Electrical system activity is measured in volts, amps, ohms, and watts.
Voltage is the measure of the potential difference between the number of surplus
electrons at one point (the  side of the circuit) and the electron deficiency at
the other end (the + side of the circuit). That difference is important because
it represents the amount of force that will be generated if the electrons move between
the two points.
Voltage is referred to as electromotive force  EMF.
Frequently, voltage is compared to pressure since it is the force that makes the
electrons
flow.
Amperage is the measure of the rate of flow. It is the volume of electrons passing
though a given point in a circuit.
Electricians live by the rule that, "It's the amps (the volume of electrons) that
kill, not the volts."
The stun guns used in law enforcement, for instance, put out
high voltage but generate only a few thousandths of an amp to create a very
unpleasant  but not fatal  experience for the person on the receiving end of the
shock.
One ampere is a current flow of one columb per second. A columb is a cubic centimeter
of electrons  approximately 6.25 million, million, million if you were to have
the time and inclination to count them.
When electrons flow, there is always some resistance. That resistance is measures
in Ohms.
One Ohm is the amount of resistance that will allow one amp of current to flow if
one volt is present. Remember, voltage is "pressure" and amperage is "volume".
Ohm's Law is one of the basic tools of electronics.
It expresses the relationship
between voltage, the current flow in amps, and the resistance in ohms. It is:
AMPS = VOLTS / OHMS
or
VOLTS = AMPS * OHMS
or
OHMS = VOLTS / AMPS
The point is that if you know two out of the three values, you can figure out the
missing value without directly measuring it.
The Watt is the measure of electrical power or the rate at which work is being done. Examples of work would include heating the wire
that creates light inside a light bulb or turning an electric motor.
One watt is equal to one amp of current flowing in a circuit at one volt.
WATTS = VOLTS * AMPS
To visualize the concept, consider the example of a typical 60 watt light bulb in your home.
WATTS(60) = VOLTS(120) * AMPS(0.5)
The illustration demonstrates that under ideal circumstances, the kind of 120 volt
circuit rated at 15 amps you would find in most homes would carry enough power to light thirty of the 60 watt bulbs used in our example. Adding more bulbs would cause the fuse to blow because the circuit was drawing too many amps.
Getting back to the subject of hair dryers; they consume a lot of power. Hair dryers
include
both a power consuming heating element and a power consuming fan motor. A typical
1800 watt hair dryer can draw the same amount of power as thirty 60 watt light bulbs,
placing a substantial load on the circuit and creating the possibility that that
the circuit will be overloaded if there are other power consuming devices in use
on the circuit when the hair dryer is turned on.
So, why don't fuses in 15 amp circuits blow every time a hair drier is turned on?
The answer is that devices are rated on the basis of the maximum amount of power
they might use, not the lower amount they may actually use at any given moment.
Typically, the maximum is approached when the device is started, after which the
power usage drops to a lower level (which also explains why electrical device failures
almost always occur the instant the device is turned on, not after it has been running
for a while).
