1. An inductor is a wire coil usually wrapped around an iron core
Inductors
Toroidal core
An inductor is a wire coil usually wrapped around an iron core

2. Self Inductance
A coil can induce a voltage in itself !!!??

3. Predict what happens when the switch closes.

4. What happens when the switch opens?

5. An inductor is designed to oppose a changing current
This is because it can induce an EMF in itself. This is called Self Inductance.

6. Direction of EMF opposes current change
Current Increasing
Increasing current
Causes increasing magnetic field
Causes increasing flux through coil
Causes induced EMF
Increasing current
Direction of EMF opposes current change
Induced EMF

7. Self Inductance A coil can induce a voltage in itself.
L is called the self inductance
Self Inductance (L) is measured in henries (H)
Circuit Symbol for inductor:
Considering resistance:

8. This induced EMF (or back EMF) opposes the increase in current, so the current rises…
SLOWLY
This is called Self Inductance
(The coil induces an EMF in itself)

9. current
I = V/R
R is the ohmic resistance of the inductor
Close switch
time

10. current
Close switch
Back EMF
Close switch

11. Imax
0.63 Imax
time
Close switch
Meaning of time constant :
The time taken for the current to increase to 63% of the maximum value.

12. Larger inductance produces larger back EMF with the same change rate of magnetic flux as in It opposes the EMF of the power supply, resulting in longer time to reach maximum current.
Larger resistance results in smaller maximum current. So it takes shorter time to reach 63% of the maximum current, resulting in smaller τ.

13. How would the graph change if:
Inductor had higher inductance?
Inductor had higher resistance (be careful)
current
Time
Close switch

14. Direction of EMF opposes current change.
Current Decreasing
Decreasing current
Causes decreasing magnetic field
flux decreasing
Causes induced EMF
Direction of EMF opposes current change.

15. This induced EMF (or back EMF) opposes the decrease in current, so the current drops…
SLOWLY
This is called Self Inductance
Key idea:
Inductors oppose a changing current but have no effect on constant current.
n.b. a real inductor has resistance

16. When the switch opens, the current drops to zero rapidly
Causes a large flux change rate
Induces a very large EMF
Causes a spark across the switch

18. Energy stored in an inductor
Current in an inductor produces a magnetic field, therefore energy is stored in the magnetic field. L: self inductance in henries (H), I: current in amps (A)

19. Inductor radio

20. Example: As shown, the coil has an inductance of 0. 20H
Example: As shown, the coil has an inductance of 0.20H. When the switch S is open, the current falls from 5.0A to zero in 1.0s.
a. What is the induced voltage?
b. What is the resistance, R, of the coil
c. What is the energy stored when S is closed?
d. What is the time constant of the coil?

22. Ex. (a) What is the steady current flowing when the switch has been closed for a long time?
(b). What is the voltage across the inductor at the instant the witch is closed?
(c) find the time constant.
(d)How long does it take for the current to reach a value of 0.021A?

23. Mutual Inductance http://phet.colorado.edu/web-pages/simulations-base.html