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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.

1.

If a 100 ohm resistor is replaced with a 200 ohm resistor, whilst the potential difference is kept constant, what happens to the current?

Stays the same

Increases

Decreases

There is not enough information

Current is inversely proportional to resistance - as resistance is doubled the current is halved

2.

The current through a component depends on which of the following?

The resistance of the component

The size of the component

The material of the component

The color of the component

A bigger resistance means a smaller current. This is not the only factor affecting the current

3.

How is the current of a resistor related to the potential difference across it?

Inversely proportional

Directly proportional

Equal

One-fifth of the potential difference

If the potential difference across a one ohm resistor is increased by one volt, the current will also increase by one amp

4.

If two resistors are connected in parallel in a circuit, which has a total electrical potential difference of five volts, what is the value of the electrical potential difference across each resistor?

2.5 V

1.25 V

5 V

10 V

When two components are connected in parallel, they are connected to the same points of the circuit, so the electrical potential difference across each one is the same

5.

An LED emits light when the current flows through it in which direction?

Forwards

Backwards

No current flow

Up and down

LED is the abbreviation for light emitting **diode** so from that, you should know that electrical charge can only flow through in the forwards direction through a diode

6.

What is the unit of resistance?

Ohm

Volt

Ampere

Coulomb

Ensure you always put units on the end of any question which asks for a numerical answer. The units of resistance are named after Ohm

7.

Which equation correctly relates voltage, resistance and current?

This is how you write down Ohm's law mathematically

8.

If two five ohm resistors are connected in series, what is the total resistance of the circuit?

5 ohms

10 ohms

2.5 ohms

7.5 ohms

Resistance of resistors in series are easy - just add up the individual values to get the total resistance

9.

If three 1.5V cells are connected in series, what is the total potential difference provided by the cells?

3 V

4.5 V

5.5 V

1.5 V

In series, just like with resistors, the voltages of the cells are added together

10.

What can be found by measuring current and voltage?

Resistivity

Resistance

The direction of true north

Magnetic field strength

Measuring current and voltage lead German scientist Georg Simon Ohm to the conclusion that is now known as **Ohm's law**