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Resistance and current

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In this section, we will look at resistance (measured in ohms) and how it is calculated in different types of circuit. We will look at how the resistance in the filament of a lamp changes when the filament heats up. You will also learn about diodes, heat-dependent resistors (called thermistors) and light-dependent resistors (LDRs).

# Resistance and Ohm's Law

An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide with the atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] of the conductorconductor: An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. A thermal conductor allows thermal energy to be transferred through it easily.. This is called resistance and it makes it harder for current to flow. These collisions make the conductor hot. It is this that makes a lamp filament hot enough to glow.

## Ohm’s Law

Resistance is measured in ohms, Ω. The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between resistance, voltage, current:

resistance = voltage / current

ohms (Ω) = volts (V) / amperes (A)

The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm’s Law. The graph shows what happens to the current and voltage when a resistor follows Ohm’s Law.

Try this calculation:

Question

Bicycles with battery operated lights often have different size bulbs for the front and rear lights. The filament in the front lamp has a resistance of 3 ohms. It takes a current of 0.6A. What voltage does it work at?

1. 0.2V
2. 1.5V
3. 1.8V
4. 5V

The answer is 1.8V. If you didn't get the correct answer make sure you have used the formula correctly.

resistance = voltage / current

We can rearrange this to:

current x resistance = voltage

So the voltage in the lamp is 3 x 0.6 = 1.8 volts

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