A student gets a charge out of a Van de Graaff generator
|The following are descriptions of demonstrations performed in the ISU Department of Physics Demonstration Road Show. Teachers, especially those at schools visited in this program, are encouraged to access this information. The discussions provided with many of the descriptions parallel those provided in a typical presentation to K-6 grade students. Only about 3/4 of the demonstrations provided below can be incorporated into a typical 50 minute presentation. Teachers are encouraged to select those demos most relevant to what they have covered or will cover in their classes. This selection is usually done during a meeting with an ISU Physics faculty member prior to a demonstration show.|
H and He Balloons
15" balloons filled with either H or He are popped with an 8" "torch" lighter. This is used as an exercise in the scientific method. In science, we make educated guesses based on observations, experiments are performed to test those guesses, the guesses are modified if necessary, and the process continues. Students are shown the balloons, and after noting that whatever is in the balloons must be lighter than air, the students are asked what might be in them. If mentioned, hot air can be discounted since there is not an obvious heat source. Heat must be continuously be added to a hot air balloon to keep it hot. Once choices are narrowed down to H and He, the students are asked to come up with an experiment to tell the difference. This results in the "flame test", since H is very flammable, while He is not.
Charge and Static Electricity
Plastic, acrylic, and glass rods are rubbed with plastic shopping bags, fur, wool and silk to transfer charge. Most matter is made up of atoms with a compact and tiny positively charged nucleus surrounded by a much larger cloud of negatively charged electrons. Opposite charges attract and like charges repel. When two materials are rubbed together, odds are that one material will be more likely to pick up extra electrons while the other will be more likely to give up electrons. The object with extra electrons will be charged negative, or minus, while the object that lost electrons will be charged positive, or plus. Black plastic when rubbed by a plastic grocery bag, fur, or wool will become negatively charged, while the plastic grocery bag, fur, or wool will be charged positive. Acrylic or glass when rubbed by silk will become positively charged while the silk will be charged negative. Attraction and repulsion is demonstrated with these materials by suspending one object in a cradle supported by fishing line and making it rotate by bringing another charged object near.
Roll the Pop Can
A charged object of either sign will attract a neutrally charged (no charge) empty pop can. The pop can is made of metal and will conduct charge freely. When a negatively charged object is brought near the can negative charge in the can will be repelled to the opposite side of the can while positive charge will be attracted to the side of the can near the negatively charged object. The amount of negative charge on the far side of the can is equal to the amount of positive charge on the near side, but the attraction between the negative object and the positive charge on the near side of the can is stronger than the repulsion between the object and the negative charge on the far side of the can because the positive charge is closer to the object. The electric force between two objects gets weaker with distance. The same sort of thing happens when a positively charged object is brought near the can.
Spin the Board
A charged object is brought near a neutrally charged long wood board that is balanced on the bottom of a curved bowl so that the board is free to rotate. No matter what the charge on the object, the board will be attracted to it and twist towards it. Materials that do not conduct charge are often polar, like wood, meaning that the molecules in the material have one end that is positively charged with the other end negatively charged. These molecules are called dipoles. When a charged object is brought near such a material a lot of the dipoles will twist so that the end with the same charge as the object is farther from the object than it used to be and the end with the opposite charge as the object is closer than it used to be. The attraction between the charged object and the oppositely charged end of a dipole is stronger than it used to be because it is closer, and the repulsion between the charged object and the end of the dipole with the same charge is weaker than it used to be because it is farther away. No matter what the charge of the charged object it will be attracted to a polar material because of the slight twisting of lots and lots of dipoles in the material.
Deflect Water Stream
A thin stream of falling water is deflected by both positive and negatively charged rods with the stream always attracted to the rods no matter what their charge. Water, like wood, is a very polar material.
Van de Graaff Generator
The Van de Graaff generator can produce voltages (charge separations) of 20,000 volts or more. It separates charge in the same fashion as with the rods and fur or silk, but much faster. A motor spins a big rubber band past metal brushes. The rubber band picks up electrons from the brushes. This makes the top of the generator positive and the base negative. This allows lots of electrostatic demonstrations with styrofoam balls with metallic paint, pieces of fur, tart pans, feathers, and pop cans. Student volunteers stand on an insulating block and hold onto the generator. They pick up lots of positive charge all over their bodies. Since like charge repels, the volunteer's clothes will "poof" out and their hair will stand on end.
Ping Pong Ball Capacitor
Two parallel metal plates are connected to the Van de Graaff generator so that they are charged oppositely. Ping pong balls painted with metal paint are tossed between the plates. The balls will pick up one charge from one plate and will be attracted to the other plate where they will lose their original charge and pick up the opposite charge. They will then be attracted to the first plate where they will again lose and gain charge. The balls bounce up and down discharging the plates while making a delightful clanging sound.
Soap Bubble Plate
A large metal plate is connected to the Van de Graaff generator and charged. Student volunteers blow soap bubbles at the plate. At first they are attracted to the plate because they are polarized just like the water in the water stream demo. Once the bubbles get close to the plate they pick up charge from the plate and are then dramatically repelled because they have the same charge as the plate.
Bi-Colored Diode Twirl
A red and green colored light emitting diode (LED) with alternating current (AC) running through it is twirled about in a circle to produce a pretty pattern of the two colors. The diode is green when current is running through it one way and red if the current runs through it the other way. With wall current the light will switch from red to green 60 times every second. We cannot normally see this, but we can if the diode is moving fast enough. As it moves around in a circle, we see a green spot followed by a red spot followed by a green spot and so on. You can joke about the electric company sending us some charge, then taking it back, while turning it off in the middle.
Magnetic Field of a Bar Magnet
The magnetic field of a bar magnet is shown by sprinkling magnetic metal filings on top of the magnet. Each filing turns into a little magnet and aligns along the magnetic field produced by the bar magnet. It is important to note that the field lines are all loops. Magnetic fields have no beginning or end. The "poles" of a magnet are just the places where most of the field lines enter or leave the magnet. The field lines close to form loops within the magnet.
Moving charges produce a magnetic field. This is shown by wrapping wires around a compass and connecting the wire across a battery. A current (moving charges) is produced in the wire which in turn produces a magnetic field. All magnetic fields arise from the motion of charges. In a permanent magnet the magnetic field is produced by the motions of electrons in the atoms of the magnet.
Wire looped around a nail makes a magnet when the wire is connected across a battery. If the connection is switched in direction the magnetic field produced by the wire switches direction. The strongest magnets we have are made in this way. Student volunteers are challenged to pull apart a powerful electromagnet powered by a single flashlight battery.
A beam of electrons is produced within a partially evacuated discharge tube. The beam is deflected when a magnet is brought near. If the magnet is flipped over, reversing the field, the direction the beam is deflected is reversed. Electric charges feel a force when they move through a magnetic field. Since magnetic fields are produced by moving charges, the magnetic force is just the way charges affect one another when there is relative motion between them.
Faraday Induction and Relativity
When the magnetic field near a conductor is changed, by moving a magnet for example, a current is produced in the conductor. This is shown by moving a bar magnet in and out of a coil of wire connected to a current meter. This is also shown with a flash coil where a current produced in a wire loop by a moving magnet makes a small light turn on. This is called induction. Not surprisingly, a current is also produced by moving the coil while keeping the magnet still. This is the same effect demonstrated with the electron beam. The charges in the wires are forced to move through a magnetic field and therefore feel a force which produces the current. You get the same effect by moving the wire or by moving the magnet. All that really matters is relative motion. This is the basic idea behind Einstein's theory of relativity. Changing magnetic fields, like that from a moving magnet, produce electric fields which make charges move. Changing electric fields, like that from a moving charge or electric current, produce magnetic fields. All that really matters is the relative changes of electric and magnetic fields.
Loops of wire are forced to move through a magnetic field. The moving charges in the coil experience a force from the magnetic field and move in a current through the wire. This is how we produce electricity at power plants. Big loops of wire are forced to move past big magnets. The wire loops are moved either moved by falling water, as in a hydroelectric dam, or by expanding steam carrying heat from something that is burned.
A motor is a generator that is run in reverse. A battery is connected across loops of wire that are near a magnet. The battery produces a current in the wire loops. The moving charges in the loops experience a force from the magnetic field which causes the loops to flip. You can also think of the loops with the electric current through them as being an electromagnet. The electromagnet is repelled by the other magnet and is made to flip.
A metal ring is placed on top of a powerful electromagnet. When the electromagnet is turned on the magnetic field rapidly changes. This makes the charges in the metal loop move in a current in the same way a moving magnet would. The current in the metal loop makes it's own magnetic field which opposes that of the big electromagnet. The ring is repelled with a large force that can shoot it 20 to 30 feet.
Powerful magnets are dropped down a metal tube and a clear plastic tube. The moving magnets create electric currents in the metal tube. These currents produce magnetic fields that oppose the field of the dropped magnets. Because of this the magnets fall very slowly through the metal tube, compared to the clear plastic tube where the magnets fall with hardly any resistance.
A powerful magnet is rolled down an inclined thick copper plate. The magnet will roll surprisingly slow because of induced electric currents in the plate. As with the Tubular Magnet demo, the induced currents produce magnetic fields that oppose the field of the rolled magnet. There is then an "induced magnet" above the rolling magnet, pulling it from above, and a second "induced magnet" below the rolling magnet, pushing it back.
The classic old sci-fi movie prop explained. A huge voltage (charge separation) creates an arc between two tall metal rods. The arc is a plasma: air molecules split into atoms and stripped of some of their electrons together with the freed electrons. The plasma is a very good conductor, while regular air is not. The plasma is also very hot. Because of this it will rise. The arc will "climb" the ladder of the two wires until it is broken near the top. A new arc will then form at the bottom.
An 80,000 volt Tesla Coil is used to shoot brilliant arcs of electricity and light up nearby flourescent tubes without wires. A Tesla Coil is a radio frequency transformer. They were originally developed to transmit electricity with the use of radio waves. Imagine a toaster or a hair dryer with short antennas instead of an electrical cord.
Regular household AC voltage is connected across a big pickle. The pickle, like us, is full of wet gushy stuff that is a very good conductor of electricity. The pickle will sputter, pop, and glow from the heat of the huge currents that flow through it. Much the same thing would happen to us if we play around with the kind of electricity that comes from wall sockets.