Do the wave
Material
Assembly
Place two strips of tape 10 cm long, 4 inches, down onto the cardboard side by side.
Place two more 10 cm long pieces of tape down on top of the first two pieces of tape, carefully leave a handle since you will be pulling the top tapes off the bottom tapes.
To Do and Notice
Rapidly pull the two top tapes of the bottom tapes.
Notice that when you pull the tapes off, they are attracted to anything in their vicinity such as your hand. (This is because the tapes became electrically charged when you pulled them off.)
Hold the tapes straight down and bring them towards each other, notice that they repel each other.
Move one tape notice that the other tape moves.
Place the sticky side of one 10 cm (4 inch) long tape against the smooth side of another of equal length, then pull the tapes apart. Notice that the tapes attract each other.
Now pull one top tape off of the tape strips on the cardboard and attach it to the edge of a table so that it hangs down.
Then place the sticky side of one tape on the smooth side of another and pull them apart.
Hold the two tapes next to each other and not touching about 10 cm from the tape hanging from the table. Notice how the tape hanging from the table deflects to one side.
Now switch the positions of the two tapes and notice how the tape hanging from the table deflects in the opposite direction.
What's Going On?
When you pull the top tapes off the bottom tapes they both get the same strong electrical charge. (Both are positively charged.)
In the 1670 model of Newton the tapes would repel because of "action at a distance." This model predicts that if you move one tape the other will move instantly.
In the 1870 model of electromagnetism by Maxwell the tapes repel because the electric charges on one tape create an electric field throughout space. This electric field from one tape then exerts a force on the electric charges of the other tape. repelling it.
Accelerate the tape side to side and you create a transverse wave in this electric field. The changing electric field creates a changing magnetic field. The changing magnetic field creates a changing electric field.
And so, an electromagnetic wave propagates from one tape to another carrying changes in the force.
A better model for electromagnetic waves is made by making a tape with a positive charge and a tape with a negative charge. You did this when you pulled apart two tapes that were stuck together. Hold these tapes side by side about 10 cm (a handwidth) apart and you have created an electric dipole. A dipole is made of equal and opposite electric charges separated by a distance. A nearby charged tape located at the perpendicular bisector of a line joining the two charges of the dipole experiences a force to one side. If you switch the positive and negative charges of the dipole the sideways or transverse force exerted on the single tape changes direction. One way to look at this is that you have created electromagnet waves of dipole radiation that traveled from the dipole to the single tape.
The wave propagates at the speed of light. Maxwell's equations can be made into a wave equation which has the speed of the wave in it. The speed of light in a vacuum depends on the electric and magnetic permeability's of the vacuum.
It's interesting that students don't worry too much about electric forces crossing a vacuum but they do worry about how light crosses a vacuum. This exploration shows that light is just a wave in the electric field. So it should easily cross the vacuum.
The speed of light is very fast, 300 million meters per second, in other units, a foot per nanosecond. So if you held your tapes a foot apart and moved one, the other would start to move a billionth of a second later. This is too fast for you to see but electronic experiments can easily measure this time delay.
Etc.
Maxwell calculated the speed of his electromagnetic waves and found that they traveled at the speed of light. He reasoned that therefor they must be light. He got lucky! We now know that gravity waves too travel at the speed of light. This means that if you accelerate the sun to the side the earth responds 8.3 minutes later, because this is how long it takes for light and gravity waves to reach the Earth from the Sun.
The waves you created in this experiment are radio waves at a frequency of abut 1 cycle per second. They are known as ELF waves. Where ELF stands for extremely low frequency. The U.S. Navy uses ELF waves to communicate with submarines underwater. Since ELF waves can penetrate deeply into sea water.
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22 May 2000 |