1. Electromagnetic Induction

2. Electromagnetic Induction
After scientists had discovered that an electric current can generate a magnetic field the logical question followed:
“If an electric current can generate a magnetic field, can a magnetic field generate an electrical current?”
Michael Faraday and Joseph Henry independently discovered they could.

3. Electromagnetic Induction
The generation of current from changing magnetic fields.

4. Faraday discovered many ways to induce a current
Faraday discovered many ways to induce a current. For example in the induction coil shown below.
What was most interesting to note was that current is induced whether the magnet moves into the coil or the coil moves over the magnet.
This showed that magnetic fields do not simply create electric currents, rather they are only generated by changing magnetic fields.

5. Another example of this comes when you move a bar magnet into or out of a hollow solenoid.
When the magnet is moved one way the current is in one direction and when it is moved the other way the current reverses.
To predict the direction of the induced current we use

6. Changing the Strength of an electromagnet….
The strength of an electromagnet depends on:
The number of turns of wire
(increase number of turns for stronger magnet)
(2) What the wire is around
(add an iron core for stronger magnet)
(3) The current through the wire
(more current makes stronger magnet)

7. Lenz’s Law is really an application of
The Law of Conservation of Energy.
Remember that we can use the 2nd Right Hand Rule to relate the poles of an electromagnet and the direction of current flow.
Thumb: Points towards North.
Fingers: Curl in direction of current flow.

8. Electric current is generated by a changing magnetic field.
In order to calculate the EMF generated we need to introduce the idea of magnetic flux.
Magnetic Flux: the amount of magnetic field lines that pass through a coil.

9. Electric current is generated by a changing magnetic field.
For a loop of wire in a magnetic field, the magnetic flux depends on:
(1) The strength of the field
(2) The area of the loop
(3) The orientation of the loop

10. Electric current is generated by a changing magnetic field.
Note that magnetic flux is at a maximum when the loop is perpendicular to the field.
And at a minimum when the loop is
parallel to the field.

12. Magnetic Flux can be calculated by:
Where:
Ф= flux
B = magnetic field
A = area
θ = orientation to field
The units of flux are Tm2 or Webers (Wb)
Ф = BAsinθ

13. EMF can be calculated by:
As we said before an EMF is produced by a changing magnetic field, specifically by a changing flux. In fact,
Where: N = number of coils
Note that the “–” sign relates to Lenz’s Law. Since the EMF generated opposes the change in flux.
ε = -NΔΦ t

15. Example:
A square loop of wire is perpendicular to a 1.50 T magnetic field. If each side of the wire is cm, what is the magnetic flux through the loop?

16. Example:
A 1.80 diameter circular coil that contains 50 turns of wire is perpendicular to a T magnetic field. If the magnetic field is reduced to zero in a time of 0.100s what is the average induced EMF in the coil?

17. Example:
A circular loop of wire radius 2.5 cm is placed in a magnetic field B = T into the page. The field is then reduced to T into the page in 0.10 s.
What is the average induced EMF?
b. Which direction does the current flow?