1. Physics 4 – March 20, 2018
P3 Challenge – A uniform magnetic field of B = 3.5 mT is directed into the page. An electron approaches this field from the left at v = 3.2 x 105 m/s. Determine the magnitude and direction of the force on the electron.
Get out p241 #37-46 for Hmk Check

2. Objectives/Agenda/Assignment
Ch 11.1 Induction
Assignment: p442 #1-13
Agenda:
HMK review
Induced motional EMF
Magnetic flux
Faraday’s Law
Lenz’s Law

3. Motional Electromotive Force (EMF)
We’ve seen that a current carrying wire creates a magnetic field.
The opposite cause and effect can also occur. A magnetic field can cause charges in a wire to move.
How? By moving a wire segment through a magnetic field.

4. Motional Electromotive Force (EMF)
Right hand rule shows that the magnetic field pushes protons up and electrons down when the rod is moved to the right.
The result is a separation of charge, which is a potential difference V… an induced EMF while the rod is moving.

5. Magnitude of induced EMF 
 = BvL where B is the magnetic field (T), v is the speed of the wire moving through the field, and L is the length of the wire located within the field.
Called a motional EMF because it requires motion to create the EMF. It must continue moving to keep the EMF.
In practice, it’s often the magnet that moves relative to a fixed wire, e.g. a fixed magnet moving in a solenoid.

6. Magnitude of induced EMF 
If there are multiple loops of wire, you can multiply this EMF by the number of loops, N, to find the overall EMF of the solenoid.
 = BvLN
Both of these forms are in the data packet

7. Power of Induced EMF
Connecting this EMF to a simple circuit, you can determine the current through a resistor, R.
IR = BvL then solve for I = BvL/R
The now current carrying wire generates a force F = BIL = B(BvL/R)L = B2vL2 /R
Recall Power = Fv = W/t
Power of the force generated by the induced EMF: P = (B2vL2 /R)v = B2v2L2 /R
Remembering  = BvL , P= 2/R This is equivalent to P = V2/R we saw before in Ch 5.2.

8. Induced EMF
Observation of this system show larger currents with:
Greater relative speed, v
Stronger magnetic field, B
Higher number of turns, N
Greater area of loop, A
Max value when magnet and turns are perpendicular, 
Something more than just the Magnetic field strength, B is operating here that involves A and 

9. Magnetic Flux, 
The amount of magnetism flowing normally (perpendicular) over an area is called the magnetic flux, .
The magnitude of magnetic flux is
 = BAcos where  is the angle relative to the normal to the area.
Unit = weber Wb, 1 Wb = 1 T m2

10. Magnetic Flux, 
Only the component of the magnetic field lines perpendicular to the plane of the area contribute to the magnetic flux.
Useful analogies: Hair within a rubberband or noodles in a loop => magnetic field lines within a given area. Flux is the number of hair, noodles or field lines.
When the magnetic field passes through many loops of a solenoid, it is call a magnetic flux linkage and  = NBAcos

11. Faraday’s Law
The induced EMF is equal to the rate of change of the magnetic flux linkage:  = N /t. Known as Faraday’s law.
 = BAcos and  is the angle to the normal to the area
Note: A changing flux creates an induced EMF, not necessarily a current. For a current it has to be placed in a circuit with a resistor.
Ex: The magnetic field through a single loop of area 0.20 m2 is changing at a rate of 4.0 T/s. What is the induced EMF?

12. Lenz’s Law
The induced EMF will be in such a direction as to oppose the change in the magnetic flux that created the current.
Pretend the loop is a solenoid with one turn and use the Right Hand Grip rule to determine the direction of a magnetic field for the current carrying wire loop that will oppose the “change direction”
Label B direction, label change in B direction, ID B direction created by current, and finally ID the direction of the current.
Consider a loop in a field directed into board a) increasing, b) decreasing
Consider a loop in a field directed out of board a) increasing b) decreasing

13. Exit slip and homework
Exit Slip – What is the magnetic flux linkage for a 3.5 mT magnetic field is present through a solenoid with 50 turns and a 6.25 cm radius?
What’s due? (homework for a homework check next class)
P442 #1-13
What’s next? (What to read to prepare for the next class)
Read 11.2 AC Power, p