## Friday, March 18, 2016

### For Classes on March 18

Happy Friday everyone!

Periods 1-2, 8-9:
Yesterday you saw something on Faraday's law for a moving hoop/changing area example. This is the case where induced voltage = B dA/dt. Today extend on this by looking at cases where a second force is trying to push a circuit through magnetism, such as dropping a metal hoop into a B-field. What you will see is that, because this process depends on speed, it ends up looking a lot like air friction and terminal velocity from last year! Weird, but true. Remember the case of you trying to swing the metal hoop through the big magnet, and you felt the forces on it trying to slow it down (this is a magnetic brake). Take good notes so you can try to make sense together of the home work problems - see if you can complete things before leaving.

Homework set:
The glider problem on page 6 is based on yesterday - use the notes on page 2 and 3, could be helpful
The 1990 problem on page 7 - notes on page 4 could be helpful
The challenge problem is on page 9! Have fun!

Periods 3-4:

Take a look at the problems for yesterday, and see if there is any consensus. Ampere's law depends on the current inside the region you are looking at, analogous to Gauss's law depending on teh charge inside the region.

One application of Ampere's law for straight wires, where B = (mu)I/(2*pi*r), is to get the force between two currents. Check out a video on the forces between two long wires with currents - they are both producing magnetism, so the wires should either attract or repel each other! Take good notes, this will be needed for some of the homework. Note that the force on currents is F = IL x B, where L is the length of a segment of the wire.

Then, take a look at a video on the initial exposure to Biot-Savart law. This is the rule that allows us to determine the magnetic field for things more exactly (Ampere is only for long wires, solenoids, and toroids, so it is limited). We will look at what a single moving particle does in terms of producing a magnetic field. Take good notes, because we will build on this.

Try the following from the packet:
Ch. 28 #8 (B-S law), 31 on page 7
AP Prob from 1983, page 11