CBSE Class 10 Science Magnetic Effects of Electric Current MCQs Set 10

Answer: (d) generator

Explanation:
1. A generator produces current by moving a coil in a magnetic field, which is electromagnetic induction.
2. A motor works on the opposite effect (force on a current-carrying conductor in a field).

Teacher's Note:
1. Generator = induction; motor = force on conductor.
2. Galvanometer and voltmeter work on the motor effect.

Answer: (c) producing induced current in a coil due to relative motion between a magnet and the coil

Explanation:
1. Electromagnetic induction is the production of an induced current in a coil when there is relative motion between the coil and a magnet.
2. The other options describe charging, electromagnetism and the motor.

Teacher's Note:
1. Changing magnetic field around a coil gives an induced current.
2. No relative motion means no induced current.

Answer: (a) generator

Explanation:
1. A generator converts mechanical energy into electrical energy.
2. A galvanometer detects current, an ammeter measures it, and a motor uses it.

Teacher's Note:
1. Generator makes current; motor uses current.
2. Galvanometer only detects small currents.

Answer: (c) the rotation of the armature causes the field through it to reverse

Explanation:
1. As the armature rotates, the flux linked with it changes from one direction to the other each half turn.
2. This reverses the induced current, so the output is alternating.
3. The field magnet itself is steady, and a plain generator has slip rings, not a commutator.

Teacher's Note:
1. An AC generator uses slip rings.
2. Reversal happens every half rotation.

Answer: (d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions

Explanation:
1. When the key is inserted, current in coil 1 rises from zero, so the field through coil 2 changes and a current is induced.
2. When the key is removed, the field falls to zero, which again induces a current, but in the opposite direction.
3. Both effects are momentary, so the answer is (d).

Teacher's Note:
1. Induced current flows only while the field is changing.
2. Increasing and decreasing flux induce currents in opposite directions.

Answer: (d) moving out of the solenoid

Explanation:
1. Current is induced only while the magnet moves; there is none when it is at rest.
2. The magnet was pushed in slowly but pulled out rapidly, and a faster change induces a larger current.
3. So the largest deflection occurs when the magnet is moving out.

Teacher's Note:
1. No motion means no induced current.
2. Faster motion gives a larger induced current.

Answer: (c) half revolution

Explanation:
1. Each half turn, the sides of the coil move in the opposite direction through the field.
2. So the induced current reverses once in every half revolution.

Teacher's Note:
1. This is why a simple generator gives AC.
2. Two reversals per full turn.

Answer: (d) electromagnetic induction

Explanation:
1. Producing a current in a coil by a changing magnetic field is electromagnetic induction.

Teacher's Note:
1. Induction: changing field gives current.
2. The magnetic effect of current is the reverse: current gives a field.

Answer: (c) the direction of the induced current

Explanation:
1. Fleming's right-hand rule tells the direction of the induced current when a conductor moves in a magnetic field.
2. It does not tell the magnitude.

Teacher's Note:
1. Thumb: motion; forefinger: field; middle finger: induced current.
2. Right hand for generators.

Answer: (d) an AC generator has slip rings while a DC generator has a commutator

Explanation:
1. An AC generator uses two slip rings, so the output reverses every half turn.
2. A DC generator uses a split-ring commutator to keep the output in one direction.
3. So (d) is correct.

Teacher's Note:
1. Slip rings: AC. Split-ring commutator: DC.
2. The magnet type does not decide AC or DC.

Answer: (a) How fast the magnet is pushed into the coil

Explanation:
1. The size of the induced current depends on how fast the field changes.
2. The speed of pushing affects the magnitude; the other factors affect only the direction.

Teacher's Note:
1. Faster motion gives a larger deflection.
2. Direction of winding or the pole entering only reverses the deflection.

Answer: (d) electromagnetic induction

Explanation:
1. Moving a conductor in a magnetic field produces a current in it for a short while.
2. This phenomenon is electromagnetic induction.

Teacher's Note:
1. Motion in a field gives induced current.
2. The deflection stops when the rod stops.

Answer: (c) only for the period during which flux changes through it

Explanation:
1. Current is induced only while the magnetic field linked with the coil is changing.
2. When the change stops, the induced current stops too.

Teacher's Note:
1. Steady field means no induced current.
2. A galvanometer kick lasts only as long as the motion.

Answer: (a) dynamo

Explanation:
1. A dynamo (generator) produces current when a coil rotates in a magnetic field, which is electromagnetic induction.
2. A galvanometer and a simple motor work on the force on a current-carrying coil.

Teacher's Note:
1. Dynamo = generator.
2. Motor effect and induction are opposite effects.

Answer: (a) AC can be transmitted without much loss of energy

Explanation:
1. AC can be sent over long distances at a high voltage and low current using transformers.
2. This keeps the loss of energy in the wires small.

Teacher's Note:
1. Power loss is I²R, so low current means low loss.
2. This is why AC is used for the mains.

Answer: (b) electromagnetic induction

Explanation:
1. Rotating a coil in a magnetic field changes the magnetic field through the coil.
2. This induces a current, so the generator works on electromagnetic induction.

Teacher's Note:
1. Generator: induction. Motor: force on a current.
2. The same principle applies to DC generators.

Answer: (d) all of the above

Explanation:
1. Anything that changes the magnetic field through the coil induces a current.
2. Moving the coil, changing the field strength and changing the orientation all do this.

Teacher's Note:
1. Key idea: change in magnetic field linked with the coil.
2. A steady field with no relative change gives nothing.

Answer: (a) a split-ring type commutator

Explanation:
1. A split-ring commutator reverses the connections every half turn.
2. This makes the output current flow in one direction only, giving DC.

Teacher's Note:
1. Slip rings: AC. Split rings: DC.
2. A stronger magnet only increases the size of the e.m.f.

Answer: (b) alternating current

Explanation:
1. A current that reverses direction at regular intervals is an alternating current.
2. Direct current always flows in one direction.

Teacher's Note:
1. AC in India has a frequency of 50 Hz.
2. It reverses direction 100 times each second.

Answer: (b) 100

Explanation:
1. In one cycle the current changes direction twice.
2. In 50 cycles per second, the polarity changes 2 × 50 = 100 times.

Teacher's Note:
1. Two reversals per cycle.
2. Frequency = cycles per second.

Answer: (c) increases heavily

Explanation:
1. In a short circuit the live and neutral wires touch, so the resistance becomes very small.
2. By Ohm's law the current becomes very large.

Teacher's Note:
1. Low resistance means a very large current.
2. A fuse protects against this.

Answer: (c) no current

Explanation:
1. Under normal working, no current flows in the earth wire.
2. It carries current only when there is a leak, and then passes it safely to the ground.

Teacher's Note:
1. It keeps the metal body at earth potential.
2. It prevents electric shock.

Answer: (b) low melting point

Explanation:
1. A fuse wire must melt quickly when the current becomes too large.
2. So it should have a low melting point, and a high resistance relative to its thickness.

Teacher's Note:
1. Fuse wire is made of a tin-lead alloy.
2. Copper is not used as it melts at a high temperature.

Answer: (b) miniature circuit breaker

Explanation:
1. MCB is the short form of miniature circuit breaker.
2. It switches off the circuit automatically when the current is too high.

Teacher's Note:
1. MCBs are replacing the older fuses in homes.
2. They can be reset after tripping.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A changing field induces current in a coil, so A is true.
2. A galvanometer with zero in the middle can deflect to either side, so R is true.
3. R does not explain why the current is induced, so (b).

Teacher's Note:
1. The deflection direction shows the direction of the induced current.
2. Zero at the centre helps detect both directions.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Rotating the coil in a magnetic field changes the flux through it and induces a current.
2. Hence the generator converts mechanical energy into electrical energy by induction; R explains A.

Teacher's Note:
1. Rotate the coil: get current.
2. This is why a generator is turned by turbines or engines.

Answer: (d) A is false, but R is true.

Explanation:
1. An induced current is produced by a changing magnetic field, not a changing electric field, so A is false.
2. The direction of induced current is given by Fleming's right-hand rule, so R is true.

Teacher's Note:
1. Look for the changed word in the assertion.
2. Induction needs a changing magnetic field.

Answer: (c) A is true, but R is false.

Explanation:
1. Induced current needs a change in the field linked with the coil, so A is true.
2. A steady flux, however large, does not give a current, so R is false.

Teacher's Note:
1. No change means no induced current.
2. Size of the field alone does nothing.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A fuse wire melts when the current becomes too large.
2. This breaks the circuit and prevents overheating and fires, so R explains A.

Teacher's Note:
1. The fuse is connected in series with the live wire.
2. Never replace a fuse by a thicker wire.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The live wire is at a high potential (220 V).
2. If a person touches it, current passes through the body to the earth and can be fatal.
3. So R explains A.

Teacher's Note:
1. Never touch live wires with wet hands.
2. Earthing and fuses are safety measures.