1. Electricity from Magnetism

2. Goal of the class
Recognize that relative motion between a conductor and a magnetic field induces an emf in the conductor.
Describe how the change in the number of magnetic field lines through a circuit loop affects the magnitude and direction of the induced electric current.
Question of the Day: How is a turbine used to create electricity?
Previous answer: Electrons are deflected by a magnetic field to the required location
Previous question: How does a CRT TV display a picture?

3. Question The loop of wire is rotating in a counterclockwise direction.
Electrons in metal are free to move.
The magnetic field is horizontal and to the left.
Students may be familiar with electromagnets but unclear about their operation. Help them clarify their ideas about how electromagnets operate.

4. Question
Will there be a force on the electrons in the left and right segments of the loop?
If so, in what direction is that force?
In which direction will the electrons flow if the two wires from the ends are connected?
Students may be familiar with electromagnets but unclear about their operation. Help them clarify their ideas about how electromagnets operate.

5. Electromagnetic Induction
Imagine a wire moving to the right as shown.
In what direction is the force on the negative charge?
Upward
This force will separate the charges.
As negative charges move upward, the wire will develop a potential difference.
Batteries use chemical energy to separate the charges and create an emf. A moving wire in a magnetic field produces a similar effect.
If the wire was connected to an external circuit such as a light bulb, the electrons would flow out of the top, through the bulb, and back into the bottom of the wire.

6. Electromagnetic Induction
Electromagnetic induction is the process of creating a current in a circuit loop by changing the magnetic flux in the loop.
This can be accomplished by moving the loop, moving the field, or changing the strength of the field.
If the magnetic flux does not change, no current is induced.
The current is increased if the loop size or magnetic field strength are increased.
You can change the flux in 1 of 3 ways
Move it in and out of B field
Rotate it in B field
Change the B field intensity

7. Lenz’s Law
As the magnet enters the coil, a force pushes the electrons around the loop, inducing a current.
The induced current creates a magnetic field that opposes the motion of the magnet.
It would be helpful to have a large coil of wire and a magnet to show students how this current and opposing magnetic field are produced. Use the right- hand rule to find the direction of current. It is best to imagine the coil moving to the right instead of the magnet to the left. Consider the top of the coil: it moves to the right and the B field is upward at that point, so the electrons would move toward the back (or the current toward the front as shown). Then have students use the other right-hand rule to find the direction of the magnetic field produced by this current. The magnetic field will come out of the coil as shown, with the north pole on the right.
The PhET website may be useful at this time.

8. Lenz’s Law
Now the magnet is being removed from the coil as it moves to the right.
This induces a current in the opposite direction.
Once again, the magnetic field in the coil opposes the motion of the magnet.
It would be helpful to have a large coil of wire and a magnet to show students how this current and opposing magnetic field are produced. Use the right- hand rule to find the direction of current. It is best to imagine the coil moving to the right instead of the magnet to the left. Consider the top of the coil: it moves to the right and the B field is upward at that point, so the electrons would move toward the back (or the current toward the front as shown). Then have students use the other right-hand rule to find the direction of the magnetic field produced by this current. The magnetic field will come out of the coil as shown, with the north pole on the right.
The PhET website may be useful at this time.

9. Faraday’s Law
The magnitude of the induced emf depends on the number of loops (N), the magnetic flux (M), and the rate of change.
M = AB cos
Talk about AP physics

10. Practice Problems
A coil with 25 turns of wire is moving in a uniform magnetic field of 1.5 T. The magnetic field is perpendicular to the plane of the coil. The cross-sectional area of the coil is 0.80 m2. The coil exits the field in 1.0 s.
Find the induced emf.
Determine the induced current in the coil if the coil’s resistance is 1.5 .
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
This problem provides a good opportunity to review the units for tesla (T), which can be expressed as (V•s)/m2 .
Answers = 30V and 20A

11. Question
Will there be a force on the electrons in the left and right segments of the loop?
If so, in what direction is that force?
In which direction will the electrons flow if the two wires from the ends are connected?
Electrons flow counterclockwise as seen from above. They exit the connection on the lower left side of the loop.
The current is in the opposite direction, clockwise (because conventional current is opposite the direction of electron flow).
This induced current creates a magnetic field that comes goes down through the center of the loop (out of the bottom of the loop and into the top). This makes the region above the loop a south pole and the region below the loop a north pole.
This magnetic field of the loops opposes the motion of the coil because the south pole of the loop is being rotated into the south pole of the permanent magnet. Lenz’s law is confirmed by the two right-hand rules, one to find the direction of current and the other to find the direction of the magnetic field caused by the current.

12. Generators
A generator is a device that converts mechanical energy into electrical energy.
Motion (KE) of a coil of wire through a magnetic field creates a potential difference in a wire, causing electrons to flow.
Power plants use falling water or high-pressure steam to spin a turbine that is connected to a generator.
Similar to the wind blowing through a fan

13. Discuss the four different positions shown
Discuss the four different positions shown. In (a) and (c), the current is zero because the wires are moving parallel to the field, so no force is exerted on the electrons. In (b) and (d), the maximum current is produced because the two side wires are moving perpendicular to the magnetic field.
Note that this produces a current that alternates direction (AC).

14. AC Generators
The current alternates as a sin curve with the maximum occurring at points (b) and (d) in the rotation.
The emfmax depends on the number of loops (N), the area of the loop (A), the magnetic field (B), and the rate of rotation ().
emfmax = NAB
Talk abot generators if there’s time.

15. Homework
Please complete questions on page 723
Q 2, 4, 7, 10, 12