Drift Velocity and Current Phenomena

It is a significant fact that all free electrons move around due to their thermal energy even if there is no current present. However as this motion is completely random then the net effect is no overall movement. To be part of a current they have to exhibit a drift velocity in a given direction Let us consider the two electron paths: Both display random motion as their thermal kinetic energy causes them to move, colliding with the fixed positive ions that make up a metal’s structure as they go. 

The first electron shows very little change in displacement, the second has moved about the same distance as the first but shows a definite displacement to the right - it has a drift superimposed on its random motion. Therefore we can say the second electron is probably part of an electrical current and the first is not. When the overall effect on all the electrons is taken into account this small drift shown by each electron provides a current, whilst when all electrons in the first example are considered the net movement of charge is zero.
Metals are good conductors (poor insulators). Electrons in the outer layers of metal atoms are free to move from atom to atom. So if one end of a piece of metal is made positive, the electrons will be attracted towards it and because they are free, they can move towards it.
Static charge only builds up on insulators. These are materials that will not allow the flow of charged particles (nearly always electrons) through them. Insulators are materials made from atoms that hold onto their electrons very strongly. The voltage across an insulator has to be extremely high before an electron is given enough energy to free itself and move through the material.
Static charge won't build up on conductors unless they are isolated because as soon as you put too many electrons in one place, they repel each other and spread out, reducing or eliminating the effect. On insulators, the charge can't spread out - so you get a noticeable effect. Semi-conductors have far fewer free electrons than metals so do not conduct as well. However, if they are given energy electrons are able to free themselves from their atom and flow, which increases their ability to conduct. Some semi-conductors are light sensitive, as the light energy is able to free the electrons. There are about 5 naturally occurring semi-conductors.

Solids, Liquids and Gases

Although in circuits we deal with electrons carrying charge, in liquids and gases other particles are also able to carry charge, such as ions in the process of electrolysis.
You can give metal objects static charge as long as the whole object is insulated from the rest of the world so that charge cannot escape from it (even though the charge is spread evenly throughout the whole metal object).
Current electricity is about moving charged particles. If you allow the charge that builds up in static electricity to flow, you get a current.
Current is the rate of flow of charge; it is the amount of charge flowing per second through a conductor.

The equation for calculating current is:



Where:

I = current (amps, A)

Q = charge flowing past a point in the circuit (coulombs, C)

t = time taken for the charge to flow (seconds, s)

So a current of 1 amp is 1 coulomb of charge flowing past a point every second.

Likewise a coulomb is the same as an ampere-second!

(Note: if you plot a graph of current flowing against time, the area under the graph will equal the charge that has moved.)

How can you get the Charge to Flow?

Well, first you need to have a conductor for it to flow through and then you need to attract or repel the charged particles to make them move. The amount of attracting or repelling you do is measured in volts and is called the voltage or the potential difference (p.d. for short).

Work is being done on these charged particles to make them move, so the voltage is a measure of the amount of energy that is provided per coulomb of charge.

1 volt = 1 joule per coulomb.

The equation for calculating voltage is:



Where:

W = amount of energy (joule, J)

V = voltage (volt, V)

Q = charge (coulomb, C)

Circuit Rules

As the charged particles flow around a circuit they don't get used up; it is the energy that the charged particles carry that decreases as they move around the circuit.

(Runners going around the 400m track run all the way round, but they lose energy as they run).

So current is not used up - if you have 12 amps leaving the battery, there will be 12 amps in the circuit and 12 amps returning to the battery.

Voltage changes as the charge moves around the circuit. The potential energy given to the charge is changed into heat energy in the circuit. An electron may leave a battery with 6 V, but will return to the battery with 0 V. This gives a change in potential of 6 V, hence the words 'potential difference'.

There are two main types of circuits you need to know about and each has two rules that make calculations simpler:

Series circuits:



In a series circuit...
the current is the same all the way around the circuit.
the voltage is divided between the components in the circuit.

Parallel circuits:



In a parallel circuit...
the current divides to travel along each loop.
the voltage is the same across each loop.

Conventional Current
Originally scientists believed that it was positively charged particles that flowed in circuits and so circuits are always labelled with the current flowing from the positive to the negative terminal of a cell in a circuit. We call this current the conventional current. The electrons are actually flowing in the opposite direction!

Conventional current is the flow of positive particles. All references to current in diagrams and questions at A-level refer to conventional current, unless it's specifically stated otherwise in the question.

Measuring Current and Voltage:

To measure current we use an ammeter. It is placed in series in a circuit to measure the amount of charge flowing through it per second. (You can compare it to a turnstile counting people into a stadium.)

To measure voltage we use a voltmeter. It is placed in parallel to compare the potential at two different points, either side of a component. It can then measure the potential difference, or voltage across the component.