1. Unit 9, Lesson 4: Magnetic Flux

2. Magnetic Flux Flux = magnetic field X perpendicular area
Φ=BAsinƟ where Ɵ is the angle between the plane and the magnetic field
Unit:
weber = tesla x square meter
1 Wb = 1 Tm2
5 min

3. Try This!
Find the flux through a circular loop of wire of radius 25 cm in a vertical magnetic field of 5x10-5 T if:
a) the loop is laying flat on the ground,
b) the loop makes a right angle with the ground,
c) the plane of the loop makes a 30O angle with the ground.
15 min

4. Brain Break!

5. Faraday’s Law and Lenz’s Law
A changing magnetic field induces a current and voltage in a coil of wire.
The magnitude of the voltage (EMF) = the number of turns X the rate of change of flux (Faraday)
The direction of the induced current always counteracts the change that produced it (Lenz)
ε = −N(∆Φ/∆t) where Φ=BA
Note: the negative sign symbolizes Lenz's law but should not be included in calculations.
10 min

6. Lenz`s Law Determines the direction of induced current.
The induced current will create its own magnetic field (an induced magnetic field).
The induced magnetic field will act to cancel out the change in the original flux and will oppose any movement that created a change in flux.
10 min

7. Examples – Lenz`s Law
Find the direction of the current in the resistor in each case:
Think: Move North away, closest end becomes South to attract it back! - 10 min
Answer: Left to right
Answer: Right to left

8. No Homework!
We will practice Lenz’s and Faraday’s Laws more next class.

9. Unit 9, Lesson 5: Understanding Faraday and Lenz

10. Warm Up – Lenz`s Law
Find the direction of the current in the resistor:
5 min
Answer: Left to right

11. Practice – Lenz`s Law
Find the direction of the current in the resistor:
a) immediately after the switch is turned on
b) a while after the switch is turned on
c) immediately after the switch is turned off
10 min
Right to left
Left to right

12. Practice - Faraday`s Law
A circular coil of wire with 100 turns and a cross-sectional area of 25 cm2 is placed in a perpendicular magnetic field of 2.5 T as shown. a) The field is increased to 5.0 T in 0.20 seconds. Find the induced EMF and the direction of the induced current. b) The loop is then moved out of the area of constant magnetic field (5.0 T) in 0.3 seconds. Find the induced EMF and the direction of the induced current.
20 min
3.1 V counterclockwise
4.2 V clockwise

13. Brain Break!

14. Motional EMF
A conductive bar cutting across a perpendicular magnetic field experiences an induced EMF that opposes its motion!
5 min
B=magnetic field strength (T)
l = length of bar (m)
v = velocity (m ̸ s)
ε = Blv

15. Motional EMF – Example
A passenger jet with a wing span of 50 m approaches the north pole at 1000 km ̸ h. Find the voltage induced across the wings and state which wing is positive. The Earth`s magnetic field is 5x10-5 T.
10 min

16. Homework:
Pg. 243 #1-4
Pg. 251 #1-3