Magnetic Effects of Electric Current
What the exam tests
Magnetic Effects of Electric Current tests magnetic field patterns, the right-hand thumb rule, electromagnets, force on a current-carrying conductor, electric motors, electromagnetic induction, generators and domestic circuits. Direction questions are common.
Core rules
- Magnetic field lines run from north to south outside a magnet and never cross; their closeness shows field strength.
- Right-hand thumb rule: hold the conductor with the thumb along the current; the curled fingers give the field direction.
- A solenoid behaves like a bar magnet. The end from which the current appears clockwise is the south pole.
- Force on a conductor: F = BIl sin θ. Fleming's left-hand rule gives the direction: forefinger = field, middle finger = current, thumb = force.
- Electric motor converts electrical energy into mechanical energy; a split ring commutator reverses current every half turn.
- Electromagnetic induction: a changing magnetic field induces a current. Fleming's right-hand rule gives the direction.
- AC reverses direction periodically; in India the frequency is 50 Hz. DC flows in one direction.
- In domestic wiring, the live wire is at high potential, the neutral is at zero, and the earth wire is a safety connection.
Worked example
A solenoid is viewed from one end, and the current appears clockwise. Which pole faces the viewer? Apply the right-hand thumb rule: curl the fingers in the clockwise direction of the current; the thumb points away from the viewer. The field enters the solenoid at the viewer's end, so that end is the south pole.
Traps that cost marks
- Using Fleming's right-hand rule for a motor; the motor uses the left-hand rule.
- Saying field lines can cross at the neutral point.
- Forgetting that the induced current direction opposes the change that produced it.
- Confusing AC and DC graphs; AC crosses zero and reverses.