Electromagnetic Induction Lenz’s discovery Faraday’s law example ways to change flux WB - EMF

Electromagnetic Induction•Lenz’s discovery•Faraday’s law

•example•ways to change flux•WB - EMF•WB Direction

Lenz’s Discovery•demo - show coil within coil

EMF (voltage) is induced by changing magnetic fields

Ways to change flux: (show) = BAcos

Change B (magnetic field)Change A (area)Change (angle - rotate)Change any combination:

= BAcos - BAcos (final) - (initial)

Magnetic Flux: = BAcos

- Magnetic Flux in WebersB - Magnetic Field in TA - Area in m2

θ - Angle twixt B and AA

Show with overhead max flux with loop, min flux

Lenz’s Law - induced current resists:•Change in flux•Motion that caused change in flux

Ways to predict direction:Magnet:

Oppose the motion – Conservation of Energy!Flux:

Lose it – replace it/Gain it – Oppose itDirection of current, energy in wire, work done

= -N t - Electromotive force (voltage)N - # windingsΔ - Change in Magnetic FluxΔt - The time elapsed

Faraday’s Law:

. . . . . . . . . . . .

50. cm

B = 1.7 T

Example: The loop is removed in .012 s. What is the EMF generated? Which way does the current flow? (N = 1)

The loop is removed in .012 s. What is the EMF generated? Which way does the current flow? (N = 1)

So the flux (out of the page) goes away. The flux is (1.7 T)(0.75 m)(0.50 m) = 0.6375 Webers (Tm2), and the EMF is (0.6375 Webers)/(0.012 s) = 53.125 V

Now for the direction, we lost flux out of the page, so we replace it (therefore resisting the change) to create flux out of the page, the loop would make the N pole out of the page, and therefore the current would go acw (anti clockwise) in the loop using the loop right hand rule.

There are other ways to predict the direction, but in this case, this is the best.

53 V, acw

Whiteboards - EMF

The approach of the magnet makes the B field inside the 3.0 cm diameter loop go from .025 T to .175 T in .0035 s. What is the EMF, direction of the current, and which electrode is + (current flows out of it)

.030 V, acw, A

The bar moves to the right at 2.0 m/s, and the loop is 1.5 m wide. What EMF is generated, and which direction is the current?

9.6 V, acw

B = 3.2 T

x x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x x

2.0 m/s1.5 m

The loop is rotated about an axis that lies in this page from its current position to an angle of 57o with the page in .0063 seconds. What EMF, what direction current?

260 V, acw

B = 5.3 T

. . . . . . . . . . . .

side view

Wrap your Rt. fingers with the current, and your thumb is the N pole

Review

Lines of flux away from N,So your thumb is the directionof flux inside the loop

WhiteboardsDirection flux in a loop

1 | 2 | 3 | 4

Which way is the flux inside the loop?

outa da page

into da page

to the left

Current goes up in the front of the coil

Current goes right in the front of the coil

to the right

Current goes down in the front of the coil

Current goes left in the front of the coil

Whiteboards – Direction of current

Which way is the current?

. . . . . . . . . . . .

Which way is the current? (When does it stop flowing?)

. . . . . . . . . . . .

B increasesinto page

x x x x x x x x x x x x x x x xx x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

Before

x x x x x x x x x x x x x x x xx x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

Loop has been rotated

Up the front

Which way is the current on the front of the coil? (up or down)

Down the front

Up the front

Which way will the current flow?

(stays fixed)

Which way is the current on the front of the coil? (right or left)

right on the front

Which way will the current flow in the circle?

Wire with current increasing

Which way will the current flow in the circle?

Wire with current Decreasing

Which way will the current flow?

Current in outer loop Decreases (ACW)

Which way will the current flow in the inner loop?

Resistance Decreases

Electromagnetic Induction Lenz’s discovery Faraday’s law example ways to change flux WB - EMF

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