You will need to know a few things about voltage and electrical current for the Electronics Information subtest of the ASVAB. Electricity isn’t as complicated as you may think. Check below to prepare for this subtest.
Measuring voltage: Do you have the potential?
A circuit is just the path of an electrical current. A very simple circuit consists of several components. For example, it may consist of a battery, one side (terminal) of which is connected by a conductor (a wire) to an on/off switch, which is connected to a lamp (a light bulb) by another wire, which is then connected back to the other side of the battery.
As long as the switch is off — which means it’s set to a position so that there’s an open (literally an open space) in the circuit — current cannot flow. When you flip the switch, there’s a short (meaning the open space has been closed), and current can flow from one side of the battery, through the closed switch, through the light bulb, and back to the other terminal of the battery, all by way of the wires connecting the components.
Voltage, which is supplied by the battery in this circuit, is the difference of the pressure between two points in a circuit. It is sometimes called the voltage drop or difference of potential. So, for instance, a 9-volt battery supplies 9 volts of electricity. To see what the voltage is anywhere in a circuit, you have to compare the voltage at that point to ground.
Ground is any part of a circuit (or other object that has electricity running through it) that measures 0 volts, such as the case of your radio, the base of a lamp, or the chassis of your car. The negative terminal of a 9-volt battery is at ground potential, so the voltage from the negative terminal to ground will measure 0 volts. The voltage from the positive terminal to either ground or the negative terminal of the battery will measure 9 volts.
To measure voltage in a circuit, you use a voltmeter or a multimeter, which has several meters in one instrument. A voltmeter has two leads. To measure voltage, you place one lead somewhere in the circuit and one lead at another location in the circuit. The voltmeter tells you what the voltage is between those two points.
A cell (a storage compartment for electricity in a battery) has a specific voltage. For example, in a particular battery, cells may be 1.5 volts. Therefore, you can figure out the number of cells a battery has by dividing the voltage of the battery by 1.5. Pretty handy stuff, huh?
Examining the current of the electrical river
Electrons are negatively charged, and they attempt to shift from one atom to the next to the next, trying to get to a positive charge, such as the positive side of a battery. They’re able to shift if the material is a conductor. But if the material is an insulator, the electrons will be much, much less able to shift because of the insulating material’s molecular structure.
Electrical current is the flow — or, more precisely, the rate of flow — of electrons in a conductor. Current flow can be expressed in terms of coulombs (abbreviated C), which measure charge. A coulomb is the amount of electricity provided by a current of 1 ampere flowing for 1 second. It’s called a coulomb because a guy named Charles de Coulomb discovered it in the late nineteenth century, and the rules say that if you discover something, someone will stick your name on it.
If 1 coulomb (about 6,241,500,000,000,000,000 electrons) flows past a specified point in 1 second, that’s a flow rate of 1 ampere (amp, abbreviated A). An ampere represents the strength of a current. For the sake of convenience, electrical currents are measured in amps. Typically current is tiny, so small that it’s measured in milliamperes; 1 milliampere is one-thousandth of an ampere. Current meters, called ammeters, measure the flow of current through a circuit.
The amount of voltage (the difference in potential) and the resistance in a circuit determine the number of amperes along a wire — or whatever you’re using to conduct the electricity from one place to another. More voltage (for instance, a higher-voltage battery) means that more amps flow in a wire (or conductor).