1) A coil of wire becomes an electromagnet when current passes through it.
2) The electromagnet interacts with a permanent magnet, causing the coil to spin.
Background
The fact that electric current flowing through a wire can create a magnetic field was discovered by accident in 1820, by a Danish professor named Hans Oersted – He accidentally left a magnetic compass near some wires when he closed the switch of a circuit, and saw the compass needle jump! We are still investigating precisely how magnets and this interaction works – quantum mechanics has a few answers – but regardless, we can already try to use this effect for something useful.
Motor operation: Imagine the Coil (Rotor) turning: when the unsanded wire is making contact to the paperclips, current flows through the coil and creates an electromagnet. One face of the coil becomes a north pole, the other a south pole. The permanent magnet attracts its opposite pole on the coil and repels its like pole, causing the coil to rotate towards it. As the coil turns, it loses electrical contact when the insulated wire touches the paperclips – the electromagnetic coil turns off. Inertia carries the coil through the rest of the turn. Then the wire leads make contact again, and the magnet pulls the opposite pole towards it again. If both faces of the wire leads were sanded, the coil would always be North/South, and the opposite pole would always face the permanent magnet – the fact that the coil’s magnet turns off while in motion allows it to keep going through the turn!
Materials
Demonstration – Electromagnet (per kit) Motor (per motor)
20 cm magnet wire 1 D cell battery
1 D cell battery (or AA + battery holder) 1 AA battery
1 2” nail 50 cm magnet wire 2 3 cm of sand paper 3 cm2 of sand paper
20 small paperclips 2 large “Jumbo” paper clips
2 strong permanent magnet
1 paper cup
20 cm scotch tape
1 piece of construction paper/paper plate on
which to sand
Electromagnet Kit
Electromagnetic Procedure Motor
To prepare before the session (assume 8 tables & families per session):
1. Build Electromagnet demonstration sets (one per table): a. The 20 cm magnet wire coiled around the nail with the both ends of the wire sanded so that the copper is bare. The nail should slip in and out of the coil easily. b. AA battery c. 5-10 small paperclips d. Scotch tape 2. Put each Electromagnet demonstration set in a container so that it can be easily handed out to each family. a. Make sure nails are de-magnetized – if they still have some residual magnetism, throw them hard at the floor to scramble the magnetic alignment! (this must be done before each session)
3. Prepare Materials for Motor sets (one set per family) a. Cut 50 cm lengths of magnet wire (red enamel-coated copper wire). b. 2 Large “Jumbo” paperclips c. D battery d. 2 Magnets (1cm diameter) (One per table:) e. Small square of sand paper f. Paper Plates (for sanding on) 4. 4. Have materials in bins or piles that are easy to distribute/replenish between sessions.
During session
• Ask participants to describe a magnet. Accept and acknowledge all reasonable answers. Mention the North & South “poles” of a magnet (not “positive & negative”, but similar).
• Introduce the first concept: A coil of wire becomes an electromagnet when current is passed through it. • Mention history about this accidental discovery by Oersted in 1820 (see background)
Pass out / point out Electromagnet demonstration kits. • Have the families try picking up the paper clips with the nail. • Ask them to try to pick up paper clips with the nail through the coil. (nothing happens) • Have the families attach the ends of the coil to each end of a battery (either holding the wires to the terminals by hand, or pushing battery into prepared battery holder). The nail will become magnetized due the current flowing through the coil, which creates an electromagnet. CAUTION: The points of contact between the coil and the battery will become hot. The families should soon realize this. They may want to tape the contacts, so they will not burn themselves. Could mention that the movement of electrons through the wire causes friction and heat.
• Have the families disconnect the battery/wires while holding up nails – paperclips should drop because the magnet turns off
Let the families explore a little and ask questions, or prod them with questions to see if they understand the first concept. For instance, do the number of coils affect the strength of the magnet? (Yes) Does the amount of current affect the strength of the magnet? (Yes)
• Introduce the second concept – We can turn a magnet ON and OFF electrically, and we can cause an object to move by having the electromagnet attract/repel a permanent magnet. Note that this time, we do not need the nail as a “surrogate” magnet (the magnetic field is still there, the nail just channeled it strongly through the iron so it looked stronger).
Making the Stator (the stationary part of the motor that does not move): Take the two Large Paperclips & bend the inner part out to form a little loop.
Tape ONE paperclip to one end of the battery – keep it kind of high up.
Show how the two paperclips will be oriented to make the stator stand & electrical contacts, but don’t tape the second paperclip yet.
Making the Rotor (the rotating, moving part of the motor): • Stress that the straightness of the wire & balance of the coil is the most important part! Try to keep the wire as straight as possible!
Leaving ~4 inches of wire projecting from either side, coil 10 times around the AA battery. Slide wire off of the battery and squish the spring-like wire into a coil. Secure the coil by passing the free ends through the middle of the coil twice. Do this for both free ends. AGAIN stress the importance of keeping the wires straight!
Projecting ends of wire must align on horizontal axis (so wire leads look like they’d go through the exact middle of the circular coil). Use demonstration coils to show families.
Bundled coil Close up of ends of wire bundling up coil
They can test the coils for alignment by holding the two wire leads and spinning the coil in their fingers – the coil should spin easily, and stay in the center if it’s aligned.
Point out that the wire has red enamel insulation – we will need to remove some to make electrical contact (so metal wire can touch metal paperclip)
Once the coil is bundled, place the coil face-flat on the paper plates and sand the top face of BOTH of the wire leads projecting from the coil so that the copper is bare on only one half of both leads. Sanding must occur on the Same face, and not on the other! Make sure the sanded side is nice and shiny. Also make sure leads are straight!
Place one wire lead through the loop in the stator’s attached paperclip. Other partner: Thread the loose paperclip’s loop onto the rotor’s free lead, and tape the paperclip to the battery. (This removes the need to bend the rotor leads)
Lastly, to actually attract & repel the electromagnet – place the 2 permanent magnets on the D battery, directly under the rotor coil.
Give it a kick-start to start the spinning, and it should keep going by itself!
Troubleshooting
1. Are the projecting wires on the coil aligned on a horizontal axis? a. Adjust coil without removing it from stator, to make wires straight & to place the center-of-weight along the axis of the wire leads. 2. Are the wire leads making contact with the paper clips? a. One can pick up the motor setup, and while watching the coil, rotate the battery (along it’s long axis) to see when the coil appears to interact with the magnets – this is the position where there is good electrical contact! b. Hold it in that position, and try kick-starting the rotor – this usually works very well! 3. Are the wires well-sanded to expose the bare copper? 4. Are the paper clips positioned so that the coil can turn on a straight axis? 5. Are the paper clips making good contact with the terminals of the battery? 6. Is the coil able to spin freely?
Notes:
The electromagnet activity can be done with battery holders as well, however, the emphasis here is that science can be done with items that are accessible and “low tech”.
Resources http://www.exploratorium.edu/snacks/stripped_down_motor/index.html http://scitoys.com/scitoys/scitoys/electro/electro.html#motor