CBSE Class 10 Science Magnetic Effects of Electric Current MCQs Set 08

Answer: (c) If magnetic field lines are parallel and equidistant, they represent zero field strength.

Explanation:
1. Statements (a), (b) and (d) are true properties of field lines.
2. Parallel and equidistant lines represent a uniform field, which is not zero.
3. So statement (c) is incorrect.

Teacher's Note:
1. Closer lines mean a stronger field.
2. Equally spaced parallel lines mean the same strength everywhere, i.e. a uniform field.

Answer: (c) anticlockwise

Explanation:
1. Looking at a face of a current loop, anticlockwise current makes that face a north pole.
2. Clockwise current makes it a south pole.
3. So the current is anticlockwise.

Teacher's Note:
1. Anticlockwise gives North; clockwise gives South (think of the letters N and S).
2. Use the right-hand thumb rule to confirm.

Answer: (a) soft iron

Explanation:
1. The core must become strongly magnetised when current flows and lose magnetism quickly when it stops.
2. Soft iron does this, while steel keeps its magnetism.
3. So the answer is soft iron.

Teacher's Note:
1. Soft iron = temporary magnet; steel = permanent magnet.
2. Brass and aluminium are not strongly magnetic.

Answer: (b) Iron

Explanation:
1. Iron is strongly attracted by magnets.
2. Mercury, gold and silver are not magnetic in this sense.
3. So the answer is iron.

Teacher's Note:
1. Iron, cobalt and nickel are the common magnetic materials.
2. Most other metals are non-magnetic.

Answer: (d) at the point of intersection there would be two directions of the magnetic force, which is impossible

Explanation:
1. The field line at a point shows the direction of the field there.
2. If two lines crossed, the compass needle at that point would point in two directions at once.
3. This is impossible, so field lines never intersect.

Teacher's Note:
1. A point in a field has only one direction of field.
2. This is a favourite reason-type question.

Answer: (d) magnetic field only

Explanation:
1. A current is a flow of charge, and moving charges produce a magnetic field (Oersted's experiment).
2. The wire as a whole is neutral, so it produces no net electric field.
3. So only a magnetic field is produced.

Teacher's Note:
1. Oersted showed that electricity and magnetism are related.
2. A neutral wire has equal positive and negative charge, so no net electric field.

Answer: (a) along the axis and parallel to each other

Explanation:
1. Inside the solenoid the field is uniform.
2. The lines are straight, parallel to each other and along the axis.

Teacher's Note:
1. Uniform field means parallel, equally spaced lines.
2. Do not confuse with the circular lines around a straight wire.

Answer: (d) Maxwell's cork-screw rule (right-hand thumb rule)

Explanation:
1. The direction of the field around a current-carrying conductor is given by the right-hand thumb rule, also called Maxwell's cork-screw rule.
2. Fleming's left-hand rule gives the direction of force, not of the field.

Teacher's Note:
1. Thumb along the current, curled fingers show the field direction.
2. Do not mix up the right-hand thumb rule with Fleming's left-hand rule.

Answer: (c) each piece will have two unlike poles

Explanation:
1. Magnetic poles cannot be isolated.
2. Each piece becomes a complete magnet with a north pole and a south pole.

Teacher's Note:
1. Poles always occur in pairs.
2. Cutting more times gives more small magnets.

Answer: (b) Electrical to magnetic

Explanation:
1. Current in the solenoid produces a magnetic field.
2. So electrical energy is converted into magnetic energy.
3. A fixed solenoid has no moving part, so no mechanical energy is produced; the answer is electrical to magnetic.

Teacher's Note:
1. Electric current produces a magnetic field.
2. Mechanical energy appears only if something moves, as in a motor.

Answer: (b) The bar will be magnetised as long as there is current in the circuit.

Explanation:
1. The field of the solenoid magnetises the soft iron by induction.
2. Soft iron loses its magnetism when the current is switched off, so it stays magnetised only while current flows.

Teacher's Note:
1. This is how an electromagnet works.
2. Steel would stay magnetised; soft iron does not.

Answer: (a) a bar magnet

Explanation:
1. A solenoid has a north and a south end like a bar magnet.
2. Its field lines resemble those of a bar magnet.

Teacher's Note:
1. Solenoid field = bar magnet field.
2. Field inside is uniform.

Answer: (d) is the same at all points

Explanation:
1. Inside a long solenoid the field lines are parallel straight lines.
2. This means the field is uniform, i.e. the same at all points inside.

Teacher's Note:
1. Parallel and equally spaced lines mean a uniform field.
2. The field is strongest and most uniform in the middle of a long solenoid.

Answer: (b) directly proportional to the current and inversely proportional to the distance from the wire

Explanation:
1. A larger current gives a stronger field, so the field is directly proportional to the current.
2. The field becomes weaker farther from the wire; for a long straight wire it is inversely proportional to the distance.
3. So (b) is correct.

Teacher's Note:
1. Field increases with current and decreases with distance.
2. The field lines are more crowded near the wire.

Answer: (c) Magnetic lines of force are far away from each other at the poles.

Explanation:
1. The field is strongest at the poles, so the lines are closest together there.
2. Statement (c) says they are far apart at the poles, which is incorrect.

Teacher's Note:
1. Crowded lines mean a strong field.
2. The lines are closest near the poles and spread out away from them.

Answer: (b) an iron piece into it

Explanation:
1. A soft iron core placed inside the solenoid becomes magnetised by the field of the solenoid.
2. The magnetised iron adds to the field, so the field becomes stronger.
3. Wood, glass and paper are not magnetic and do not help.

Teacher's Note:
1. Solenoid with a soft iron core is an electromagnet.
2. Using more turns or more current also increases the field.

Answer: (a) from south to north

Explanation:
1. Outside a magnet the lines go from north to south.
2. They are closed curves, so inside the magnet they go from the south pole to the north pole.

Teacher's Note:
1. Field lines never start or stop; they form closed loops.
2. Outside: N to S. Inside: S to N.

Answer: (c) Oersted

Explanation:
1. Hans Christian Oersted noticed that a compass needle is deflected when current flows in a nearby wire.
2. This showed that a current produces a magnetic field.

Teacher's Note:
1. Oersted: current gives magnetism. Faraday: magnetism gives current.
2. Fleming gave the hand rules.

Answer: (a) north pole is almost equal to that at the south pole

Explanation:
1. The magnetic force is strongest at the two poles and equally strong at both.
2. So nearly equal amounts of filings cling to the two poles, and very little at the middle.

Teacher's Note:
1. Poles are the strongest parts of a magnet.
2. The two poles of a bar magnet have equal strength.

Answer: (b) finished just after

Explanation:
1. A piece of soft iron near a magnet is magnetised by induction only as long as the magnet is present.
2. When the magnet is removed, the induced magnetism disappears at once.

Teacher's Note:
1. Induced magnetism is temporary in soft iron.
2. Steel would retain some magnetism.

Answer: (a) The field lines are directed from N to S inside the magnet.

Explanation:
1. Inside a magnet the field lines go from the south pole to the north pole, not N to S.
2. So statement (a) is incorrect.

Teacher's Note:
1. Outside: N to S. Inside: S to N.
2. The tangent at a point gives the field direction.

Answer: (c) a compass needle

Explanation:
1. A compass needle is a small magnet that turns when placed in a magnetic field.
2. Its deflection shows that a field is present.

Teacher's Note:
1. Compass = small pivoted magnet.
2. Its north end shows the field direction.

Answer: (a) gets reversed in direction

Explanation:
1. The field direction is given by the right-hand thumb rule.
2. If the current reverses, the direction of the field also reverses; its strength stays the same.

Teacher's Note:
1. Compass deflection also reverses.
2. Strength depends on the size of the current, not its direction.

Answer: (c) Both (I) and (III)

Explanation:
1. Field lines are imaginary lines, so (I) is true.
2. They are closed curves, so (III) is true.
3. They have direction as well as magnitude, so (II) is false.
4. They emerge from the north pole, so (IV) is false.

Teacher's Note:
1. Field lines are drawn for convenience.
2. They start from N outside the magnet.

Answer: (b) heart and brain

Explanation:
1. Tiny electric currents flow in the heart and the brain.
2. Any current produces a magnetic field, so these two organs produce magnetic fields.

Teacher's Note:
1. MRI uses magnetic fields to image the body.
2. Signals in nerves and the heart are electrical.

Answer: (d) both (a) and (b)

Explanation:
1. A larger current gives a stronger field.
2. More turns also give a stronger field.
3. Heat produced does not decide the field, so the answer is both (a) and (b).

Teacher's Note:
1. A soft iron core also strengthens the field.
2. Strength grows with current and turns.

Answer: (d) All of the above

Explanation:
1. Field lines form closed, continuous curves.
2. They never cross one another.
3. They are closest together near the poles, where the field is strongest.
4. So all three statements are properties of field lines.

Teacher's Note:
1. Closeness of lines shows strength.
2. Learn all the properties together.

Answer: (a) zero

Explanation:
1. The attraction of a magnet is greatest at its poles.
2. At the centre, the effects of the two poles cancel, so the magnetism is practically zero.

Teacher's Note:
1. Iron filings cling mostly at the ends of a bar magnet.
2. Very few filings stay at the middle.

Answer: (a) the upper face

Explanation:
1. Anticlockwise current as seen from a face makes that face a north pole.
2. From above the current is anticlockwise, so the upper face is the north pole.

Teacher's Note:
1. Anticlockwise: N. Clockwise: S.
2. The two faces always have opposite poles.

Answer: (c) right-hand thumb rule

Explanation:
1. The right-hand thumb rule gives the direction of the magnetic field around a straight current-carrying wire.
2. The other rules are for force or for induced current.

Teacher's Note:
1. Thumb along the current, fingers show the field.
2. Do not confuse it with Fleming's rules.