1. Magnetic Forces on Conductors
SPH4U

2. Force on a Conductor
also applies to charged particles moving through conductors. The force is on the entire conductor:

3. Force on a Conductor
also applies to charged particles moving through conductors. The force is on the entire conductor:

4. Force on a Conductor
also applies to charged particles moving through conductors. The force is on the entire conductor:

5. Force on a Conductor
also applies to charged particles moving through conductors. The force is on the entire conductor:
length of the conductor

6. Example 1
A wire in the armature of a motor is 25 cm long and perpendicular to a uniform magnetic field of 0.20 T. Calculate the force exerted on the wire when it carries a current of 15 A.

7. Example 1
A wire in the armature of a motor is 25 cm long and perpendicular to a uniform magnetic field of 0.20 T. Calculate the force exerted on the wire when it carries a current of 15 A.

8. Example 1
A wire in the armature of a motor is 25 cm long and perpendicular to a uniform magnetic field of 0.20 T. Calculate the force exerted on the wire when it carries a current of 15 A.

9. Field of a Conductor
Remember that this force exists because the conductor’s magnetic field is interacting with the external magnetic field.

10. Field of a Conductor
Remember that this force exists because the conductor’s magnetic field is interacting with the external magnetic field. What is the magnitude of the conductor’s magnetic field?

11. Field of a Conductor
The field of a conductor is directly proportional to the current and inversely proportional to the distance from the conductor:

12. Field of a Conductor
The field of a conductor is directly proportional to the current and inversely proportional to the distance from the conductor:

13. Field of a Conductor
The field of a conductor is directly proportional to the current and inversely proportional to the distance from the conductor:
m0 is the permeability of free space:
m0 = 4p x 10-7 T∙m/A

14. Field of a Conductor
The field of a conductor is directly proportional to the current and inversely proportional to the distance from the conductor:
m0 is the permeability of free space:
m0 = 4p x 10-7 T∙m/A
This is Ampere’s Law for a straight conductor.

15. Example 2
Calculate the magnetic field strength 3.5 cm from a long, straight conductor with a current of 1.8 A.

16. Example 2
Calculate the magnetic field strength 3.5 cm from a long, straight conductor with a current of 1.8 A.

17. Example 2
Calculate the magnetic field strength 3.5 cm from a long, straight conductor with a current of 1.8 A.

18. Field of a Solenoid
For a solenoid of N turns and length L, the magnetic field in the core of the solenoid is:

19. Field of a Solenoid
For a solenoid of N turns and length L, the magnetic field in the core of the solenoid is:
You are adding the contributions of N loops, each of 2pr circumference.
The field is spread out over a distance L.

20. Example 3
What is the magnitude of the magnetic field in the core of a solenoid 5.0 cm long, with 300 turns and carrying a current of 8.0 A?

21. Example 3
What is the magnitude of the magnetic field in the core of a solenoid 5.0 cm long, with 300 turns and carrying a current of 8.0 A?

22. Example 3
What is the magnitude of the magnetic field in the core of a solenoid 5.0 cm long, with 300 turns and carrying a current of 8.0 A?

23. Electromagnetic Induction
Faraday’s Law of Electromagnetic Induction states that current can be induced to flow through a conductor in a changing magnetic field.

24. Electromagnetic Induction
Faraday’s Law of Electromagnetic Induction states that current can be induced to flow through a conductor in a changing magnetic field.
Why does the field need to be changing?
Because there is no energy in a static field.

25. Transformers
This is the principle behind transformers: a changing magnetic field produced by alternating current in the primary coil induces current flow in the secondary.

26. Lenz’s Law
What determines the direction of the current?

27. Lenz’s Law
What determines the direction of the current? The current must produce a field that resists the motion.

28. Lenz’s Law
Why?

29. Lenz’s Law
Why? Conservation of energy: the electrical energy comes from the work done in pushing the magnet into the coil against the opposing force.

30. Lenz’s Law
(If the current produced a field that attracted an approaching magnet, it would accelerate it, creating a stronger field and then more acceleration, etc.; we cannot have infinitely spiralling energy increases.)

31. Diamagnetism

32. More Practice Textbook questions: p. 409 – 410 #3, 4 p. 411 #8