Magnetic force on a current carrying wire - Jumping wire physics demonstration [video]

Abstract

Magnetic Force on a current carrying wire Hi. I’m Dr. Bruce Denardo here in the Physics Department of the Naval Postgraduate School in Monterey, California. In another video, we demonstrated the fundamental magnetic force on a charged particle that is moving in a magnetic field. In this video, we will explain and demonstrate the magnetic force on a wire that is carrying an electric current. Surprisingly, the reverse demonstration can be done, which we will also explain and do. 2. JUMPING WIRE Suppose that we have a current-carrying wire in a magnetic field. We know that there are magnetic forces on the moving charges. Because these charges are confined to the wire, there will be a force on the wire. For a straight wire that is perpendicular to a uniform (or spatially constant) magnetic field, the fundamental magnetic force law tells us that the force is perpendicular to the field and the wire, with direction given by the right-hand rule. The magnitude of the force is the product of the current, length of wire, and magnetic field. Here is a demonstration of this effect. A segment of a wire passes between the pole faces of a permanent magnet. The magnetic field points from the north to the south poles. The wire is connected to a dc power supply. Positive current will flow from the red to the black terminal. By the right-hand rule, the predicted magnetic force is then outward. Let’s do the demonstration. I will turn on the current from the power supply, which will cause roughly 5 amps flows through the circuit. You can see that there is a force that is indeed outward. If the current is reversed, the force reverses. If, instead, the magnetic field is reversed, the force also reverses. This is all just what the magnetic force law predicts. 3. REVERSE JUMPING WIRE In the previous demonstration, we saw that putting a current through a wire in a magnetic field causes a force on the wire, which causes the wire to move. This demonstration is a standard one, and is often called “the jumping wire.” Some of you may have seen this. But we are now going to do a demonstration that you have probably not seen. Many physical phenomena are reversible (the technical name is reciprocal). Is the jumping wire demonstration reversible? That is, instead of putting a current through a wire to cause it to move in a magnetic field, what if we move the wire in the magnetic field? Will a current be generated? We consider the case where wire is moving perpendicular to the magnetic field. There are moving charges in the wire, but they now experience a magnetic force that is along the wire. This generates a voltage across the wire. If the circuit is closed, there will be a current! The jumping wire demonstration should thus be reversible! Can we demonstrate this? Note that the wire forms a pendulum here. If we start this pendulum oscillating, the wire will move back-and-forth in the magnetic field. So, we just connect the wire leads to an oscilloscope and look for a voltage. It appears that there may be a signal there, but there is too much noise. Part of this is due to the wire leads acting as an antenna loop that picks up electromagnetic noise. If the leads are twisted together, we get a significant reduction of noise, as you can see. To further reduce the noise, we use a preamplifier that has built-in filters. I have set the filters to remove most of the noise below and above the pendulum frequency. I have also set the gain to 1000. In the end, you can see that we have greatly increased the signal-to-noise ratio! When the amplitude of the pendulum is large, note that a significant voltage only occurs when the bottom wire is in the magnetic field. As the amplitude of the pendulum decreases due to damping, and I have to increase the gain on the oscilloscope, note that the bottom wire spends more and more of its time in the magnetic field. The signal becomes more and more sinusoidal, which it should! 4. CONCLUSION Putting an electric current through a wire in a magnetic field can cause the wire to move. This is the well-known “jumping wire” demonstration. The force is due to the fundamental magnetic force on the moving charges. The result is that we have a motor, which is any device that converts electrical energy into mechanical energy. Remarkably, the reverse process can also occur. Moving a wire in a magnetic field can cause a voltage to occur across the wire. This voltage can be used to drive an electrical current. The result is that we have a generator, which is any device that converts mechanical energy into electrical energy. Simple physics demonstrations such as these fundamentally contributed to the Industrial Revolution! Physics lecture demonstrations are always fascinating, and the quest for them never ends. This is the Physics Department of the Naval Postgraduate School, and I’m Dr. Bruce Denardo. Thank you.