Names:
In-Class Activity for Physics 7: Faraday’s Electromagnetic Lab
1. On your laptop, open a web browser and go to the PhET site (http://phet.colorado.edu/en/simulation/faraday) to find the
Faraday’s Electromagnetic Lab simulation. Click “Run now!” to start. When you first open the simulation, the Bar Magnet tab
should be active, and you should see a bar magnet and a compass on the screen. (If not, click the Bar Magnet tab to select it.)
We will not be using the compass right now, so uncheck the “Show compass” box. Then drag the bar magnet to the center
of the screen. The display should now look like the picture below–except that some additional labels, A, B, and C, have been
added to the picture.
Check the box that says “Show field meter,” which allows you to measure the magnetic field at any location. The meter
shows the magnetic field magnitude B, the components Bx and By , and the direction angle θ. Right now, we will focus on B,
which tells us the strength of the magnetic field.
a) Measure and record the values of B at the approximate locations marked A, B, and C in the picture. For which region is
the magnetic field the strongest? Why do you think this is?
Now select the Pickup Coil tab, so that your display looks like this:
Click on the magnet and drag it to move it around. Try placing the magnet in different locations: far from the coil, close to
the coil, with the north or south pole right in the middle of the coils, with the center of the magnet right in the middle of the
coils... As you do this, watch the behavior of the light bulb attached to the coils.
b) Is there any location for which the light bulb turns on while the magnet is stationary? If so, where? What about while
you are moving the magnet? Try to explain your observations in terms of the physics we have learned.
Now let’s try moving the magnet in a more systematic way. Start with the magnet at the left side of the screen; then drag it
slowly and steadily toward the loop, through it, and out the other side. Try to keep the magnet moving at a constant speed
the whole time. Watch the light bulb.
c) Does the light bulb turn on? If so, does it stay about equally bright the whole time, or are there some times when it
is noticeably brighter? If there are any changes, where is the magnet located when the bulb is brightest? Try to explain your
observations in terms of magnetic flux.
d) Predict how, if at all, your observations would be different if you moved the magnet more quickly through the coil.
Explain your reasoning.
Test your prediction by moving the magnet more quickly through the coil.
e) Was your prediction correct? If not, what did happen, and why? Does it make any difference which direction you move
the magnet through the coil (left or right)? Why is this?
Now select the Transformer tab in the simulation. This gives you an electromagnet–a coil hooked up to the DC battery–along
with the original coil with a light bulb and no battery. Try moving the electromagnet around and placing it in different locations,
as you did with the bar magnet (inside the other coil, far from the coil...)
e) What happens while the electromagnet is moving, and while it is stationary at various locations? What do you need to do
to get the bulb to light? Explain why. Is there any difference from your observations with the bar magnet? Why or why not?
Move the electromagnet just to the left of the coil and leave it there. At the top right of the screen, you should see “DC” (with
a picture of the battery) selected under “Current source.” There is also an option to select an AC source. Before doing this,
make a prediction:
f) Predict what will happen when you replace the battery with an AC source (keeping the electromagnet stationary to the
left of the coil). Will the bulb light? If so, will its brightness stay constant or change? Explain your reasoning.
Keep the electromagnet just to the left of the coil, as shown below, and change the current source from DC to AC. Don’t move
the electromagnet; watch what the bulb does.
g) Was your prediction from part (f) correct? If not, explain what did happen and why.
The blue box representing the AC current supply shows a graph of the current versus time. You can adjust the maximum current
value using the vertical slider, and the frequency by using the horizontal slider. First, try adjusting the frequency; move it up
and down and watch what happens to the bulb.
h) As you increase the frequency, does the average brightness of the bulb increase, decrease, or remain the same? Explain
why.
Now turn the frequency all the way down to its lowest level (5%). This allows your eye to follow the changes in the current
graph easily. The red vertical line shows the value of the current at each instant.
i) Does the bulb’s brightness vary in time? If so, when is it brightest: when the current is at its most positive, at its most
negative, near zero, or some other time? Why do you think this happens?
Adjust the frequency up to its highest level (100%). At the bottom right of the screen, you will see a box describing the Pickup
coil properties, including the number of loops, which can be set to 1, 2, or 3 (it should initially be on 2).
j) Change the number of loops, and look at the bulb in the cases of 1, 2, and 3 loops. How, if at all, does the bulb’s average
brightness seem to depend on the number of loops? Why does this happen?
k) Now use the Pickup coil properties box to change the area of the coil loops (keeping the number of loops fixed at 2). How,
if at all, does the brightness of the bulb seem to depend on the loop area? Why does this happen?