Homopolar Motor (Jason Aiken)
How to build a simple electric motor
Principle(s) Investigated: Lorentz force, electric circuit, batteries, magnets
NGSS Cross Cutting Concepts
2. Cause and effect: Mechanism and explanation. Events have causes, sometimes simple, sometimes multifaceted. A major activity of science is investigating and explaining causal relationships and the mechanisms by which they are mediated. Such mechanisms can then be tested across given contexts and used to predict and explain events in new contexts.
- Completing the circuit causes the nail to spin.
3. Scale, proportion, and quantity. In considering phenomena, it is critical to recognize what is relevant at different measures of size, time, and energy and to recognize how changes in scale, proportion, or quantity affect a system’s structure or performance.
- A 1.5V battery is used.
- We do not measure the strength of the magnet. There are several ways that we might in the future measure the magnetic moment, the total magnetic flux it produces, or the local strength of magnetism (magnetization).
- We do not measure the output current (or voltage drop) of the battery.
- We do not measure the rpm of the motor.
6. Structure and function. The way in which an object or living thing is shaped and its substructure determine many of its properties and functions.
- We make a drawing of the structure of the system.
7. Stability and change. For natural and built systems alike, conditions of stability and determinants of rates of change or evolution of a system are critical elements of study.
- The nail or screw is suspended in a stable state. We complete the circuit and cause a change (the nail spins).
Material:
- About six inches of copper wire
- The wire does not have to be copper, but it must be a material which is not attracted to a permanent magnet.
- A small but strong magnet
- For safety, use a small magnet. I am using a 8.1mm diameter 3.0 mm thick Neodymium Iron Boron (NdFeB) magnet. These are commonly called neodymium magnets. I don’t recommend using a Samarium Cobalt magnet, because SmCo magnets are too brittle. Students might be injured if a SmCo magnet breaks.
- One nail or screw
- It must be the right combination of length and weight, so the magnetic force holds it hanging below the battery.
- One 1.5 V battery
- You can try AAA, AA, C, or D.
Safety precautions:
Do not run the motor for more than a 10 seconds
The battery and wire will heat up.
Do not leave the battery shorted
You could start a fire.
Do not touch wires outside of this activity
We can touch these wires because we know the battery only puts out 1.5V.
Powerful attraction forces can cause serious injury
Neodymium magnets are powerful. The force between magnets can often be surprising to those unfamiliar with their strength. Fingers and other body parts can be pinched between two magnets. With larger magnets, injuries of this type can be severe.
Therefore, only use small magnets in class.
Neodymium magnets are not for small children
Neodymium magnets are not toys. Small magnets can pose a choking hazard. If multiple magnets are swallowed, they can attach to one another through intestine walls. This can cause a severe health risk which may require immediate emergency surgery.
Neodymium magnets can affect pacemakers
The strong magnetic fields near a neodymium magnet can affect pacemakers, ICDs and other implanted medical devices. Many of these devices are made with a feature that deactivates it with a magnetic field. Therefore, care must be taken to avoid inadvertently deactivating such devices.
Neodymium magnets are brittle and fragile
Neodymium magnets are made of a hard, brittle material. Despite being made of metal, and the shiny, metallic appearance of their nickel plating, they are not as durable as steel. Neodymium magnets can peel, chip, crack or shatter if allowed to slam together. Eye protection should be worn when handling magnets, since shattering magnets can launch small pieces at great speeds.
Strong magnetic fields can interfere with compasses and navigation
Magnetic fields can influence compasses or magnetometers used in air transport. They can also affect internal compasses of smartphone and GPS devices.
Procedure:
Do not run the motor for more than a 10 seconds. The battery and wire will heat up.First created in 1821, the homopolar motor is easy to experiment with.We can make a simple homopolar motor with a screw, a battery, a wire, and a magnet. The magnet is on top of the screw head. The screw and magnet are held to the battery by the magnet's attraction. The screw and magnet spin, with the screw tip acting as a bearing.Options:
Turn over the magnet to spin the nail in the opposite direction.
Turn over the battery to spin the nail in the opposite direction.
Student prior knowledge:
This experiment can be done by students who have little understanding of concepts like completing an electrical circuit, vectors, forces, cross products, or magnetic fields. In this case the experiment can be used to introduce these topics, or a number of other topics including: electric batteries, electrical conductors, magnets, NdFeB magnets, EMF, electromagnetism, or the electron.
The more advanced student should have some understanding these terms:
Electron
Electrical circuit
Vector
Force
Magnetic field
Electric battery
Explanation (for an AP physics student):
The name homopolar indicates that the electrical polarity of the conductor and the magnetic field poles do not change (i.e., the motor does not require commutation). I don't use the term monopolar when describing this motor, because I think the term monopole should be reserved for a hypothetic magnetic monopole. In reality, even though all magnets are said to have a north pole and a south pole, these two poles cannot be separated from each other. For now, magnetic monopoles do not exist.
Ohm's Law comic
When we connect the wire to complete the circuit, there is very little resistance to the flow of current. We have created an intentional short circuit (a circuit with very low electrical impedance). The battery puts out as much current as it can while its voltage drops. I measured an old AA battery putting out 7 amps for the first few seconds, then dropping to 6 amps for a few seconds. I stopped the test. A new AA battery might put out 9.5 amps for a short time.
The Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force F:
The little arrows denote vectors
F = force on a moving charge
q = magnitude of charge
E = electric field
v = velocity of charge
x = cross product
B = magnetic field
In our experiment, we can ignore the electric force (effects of an electric field), and reduce the equation to only the magnetic force:
The implications of this expression include:
1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B.
2. The magnitude of the force is F = qvB sinθ where θ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero.
3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product.
The diagrams above help us understand the force on a moving positive charge. The force is in the opposite direction for a negative charge moving in the direction shown (often called left-hand rule). One fact to keep in mind is that the magnetic force is perpendicular to both the magnetic field and the charge velocity, but that leaves two possibilities. The right hand rule just helps you pin down which of the two directions applies.
In our case, we have not measured the polarity of our magnet, so we do not know the direction of the force until the motor begins to spin.
Questions & Answers:
1. Can an electron at rest be put in motion by a magnetic field?
Looking only at the equation for the Lorentz force, we would answer, "No. However, you can set a resting electron into motion with an electric field." But thinking more broadly, we would answer, "There is no such thing as an 'electron at rest'." We don't know of any mechanism to stop an electron, thereby bringing it to rest. We could discuss if an "electron at rest" would violate the Heisenburg uncertainty principle - I think it would. We could also discuss if wave particle duality has any meaning for a theoretical electron at rest.
2. Is it necessary for the magnet to move?
No. It is not necessary for the magnet to move, or even to be in contact with the rest of the motor; its sole purpose is to provide a magnetic field that will interact with the magnetic field induced by the current in the wire.
3. Does the current moving up the nail have much of an effect?
No. By experimentation, we can see that touching the wire to the bottom of the magnet does not start the motor spinning. We would have to do a computer simulation of our experiment to see just how much torque is applied to the nail from the current traveling up the nail.At first glance, students might think that the magnetic field induced by the current going up the nail causes the nail to spin. It is true that when current flows in a wire, a magnetic field is generated (as pictured). But, going back to Lorentz force equation, we remember that the force on a charged particle moving through a magnetic field is zero if the charge is moving with uniform velocity parallel to the direction of a uniform magnetic field. It continues to move with uniform velocity v along a straight line parallel to the field. Our nail has thickness, and we can imagine the field lines starting to curve while still inside the nail. This would create a cross product that is small, but not zero.
Applications to Everyday Life:
If you have a car with an internal combustion engine, then you very likely have an electric motor to start your engine.
Electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor. In certain applications, such as in the transportation industry with traction motors, electric motors can operate in both motoring and generating or braking modes to also produce electrical energy from mechanical energy.
Your blender, juicer, vacuum cleaner, and garbage disposal all have electric motors.
Similar to an electric motor, the conversion of mechanical energy into electrical energy is done by an electric generator.
Videos:
24 second video explanation:
Alternate design of homopolar motor
We can attach the magnet to the battery and allow the conducting wire to rotate freely while closing the electric circuit by touching both the top of the battery and the magnet attached to the bottom of the battery.
I made one:
Supplies:
Procedure:
Explanation:
