Electricity and magnetism are intently related.
We use moving magnets to generate electric
current, as with these turbines, and electric
current to make magnets.
Here I've set up a powerful magnet with a
strip of aluminum foil running perpendicular
to the magnetic field lines. Remember these
from the last episode? Well, I've set up a
power supply to send a current through the
foil, and as you can see such a current feels
a force in the presence of a magnetic field.
If I change the direction the current flows,
I change the direction of the force. Here
it pushes the foil up. Let's see that again.
First down, switch the wires, and up.
To help remember in what direction this force
works, scientists have devised hand rules.
Basically, a charge moving through a pre-existing
magnetic field will feel a force.
To determine the direction for a negative
charge, use your left hand.
Point your thumb in the direction the charge
is moving.
Then, your fingers in the direction of the
magnetic field.
And your palm tells you the direction of the
force.
For positive charges, we use our right hand.
Direction of motion.
Direction of the magnetic field.
Direction of push.
The strength of this force varies due to how
the charge's motion lines up with the magnetic
field lines. If it's moving in the same direction
or the opposite, there's no force. Ninety
degrees different, big force. In the middle,
medium force.
To review, for negative charges we use our
left hand. The thumb points in the direction
the charge is moving, the fingers in the direction
the magnetic field is pointing, and your palm
in the direction of the force.
For positive charges, use your right hand.
Again, the thumb points in the direction the
charge is moving, the fingers in the direction
of the magnetic field, and your palm in the
direction of the force.
Now let's see how these rules can help us
figure out how our foil will behave.
Here we're using direct current, which we
know to be the movement of a negative electrical
charge, namely electrons. So we are going
to use our left hand. We know that the current
flows in this setup from the black wire to
the red wire. So we point our thumb to our
left. We can see that the north pole is at
the top of the screen and the south pole at
the bottom of the screen. So we take our fingers
and point them out of the screen. This leaves
our palm pointing down, and that should be
the direction of the force. Let's see.
Okay.
Let's switch the direction of our wires, changing
the direction the current flows.
Exactly as we expect. Since our palm now points
up, so too the thing moves up. If we try it
over and over again, we get the same result.
