## Due before class Wednesday, Jul 11th

Physics 106, Summer 2012

Did you complete the reading assignment?

Yes
No

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?
Yes
No

## KEY CONCEPTS:

1) Magnetic flux is the number of magnetic field lines that flow through a specified area in a unit of time and describes the power of a magnetic field in the specified area.

2) Magnetic flux is a scalar quantity.

3) Faraday's law of induction states: The emf induced in a closed circuit is equal to the rate of change of the magnetic flux through the circuit. In simpler words, when the magnetic field around a wire changes, an emf will cause a current to flow through a wire.

Flux is a scientific and mathematical term used in more than just electromagnetism. To gain a better understanding of what flux is physically, go to the following link: http://leonid.arc.nasa.gov/estimator.html

Exercise 1

The graph on the link can be hard to understand because it does not show units. The y-axis on the graph is the number of meteors that will be seen in the sky. The x-axis on the graph is the time of day in military time. When you combine them both you get something called Zenithal Hourly Rate (ZHR) which is a fancy term for how many meteors you will see in the sky per hour. On November 18, 2009 there will be a huge meteor shower that can be seen in Beijing, China. Change the settings below the graph to 13 Lenoids for shower, Beijing, China for location, downtown for conditions, and November 17-18, 2009 for the date.

What is the meteor shower flux for these settings at 6 am?

You will not see anything.

Now change the conditions setting to  mountain top. What is the meteor shower flux for these settings?

You will not see anything.

ZHR is a strange unit only used in astronomy. What is the standard unit of flux?

Joules/meter
Joules/meter2
Watts/meter
Watts/meter2

Exercise 2

Now that you have a better understanding of flux, we can apply it to magnetism and Faraday's law. Faraday’s law uses flux by thinking of the field lines in an area (like a wire loop). Go to the following applet: http://phet.colorado.edu/sims/faradays-law/faradays-law_en.htm

Click on the button to show the field lines. When the light bulb is not lit, there is no current in the circuit. This means there must be some “changing flux” to make the current. You can tell how much emf is induced by looking at the brightness of the light bulb or by looking at the voltage meter in the bottom right.

Where is the greatest flux on a bar magnet? Compare this to the question about the strength of the magnet from reading quiz 10. Does this make sense?

Drag the bar magnet through the coil. What happens to the light?

It turns on
Nothing

Try dragging the bar magnet though the coil very fast and again very slowly. Does the speed at which a magnetic field travels through a wire loop change the emf induced in the loop?

Yes, when the magnet goes faster there is a greater emf induced.
Yes, when the magnet goes faster there is a smaller emf induced.
No

Now drag the bar magnet through the coil (make sure the magnet is inside the coil) and compare it to dragging the bar magnet outside, but right next to the wire loop. Which way induces a greater emf? Why?

Click on the '2 Coils' option at the top of the screen. The second coil has a few less wire loops in it. Drag the bar magnet at the same speed through both coils. Which one induces a greater emf? Why?

FEEDBACK

The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:

1) If you have a specific question or concern and would like an individual answer you will need to come by the office or send an email.
2) If you really want to get an anonymous comment to me before the lecture, it must be submitted before 11am that day.

Was there anything that you didn't understand in the reading assignment?  What was confusing to you?