Electrical devices

We have studied several electrical devices in this section.

A light bulb turns electrical energy into heat and light no matter which way the current flows.

The behavior of a motor does depend on the direction of the current: when you reverse the current, it turns the other way.

The buzzer in your kit is a very sophisticated device -- there is even a transistor in there -- but fundamentally it also is a kind of electrical magnet. Transistors only work with one direction of current, and so this device is only works when the battery is connected the right way.

DC and AC

In circuits powered by batteries the current must always flow the same direction, from plus to minus in the circuit outside the battery (which means from minus to plus inside the battery). The electrical components we have studied -- motors, light-emitting diodes, and buzzers -- were all designed for use with batteries, and thus frequently are sensitive to the direction of the current.

The power grid that moves electrical energy from a power plant to your house uses alternating current, that changes direction 120 times per second (the current is going the same way 60 times per second, and so we say it is 60-cycle or 60 Hertz). Light bulbs and other devices that turn electrical power into heat are insensitive to the direction of current and work the same way on AC or DC. Most devices that you plug into the wall actually run on DC and have to contain a circuit to convert AC to DC. Sometimes this can be done simply and inexpensively with a circuit element that only lets current flow one way; but a significant part of the cost (and weight) of a good sound system is the circuitry that eliminates all trace of the 60 cycle variation (which would be audible as a low pitched hum).

The reason for using AC is that this makes it easier to change voltage levels (from hundreds of thousands of volts to the household 115 V) and to connect one part of a circuit to another using transformers (which only work on AC). Although changing direction 120 times a second seems very fast to you and me, from the point of view of the electrons it is very slow: during the 8 milliseconds that the current is flowing one direction, the system behaves pretty much the same as if it were being operated at DC. We can understand household wiring perfectly well if we think of it in terms of DC, with the power company playing the role of a 115 V battery.

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