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Fields fill the space between matter and they determine how it is that bits of matter can exert forces on other bits of matter at a distance. There are several different fields in nature, and their reality is demonstrated by our observation of the forces with which they are associated. So, for example, gravitational fields determine how it is that objects with mass are attracted together by a gravitational force. Electric fields determine how it is that objects with electric charge are attracted together by an electric force, if they have opposite electric charge, or repelled from each other, if they have the same electric charge. Interestingly, unlike an electric field, a magnetic field only comes into play when electric charges are moving. Magnetic fields determine how it is that electric currents, composed of moving electric charges, exert forces on other electric currents. Consider, then, two parallel wires, each with an electric current flowing in the same direction. By virtue of the magnetic field, they will be pulled toward each other, they experience an attractive force. If the currents are flowing in the opposite direction, then there will be a repulsive force between the wires. More generally, magnetic fields are generated by electric currents, the motion of electric charges, and, conversely, electric currents and the motion of electric charges can be induced by time-dependent magnetic fields. In fact, an electric generator works by the motion of magnetic fields.

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