Circuits are electrical systems that provide a path for current to flow. If a circuit is broken, for example by using a switch, current is no longer able to move through it. If there is a short circuit, an electrical system will not work properly. A short circuit is a fault in a system that links two parts of a circuit that should not be linked together. In other words, it is a pathway that takes the electricity in the wrong direction.
Electrical circuits can have a wide range of applications; controlling your central heating; playing a door chime when the bell is pressed; inside computers and smartphones; controlling a car engine, and many, many more. They all follow the same straightforward laws, and it is how the components are arranged together that can make them do some pretty complicated things.
When studying electricity, one of the simplest relationships that you need to know about is Ohm's law. This is about the relationship between current, resistance and voltage when a component is kept at a constant temperature. In a component that follows Ohm's law, the current and voltage are directly proportional, so if you double the voltage, you get double the current and vice-versa. If you draw a graph of current versus voltage, it turns out to be a straight line. If you take measurements using a filament light bulb, you will find that the graph has a straight section but eventually becomes curved - it does not obey Ohm's law because it is designed to get hotter as more current passes through it - remember, Ohm's law only works if the temperature of the component stays constant. You also need to be able to recognize the graph for a diode.
Electrical current is a flow of electrons. These have negative electrical charge so you may also see current defined as a flow of charge through a conductor. As they pass through a conductor, the electrons collide with the atoms of the conductor which makes it harder for them to pass. This is electrical resistance. Resistance has the units of ohms. A larger resistance in a component means that electrons have more difficulty in trying to pass through it. Several factors affect the resistance of a component, the material from which it is made, how long it is and how thick it is. Take for example a wire. A long wire has more resistance than a short one. A thick wire has less resistance than a thin one and a wire made from iron would have a greater resistance than one made from copper.
Resistors are components that are designed to have a specific value of resistance to electrical current. When resistors are connected into a circuit, it can be done in series (one after the other) or in parallel (side by side). When working out the total resistance of resistors in a circuit, it is very easy when they are in series - just add up the resistances and you have the answer. In parallel, it is a bit more tricky, the main things that you need to remember are that the potential difference across each component is the same, the total current through the whole circuit is the sum of the currents through the separate components and that the total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor.
