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Use Super-strong Magnets to Make a Simple Motor

Difficulty
Time Required Average (6-10 days)
Prerequisites Some familiarity with basic electronics and physics would be helpful, but is not required.
Material Availability Specialty items are required. See the Materials and Equipment list for details.
Cost Average ($50 - $100)
Safety Minor injury is possible. Wear safety goggles at all times. Be sure to read the important safety notes at the beginning of the Experimental Procedure before you begin.

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.

Credits

David B. Whyte, PhD, Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Use Super-strong Magnets to Make a Simple Motor" Science Buddies. Science Buddies, 23 Oct. 2014. Web. 24 Oct. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p065.shtml>

APA Style

Science Buddies Staff. (2014, October 23). Use Super-strong Magnets to Make a Simple Motor. Retrieved October 24, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p065.shtml

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Last edit date: 2014-10-23

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. 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.



Electricity science  project <B>Figure 1.</B>The panel on the left shows the motor and the laser tachometer. The motor consists of a small screwdriver, a length of copper wire, a C battery, a piece of reflective tape (for the laser tachometer reading), and neodymium batteries. The panel on the right focuses on the axis through the magnets.

Figure 1. The panel on the left shows the motor and the laser tachometer. The motor consists of a small screwdriver, a length of copper wire, a C battery, a piece of reflective tape (for the laser tachometer reading), and neodymium magnets. The panel on the right focuses on the axis through the magnets. The battery and the magnet spin because of a tangential force created by the flow of a current through the magnet. The magnitude of the force is given by the product of the current, I, the length, L (which, in this case, equals the radius of the magnet), and the magnetic field strength, B.



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:

F = qvB

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:

v = L/t

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:

F = qLB/t = (q/t)LB

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:

I = q/t

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:

F = ILB

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?

Bibliography

You can also try searching YouTube for videos of homopolar motors. Some versions have the wire moving and the magnet staying fixed.

Materials and Equipment

  • C batteries (4)
  • Permanent marker
  • Laser tachometer; available from Amazon.com at www.amazon.com, Neiko model # 20713A
  • Ruler, metric
  • Scissors
  • Small Phillips head screwdriver, or a screw or nail, but the screwdriver is easier to hold.
  • Wire, copper, 18-gauge, 10-cm (6-inches); available at most hardware stores. Copper should be used because it is not magnetic and it has very good electrical conductivity.
  • Wire cutters
  • Neodymium magnets, 5/8-in. diameter x 3/16-in. thick, grade N42, nickel plated, axially magnetized (5); available from K&J Magnetics, Inc. at kjmagnetics.com, catalog # DA3.
    • Note: Read the FAQs section of the K&J website for hints on working with neodymium magnets.
  • Lab notebook
  • Helper
  • Graph paper

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Experimental Procedure

Safety Notes about Neodymium Magnets:

  • Handle magnets carefully. Neodymium magnets (used in this science project) are strongly attracted and snap together quickly. Keep fingers and other body parts clear to avoid getting severely pinched.
  • Keep magnets away from electronics. The strong magnetic fields of neodymium magnets can erase magnetic media like credit cards, magnetic I.D. cards, and video tapes. It can also damage electronics like TVs, VCRs, computer monitors, and other CRT displays.
  • Keep magnets away from young children and pets. These small magnets pose a choking hazard and can cause internal damage if swallowed.
  • Avoid use around people with pacemakers. The strong magnetic field of neodymium magnets can disrupt the operation of pacemakers and similar medical devices. Never use neodymium magnets near persons with these devices.
  • Use the magnets gently. Neodymium magnets are more brittle than other types of magnets and can crack or chip. Do not try to machine (cut) them. To reduce the chance of chipping, avoid slamming them together. Eye protection should be worn if you are snapping them together at high speeds, as small shards may be launched at high speeds. Do not burn them; burning will create toxic fumes.
  • Be patient when separating the magnets. If you need to separate neodymium magnets, they can usually be separated by hand, one at a time, by sliding the end magnet off the stack. If you cannot separate them this way, try using the edge of a table or a countertop. Place the magnets on a tabletop with one of the magnets hanging over the edge. Then, using your body weight, hold the stack of magnets on the table and push down with the palm of your hand on the magnet hanging over the edge. With a little work and practice, you should be able to slide the magnets apart. Just be careful that they do not snap back together, pinching you, once you have separated them.
  • Wear eye protection. Neodymium magnets are brittle and may crack or shatter if they slam together, possibly launching magnet fragments at high speeds.

Preparing the Experimental Setup

  1. Caution: Because the homopolar motor has moving parts, you should wear safety goggles at all times. Label the batteries 1–4 with the permanent marker.
  2. Attach a small piece of reflective tape to each battery.
    1. The reflective tape is included with the laser tachometer that is listed in the Materials and Equipment section.
  3. Cut about 10 cm of copper wire.
  4. Wrap one end of the wire around the screwdriver four or five times, about 3 cm from the tip. See Figure 1, above.
  5. Attach one neodymium magnet to the positive side of the battery labeled 4. The positive side is marked with a "+" on one end. Center the magnet over the battery.
  6. Attach four magnets on the negative side of the C battery.
    1. These magnets should have the same polarity as the magnet that is already attached.
    2. Center the magnets.
  7. Touch the tip of the screwdriver to the single magnet attached to the positive pole of the battery.
  8. Hold the battery up so that the battery and magnets are hanging from the screwdriver.
  9. Touch the free end of the copper wire to the lower stack of batteries.
    1. Touch the magnet in the stack that is closest to the battery.
  10. The battery should start to spin.
  11. Measure the rate of rotation using the laser tachometer.
    1. Follow the directions that came with the tachometer to learn how to operate it.
    2. Have your helper measure the rate and record it in your lab notebook.
  12. Avoid running the motor continuously. The battery should not get hot.
  13. Remove the magnets from the battery labeled 4.

Measuring the Maximum Rate of Rotation with Additional Magnets

  1. Attach one neodymium magnet to the positive side of the battery labeled 3.
  2. Attach three magnets on the negative side of the C battery labeled 3.
    1. These magnets should have the same polarity as the magnet that is already attached.
  3. Repeat steps 6–13 of the previous section to obtain the maximum rate of rotation with three neodymium magnets attached to the negative pole of the battery.
  4. Remove the magnets from the battery labeled 3.
  5. Attach one neodymium magnet to the positive side of the battery labeled 2.
  6. Attach two magnets on the negative side of the C battery labeled 2.
    1. These magnets should have the same polarity as the magnet that is already attached.
  7. Repeat steps 6–13 of the previous section to obtain the maximum rate of rotation with two neodymium magnets attached to the negative pole of the battery.
  8. Remove the magnets from the battery labeled 2.
  9. Attach one neodymium magnet to the positive side of the battery labeled 1.
  10. Attach one magnet on the negative side of the C battery labeled 1.
    1. The magnet should have the same polarity as the magnet that is already attached.
  11. Repeat steps 6–13 of the previous section to obtain the maximum rate of rotation with one neodymium magnet attached to the negative pole of the battery.
  12. Perform the entire procedure two more times. Performing three trials ensures that your results are accurate and repeatable.

Graphing Your Results

  1. Average the maximum rates of rotation for one, two, three, and four magnets.
  2. Make a graph with the number of magnets on the x-axis and the maximum rate of rotation on the y-axis.
  3. Discuss the factors that affected the speed of rotation you observed.

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Variations

  • Try other sizes of magnets. How does the diameter affect the performance?
  • With four magnets on the negative terminal, measure the rate of rotation when you touch the copper wire to each of the four magnets. What do you predict will happen?
  • Devise a stand for the spinning motor so that you do not have to hold it.
  • Use a multimeter to measure the current in the circuit.
  • Devise a way to measure the Lorentz force.
  • Identify sources of unwanted variation in the results and design a procedure that avoids them.

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