1. Physics 2112 Unit 16
Concept:
Motional EMF

2. Really odd formula coming:
The Plan
Really odd formula coming:
Spend a bit of time seeing that it makes sense (See prelecture for additional info.)
Do two examples problem in detail.
Move on to use formula.

3. Motional EMF
Motion of a conductor through B field can cause potential difference. (Remember Hall Effect clicker question?) B
X X X X X X X X + - F
X X X X X X X X + - + - E + - E FB FE
X X X X X X X X v
X X X X X X X X
X X X X X X X X L

4. CheckPoint 1
Two identical conducting bars (shown in end view) are moving through a vertical magnetic field. Bar (a) is moving vertically and bar (b) is moving horizontally
Which of the following statements is true?
A motional emf exists in the bar for case (a), but not for case (b)
A motional emf exists in the bar for case (b), but not for case (a)
A motional emf exists in the bar for both cases (a) and (b)
A motional emf exists in the bar for neither case (a) nor case (b)

5. CheckPoint 2A
A conducting bar (green) rests on two frictionless wires connected by a resistor as shown. The entire apparatus is placed in a uniform magnetic field pointing into the screen, and the bar is given an initial velocity to the right.
x x x x x x x x
The motion of the green bar creates a current through the bar
going up
going down

6. CheckPoint 2b
A conducting bar (green) rests on two frictionless wires connected by a resistor as shown. The entire apparatus is placed in a uniform magnetic field pointing into the screen, and the bar is given an initial velocity to the right.
x x x x x x x x
The current through this bar results in a force on the bar
down up
right
left
into the screen
out of the screen

7. Example 16.1
A 10cm conducting bar (green) rests on two frictionless wires connected by a resistor R = 50W. The entire apparatus is placed in a uniform magnetic field of 0.5 T pointing into the screen.
x x x x x x x x
The bar is pulled to the right by a force, F, at a velocity of 8m/sec.
What is the current through the resistor?
What is the force required to keep the bar moving at this constant velocity?

8. Checkpoint 2
x x x x x x x x
A wire loop travels to the right at a constant velocity. Which plot best represents the induced current in the loop as it travels from left of the region of magnetic field, through the magnetic field, and then entirely out of the field on the right side.

9. Checkpoint 3
A conducting rectangular loop moves with velocity v towards an infinite straight wire carrying current as shown.
In what direction is the induced current in the loop?
clockwise
counterclockwise
There is no induced current in the loop

10. Generator: Changing Orientation
On which legs of the loop is charge separated?
Top and Bottom legs only
Front and Back legs only
All legs
None of the legs
Parallel to top and bottom legs
Perpendicular to front and back legs

11. Generator: Changing OrientationL w
At what angle q is emf the largest?
A) q = 0
B) q = 45o
C) q = 90o
D) emf is same at all angles
Largest for q = 0 (v perp to B)
= v

12. Checkpoint 4
A rectangular loop rotates in a region containing a constant magnetic field as shown.
The side view of the loop is shown at a particular time during the rotation. At this time, what is the direction of the induced (positive) current in segment ab?
from b to a
from a to b
There is no induced current in the loop at this time

13. Example 16.2 (Loop moving from wire)
A rectangular loop (h = 0.3m L = 1.2 m) with total resistance of 5W is moving away from a long straight wire at a velocity of 9 m/sec. The wire carries total current 8 amps.
What is the induced current in the loop when it is a distance x = 0.7 m from the wire? v I h x L
Conceptual Analysis:
Long straight current creates magnetic field in region of the loop.
Vertical sides develop emf due to motion through B field
Net emf produces current
Strategic Analysis:
Calculate B field due to wire.
Calculate motional emf for each segment
Use net emf and Ohm’s law to get current

14. Net Force on the loop
B into page
Note that the loop is moving with a constant velocity.  Fnet = 0.
What are the magnetic forces on the four edges of the loop? I v I h x L
Something must be doing positive work on the loop to keep it moving at constant v.

15. Faraday’s Law Putting it Together Notice the minus sign!!!
Change Area of loop
Change magnetic field through loop
Change orientation of loop relative to B
Faraday’s Law
Notice the minus sign!!!