posted on 30 Jan 2013 by guy
last changed 23 Apr 2014
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ages: 10 to 99 yrs
budget: $3.00 to $10.00
prep time: 15 to 60 min
class time: 15 to 60 min
This lesson shows how to construct and operate a modern version of the first electric motor, invented by Michael Faraday. It serves as an excellent demonstration or hands-on activity to show the basic principles behind the operation of an electric motor.
optional equipment: modeling clay, knife switch, 9 volt battery connector, coat hanger
subjects: Chemistry, Engineering, Physics
keywords: Faraday, motor, homopolar, Neodymium, salt, water
Fig. 1: The first electric motor by Michael Faraday. From the Quarterly Journal of Science, vol. xii, 1821.
In 1821, the English scientist Michael Faraday designed and built the first electric motor. His design is one in the class of homopolar motors (see the lesson on Minimalist Motors for more discussion) and is one of the simplest motor designs to date. His original version used a copper rod that rotated in a pool of mercury around a central magnet. Although it's easy enough to build Faraday's original design, mercury is a toxic substance that we prefer to keep out of the classroom. In this lesson, we construct a less toxic modern version of Faraday's motor out of everyday materials. For basic principles of operation describing how this and other motors work, please see our lesson on A Survey of Simple Electric Motors.
How-to video by Arbor Scientific for a simple Faraday motor.
There are many implementations of Faraday's motor available; one of the best setups I've seen is by Arbor Scientific, shown in the video above. The materials you will need are:
- two plastic 2-liter soda bottles (or one soda bottle and one clear glass)
- connecting wires (alligator clip leads preferred)
- one or two 9V batteries
- strong disk magnets (preferably Neodymium magnets)
- modeling clay (optional)
- stiff, solid core wire (14 to 10 gauge — 1.5 to 2.5 mm diameter) or coat hanger for the wire in the straw
- stiff, solid core copper or aluminum wire (18 to 15 gauge) for the hanging rod
- aluminum foil
- two small paper clips
- a plastic soda straw
- optional switch
- optional battery caps (see Figures 2 and 3)
The basic construction steps are outlined in the video. I add only a few comments here.
The solid copper or aluminum wire is sometimes a little difficult to come by in the thicker gauges required for the wire inside the plastic straw, and can't always be found in the local hardware store. Look for solid core grounding wire, or galvanized utility wire. Another option is copper or aluminum Bonsai tree training wire. Still another option is a piece of steel wire from a coathanger. Use a coathanger with a cardboard tube across the bottom since the end of that wire already has a nice hook in it, and the cardboard tube can replace the plastic straw.
The hanging copper rod must be non-magnetic (copper or aluminum) and should be cut to hang just above the bottom of the container. It should reach down past the sides of the magnet stack. This wire can be thinner. Anything from 18 to 15 gauge (1 to 1.5 mm diameter) should work.
The video suggests cutting the bottom off of a 2-liter soda bottle to act as the container for the pool of saltwater. You may wish to use a taller container in order to explore what happens as more salt water is added (see the discussion in "questions to ponder" below). In that case, cut just the top off the bottle and use a tall container; it can always be cut down later. The 2-liter bottle that serves as the stand should be mostly filled with water to weigh it down.
When preparing the saltwater, keep in mind that more salt generally supports more current, up to a point. At room temperature, roughly 20% salt by weight makes a saturated solution. You may wish to start with less salt, or no salt, to see what happens as additional salt is added to the solution (see the discussion in "questions to ponder" below). Fill the container to about the level of the top of the magnets or a little higher for best results.
The thickness of the disk magnets does not matter significantly; there should be enough of them to form a stack 2 cm or more in height. The magnet diameter needs to be small enough so that the copper rod revolves around it without touching. A diameter between 3 and 5 mm (1/8 to 1/4 inch) works well.
The modeling clay is only used to support the stack of magnets on the uneven bottom of the soda bottle. You can do without the clay if you stick one of the magnets underneath the soda bottle to hold the other magnets in place. If you are using a clear flat-bottom glass instead, you may not need even that. Alternatively, chewing gum may work, which has the advantage of engaging an entire class in the preparation of the gum. And NO, they cannot have the gum back after it has been used in the experiment.
Alligator leads are very convenient for quick connections, and are available from many sources, but tape and flexible insulated wire works just as well.
You may wish to use a switch in the circuit, like the single-pole, single throw knife switch shown in the video. It is not necessary however; to turn off the motor you can always disconnect a wire.
Connect the circuit and watch it go.
If you have connected the circuit as shown in the video, electrons will flow from the negative terminal of the battery, through through the paper clips and the copper rod. That electric current will be transferred through the saltwater, to the aluminum foil (though not as free electrons — see the relevant discussion in "questions to ponder" below), and back to the positive terminal of the battery. As the electrons flow down the copper rod, they feel a force from the magnetic field generated by the stack of magnets.
This force derives from the notion that an electric current in a wire produces a magnetic field (a fact discovered by Hans Christien Ørsted and André-Marie Ampère in 1820, and described by Ampère's Law as the principle behind the electromagnet). A current-carrying wire therefore feels a force from any nearby magnet in the same way that two magnets feel forces from each other. The force felt by a current in a magnetic field is called the Lorentz force (see the lessons on Classroom Rail Gun and A Survey of Simple Electric Motors for more on the Lorentz force), and was derived by the Dutch scientist Hendrik Lorentz in 1892. In our Faraday motor, the electrons moving down the copper rod feel a Lorentz force from the magnets, which pushes them in a clockwise or counter-clockwise direction around the stack, depending on whether the magnets are arranged with north poles up or down.
For the geek who wants to precisely calculate the direction of the Lorentz force, the Java demo created by the clever folks at the National High Magnetic Field Lab shows the setup of the Faraday motor with the magnetic field lines drawn in. Knowing the magnetic field, the current direction, and the right hand rule (a useful mnemonic for determining the direction of a Lorentz force), one can determine exactly what direction the force is pointing.
Students should be encouraged to modify the parameters of the motor to see how various changes affect the operation of the motor. Some suggestions are:
- add more salt to the solution
- use fewer or more magnets
- reverse the leads on the battery
- turn the magnets upside down
- use a 1.5 volt D cell battery instead of a 9 volt battery
- use two 9 volt batteries connected in series (see Figure 2)
- use two 9 volt batteries connected in parallel (see Figure 3)
- add more saltwater to the container
questions to ponder
- What happens if we turn the magnets over or reverse the leads on the battery?
Naturally, if we turn the magnets over, we reverse the direction of the magnetic field lines. If we reverse the leads on the battery, we reverse the direction of current flow. In either case, we reverse the direction of all the forces. The motor will run backwards.
- What happens if we use more batteries?
Keep in mind that the force on the movable rod depends on the strength of the magnet stack and the electric current flowing through the rod. More current leads to more force, and a faster motor. If we connect two batteries in series, we produce twice the voltage and push more current through the circuit. The motor runs faster, and in a larger diameter circle.
Note for geeks: In truth, the answer to this question depends on how we connect the batteries. The resistance in the circuit is dominated by the resistance of the saltwater, which may be a few hundred ohms. This is much more than the internal resistance of a 9 volt battery, which is usually around 1 or 2 ohms, or any other conductor in the circuit, which is typically a tiny fraction of an ohm. That's why, when we connect the batteries in series, the voltage is doubled, but the resistance hardly changes at all, even though there's a little extra resistance from the second battery. The net effect is that the current doubles (as determined by Ohm's law), and the copper rod spins faster and in a larger circle. On the other hand, if we connect two batteries in parallel, the voltage remains the same (9 Volts), and the resistance hardly changes — it is still dominated by the salt water. With two batteries in parallel, the motor speed does not change.
- What happens if we fill up the container with salt water?
An electric current generally tries to take the shortest path through a conducting medium. When the current is traveling through the water between the hanging rod and the aluminum foil, it tends to travel near the surface. In particular, the current in the hanging rod leaves the rod (or enters the rod) as soon as the rod touches the water. Current does not travel much further down in the portion of the rod that is deeply immersed. If we fill up the container so that most of the hanging rod is immersed, not much of the rod will carry a current, and the part that does will be high above the magnets. Therefore not much of the rod will feel a Lorentz force. The motor should slow down.
- What makes the electric current in the wires?
Usually, current flows through a conductor because some of the electrons in conductors are only very weakly bound to their atoms. This is true of most metals. When a copper wire is connected between the negative and positive terminals of a battery, the electrons in the wire are repelled from the negatively charged terminal of the battery (two charges of the same sign repel each other) and attracted to the positively charged terminal of the battery (two charges of opposite sign attract each other). Consequently electrons move from the negative terminal, through the wire, to the positive terminal. This flow of electrons is the electric current.
There is one sticky point. By convention, the direction of the standard electric current is the direction of flow of positive charges. Since electrons are negatively charged, the direction of the standard current is defined to be opposite the direction of the movement of the electrons. The definition of the positive and negative charge was made before scientists actually knew what the charge carrier was in conductors. Once they discovered it was the electron that moved inside a conductor, and that the electron was negatively charged, it was too late to change. Nowadays, this secret convention, and others like it, helps provide job security to physicists and engineers.
- What makes the electric current in the water?
This current is much more complicated than the current in the copper wires. In truth, free electrons do not flow through the water. Nonetheless, the electric current can travel through the water even without a flow of electrons. All we need is for electrons to be taken away from the copper wire and surrendered to the aluminum foil (or vice versa, depending on how the battery is connected). In a saltwater solution, table salt (a.k.a. sodium chloride — NaCl) dissolves into ions Na+ (a Sodium atom with an electron removed) and Cl- (a Chlorine atom with an extra electron attached). In addition, a small fraction of the water molecules also disassociate into ions: H+ and OH-. In our Faraday motor, the excess electrons in the copper rod combine with H+ ions in solution to make neutral hydrogen gas. When we connect the motor we can start to see hydrogen bubbles form on the copper rod. At the aluminum foil, some chlorine ions Cl- surrender their extra electrons to the positively charged alumium and form neutral chlorine gas, which will also form bubbles. The left over Na+ ions combine with the left over OH- ions to form neutral Sodium Hydroxide (NaOH) in solution. At this point, more water molecules can disassociate and the process can continue until all the sodium and chlorine ions are used up. The net effect is that electrons are taken away from the copper rod and delivered to the aluminum foil, just as if they flowed from one to the other.
- Why do bubbles form on the aluminum foil and the copper rod?
Chlorine gas is being formed at the aluminum foil (assuming it is connected to the positive terminal of the battery) and hydrogen gas is being formed at the copper rod. See the discussion above on how electric current flows in saltwater.
If the motor is not working, the first thing to check is all the electrical connections. The connections at the paper clips are especially prone to disruption from oil or grime and may need to be cleaned with alcohol. Try to use new paper clips, and sand or file the interior surfaces if necessary. Check the resistance with an ohmmeter if you have one. If neither the aluminum foil nor the hanging copper rod are producing bubbles, a connection is broken somewhere.
The electrical connections in the water are also a bit unreliable. As gas bubbles form on the aluminum foil and the copper rod, they can disrupt the connection. Periodically stirring the water or tapping the copper rod on the side of the container to dislodge bubbles may help. Use several layers of aluminum foil to ensure a large surface area.
If the circuit is not complete and you suspect the connection at the paper clips, you might try replacing the paper clips with several strands of fine copper wire. You can tie or solder the wire on tightly at both ends to make a good connection, but it will still be flexible enough to allow the hanging copper rod to revolve freely.
If the magnet stack is too short and does not reach up to the bottom of the hanging wire, the motor will not work. In this case, the wire is hanging along the axis through the magnets, and the current is parallel to the magnetic field lines above the magnet, so there is no Lorentz force. The wire should hang down beside the magnets.
If the container is too full of salt water, the motor may not work. The water should be deep enough to make good connection with the hanging copper rod, but it should not be much above the top of the magnets.
The Faraday motor works best after students have seen and understood electromagnets and can appreciate the interaction between a magnet and a current in a wire. Check out Basic Electromagnets for some ideas. The lesson Classroom Rail Gun also demonstrates the Lorentz force in a very simple way that helps prepare students to figure out the operation of more complex motors. More precise visualization of the Lorentz force can be achieved if the students already know about magnetic field lines. See Iron and Magnets for an introductory lesson.
To initiate more discussion about where the current flows through the water, have the students start with a minimum amount of saltwater to get the motor running, and then slowly add more saltwater. When the water level is well above the top of the magnets, the copper rod should quit moving, even though bubbles are still forming on the rod near the surface of the water. (If you can't easily see bubbles forming on the copper rod, make sure the negative terminal of the battery is connected to the copper rod.) With bubbles forming near the surface of the water, but not further down on the copper rod, students will start to see that the water is only interacting with the current near the surface. See the discussion above in "questions to ponder" for more details. Siphon off some of the water to get the motor running again.
For the teacher who wants to get into the chemistry of saltwater electrolysis, one good way to introduce the topic is to have the students start with pure tap water and slowly add salt. The motor will not work with pure tap water; there aren't enough ions in tap water to support a significant current. Once enough salt has been added to start the electrodes bubbling and the motor running, a discussion on how electric current flows through saltwater can ensue (see "questions to ponder" above).
To jumpstart a discussion on basic electronics of batteries and circuits, including series and parallel connections, just have the students replace the 9 V battery with two 9 V batteries in series and watch what happens. The electrodes will start bubbling more vigorously, and the rod will start moving faster through the water. Have the students measure the time it takes for a revolution and estimate the diameter of the circle to calculate the speed.
John Jenkins has compiled a beautiful visual history of motors showing a number of early motors, including several versions of Faraday's design, at http://www.sparkmuseum.com/MOTORS.HTM .