1. Ampere’s Law in Magnetostatics
Biot-Savart’s Law can be used to derive another relation: Ampere’s Law
The path integral of the dot product of magnetic field and unit vector along a closed loop, Amperian loop, is proportional to the net current encircled by the loop,
Choosing a direction of
integration.
A current is positive if it
flows along the RHR normal
direction of the Amperian
loop, as defined by the
direction of integration.

2. Magnetization and “Bound” Current in Matter
Strong externally applied field B app aligns the magnetic moments in matter. (M) Magnetization

3. Hysteresis for a Ferromagnet
Lack of retraceability shown is called hysteresis.
Memory in magnetic disk and tape
Alignment of magnetic domains retained in rock (cf. lodestones)
Area enclosed in hysteresis loop
Energy loss per unit volume
hard magnet: broad hysteresis loop (hard to demagnetize, large energy loss, high memory)
soft magnet: narrow hysteresis loop (easy to demagnetize,…)

4. BRIDGE OF NAILS

5. Bar magnet approaches coil
Induction v S N
Bar magnet approaches coil
Current induced in coil
Current in opposite direction v N S
Reverse poles of magnet N S
Bar magnet stationary
No induced current v S N
Coil moving around bar magnet
Same currents induced in coil
What’s in common?: Change of Magnetic flux = EMF!

6. Magnetic Flux
(N turns)

7. MAGNETIC BRAKING 6D08

8. Faraday’s Law of Induction
The magnitude of the induced EMF in conducting loop is equal to the rate at which the magnetic flux through the surface spanned by the loop changes with time. N
Minus sign indicates the sense of EMF: Lenz’s Law
Decide on which way n goes
Fixes sign of ϕB N
RHR determines the positive direction for EMF

9. Induced Electric Field from Faraday’s Law
Rewrite Faraday’s Law in terms of induced electric field:
This form relates E and B!
Note that for E fields generated by charges at rest (electrostatics) since this would correspond to the potential difference between a point and itself. => Static E is conservative.
The induced E by magnetic flux changes is non-conservative.

10. JUMPING JACK 6D11

11. How to use Faraday’s law to determine the induced current direction
define the direction of ; can be any of the two normal direction, e.g point to right
determine the sign of Φ. Here Φ>0
determine the sign of ∆Φ. Here ∆Φ >0
determine the sign of Δϕ ind using faraday’s law. Here Δϕ ind <0
RHR determines the positive direction for EMF
If Δϕ >0, current follow the direction of the curled fingers.
If Δϕ <0, current goes to the opposite direction of the curled fingers. N

12. Conducting Loop in a Changing Magnetic Field
Induced EMF has a direction such that it opposes the change in magnetic flux that produced it.
approaching
moving away
Now the demonstration again
Magnetic moment created by induced currrent I attracts the bar magnet.
Magnetic moment created by induced currrent I repels the bar magnet.
Force on ring is attractive.
Force on ring is repulsive.

13. Faraday’s and Lenz’s Laws
At 2, ΦB is increasing into page. So emf is induced to produce a counterclockwise current.
At 4, ΦB in decreasing into page. So current is clockwise.
At 1, 3, and 5, ΦB is not changing. So there is no induced emf.

14. Motional EMF of Sliding Conductor
Induced EMF:
Lenz’s Law gives direction
Faraday’s Law
FM decelerates the bar
This EMF induces current I
Magnetic force FM acts on this I

15. Ways to Change Magnetic Flux
Changing the magnitude of the field within a conducting loop (or coil).
Changing the area of the loop (or coil) that lies within the magnetic field.
Changing the relative orientation of the field and the loop.
generator
motor

16. Other Examples of Induction
EMF induced in Coil 2 + -
EMF is induced again + -
Switch has been open for some time:
Nothing happening
Switch is just opened:
Switch is just closed:
EMF is induced in coil + -
Switch is just closed:
Back emf (counter emf)