1. Using the “Clicker” If you have a clicker now, and did not do this last time, please enter your ID in your clicker. First, turn on your clicker by sliding the power switch, on the left, up. Next, store your student number in the clicker. You only have to do this once. Press the * button to enter the setup menu. Press the up arrow button to get to ID Press the big green arrow key Press the T button, then the up arrow to get a U Enter the rest of your BU ID. Press the big green arrow key.

2. Motional emf Motional emf is the voltage induced across a conductor moving through a magnetic field. If a metal rod of length L moves at velocity v through a magnetic field B, the motional emf is: as long as the velocity, field, and length are mutually perpendicular. In which direction do positive charges deflect, if they move with a metal rod to the right in a magnetic field directed into the page?

3. Motional emf In which direction do positive charges deflect, if they move with a metal rod to the right in a magnetic field directed into the page? Use the right-hand rule associated with. Positive charges deflect up, and negative charges deflect down.

4. Acting like a battery The moving rod can act like a battery if we connect it up in a circuit, like so. The rod is placed on a pair of conducting rails that are separated by a distance L. The rails are connected at the left end by a resistor of resistance R - assume the resistance of the rod and rails is negligible compared to R. There is a uniform magnetic field of magnitude B directed into the page.

5. Direction of the induced current? If the rod (in red) is moved to the right, will there be an induced current? If so, in what direction is it? 1. Clockwise 2. Counterclockwise 3. There is no induced current

6. Apply the pictorial method BeforeAfter The simulation draws the Before and After pictures for us. To oppose the change, the loop needs to create field lines out of the page, requiring a counterclockwise induced current.

7. Acting like a battery The rod is initially at rest, but is then subjected to a constant force F directed right. Neglect friction between the rod and the rails. What happens? 1. The bar experiences a constant acceleration, and the speed increases at a constant rate 2. The changing flux gives rise to another force in the same direction as F that accelerates the bar even faster than F would by itself. 3. The changing flux gives rise to another force opposite in direction to F that causes the bar to reach a terminal (constant) velocity 4. The changing flux gives rise to another force opposite in direction to F that causes the bar to come to rest

8. Acting like a battery The faster the rod goes, the larger the current induced in the loop consisting of rod, rail, resistor, rail. This current gives rise to a force of magnitude ILB opposing the applied force F. When the forces are equal and opposite, there is no net force, so the rod continues to move at a constant velocity. F ILB

9. Eddy currents An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. By Lenz's law, the swirling current sets up a magnetic field opposing the change. In a conductor, electrons swirl in a plane perpendicular to the magnetic field. Eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. In many applications the loss of useful energy is not particularly desirable, but there are some practical applications such as train brakes.

10. Eddy current application: train brakes During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion. Draw a set of pictures for the green region, to show the direction of the induced current in the green region as it enters the magnetic field.

11. Eddy current application: train brakes During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion. In which direction is the force on this induced current?

12. Eddy current application: train brakes During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.

13. Eddy current question To stop (or slow down) the train, an electromagnet is turned on, passing a magnetic field through sections of the train's wheels. The eddy currents set up in the wheels act to slow the train down. What would happen if the direction of the magnetic field was reversed? 1. The train would still slow down. 2. The train would speed up.

14. Eddy current question If the field goes the other way, the eddy currents also reverse direction. Reversing both the field and the current gives a force ( F = ILB ) in the same direction – the train still slows down.

15. Whiteboard