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插图：2. Electronic CircuitsCurrent flow takes place in electronic circuits. A circuit is a path for electric current flow. Electric current flows only when it has a complete, or closed-circuit, path. There must be a source of electrical energy to cause current to flow along a closed path. 4 The electrical energy is converted into more useful energy, for example, the light energy.Electric current cannot flow if a circuit is open. An open circuit does not provide a complete path for current flow. Free electrons of the conductor would no longer move from one atom to another. An example of an open circuit is a "burned-out" light bulb. Actually, the filament (the part that produces light) has become open. The open filament of a light bulb stops current flow from the source of electrical energy. This causes the bulb to stop burning, or producing light.Another common circuit term is a short circuit. A short circuit, which can be very harmful, occurs when a conductor connects directly across the terminals of an electrical energy source. For safety purposes, a short circuit should never happen because short circuits cause too much current to flow from the source. If a wire is placed across a battery, a short circuit occurs. The battery would probably be destroyed and the wire could get hot or possibly melt due to the short circuit.1.3.2 VoltageWater pressure is needed to force water along a pipe. Similarly, electrical pressure is needed to force current along a conductor. If a motor is rated at 220 V, it requires 220 V of electrical pressure applied to the motor to force the proper amount of current through it. More pressure would increase the current flow and less pressure would not force enough current to flow. The motor would not operate properly with too high or too low voltage. An electrical energy source such as a battery or generator produces current flow through a circuit. As voltage is increased, the a- mount of current in the circuit is also increased. Voltage is also called electromotive force (EMF).

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