IGCSE Online Tutorial - Jesmul Sir's Portal # IGCSE Online Tutorial - Jesmul Sir's Portal

Basic circuits

## Current, voltage and resistances

The main problem with electricity is that everyone uses it all the time, so they think they know how it works already. It can be easy, particularly if you get the main terms right:

Current - This is a measure of the flow of electrons around a circuit (measured in Amperes or Amps (A))

Voltage - This is a measure of how much energy the electrons are carrying around to the things in the circuit (measured in Volts (V))

Resistance - This is a measure of how hard itis for the electrons to travel through a part of the circuit (measured in Ohms (Ω))

The main worries are the first two - people often get them mixed up. Make sure you learn them!

That's all very well, but what is a circuit and what is an electron? Well,a circuit is a collection of components wired to a battery or power supply, which pushes the small packets of charge, usually electrons, around it.

Analogies - ("it's a bit like...").The problem with electricity is that you can't really see what's going on,so people come up with different ways of thinking about it. You might have heard people talk about water flowing (current?) due to pressure (voltage?)and meeting narrower pipes (resistance?). Or, there's the idea of cars, where resistors are muddy tracks instead of motorways, and you see how much petrol is used up to find the voltage. Pick whatever works for you, but, remember that they are only models and are not the whole story.

Direction Problem!

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 (how confusing is that!)

## Circuits

Simple Measurement Circuit
This shows how to connect a Voltmeter for measuring voltage and an Ammeter for measuring current. Thinking about how this works may help you understand voltage and current a little better.

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. Potential is to do with energy in physics, so what the voltmeter does is compare the energy difference between two points in a circuit, to see how much has been used up. 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).

Measuring voltage and current can be used to measure resistance (see formulae section).

Series Circuits
You should have come across the difference between series and parallel circuits many times, but that does not mean to say that you can't get confused! Series circuits are a bit pants, really. Any break in a circuit causes the whole thing to go down, so one faulty Christmas Tree light can cause the whole lot to go out (and then how do you figure out which one
blew?).

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.

Parallel Circuits
More useful and more common, but a little harder to get your head round. The main deal is that each component is directly connected to the battery (follow the wires round the circuit to check), so they each get the full voltage.

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 (because it's harder for current to flow).

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. This is because you are giving the current more ways to go.

Nasty Circuits

If you have a combination of series and parallel in one circuit take your time and analyse it bit by bit. Don't be ashamed to move your finger around the wires to see how it connects - I still do!!! Practise questions if you are not confident with this, but it is rare to get given anything really complicated at GCSE.

## Circuit symbols

Cells and Batteries: strictly speaking, one cell represents 1.5V and you make a battery by adding more cells, so the left hand battery would be 3V (2 x 1.5). This is nit picking a bit though, often people just draw a cell and then write '6V' above it or something, which is usually fine.

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 (turn a knob or move a slider to 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. In the diagram below, only LED (B) will light. An 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!

General Note: All formulae are given in each possible arrangement, but it is easier to learn the first one and know how to rearrange them! Try the triangle method shown for Ohm's law on the other equations you learn.

Ohm's Law

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:
V is Voltage in Volts, I is Current in Amps and R is resistance in Ohms.

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.
More Equations

Charge (Q) in Coulombs (C) and Time(t) in seconds(s):
Energy (E) measured in Joules (J):
Power (P) measured in Watts (W):
Here are the formulae you need in 'triangle' form:
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. Always use the base unit if unsure. Check you know the information below. Base units are given in the topics, the ones to watch for are time (seconds) and mass (kilograms not grams).

 Prefixes: Name: Value: Example: 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