Electromagnetism

Question 1

Electromagnetism

Answer 1

The magnetic effect produced by a current

Question 2

Strength of magnetic field

Answer 2

Increases with decreasing distance from the wire

Increases with increasing current

Question 3

Magnetic field in solenoid

Answer 3

Magnetic field is stronger inside the solenoid, it is almost uniform.

Strength of magnetic field
- increases with increasing current,
- increases with the number of
turns per unit length of the
solenoid
- increases when a soft iron core is
placed within the solenoid.
(this is because the iron core concentrates the magnetic field lines within it)

Question 4

EM in Circuit breaker

Answer 4

2. When the current is within the limit:
   - The solenoid’s magnetic field is not strong enough to attract the soft iron latch.
   - The interrupt point remains closed and the circuit is closed.
3. When there is a sudden surge in current (e.g. short circuit or overloading):
   - The solenoid becomes a very strong electromagnet that attracts the soft iron latch.
   - The spring is released which pushes the safety bar outwards.
   - The interrupt point is now open and circuit becomes broken.
4. After the fault is repaired, the circuit breaker can be reset by pushing the safety bar button.

Question 5

Motor effect

Answer 5

Motor effect:

A current-carrying conductor experiences a force, when it is placed within the influence of an external magnetic field.

Question 6

Why a current-carrying conductor in a magnetic field experiences a force:

Answer 6

• The magnetic fields of the magnet and the conductor (e.g. wire) interact.
• The interaction of the two magnetic fields causes a force to act on the conductor (from the stronger field to the weaker field).

Question 7

Why a current-carrying coil in a magnetic field experiences a turning effect:

Answer 7

1. The magnetic fields of the magnet and the current- carrying coil interact with each other
2. The interaction of the two magnetic fields causes a pair of equal and opposite forces to act on opposite sides of the coil
3. This produces a resultant moment on the coil about the axis

Question 8

Split-ring commutator

Answer 8

Purpose: To reverse the direction of
the current in the coil every time it passes the vertical position (whenever the commutator changes contact from one brush to another) →This ensures that the coil always turns in the same direction (forces act to provide same turning effect).
→ Otherwise, the coil will stop turning at the vertical position if the current is not cut off (the forces will not provide a turning effect then as the resultant moment at the vertical position is zero).

Question 9

Two carbon brushes

Answer 9

• Purpose: To ensure good electrical
  contact between the circuit and the
  coil
  →Carbon is a good electrical conductor. →Brushes minimise friction losses.

Question 10

To increase the turning effect of the coil

Answer 10

insert a soft iron core or cylinder in the coil
(to concentrate the magnetic field lines to make the magnetic field stronger)
- increase the strength of the permanent magnets
- increase the number of turns per unit length on the coil
- increase the current in the coil