Ampere's
law
Using EMField
- Download and run the program EMField,
which you will find on the Files page of the class website.
- From the Sources switch,
choose 2D line currents (magnetic). You need to click on the little ball at the end of the "switch" to change its position.
- From the Measure Type pull-down
menu, choose Vectors.
Drag a current-carrying
wire, with current -5A (hollow green circle) down from the source square at the top of the screen and place it
somewhere near the middle of the screen.
- Experiment with dragging
the mouse to see magnetic field vectors. Click to deposit a vector on the
screen.
If you want to clean up the screen, click on the Clear button.
- Deposit field vectors
in a radial pattern around the current, at several distances from the wire.
Is conventional current flowing into or out of the screen? (Use your right
hand).
- Move the wire. What
happens?
- Drag the existing wire back onto its source box to delete it,
and replace it with a wire carrying a current of +5A.
- Again deposit field
vectors in a radial pattern around the current. Is this current into or
out of the screen?
Ampere's Law
- Leave the +5A current
on the screen.
- From the Measure Type pull-down menu, choose Ampere's Law
- Place the mouse near
the current. Drag the mouse in a counterclockwise direction completely around
the current. The line shows
- Is the result positive
or negative?
- Repeat this, making
a circle of much larger radius around the current. Why is the result for
the same ?
- Drag the mouse clockwise
around the current. What is different?
- By convention, we take
a counterclockwise path, and interpret a positive result as corresponding
to conventional current out of the page.
The effect of currents
not inside the path
- Clean up the screen,
leaving a +5A current on the screen.
- Drag the mouse in a
circular path near but not enclosing the charge.
- Explain why
comes out to be zero.
- Experiment with several
currents, and try paths that enclose some, all, or none of them.