1. Electromagnetic Forces and Fields
Lesson 2 : Wires carrying current
close to each other.

2. Magnetic Flux Density
A – higher density of field lines, greater B, greater Fm
B – lower density of field lines, lesser B, lesser Fm
Greatest density of field lines is near the poles of a magnet A B

3. Magnetic lines of force are continuous
The direction of a magnetic field line outside a magnet is N to S and inside is from S to N

4. Earth’s Magnetic Field
The Earth’s magnetic field is similar to a bar magnet
The field is the result of the fluid motion of iron within the earth’s core (read text for new theories)

5. 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

6. 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

7. 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.

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

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

10. Example
Determine position of each compass

11. Solution

12. 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

13. Electric Bell

14. Electromagnetic Relay
Changes current to another circuit

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

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

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

18. 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)

19. 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.

20. 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.

21. Solution
What is the direction of the force?

22. 3rd Left Hand Rule

23. Example Diagram