SCEN 102 - 2000
This page contains some information about the magnetism lab, which was conducted the week before spring break in 2000. The items were also on display the week after spring break. What's here are the captions which were in the museum and some additional explanatory information.
"Space Wheel" Toy
There are several of these toys on display. Some of them have the electronic insides intact and some have them taken out. Please do not take them apart, though you can pick them up and handle them. You can inspect the insides of one of them, where we have removed the bottom cover, battery,and other internal apparatus.
TO DO AND NOTICE:
Place the rotor -- the wheel with three extensions -- on the end of several of the "space wheels." Start the rotor rolling on each one at the same time. It will help you figure things out of one of the rotors is placed on the track which has no battery in the bottom.
See how long the wheels keep rolling in these apparatuses.
You may wish to use one of the compasses that we have to investigate what's going on in the device where the wheel rolls for longer.
If you have one of these devices which is perfectly level and kept clean, it is possible for the rotor to keep rolling for months.
WHAT'S GOING ON?
Why do the rotors in some of the devices keep rolling for longer than in some other devices?
What does the battery in the base of the space wheel do?
THE EXPLANATION:
In the bottom of both of these devices is a 9-volt battery connected to a circuit and an electromagnet. The circuit is a bit complex and I'm not prepared to go in the direction of explaining exactly how it works. What does happen is that when the circuit senses a magnetic field,
from the rotor, it turns on the electromagnet in such a way as to keep the motion going. When the electromagnet goes on, compasses will start moving.
The main idea is conservation of energy. Perpetual motion machines can't exist, because friction will always cause a moving object to slow down. But what happens here is
Electrical energy from the battery --> Gravitational Potential Energy <--> Kinetic Energy --> via friction and air resistance, heating up the air in the room.
If the battery is continuing to supply more energy, then the usual loss of energy to friction can be compensated for. The wheel will stop rolling when the battery runs out, of course.
Kinetic Art apparatus
Observe the circling balls which keep moving, moving, and moving. Does this apparatus violate the conservation of energy? Use a compass to probe what is going on in the base of the apparatus and see if you can figure out what is going on.
THE EXPLANATION
The explanation of this device is quite similar to that of the "space wheel" toy. Indeed, the circuitry in its base looks virtually identical. Each of the silvery balls contains a magnet in it, and when this magnet passes by the base of the device, it is accelerated because the battery turns on an electromagnet.
DC and AC Generators
TO DO AND NOTICE:
Spin the coils in each of these generators and notice what happens to the meter.
WHAT'S GOING ON
Ask yourself, and answer, the following questions:
How are the DC and AC generators different? Examine the construction of each one and find out why one of the devices generates AC and one generates DC.
How good is the DC generator? Does it generate a steady current?
EXPLANATION
What's going on is that in the AC apparatus, one end of the coil spins past the north pole of the magnets, and then a half cycle later (which is really a fraction of a second) it spins past the south pole. Because each end of the coil is attached directly to the meter, the meter
oscillates around zero.
In the DC apparatus, each end of the coil is attached to a split ring, and if it is adjusted correctly, it turns an alternating current into something resembling a direct current because it sends it into a different wire each half cycle.
What I want the students to get out of this is an understanding of how AC and DC are different.
THE MYSTERIOUS ALUMINUM TUBE
To do and notice:
Take the two blue weights and drop them through the aluminum tube. Compare the rates at which they fall. What is different about those two blue weights?
Some hints:
Hint no. 1: Think about the other materials that are in this room. Can you demonstrate that one of the blue weights is a magnet? How would you prove that?
Hint no. 2: Changing currents create magnetic fields, and changing magnetic fields create currents. Where are the currents being created?
Hint no. 3: The tube is made of metal (specifically, of aluminum). Can currents flow in the tube?
Hint no. 4: Draw an energy flow chart that can help you describe what's going on.
You might wish to compare this experiment to the "eddy current apparatus" in order to figure out what is going on.
EDDY CURRENT APPARATUS (swinging copper plates)
TO DO AND NOTICE:
Swing the pendulum, with a copper plate at the bottom, through the poles of the great big magnet. Switch the copper plate around so that the side with slots in it swings through the poles of the magnet, and compare its behavior to its behavior when the solid side swings through the magnet.
WHAT'S GOING ON:
Hint no. 1: Changing currents create magnetic fields, and changing magnetic fields create currents. Where are the currents being created?
Hint No. 2: Where can currents flow in a solid copper plate? Can currents flow in these same places in a copper plate which is not solid?
EXPLANATION:
The copper plate stops swinging when the solid end swings through the poles of the magnet. The reason for this is that the motion of a conductor through a magnetic field generates an electrical current, in the copper plate (Hint No. 1). Because the copper plate is solid, the current can flow over some distance and build up. (Hint no. 2). Energy is used to make these currents flow and the only place the energy can come from is the motion of the pendulum. The energy dissipated in the swirling currents cause the swinging copper plate to come to a stop.
When the slotted end of the copper plate swings through the magnet, the slots prevent the current from building up. The current can no longer flow over a large distance, and so its intensity is much reduced. As a result, less energy is dissipated, and the plate keeps moving for longer.
JUMPING RING APPARATUS
This was the big coil which was plugged into AC power which caused rings to jump into the air.
TO DO AND NOTICE:
Play with this apparatus, noticing which rings jump into the air and which rings do not jump into the air.
WHAT'S GOING ON:
(Hint no. 1): When the switch is turned on, electrical current flows through the large coil. Because this current is alternating current, the large coil generates changing magnetic fields.
(Hint no. 2:) Changing magnetic fields create electrical currents.
EXPLANATION:
Why do the rings jump? (Short version). When you turn the coil on, changing currents generate changing magnetic fields. These changing magnetic fields then generate changing currents in the copper ring. Since the copper ring is a ring, it acts like an electromagnet -- and the iron core which passes through the center of the ring makes it a more powerful electromagnet. The interaction between the magnetic field from the ring and the magnetic field from the coil produces a force, which causes the ring to jump.
Why does the lighter ring jump less high? Because there is less current in it, so it is a weaker electromagnet and does not produce as great a repulsive force. Why is it important that the rings have no gaps in them? If there are gaps, no current flows.