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.

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

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,…)

BRIDGE OF NAILS

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!

Magnetic Flux (N turns)

MAGNETIC BRAKING 6D08

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

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.

JUMPING JACK 6D11

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

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.

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.

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

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

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)