Understanding Volts, Amps, and Ohms in Physics
Units of Electric Current, Resistance, and Potential Difference
People frequently refer to volts, amps, and occasionally ohms, but what are they? What does it mean to refer to a 12 volt battery or a 30 amp circuit breaker?
Electric Charge
The fundamental quantity of electricity is the electric charge. Electric charge is measured in units of coulombs. A single electron or proton has a charge of 1.6E-19 coulombs. Other electrical units, such as volts, amps, and ohms, are defined in terms of coulombs and other fundamental units.
Electric Current
Physicists use electric current to measure movement of electric charges. They define the electric current at a specific point as the amount of charge that moves by that point each second. If a wire carries a greater electric current, then it has more electric charge, electrons in this case, moving through the wire.
Because it is a charge divided by a time the units for electric current are coulombs per second. Physicists define a coulomb per second as an ampere or amp for short.
A 30 amp circuit breaker is designed to trip when more than 30 amps of electric current pass through the circuit. That would mean more than 30 coulombs of electric charge are moving through the circuit breaker each second.
Electric Potential Difference
Electric potential difference is always measured as the difference between two points. The electric potential difference is the difference in electrical potential energy between the two points for 1 coulomb of electric charge. That potential energy also equals the amount of work required to move 1 coulomb of charge between the two points.
Volts are the units of electric potential difference. A 12 volt battery has an electric potential difference of 12 volts between its positive and negative terminals. That means a charge of 1 coulomb changes its potential energy by 12 joules moving between the terminals, or 12 joules of work is needed to move the charge against the electrical forces.
In terms of more fundamental units, a volt is a joule per coulomb.
Electrical Power
Notice that a volt is a joule per coulomb and that an amp is a coulomb per second. Hence multiplying a volt times an amp gives a joule per second, and the coulombs cancel. A joule per second is a watt, which is a unit of power.
Hence the power, or wattage, in an electrical circuit is the voltage multiplied by the current in amps. For example a 120 volt household circuit carrying 30 amps has 3600 watts of power. As this power increases the wires get hot. A wire carrying more current than it can handle poses a fire risk. That is why circuit breakers and fuses are designed to break the circuit if the current becomes too high.
Electrical Resistance
When an electric current flows, there is some resistance to this current. A good electrical conductor, such as copper wire, will have a low resistance. Current flows easily. A good electrical insulator, such as the rubber material surrounding wires, has a very high resistance. Current does not flow easily.
Physicists measure electric resistance in units of ohms. An ohm is a volt divided by an amp (volt per amp).
The current (I), potential difference (V), and resistance (R) of an electric circuit are related by Ohm's law: V=IR.
Volts, amps, and ohms are common units for electrical measurements both in household circuits and in physics.
Further Reading
Hecht, E., Physics: Algebra/Trig (Brooks/Cole 1998)
Digital Multimeter for Electrical Measurements
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