Interactive Lecture Demonstration 7 – Magnetic Forces on Currents

The purpose of this activity is for you to practice applying the Second Right Hand Rule (sometimes called the Palm-Push Rule). That rule tells the direction of the force that a magnetic field makes on a moving charge or current. The rule is: using your right hand, point your fingers along the direction of the magnetic field (think "fingers go with field), and point your thumb in the direction of the velocity of the charge (i.e. the direction it is moving). Your palm will then be facing (think "palm pushes") in the direction of the force made by the field on a positive charge. If the charge is negative, the force on the charge is in the direction opposite to the way that your palm faces.

Another way of stating the Second Right Hand Rule is that the magnetic force on a moving charge is always perpendicular to both the direction of the magnetic field and the direction of the velocity of the charge. For a positive charge, the directions of velocity, field and force form an x-y-z coordinate system, with velocity representing the x-direction, magnetic field the y-direction and magnetic force the z-direction. For a negative charge, the force is in the negative z-direction (opposite the direction of the force on a positive charge).

In each of the following situations, predict the direction of the magnetic force on the current. We will then do the experiment to test if your prediction was correct.

Situation 1: An oscilloscope faces you, with the electron beam moving directly towards you and hitting the center of the oscilloscope screen. The north pole of a bar magnet is brought near the oscilloscope so that the axis of the bar magnet is along the direction of motion of the electrons. What will happen (will the electron beam be deflected up, down, to the right, to the left, ??)

Situation 2: Now the bar magnet is turned so that its N pole is on your left as you face the oscilloscope screen, its S pole is on your right. What will happen to the electron beam when the bar magnet is brought near the screen ?

Situation 3: Now the bar magnet is reversed – its S pole will be on your left as you face the oscilloscope screen, its N pole is on your right. What will happen when the magnet is brought near the screen ?

Situation 4: A Crooke's Tube (with a cathode at the left end as you face it, and an anode at the right end) has a high voltage (supplied by a Tesla coil) between the cathode and anode. Electrons will be accelerated from the low-voltage cathode towards the high-voltage anode. You will see the beam of electrons, because it will glance off a metal plate that is coated with phosphorescent paint. A bar magnet will be brought near, with the N pole facing directly away from you (and perpendicular to the direction of motion of the electrons). What will happen ?

Situation 5: A long row of U magnets forms a "tunnel," inside which a strip of aluminum foil sits. All of the N poles of the U magnets are on the side of the tunnel closest to you, all of the S poles are on the side that's farther from you. An electric current will be passed through the aluminum foil, moving to your right. What will happen ?

Situation 6: Two strips of aluminum foil hang side by side. An electric current will be passed through both wires. Current will move upward through both wires. What will happen ? To decide, look first at what will happen to the wire on the right. Apply the First Right Hand Rule (that we learned yesterday) in order to tell the direction of the magnetic field that the left wire makes at the location of the right wire. Then apply the Second Right Hand Rule to decide the direction of the force made by that field.

You could go through the same chain of logic to tell the direction of the force made by the right wire on the left wire, but there is a simpler way to decide that question – use Newton's Third Law !

Situation 7: Setup is the same as in Situation 6, except that now the current will run upward through the left wire but downward through the right wire. What will happen, and why ?

Now use the formula for the magnitude of the magnetic force on a current-carrying wire, and the formula for the magnitude of the magnetic field created by a long straight wire, to calculate the force that one of the hanging aluminum strips makes on the other one. You will need to measure/estimate some quantities.