Tutorial 1 What is Electricity?
To understand transfers of charge between insulating objects.
Electric charges are very important in all of nature and beyond. Charged particles produce a force-field called an electric field. Charged objects use this force field to attract other charged objects, or repel other charged objects. Charges are responsible for a whole range of things that happen in physics, from the forces that hold you up in your chair as you read this, to the circuits in the computer that you are reading this from. Light from the Sun occurs because of charged particles called electrons.
There are two main charged particles to consider:
All matter is made up of protons, neutrons, and electrons. The neutron has zero charge and is found in the nucleus of an atom. The proton has a positive charge and is found in the nucleus.
The electron has a negative charge and is found orbiting the atom. If the numbers of protons and the numbers of electrons are the same, the object has zero charge. That doesn't mean that there is no charge; it means that the charges add up to zero. An object that has zero charge is called neutral. If there are different numbers of protons and electrons, the object is charged, which means it can make an electric force field to attract or repel other charged objects.
Protons are in the nucleus and NEVER move. Only electrons can move. Therefore:
If we separate positive and negative charge from each other, we have to do a job of work. This is because they are pulling against us to get back together again. In physics, we say that we have used energy to do that job of work. But that's not the end of it. The two separated charges can come back together, and we can get them to do a job of work as they come back together. We refer to this energy as a potential difference or voltage.
We use these ideas in static electricity which arises from separation of charge in an insulating material. The material has to be insulating, because if we used a conductor like a metal, the negative charges and positive charges would come back together immediately.
Let's suppose we rub a polythene rod with a cloth:
Electrons from the cloth are rubbed onto the polythene rod. This gives the polythene rod a negative charge.
Now suppose we charge an acetate rod with the cloth:
This time we rub electrons off the rod onto the cloth. The rod becomes positively charged, and the cloth becomes negatively charged.
If we hang the rods so that they can swing freely, we observe the following:
A charged polythene rod brought close to another charged polythene rod repels;
A charged acetate rod brought close to another charged acetate rod repels;
A charged polythene rod brought close to another acetate polythene rod attracts;
We can conclude from this simple experiment that:
like charges repel; unlike charges attract.
Voltages in static electricity tend to be high. You can easily get voltages of 5000 V on your jumper on a dry day, or 30 000 V from a nylon carpet. These can give sparks and a small shock, but the currents are so tiny that they cannot harm you.
There are many demonstrations in Physics that can be used to show static electricity, for example the Van der Graaff generator
Static electricity has uses, for example, in the photocopier. It can also be a source of hazards, for example sparks that can occur when an aeroplane is being refuelled. This could be very dangerous. Beyond being a physics curiosity for the electrical engineer, static electricity is of limited concern beyond the fact that static electricity can do a lot of damage to delicate electronic components.
When we get charges to flow in metal wires, we can do rather more than show physics curiosities; we can do something useful with it. We will look at current electricity, the electricity that runs through wires made of conducting materials. For current electricity to flow, we need:
a complete circuit;
a source of voltage (e.g. a battery).
All metals are conducting materials, as is the non-metal carbon. Silicon and germanium are called semi-conductors which means that they can conduct electricity under certain circumstances. Some compounds can be made to conduct electricity under certain circumstances as well.
Insulating materials keep parts of the circuit separate, for example the positive and negative terminals of a battery. If the two are not kept separate, there will be a short circuit. This can cause an electrical explosion or a fire. The picture shows damage due to a short circuit.
Picture by Gabriel Acquistapace, Wikimedia Commons
We don't have to be good artists to draw good circuit diagrams. They are useful because:
they show how the components are laid out;
they show how the components are connected to each other;
they show what is NOT connected together, which is just as important as what is;
anyone can understand how the circuit works.
This is a simple circuit for a torch.
To make sense of circuit diagrams you must learn the following symbols:
It is also important that you know not just how parts of the circuit are connected, but also how parts are NOT connected:
The connection between wires is shown by the black circle, which can be thought of as representing a blob of solder. Connections are called junctions.
Quantities in Electricity
Electricity has two important measurements from which other measurements can be worked out:
Potential Difference or voltage
Current is a flow of charge. We measure charge not in the total number of electrons, but in packets called coulombs (C). (You buy a kilo of sugar, not 1 000 000 crystals of sugar.)
1 C = 6 × 1018 electrons
You can see why we don't count the number of electrons flowing.
Current is measured in ampères or amps (A). If 1 coulomb of charge flows every second, a current is 1 amp.
1 A = 1 C/s
From this we can write an equation that links current and charge.
charge (C) = current (A) × time (s)
In Physics Code:
Q = It
In triangle form:
Make sure that time (t) is in seconds. You know how many seconds there are in a minute, and how many there are in one hour, don't you?
|A current of 5 amps flows for 5 minutes. How much charge has flowed?|
A Model for Voltage
Voltage is the "electrical pressure" in a circuit. (The correct definition is energy per unit charge, but you may find that a difficult concept at this stage.) Look at this picture of a water circuit:
The pump pumps water at a certain pressure. The work that can be got out of the load depends on two things:
The number of litres of water passing every second;
The pressure the water is under.
This model can apply to an electric circuit:
The pump is the battery;
The load is the load (e.g. bulb, motor, etc);
The water flow is the current;
The pressure is the voltage.
There are all sorts of other models to explain voltage. Ask your tutor.
Potential difference is the more "formal" term for voltage. I tend to use voltage in these notes as well.
Potential difference is defined as:
the energy per unit charge turned from electrical energy into other forms of energy
In other words, potential energy is defined in terms of joules per coulomb.
We can write this as an equation:
potential difference (V) = energy (J) ÷ charge (C)
In Physics Code we write:
V = E ÷ Q
In triangle form:
|Use your answer to Question 1 to work out how much energy is transferred if the potential difference is 20 volts.|
|Do the interactive matching exercise to link electrical quantities with their units|
Sources of Voltage
Three obvious sources of voltage are:
Power pack, which you use in your physics practicals.
Strictly speaking a battery consists of two or more cells, although to describe a single cell as a battery is perfectly OK.
The positive terminal of a cell is represented by the long line on the symbol; the negative is shown by the short line.
Conventional current flows from positive to negative. Although we know that positives don't move and electrons (negatives) do, the early physicists got it wrong. To get round it physicists have brought out the idea of conventional current. In these notes and all text books, we will regard all currents as conventional.
Each cell gives out a voltage of 1.5 volts. So two batteries wired in series as below will give out a voltage of 3.0 V.
What is the voltage from these combinations of cells?