Magnetic Effects of Electric Current – Class 10 Science notes, important questions and MCQs

Chapter overview

An electric current produces a magnetic field, and a magnetic field can push a current-carrying wire. These effects run electric motors, while changing magnetic fields produce current in generators.

Magnetic field of a current

• Field lines form closed loops; they never cross; closer lines mean a stronger field.
• Straight wire: concentric circles; direction by the right-hand thumb rule.
• Solenoid: field like a bar magnet; with a soft-iron core it becomes an electromagnet.

Force on a conductor

• A current-carrying wire in a magnetic field feels a force (Fleming's left-hand rule).
• Force is maximum when the current is perpendicular to the field.
• The electric motor uses this effect.

Domestic circuits

• Live wire (red/brown), neutral (black/blue), earth (green); India supplies 220 V AC at 50 Hz.
• Earthing protects from shocks through metal bodies.
• Fuse and MCB protect against overloading and short circuits.

Key terms

Solenoid

a long coil of insulated wire

Electromagnet

a temporary magnet made by a current in a coil around soft iron

Short circuit

live and neutral wires touching directly, causing a large current

Formulas

• Right-hand thumb rule: thumb = current, fingers = field
• Fleming's left-hand rule: First finger Field, seCond finger Current, thuMb Motion

Important questions with answers

Quick revision

• Current produces a magnetic field.
• Motor = left-hand rule.
• Earth wire carries leaked current safely to the ground.

Exam tips

• Field inside a solenoid is uniform.
• Overloading happens when too many appliances run on one socket.

Practice MCQs

1. Magnetic field lines around a straight current-carrying wire are

   1. straight lines
   2. concentric circles
   3. parabolas
   4. random
   Show answer

   Answer: (b) concentric circles. Concentric circles.

2. The colour of the earth wire is usually

   1. red
   2. black
   3. green
   4. blue
   Show answer

   Answer: (c) green. Green.

3. AC supply in India has a frequency of

   1. 60 Hz
   2. 50 Hz
   3. 100 Hz
   4. 220 Hz
   Show answer

   Answer: (b) 50 Hz. 50 Hz.

4. The direction of force on a current-carrying conductor in a magnetic field is given by

   1. right-hand thumb rule
   2. Fleming's left-hand rule
   3. Ohm's law
   4. Snell's law
   Show answer

   Answer: (b) Fleming's left-hand rule. Fleming's left-hand rule.

5. A soft-iron core inside a solenoid

   1. weakens the field
   2. reverses the field
   3. stops the current
   4. makes it an electromagnet
   Show answer

   Answer: (d) makes it an electromagnet. Soft iron is easily magnetised.

6. Which device protects a circuit from overloading?

   1. switch
   2. voltmeter
   3. fuse
   4. bulb
   Show answer

   Answer: (c) fuse. Fuse melts at high current.

7. The force on a wire is maximum when the angle between current and field is

   1. 0°
   2. 45°
   3. 90°
   4. 180°
   Show answer

   Answer: (c) 90°. Perpendicular.

8. Why do magnetic field lines never cross?

   1. they are always parallel
   2. they are imaginary
   3. there would be two directions of field at one point
   4. they are curved
   Show answer

   Answer: (c) there would be two directions of field at one point. Field has one direction at each point.