1. Current in a Magnetic Field
Chapter 27

2. To show that a Current-Carrying Conductor in a Magnetic Field experiences a Force
Send a current through the tinfoil.
The foil will move forwards. Reverse the current and the foil will move backwards.
Conclusion: A current-carrying conductor in a magnetic field experiences a force.

3. The moving coil meter and the moving coil loudspeaker are based on the principle that a current-carrying conductor in a magnetic field experiences a force.

4. A simple d.c. Motor is based on the principle that a current-carrying conductor in a magnetic field experiences a force.

5. Why would you expect a current-carrying conductor placed in a magnetic field to experience a force?
A current-carrying conductor has a magnetic field around it due to the current.
When this conductor is placed in another magnetic field, the two magnetic fields interact (push off each other!).
This causes the force on the current-carrying conductor.

6. What is the Direction of the Force on a Current-Carrying Conductor in a Magnetic Field?
The direction of the force is always:
 Perpendicular to the current
 Perpendicular to the magnetic field
NOTE: A Current-Carrying Conductor in a magnetic field experiences no force if the conductor is parallel to the magnetic field.

7. Fleming’s Left-Hand Rule: If the thumb, first finger and second finger of the left hand are held at right angles,
with the first finger in the direction of the magnetic field and the second finger in the direction of the current, then the thumb points in the direction of the force.

8. What determines the Size of the Force on a current-carrying conductor in a magnetic field?
The size of the Current
The Length of the conductor
How strong the Magnetic Field is
The Angle between the conductor and the magnetic field

9. Magnetic Flux Density (B)
Accurate experiments show that if a conductor of length l, carrying a current I is placed at right angles to a uniform magnetic field it experiences a force F where:
F  I and F  l
It follows that: F  I l
 F = I l B where B is a constant.
The value of B depends on how strong the magnetic field is.
In a strong magnetic field B is large and in a weak field B is small.
Thus B is a measure of how strong the magnetic field is.
B is called the Magnetic Flux Density.

10. Define Magnetic Flux Density
At a point in a magnetic field the Magnetic Flux Density ( B ) is a vector whose:
direction is the direction of the force on a north pole placed at that point
magnitude is the value of B from the equation F = I l B
or put another way:
The magnetic Flux Density (B) at a point in a magnetic field is a vector whose:
magnitude is equal to the force that would be experienced by a conductor of length 1 m carrying a current of 1 A at right angles to the field at that point. Its direction is the direction of the force on a north pole placed at that point.

11. What is the SI Unit of Magnetic Flux Density?
The SI unit of magnetic flux density is the tesla (T)

12. If the conductor is not perpendicular to the field resolve the B into two perpendicular components - one parallel to the conductor and the other at right angles to the conductor. It is the component of B that is perpendicular to the conductor that causes the force on it. The parallel component has no effect on the wire.
F = B I l Sin 30o

13. The coil is free to rotate about the axis.
Convince yourself that the directions of the forces on the sides of the coil are correct and that the coil will begin to rotate.

14. To Show the Force on a current-carrying coil in a magnetic field
Use the equipment above. The coil is free to rotate about the axis.
When the current is switched on the coil starts to rotate as shown.

15. A Beam of Electrons in a cathode ray tube is an Electric Current
A beam of electrons in a cathode ray tube moves in a vacuum.
The beam passes close to a fluorescent screen and shows up as a beam of light.
The moving electrons have negative charge and thus are an electric current.
They, therefore, have a magnetic field around them.

16. Force on a Moving Charge in a Magnetic Field
This magnetic field, due to the beam of moving charges (the electrons), will interact with any other magnetic field placed near it.
The picture shows the beam of electrons deflecting due to the presence of a bar magnet.

17. F = q v B The Size of the Force on a Moving Charge in a Magnetic Field
A charge of q coulombs moving with a speed of v metres per second at right angles to a magnetic field of flux density B teslas experiences a force of F newtons, given by;
F = q v B

18. A charged particle moving at constant speed enters a uniform magnetic field and moves at right angles to the field. Explain why the particle moves in a circle.
When the charged particle enters the magnetic field there is a force on it.
The force is at right angles to its direction of motion. Therefore its speed does not change. Only its direction of motion changes.
The force on it has a constant magnitude
(F = q v B.).
As it turns the force always remains at right angles to the direction of motion.
Thus the particle moves in a circular path.

19. Electric Current and Electric Charge
An Electric Current is a flow of charge.
The ampere (A).
The coulomb (C).
1 coulomb is the amount of charge that passes any point in a circuit when a current of 1 ampere flows for 1 second.
What is an Electric Current?
What is the SI Unit of electric current?
What is the SI Unit of electric charge?
Define the coulomb.

20. Electric Current and Electric Charge
What is the relationship between Electric Current and Electric Charge?
The current (I) is the amount of charge (Q) passing per second
Q = I t
Where: Q is charge gone past
I is the steady current
t is the time taken.

21. Magnetic Forces between Currents
Two parallel conductors carry current in opposite directions.
Each current creates a magnetic field around itself.
The magnetic fields interact with each other and cause a force on each conductor, pushing the conductors apart.
If the conductors carry current in the same direction the force between them is attractive.

22. State the principle on which the definition of the ampere is based.
The definition of the ampere is based on the principle that:
Two current carrying conductors exert a force on each other due to their magnetic fields.

23. The Ampere The ampere is that current which:
if maintained in two infinitely long parallel wires, is of negligible cross section
is placed 1 metre apart in a vacuum
would produce a force on each wire of 2 × 10-7 newtons per metre of length

24. Experiment to demonstrate the principle on which the definition of the ampere is based
Use the equipment as shown.
Send a current through the parallel strips of tin foil.
The foil strips will be seen to move away from each other.
Conclusion: There is a force between current carrying conductors due to their magnetic fields.