Here you can find out more about magnetic fields due to a current carryin wire, Lorentz force and Fleming's left-hand rule.

1. Chapter 21
Electromagnetism

2. At the end of the chapter,
you should be able to
 Describe experiments to show the force on
a current-carrying conductor, and on a
beam of charged particles, in a magnetic
field, including the effect of reversing
 the current,
 the direction of field.

3. At the end of the chapter,
you should be able to
 State the relative directions of force, field
and current when any two of these
quantities are at right angles to each other,
using Fleming’s left-hand rule.

4. At the end of the chapter,
you should be able to
 Explain how a current-carrying coil in a
magnetic field experiences a turning effect
and that the effect is increased by
increasing
 the number of turns on the coil,
 the current.

5. At the end of the chapter,
you should be able to
 Discuss how this turning effect is used in
the action of an electric motor.
 Describe the action of a split-ring
commutator in a two-polt, single-coil motor
and the effect of winding the coil on to a
soft-iron cylinder.

6. Magnetic Effect of a Current
 Electric current produce a magnetic field
even if the wire itself is not made of
magnetic material.

7. Magnetic Effect of a Current
Magnetic field pattern due to a straight wire.
 The direction of the magnetic field can be
determined by the Right-hand rule.
If you imagine that the conductor is
held in the right hand, with the
thumb pointing in the direction of
the current flow, then the fingers
curl the direction of the magnetic
field

8. Magnetic Effect of a Current
Magnetic field pattern due to a straight wire.
 The direction of the magnetic field can be
determined by the Right-hand rule.
current
Current flowing out of paper

9. Magnetic Effect of a Current
Magnetic field pattern due to a straight wire.
 The direction of the magnetic field can be
determined by the Right-hand rule.
current
Current flowing into the paper

10. Magnetic Effect of a Current
Magnetic field pattern due to a flat coil

11. Magnetic Effect of a Current
Magnetic field pattern due to a flat coil
 To increase the strength of the magnetic field
at the centre of the flat coil.
- Increase the current in the coil.
- Increase the number of turns of
the flat coil.

12. Magnetic Effect of a Current
Magnetic field pattern of a solenoid

13. Magnetic Effect of a Current
Magnetic field pattern of a solenoid
 To increase the strength of the magnetic field
of a solenoid.
- Increase the current in the solenoid.
- Increase the number of turns
per unit length of the solenoid.
- Using a soft-iron core within the
solenoid.

14. Quiz
 Describe an experiment that will allow you
to observe the magnetic field patterns
produced by a current-carrying straight
wire.

15. Applications of a Magnetic Effect
of Current
 circuit breaker
A ______________ is used to protect the
appliance from excessive current flow.

16. Applications of a Magnetic Effect
of Current
 current
When a larger _______ flows through the
circuit breaker, the magnetic field produced
stronger
by the solenoid becomes ___________.
 The solenoid becomes a ______________
strong electromagnet.
attract
It is now able to ___________ the latch and
the spring decompresses. The spring
pushes the safety bar out of the interrupt
switches off
point and ___________ the circuit.

17. Applications of a Magnetic Effect
of Current
 When the reset button is pushed, the spring
compresses and the safety bar returns to
close
_______ the interrupt point. The spring
stays compressed due to the soft iron
latch
___________ and current can flow through
the circuit breaker again.

18. Applications of a Magnetic Effect
of Current
 A circuit breaker is not suitable for use with
high voltage electricity because the electric
flow
current can _______ across the small
opening in the interrupt point when the
potential difference is sufficiently
high
___________.

20. Quiz
 State the effect of a larger current on the
magnetic force between the solenoid and
the soft iron latch in a circuit breaker.
The magnetic force will be stronger.

21. Force on a Current-Carrying
Conductor in a Magnetic Field
Investigation of the force on a
current-carrying conductor in a
magnetic field (Lorentz Force)
(Textbook, Page 422)

22. Force on a Current-Carrying
Conductor in a Magnetic Field
 Apparatus
 Stiff wire, string permanent magnets,
9V d.c. power supply.

23. Force on a Current-Carrying
Conductor in a Magnetic Field
 Procedures:
 Bend a stiff wire ABCD into the shape
of a swing as shown in the figure.

24. Force on a Current-Carrying
Conductor in a Magnetic Field
 Procedures:
 Place the magnet over the wire BC as
shown.

25. Force on a Current-Carrying
Conductor in a Magnetic Field
 Procedures:
 Switch on the current. Observe the
direction in which the wire is swung.

26. Force on a Current-Carrying
Conductor in a Magnetic Field
 Procedures:
 Reverse the direction of the current by
switching the polarity of the dry cell. In
which direction is the swing flung
now?

27. Force on a Current-Carrying
Conductor in a Magnetic Field
 Observations:
 With the current flowing in the
direction A --> B --> C --> D, the wire
is observed to swing outwards from
the magnet.
 If the current is reversed, the swing of
the wire will be reversed (inwards)

29. Force on a Current-Carrying
Conductor in a Magnetic Field
 In conclusion to the experiment, we can
say that
 A force acts on the current-carrying
wire when placed in a magnetic field.
 The force acts at right angle to both
the current direction and the direction
of the magnetic field.

30. Force on a Current-Carrying
Conductor in a Magnetic Field
 In conclusion to the experiment, we can
say that
 When the direction of current (or
magnetic field) is reversed, the
direction of the force on the wire
is also reversed.

31. Fleming’s Left-hand Rule
 The direction of the force can be
deduced by using this rule.
Motion of force
 thuMb - direction ofMotion of the wire.

32. Fleming’s Left-hand Rule
 The direction of the force can be
deduced by using this rule.
Motion of force Magnetic Field
(N-S direction)
 Forefinger - direction of magnetic Field

33. Fleming’s Left-hand Rule
 The direction of the force can be
deduced by using this rule.
Motion of force Magnetic Field
Current
 SeCond finger - direction of Current
(Conventional current)

34. Force on a Current-Carrying
Conductor in a Magnetic Field
 To explain the force exerted on the wire, we
S
need to consider the combined magnetic fields
due to the current flowing through the straight
wire and the magnet.
N

35. 22.1 Force on a Current-Carrying
Conductor in a Magnetic Field
 The wire moves because the magnetic field
of the permanent magnets reacts with the
magnetic field of the current in the wire.
S
N

36. 22.1 Force on a Current-Carrying
Conductor in a Magnetic Field
 The two fields acting in the same direction
combine to give a stronger field, but the two
field opposing each other combine to give a
weaker field.
weaker field
stronger field

37. 22.1 Force on a Current-Carrying
Conductor in a Magnetic Field
 Hence the unbalanced fields on both sides
produces a force that exerts on the wire.
weaker field
stronger field

38. Force on a Moving Charge in a
Magnetic Field
 Fleming’s left-hand rule can be applied to all
moving charges.
Force Magnetic Field
Current
(conventional
current)
 Note that the conventional current (flow
of positive charges) travels in an opposite
direction to that of the electron flow.

39. Moving-coil loudspeaker
 It consists a movable coil attached to the
large cone.

40. Moving-coil loudspeaker
 The cone fits loosely over the centre of a
cylindrical permanent magnet so that the
coil is in a strong magnetic field.

41. Moving-coil loudspeaker
 The wire of the coil is at right angles to
the field.

42. Moving-coil loudspeaker
S
 If the current flows in the direction shown
about, the coil will move out of the page.

43. Moving-coil loudspeaker
S
 If the direction of the current reverses, the
coil will move in the opposite direction
(into the page).

44. Simple Circuit Breaker

45. Simple Circuit Breaker
• When the live wire carries the usual
operating current the electromagnet is not
strong enough to separate the contacts.

46. Simple Circuit Breaker
• If something goes wrong with the
appliance and a large current flows
the electromagnet will exert a strong
magnetic force to separate the contacts
and break the circuit.
The spring then keeps the contacts apart.

47. Simple Circuit Breaker
• After the fault is repaired, the contacts can
then be pushed back together
by lifting a switch on the outside of the
circuit breaker.