched on and off, and its strength can be controlled by changing the current/turns.

✦ Answer: a current-controlled magnet; advantage = switchable and adjustable strength.

Question 7

Explain the working of an electric motor and state the function of the split-ring commutator.

Accepted Answer

Step 1 — A current-carrying coil is placed in a magnetic field.

Step 2 — By Fleming's left-hand rule, the two sides of the coil experience equal and opposite forces, producing a turning effect (torque) that rotates the coil.

Step 3 — After each half rotation the split-ring commutator reverses the current direction in the coil, so the force keeps acting in the same rotational sense and the coil continues to spin one way.

✦ Answer: forces on the coil sides rotate it; the commutator reverses current every half-turn to keep rotation continuous.

Question 8

State the conditions under which a current is induced in a coil, and name the rule for the direction of the induced current.

Accepted Answer

Step 1 — A current is induced when the magnetic field linked with the coil CHANGES — by moving a magnet in/out of the coil, or moving the coil in a field, or changing the current in a nearby coil.

Step 2 — The direction of the induced current is found by Fleming's right-hand rule (Forefinger→Field, Thumb→motion, Middle→induced Current).

✦ Answer: a changing magnetic flux induces current; direction by Fleming's right-hand rule.

Question 9

Differentiate between an AC generator and a DC generator. Why is AC preferred for transmission, and what is the frequency of AC in India?

Accepted Answer

Step 1 — AC generator uses slip rings → output current reverses direction periodically (alternating current). DC generator uses a split-ring commutator → output is unidirectional (direct current).

Step 2 — AC is preferred for transmission because its voltage can be easily stepped up (to reduce current and hence I²R losses) and stepped down using transformers.

Step 3 — In India, AC has a frequency of 50 Hz (it reverses 50 times per second).

✦ Answer: slip rings (AC) vs commutator (DC); AC transmits efficiently via transformers; 50 Hz in India.

Question 10

Name the three wires in a domestic circuit and their colours. Explain the difference between overloading and short-circuiting, and how a fuse protects the circuit.

Accepted Answer

Step 1 — Live (red/brown, ~220 V), Neutral (black/blue, ~0 V), Earth (green, safety wire to ground).

Step 2 — Overloading: too many appliances draw a large total current. Short-circuiting: live and neutral touch directly so resistance ≈ 0 and the current becomes very large.

Step 3 — Both cause excessive heating (H = I²Rt). A fuse (a thin, low-melting wire in the live wire) melts when the current exceeds a safe value, breaking the circuit and preventing fire/damage.

✦ Answer: live/neutral/earth with colours; overload = too much load, short circuit = live–neutral contact; fuse melts to break the circuit.

Practice — Magnetic Effects of Electric Current

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