Abstract

In this science project, you will build what might be the world's simplest motor. It has just four basic parts: magnets, a battery, a screwdriver, and a short piece of wire. It takes only minutes to assemble, but it provides a wonderful device to explore how electricity and magnetism combine to produce a fast-spinning motor.

Objective

The objective of this science project is to make a very simple homopolar motor and to determine how the size of a neodymium magnet affects its rate of rotation.

Introduction

An electric motor is a device that uses electrical energy to produce kinetic energy. In a toy car, for example, the electrical energy in the battery is converted into the kinetic energy that spins the wheels and makes the car move forward. Electric motors work by taking advantage of the interaction of magnetic fields and current-carrying conductors. In this science project, you will build what might be the world's simplest motor.

The motor has four parts: a battery, a piece of copper wire, a small screwdriver, and neodymium magnets. Neodymium magnets are much stronger than the "normal" magnets you might have on your refrigerator. The neodymium magnets will be attached to both sides of the battery, and the battery will be suspended from the point of the screwdriver (see Figure 1). Because the magnet is so strong, the attractive force is sufficient to hold the suspended weight of the battery and the magnet (note: you can read more about magnets in the Science Buddies Electricity, Magnetism, & Electromagnetism Tutorial). The battery will start to spin when the wire is connected between the screwdriver and the edge of the stack of magnets below the battery, producing a current. A laser tachometer will measure the rate of rotation. See the setup in Figure 1.

In order for the battery and magnets to move, there has to be a force acting on them. The force that causes the battery and magnets to move results from the interaction of the magnetic field produced by the magnets with the current that flows through the wire, the magnets, and the battery as the wire touches the magnets. You would usually avoid connecting the two poles of a battery with a wire, since this discharges the battery very quickly. However, in this case, it is desirable to have a large current to maximize the interaction of the magnetic field with the current.

The force acting on the magnet to make it spin is called the Lorentz force. The Lorentz force can be more precisely defined as the force, F, acting on a particle with an electric charge, q, and moving with a velocity, v, in a magnetic field with strength B. The equation for the Lorentz force when the magnetic field is perpendicular to the current is shown below in Equation 1:

Equation 1: This equation states that the force is equal to the product of the charge, q, the velocity, v, and the magnetic field strength, B.

• F = Lorentz force, in newtons (N)
• q = Charge, in coulombs (C)
• v = Velocity, in meters/second (m/s)
• B = Magnetic field, in teslas (T)

How do you convert this equation, which describes the force on a moving charge, to one that uses familiar electronic terms, such as current? The velocity term seems especially troubling, since there's no way to measure it, but you can get rid of the velocity term by replacing it with the distance at which the charge travels (L) divided by time (t).

Equation 2: In words: Velocity = Distance, L, divided by time, t

• v = Velocity of the charge, q, in meters/second (m/s)
• L = Length that the charge travels, in meters (m)
• t = Time for charge to travel the distance L, in seconds (sec)

Substitute Equation 1 into Equation 2 to get Equation 3:

Equation 3: In other words, the Lorentz force equals the product of the charge, the length the charge travels, and the magnetic field strength, divided by the time it takes the charge to move the distance L.

• F = Lorentz force, in newtons (N)
• q = Charge, in coulombs (C)
• L = Length that the charge travels, in meters (m)
• B = Strength of the magnetic field, in teslas (T)
• t = Time for charge to travel the distance L, in seconds (sec)

Current, I, is defined as charge per unit time:

Equation 4: This equation states that if you were to look at a point in the wire (or screwdriver or magnet or battery) and measure how much charge passed through over a certain time period, the current at that point would equal the charge divided by the time.

• I = Current, in amperes (A)
• q = Charge, in coulombs (C)
• t = Time, in seconds (sec)

Substituting Equation 4 into Equation 3 gives us the Lorentz equation in familiar electronic terms.

Equation 5: This equation states that the force on an object of length L, carrying a current I, in a magnetic field B equals the product of the current, the length of the current, and the strength of the magnetic field.

• F = Lorentz force, in newtons (N)
• I = Current, in amperes (A)
• L = Length of current carrier, in meters (m)
• B = Strength of the magnetic field, in teslas (T)

As you can see in Figure 1, the force is directed perpendicular to the edge of the magnet. This tangential force causes the magnet to start spinning.

For this electronics science project, you will assemble the motor described above. This kind of motor is referred to as homopolar, because unlike regular electric motors, it does not have alternating polarity. You will determine how adding more magnets affects the rate at which the motor spins. The rate of spin can be accurately measured using a laser tachometer. The tachometer measures the rate at which laser light is reflected back to it from reflective tape attached to the spinning battery.

Terms and Concepts

• Electrical energy
• Kinetic energy
• Magnetic field
• Current-carrying conductor
• Neodymium magnet
• Rate of rotation
• Tangential force
• Current (I)
• Magnetic field strength (B)
• Force
• Lorentz force
• Electric charge (q)
• Velocity (v)
• Homopolar
• Right-hand rule
• Newton's third law

Questions

• What will happen to the direction of spin if you flip the direction of the poles of the magnet?
• The magnetic field strength can also be measured in a unit called a gauss. How many gauss are in 1 tesla?
• What are the units for a tesla? Hint: Solve Equation 3 or 5 for B.
• Based on your research, what is the right-hand rule for determining the direction in which the motor will spin?

Materials and Equipment

• C batteries (4), available from Amazon
• Permanent marker
• Laser tachometer; available from Amazon, Neiko model # 20713A
• Ruler, metric
• Scissors
• Small Phillips head screwdriver, or a screw or nail, but the screwdriver is easier to hold.
• Uninsulated 18-gauge copper wire (10 cm), available at most hardware stores or Amazon. Copper should be used because it is not magnetic and it has very good electrical conductivity.
• Wire cutters, available at a hardware store or Amazon
• Neodymium magnets, 5/8 inch diameter x 3/16 inch thick, grade N42, nickel plated, axially magnetized (5); available from K&J Magnetics.
  • Note: Read the FAQ section of the K&J website for hints on working with neodymium magnets.
  • Note: Neodymium magnets of slightly different dimensions and grades are also available from Amazon. This project should still work with similar magnets, but Science Buddies has only tested the K&J magnets, so cannot guarantee performance with other magnets.
• Lab notebook
• Helper
• Graph paper

Experimental Procedure

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Electricity & Electronics Project Ideas
• My Favorites

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

APA Style