Electric Forces
Electric forces act between charged objects or particles such as electrons and protons. Objects with opposite charges—positive and negative—attract one another. Objects with like charges repel one another. Figure 19 shows that clothes often cling together when they are removed from a dryer. Some clothes, such as cotton socks, lose electrons easily and become positively charged. Other clothes, such as polyester shirts, gain electrons easily and become negatively charged. Because the oppositely charged particles attract one another, the clothes cling together.
Magnetic Forces
Magnetic forces act on certain metals, on the poles of magnets, and on moving charges. Magnets have two poles, north and south, that attract each other. Two poles that are alike repel each other. If you have handled magnets, you know that when opposite magnetic poles are brought close, they almost seem to jump together. On the other hand, when two similar poles approach each other, you can feel them pushing apart.
Figure 20 shows a child's wooden train set whose cars are linked with magnets. Each car has a north pole on one end and a south pole on the other. Children quickly learn that if a train car won't stick to the one in front of it, the car must be turned around.
What type of force acts between particles with the same electrical charge?
Nuclear Forces
Think about the nucleus of an atom, with its protons crammed into an incredibly small space. Because protons are positively charged, you would expect that an electric force of repulsion would break the nucleus apart. Scientists believe the nucleus would fly apart if there were not another, much stronger, attractive force holding the protons within the nucleus.
Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together. The strong nuclear force overcomes the electric force of repulsion that acts among the protons in the nucleus. The weak nuclear force is involved in certain types of radioactive processes.
Strong Nuclear Force
The strong nuclear force is a powerful force of attraction that acts only on the neutrons and protons in the nucleus, holding them together. The range over which the strong nuclear forces acts is approximately equal to the diameter of a proton (10-15 m). Although this force acts over only extremely short distances, it is 100 times stronger than the electric force of repulsion at these distances.
Figure 19 Clothes often acquire electric charges in the dryer. Clothes with opposite charges tend to cling together.
Figure 20 A magnetic force of attraction holds the two train cars together.
Applying Concepts How are the two magnetic poles of the magnets related?
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