Class 10 Science, Chapter 12 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
Why do two magnetic field lines never cross each other?
(b) The field has one direction at each point
If two lines crossed, the compass needle at that point would have to point two ways at once, which is impossible.
What is a magnetic field line?
(c) A representation of the magnetic field, not a physical object
A magnetic field line is a representation used to show the direction and strength of a magnetic field; it is not a physical object in space.
What shape do the magnetic field lines around a straight current-carrying wire have?
(c) Concentric circles centred on the wire
The magnetic field lines around a straight current-carrying wire are concentric circles centred on the wire.
What happens to the strength of the magnetic field around a straight current-carrying wire as the distance from the wire increases?
(d) It becomes weaker
The magnetic field around a straight current-carrying wire becomes weaker as the distance from the wire increases.
A straight wire carries current downward. Which hand and which part of it gives the direction of the magnetic field around the wire?
(a) Right hand; curled fingers
Point the right thumb along the direction of the current. The curled fingers then show the direction of the magnetic field.
A long straight wire carries current into the page. What is the direction of its magnetic field near the wire?
(b) Clockwise around the wire
Current into the page produces a magnetic field directed clockwise around the wire, as given by the right-hand thumb rule.
At the centre of a current-carrying circular loop, how do the fields from different parts of the loop combine?
(c) They reinforce, all pointing the same way
Every part of the loop carries current in a consistent sense around the ring. Each part's field at the centre points the same way, perpendicular to the loop's plane. So the fields add up instead of cancelling.
Current in a circular loop appears clockwise when viewed from one face. Which pole does that face behave as?
(c) South
Viewed from a face, anticlockwise current makes that face north and clockwise current makes it south. Since the current is clockwise, that face behaves as a south pole.
How is magnetic field strength distributed across the interior near the centre of a long current-carrying solenoid?
(a) Nearly constant and nonzero
Near the centre of a long current-carrying solenoid, the interior magnetic field is nearly uniform and nonzero. Its field lines are approximately parallel and equally spaced.
A solenoid carries current, and the direction of the current through it is reversed. What happens to its magnetic poles?
(c) The north and south ends swap
Reversing the current direction reverses the solenoid's magnetic poles, so the end that was north becomes south and the end that was south becomes north.
In Fleming's left-hand rule, what does the thumb show?
(d) Force
In Fleming's left-hand rule, the forefinger shows the magnetic field, the middle finger shows the current, and the thumb shows the force on the conductor.
What must a current-carrying wire have, besides current, to feel a magnetic force from a nearby magnetic field?
(c) A current in the wire and an external magnetic field that is not parallel to it
A wire feels a magnetic force only when current flows through it and the field is not parallel to the current. A magnetic field with no current, or a current parallel to the field, produces no force.
In Fleming's right-hand rule, what does the middle finger show?
(b) The induced current
In Fleming's right-hand rule, the thumb shows the motion, the forefinger the field, and the middle finger the induced current.
In the red-black-green domestic wiring convention, which wire is normally at high potential relative to earth?
(d) Red wire
The live wire is at high potential relative to earth and has red insulation in this colour convention. The black neutral wire is near earth potential, and the green earth wire connects exposed metal parts to earth.
The live wire touches the neutral wire directly, so the circuit resistance suddenly becomes very small. What happens to the current in the circuit?
(d) It rises sharply
When live and neutral touch directly, circuit resistance drops sharply. By Ohm's law, current rises sharply for the same voltage.
Wrong
The compass deflects the same way after reversing the current.
Why it feels right. The wire stays in the same position.
Right
The field direction around the wire depends on the direction of current.
Worked example
