Moving charges generate magnetic fields, which can affect other charges. Electric current is an example of a moving charge, and batteries act as pumps to push current around a circuit. Electromagnetic fields produced by circuits are used in modern technology. Moving charges are affected by magnetic fields, and transformers and power plants use this effect to generate electricity. Assembling charged particles requires work, and a battery can be understood as a pair of springs.
A moving charge is an informal way of referring to a charged object that changes its position relative to a particular observer. Moving charges differ significantly from stationary ones in that they generate magnetic fields. Because an object can move relative to one observer while remaining stationary relative to another, it is possible for different observers to measure different values for the same magnetic field. This observation was one of the reasons that prompted Albert Einstein to formulate his special theory of relativity in 1905.
Electric current is a common example of a moving charge. When a wire is connected to a battery, the electric field maintained by the battery naturally causes the electrons in the wire to move. While the electrons are responsible for carrying the charge along the wires, the larger ions move the charge within the battery. Negative ions move from a battery’s positive terminal to its negative terminal, and positive ions move in the opposite direction. In this way a battery acts like a pump that pushes current around a circuit.
The movement of charges in the circuit generates a magnetic field. Although schoolchildren are often given the impression that the power of an electrical circuit lies in the movement of electrons in the wire, the usefulness of a circuit in all but the most basic applications lies in the electromagnetic fields it produces. Modern technology uses these fields in myriad ways, most notably to power electromagnets critical to the operation of electric motors.
Moving charges are not only the cause of magnetic fields, they are also affected by them. A magnetic field causes a moving charge to bend, and the higher its velocity, the greater the force the magnetic field exerts on the moving charge. In this way, the current in one circuit can affect the current in another. Transformers use this effect, using current in one wire to generate a different current in another. Power plants use a variation of this idea to generate electricity.
Electrically charged particles exert force on each other, so work is required to move them relative to each other even in the absence of magnetic fields. The work required to assemble a set of charges is equal to the electrical energy of the system. Two positively charged objects repel each other and the effort required to push them towards each other can be compared to the energy required to compress a spring. A battery can be understood qualitatively as a pair of such springs, and chemical reactions inside the battery cause charges of similar sign to build up at both ends.
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