1. Electromagnetic Forces and Fields
Lesson 2 : RULES for Magnetic Fields.

2. Magnetic Field of Current Carrying Wire
The shape of the magnetic field produced by the moving charges in a wire is circular. It grows weaker as one moves away from the wire.
What is the direction of the field around the current carrying wire?

4. First Left Hand Rule
For the direction of the B around a current carrying wire we let the thumb of the left hand indicate the direction of electron flow. The finger tips of the hand that circle the wire indicate the direction of the B field.

5. First Left Hand Rule Left hand is used for negative charges
Right for positive charges

6. First Left Hand Rule

7. Conventional Current
The flow of positive charge is referred to as conventional current. Some textbooks use this and talk in terms of right hand rules rather than left hand rules.

8. Magnetic Fields in Solenoids
When current flows through each loop of wire in a coil a magnetic field is set up in such a way that it makes the coil act like a single bar magnet.
- the magnetic field around a straight wire is very weak
- to make the field stronger, we can bend the wire into many loops

9. Solenoid the current goes in the same direction for each wire
the magnetic field of each wire adds up to create a much stronger field

10. Second Left Hand Rule
Fingers curl around the coil in the direction of the electron flow.
Outstretched thumb points to the end of the coil that acts as the north end of a fixed magnet.

11. Example:
Is the direction of the current shown, e¯ flow or conventional current?

12. Solution Need to use right hand to B field direction as shown:
∴ is conventional current.

13. Example
Determine position of each compass

14. Solution

15. Electromagnets
A solenoid can be made to be a much stronger magnet by placing a ferromagnetic material (usually steel) inside the coil.
Electromagnets have many uses.
Magnetizing ferromagnetic material
Bells
Relay switches

16. Electric Bell

17. Electromagnetic Relay
Changes current to another circuit

18. Magnetic Effects of Current
Ampere investigated the interaction between two current-carrying wires parallel to each other.
What is the effect on the wires?

19. Answer
The direction of the fields is different therefore the fields attract each other. If the field directions are the same they would repel.

20. Current Current is the rate at which e¯ flow through a wire.
Units: C/s or ampere (A)
Symbol: I

21. Current and B field
The B field around a current carrying wire depends in part on the current in the wire.
F = magnetic force (N)
I = current (A) or (C/s)
l = length (perpendicular to B)
B = magnetic field strength in tesla (T)

22. Definition of an Ampere
If two wires, each one meter long and one meter apart, carry current so that the force between the two wires is x 10-7 N, the current in each wire is one ampere.

23. Example
A 30 cm long wire is placed in a 0.50 T B field that is directed out of the page. The e¯ flow is to the right, with a current of 1.2 A. Determine the force acting on the wire.

24. Solution
What is the direction of the force?

25. 3rd Left Hand Rule

26. Example Diagram