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Consider two magnets attracting each other from some distance apart. If one magnet moves up, the other magnet is also pulled up. The question is how? How does the force get from one magnet to the other? If one magnet were to vanish, would the force on the other magnet instantly vanish? Suppose the two magnets were a kilometer apart. The force would be much smaller but the question of time remains. Would one magnet instantly respond when the other moved or would there be some time delay? 
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Consider a similar situation with gravity. Suppose the Sun were to instantly vanish; would the Sun’s gravitational pull on the Earth stop immediately? The answer is no! Earth would continue to “feel” the Sun’s gravity for 8.3 min. Why? Why does it take 8.3 min for Earth to “notice” that the Sun has vanished?
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The reason is that the Sun does not act on the Earth directly. Instead the Sun creates a gravitational field in space. The field acts on the Earth to keep the planet in a circular orbit. The Earth would still be affected by the Sun’s gravity because the Sun’s gravitational field is already there, in space, at Earth’s orbit. Any change in the field travels at the speed of light, so the time interval of 8.3 min is also the time it takes the lack of a gravitational field to reach Earth’s orbit.
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A force field is an organization of energy in space that creates a force on any receptive matter that passes within its influence. Although force fields are popular in science fiction, some force fields are nonetheless real. Gravity is one example. The Sun’s gravitational field creates forces on each planet in the Solar System. The gravitational field travels at the speed of light, and therefore changes to the field also travel at the speed of light. The force between magnets also acts through a field, the magnetic field. The presence of magnetic poles creates a magnetic field that extends outward in the surrounding space. Other magnets “feel” forces through their interaction with the magnetic field. It is the field that creates forces, not the magnets interacting directly.
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It takes work to move the north poles of two magnets close to each other because they repel each other. If you then release the two magnets, they move apart. Their magnetic field can be thought of as storing potential energy that is converted into kinetic energy when the two magnets are released. In a similar way, potential energy is stored in gravitational fields and electric fields.
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The potential energy associated with a field can be either positive or negative. In the case of the two north poles of magnets moved close to each other, this configuration contains positive potential energy, which is then converted into kinetic energy when the two magnets are released and move away from each other. A north pole and a south pole moved close to each other have a negative potential energy; if you release them, they move even closer to each other! You will learn more about electric potential energy later in this chapter. 
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When you rub your two hands together, you feel the force of friction between them. This is called a contact force. Contact forces involve direct interaction of matter. Gravitation and magnetism, by contrast, are capable of acting through empty space without any contact between objects. Both these noncontact forces act through fields. The electric force between two charged objects is a third example of a noncontact force that acts through a field. In fact, the interaction of two particles can be thought of as occurring in two steps: 1. the creation of the field and 2. the field interacting with matter.
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Which of the following are noncontact forces and create fields? - the magnetic force
- the normal force
- the gravitational force
- I only
- II only
- II and III only
- I and III only
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The correct answer is d, I and III only. Magnetic and gravitational forces can act over distances and create fields, whereas the normal force can only act where two surfaces are touching. 
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