S-Cool Revision Summary
Static charge is a charge that can't move. There are two kinds positive (+)and negative (-).
All atoms contain positive particles (protons)and negative particles (electrons)but because they contain the same number of protonsand electronsthey have no overall charge.
Static electricity is caused by an atom having too many or too few electrons (e-).
A Van de Graaff Generator is a machine that generates huge amounts of static charge, by rubbing electrons off a roller and depositing them on the metal dome.
Induction and Earthing
The basic rule you need to know is that Like charges repel and Opposite charges attract.
Induction - This is the effect caused when a charged object causes electrons in another object to move. This causes the uncharged object to become attracted to the charged object.
Earthing - If enough charge builds up on an insulator, the charge can leap the gap, causing a spark. This can be prevented by discharging the object, gradually. This is called earthing.
Static electricity is used in many useful machines like photocopiers and smoke stacks (to remove pollution from the smoke).
If clouds get charged up enough, you get lightning, the biggest spark of all. Static can also be dangerous when refueling aircraft. The fuel rubs against the side of the hose and lots of charge builds up. If the plane isn't earthed, the spark can blow the plane up.
Current, Voltage and Resistances
Current - This is a measure of the flow of electrons around a circuit (measured in Amperes or Amps).
Voltage - This is a measure of how much energy the electrons are carrying around the circuit (measured in Volts).
Resistance - This is a measure of how hard it is for the purple to travel through a part of the circuit (measured in Ohms).
Current flows from the positive (+ve) terminal of the battery to the negative (-ve). This is called conventional current flow. The problem is, electrons are negatively charged, so they want to get away from the -ve and go to the +ve. So if electrons are going left to right, you say that the current is going right to left.
Simple Measurement Circuit
An ammeter needs to measure the flow of charge, so it is in series. This means that all the charge has to flow through it and can be counted. It also means that an ammeter needs to have a very low resistance.
A voltmeter measures voltage across a component, which you may have heard as potential difference. This means it is in parallel and it also needs a high resistance (otherwise all the current would flow through the meter instead of the component).
Current in series: same all the way round (all the current has to flow through everything).
Voltage in series: voltages across each component add up to the total voltage supplied by the battery, as they have to share the voltage between them [(A) = (B) + (C) in the diagram]. Higher resistances will need more of the voltage.
Final point - resistors in series: To work out the total resistance of two resistors, just add them together. This is because the current has to go through both of them.
Voltage in parallel: all voltages the same.
Current in parallel: the current is shared out between the branches, but recombines near the battery. In the diagram (A) = (B) + (C) = (D). How much current each branch gets depends on the individual resistors - bigger resistance = lower current.
Resistance in parallel: you don't normally have to work out numbers, but the rule of thumb is that the total resistance of two resistors in parallel is less than the lowest individual resistor.
Cells and Batteries: strictly speaking one cell represents 1.5V, but of you write the voltage above it (e.g. '6V'), most people will understand the cell has 6 volts.
Power Supplies: come in all shapes and sizes, just label them as you want.
Switches: several types, I've shown the main two that you will come across
Lamps/Bulbs: either symbol could be used - it doesn't matter.
Resistors: a few types - Fixed, Variable (you can change the resistance), Thermistor (as it gets hotter, its resistance decreases) and Light Dependent Resistor or LDR (the more light that shines on it, the lower its resistance gets).
Diode: A diode is like an electrical valve, it only lets current flow one way. If it is connected with the arrow pointing to the negative terminal, current can easily flow, if it is the other way round, it will block the current.
A LED or Light Emitting Diode is just the same except it gives off light...
Ohmmeter: is connected directly to a resistor, of any kind, to find its resistance (no other circuit is used with it)
Check in your syllabus to see if there are anymore you need to know!
Know Your Formulae
The law actually says that the resistance of a metal conductor is the same whatever the current - unless it's getting hotter. However most people think of these equations when the law gets mentioned:
Triangle method for rearranging a formula:
Take the equation into a form where there is only multiplication and not division (V=IR in this case). The V goes on the top and the I and R slot in the bottom. Cover the one you want to know, and the other side of the equation will reveal itself.
Below are equations for Voltage, Charge, Energy and Power shown in the triangle format:
Voltage (V) in Volts, Current (I) in Amps and Resistance (R) in Ohms.
Charge (Q) in Coulombs, Time (t) in seconds and Power (P) in Watts.
Always remember to show all your working out, including writing the formula properly (not just the triangle!) and checking your units (e.g. check for mV or kW instead of V or W)
Prefixes: These are little letters added to units to make them a different size, but always use the base unit if unsure. Base units are given in the topics, the ones to watch for are time (seconds) and mass (kilograms not grams).
|M||Mega||x 1,000,000||1MW = 1,000,000W|
|k||kilo||x 1,000||1kg = 1,000g|
|c||centi||÷ 100||1cm = 0.01m|
|m||milli||÷ 1,000||1ms = 0.001s|
Mains Supply (AC and DC)
We use two main sorts of electrical supplies, DC and AC.
DC - This is Direct Current. The current flows in one direction only and has a consistent value. Provided by batteries or DC adaptors/transformers that plug into the mains supply.
AC - This is Alternating Current. The current flows first one way then the other at a frequency of 50Hz. AC is what comes out of the mains sockets, usually at around 240V.
The Ring Main
This is the name given to the circuit in your home. You only need to know that it is a parallel circuit and that the lighting circuit is separate from the circuit for sockets.
The National Grid
This is the circuit that carries electricity all around the country, from the power stations to homes and businesses.
Producing the Power
Energy is produced by burning fuel which turns water to steam, this drives a turbine, which make electricity via a generator.
This electricity is a very high voltage and is passed over the National grid to a step down station then passed straight to your home.
Why The High Voltage?
High voltage is used over the National grid, to keep current low. This stops energy being wasted.
Energy and the Cost
The kilowatt-hour is the common unit used by energy companies to measure electricity. This is a unit of energy not power or time. It is the amount of energy if a 1kW appliance was left on for 1 hour.
1kWh of electrical energy costs around 6p, though it may change depending on your supplier. So multiplying the number of Kilowatt-hours you use by the unit cost (approx 6p), give you the total cost of the electricity you use.
A common question is to give you a picture of domestic bliss and get you to identify the hazards, such as the person sticking their fingers in the toaster. Things to look for are:
water near appliances,
too many double plugs/adaptors,
Just use your common sense and you should get some easy marks!
Wiring a Plug
One big problem used to be wiring plugs. By law now, all new appliances are fitted with one already, which helps, but you do need to know what's going on inside there:
Fuses help protect the circuit against faults. The key thing is to get the wire just thick enough to carry the current you want, but thin enough to melt if there is a current surge.
Common sizes are 3, 5 and 13Amp fuses, but there are many others. Always choose one slightly higher than the current rating of the appliance, so that it doesn't blow under normal conditions.
Fuses are not always effective at protecting you, so circuit breakers are also used. They automatically compare the current entering and leaving the circuit and even if there is the tiniest difference they 'trip' off.
The Earth (yes, I do mean our planet) is very good at soaking up loose charge. The earth in your house is probably connected to the plumbing (goes to ground) or a large metal spike in the ground somewhere.
If something is completely cased in an insulator, like plastic, it is said to be double insulated, and does not need earthing. You can't get a shock from the case!