The second type of electricity is called current electricity. This is the type that powers electric lights, many devices and even your computer. It is current electricity that will shock you if you were to bite into an electrical cord when it's plugged in. Electrical current flows through conductors- materials which allow electron flow. Most conductors are metals. Materials which do not easily allow electron flow are called insulators.

Electric current is used in what we call electric circuits. A circuit has two main components. One is a "charge pump" which increases the potential difference, such as a battery or generator. The other is a device which reduces the potential energy of the charges, such as a motor or other "load". A load is what we call any device which uses electricity. Any device which is a load, and reduces the potential energy of the flow of electrons is called a resistor. This is because it resists the flow of the current.

Electric current is the transfer of energy, and energy is used for work. It is that simple. The energy carried by a current depends upon two things. It depends on the amount of charge transfered, and the potential difference across which it moves. To calculate the energy of a circuit, we use the formula:


E=qV

You should realize by now that q is charge in coulombs, V is potential difference in volts, and E is energy in Joules (J).

Scientists measure the rate at which the current flows. The unit of the rate of flow of electric current is the Ampere, or Amp (A). One Ampere equals one coulomb of charge per second.

1 A = 1 C / 1 s

A device which we commonly use to measure current flow is called an ammeter.

Suppose we had a circuit, with a potential difference of 1000 volts, and a current of 0.1 Amps. Why would there be such a low rate of flow? With a such high potential difference, the electric current should readily flow through the circuit. The answer is that there must be a resistor between the conductors of the circuit. As we mentioned, resistors resist the flow of current. They are often made out of insulators which do not easilly transfer electrons. Resistance is measured in units called Ohms. The symbol for the Ohm is the greek letter Omega ( ). Resistance in Ohms is calculated using a formula called Ohm's Law. Ohm was a famous scientist, much like Ampere or Coulomb. Ohm discovered the relationship between current, potential difference and resistance. He said that "The ratio of the voltage to the electric current in a circuit gives the resistance." This relationship is simply:


R = V / I

In Ohm's Law, Resistance is given the letter R, Potential Difference is V, and Current is I.

Ohm's Law is probably the most important equation in this unit, and you should readily be able to use it in any of it's three forms. The other two are:

V = IR

and

I = V / R

We have learned that electricity is measured in two respects- the rate of flow (I) and the potential difference (V). For convenience and other reasons as well, scientists sometimes measure electrical Power, or it's capability to do work. Power is measured in Watts. To calculate power, you use a simple equation:


P = VI

Since you already know Ohm's law, we can substitute it for v, and get another popular Power formula,

P = (IR)I

or

P=I^2R

This power equation can be used to find the power dissipated (used) by a resistor in a circuit. Resistors commonly dissipate power as heat energy. So if you need to find the heat energy of a resistor, we can combine two more formulas:

E=Pt

and

P=I^2R

to get


E=I^2Rt

This last formula solves for energy using time, current and resistance, and can be very useful to you.

We promised that the equation for energy lost by a resistor (in the form of heat energy) would be useful. Well, it is useful in real life. Electricity is transfered to our homes across long, thick, conducting cables. This electricity is sent to you at extremely high voltages, like 25 000 volts, and at very low levels of current, like 0.1 amps. In addition, the wires have very, very low resistances. Can you figure out why?

The answer is simple. We can't have our wires heating up, and melting or causing fires. We know that heat energy is dependant on resistance and current, but not voltage. So as long as we keep both resistance and current very low, and we keep voltage very high, we can safely transfer electricity without massive heat losses.

In a series circuit, we have a power supply, and one or more resisting loads (resistors) all in a single loop. They are all "single file."

Imagine a pipe with flowing water. In a series circuit, each resistor is like a small constriction in the pipe. In each resistor, the flow will be the same. Therefore, in series, the current is the same throughout. However, even though the current is constant, the voltage is additive. What this means is, the total voltage of the circuit is equal to the sum of the voltages across each individual resistor.

V = IR

Vtotal = IR + IR + IR ...



Like voltage, in a series circuit resistance is additive.

Rtotal = R + R + R

It is important to remember that voltage and resistance in a series circuit are additive.

In a parallel circuit, we also have a power supply, but we have two or more resistors. They are not "single file," but instead the circuit branches off and then re-connects. Take a look at the diagram below.

We can think of the pipe analogy again. Imagine a water-filled pipe. In each branch of current, the pipe also branches. In each branch, there is less pressure so the water moves more slowly. Current is additive in a parallel circuit. Voltage, on the other hand, is constant, since all of the "branches" come together in the end.

Resistance in parallel is a little more tricky. You need to use a special formula.

1/Rtotal = 1/R + 1/R +1/R ...

Notice that we use the inverses of the resistances, then add them. This does not find the total resistance, but instead it finds 1 over the total. Once you have solved the equation above, you need to take it's inverse. Rtotal = 1/ (1/Rtotal)