Class 10 Science Chapter 12: Important & Previous Year Questions – Bihar Board

Correct answer: (A) Maxwell

The right-hand thumb rule for the field of a straight current-carrying wire is also called Maxwell's corkscrew rule. Fleming's rules are for force and induced current; Oersted discovered the magnetic effect of current (lesson 3).

Hold the current-carrying straight conductor in the right hand so that the thumb points along the current. Then the fingers wrapped around the conductor give the direction of the magnetic field lines. It is also called Maxwell's corkscrew rule (lesson 3).

• Dynamo (electric generator): a device that converts mechanical energy into electrical energy.
• Principle: electromagnetic induction — when a coil rotates in a magnetic field the field linked with it changes and an induced current flows. Direction by Fleming's right-hand rule.
• Parts: rectangular coil ABCD between the poles (N, S) of a strong magnet; axle; slip rings (for AC) or a split ring (for DC); carbon brushes; galvanometer in the outer circuit.
• Working: turning the axle moves arm AB up and CD down — current is induced in both; after half a turn the arms swap and the current reverses. Slip rings give alternating current (AC); a split ring gives one-way direct current (DC).
• (Diagram: label the poles, coil ABCD, rings, brushes, galvanometer.) This part is not in the current NCERT book; the Bihar Board asks it (lesson 7).

Correct answer: (A) Oersted

Hans Christian Oersted saw in 1820 that a compass needle turns near a current-carrying wire (lesson 1).

Correct answer: (A) Imaginary

Field lines are an imaginary way to show a field — iron filings only show their pattern (lesson 2).

Correct answer: (A) Generator

A generator makes current from mechanical energy; a motor does the reverse — electrical to mechanical (lesson 7).

Correct answer: (B) decreases considerably

In a short circuit the live and neutral wires touch directly, the resistance falls greatly, so the current rises sharply (NCERT exercise 2 asks this current) (lesson 8).

When the magnetic field linked with a coil (conductor) changes — by moving a magnet towards or away from the coil, or by moving the conductor in a field — an induced potential difference and induced current are produced in the coil. This is electromagnetic induction (Faraday). The direction is given by Fleming's right-hand rule (lesson 7).

If a current-carrying straight conductor is held in the right hand with the thumb along the current, the fingers curled around it give the direction of the magnetic field. (Also called the right-hand thumb rule or the corkscrew rule.) (lesson 3)