Resistance
Science: KS3, topic 3.4.8
Resistance
If you resist something, you try to stop it happening. Resistance happens in a circuit when a part resists the flow of current.
Resistance
- Most conductors restrict the flow of current in a circuit.
- To reduce current, increase the resistance.
- Insulators will have a very high resistance.
- So a current will not flow through them.
Wire resistance
- There is more resistance in a wire if:
- The wire is longer.
- The wire is thinner.
Tasers
- Some UK police officers carry a taser.
- A taser is used as a non-lethal method of restraining a violent person.
- A taser can deliver a potential difference of 50,000 V.
Taser safety
- The taser is not deadly as it has high resistance.
- The high resistance means the taser delivers a small current, around 0.002 A.
- The small current is enough to make a suspect's muscles relax.
Calculation
- Resistance is measured in ohms.
- The symbol for ohms is the Greek omega symbol (Ω).
- Resistance can be calculated using the equation:
- Resistance (Ω) = potential difference (V) ÷ current (A)
Ohm's law
- A resistor can obey Ohm's law.
- The current in the resistor is directly proportional to the potential difference.
- If you double the potential difference, the current will double.
Proportional graph
- A directly proportional relationship gives a straight line on a graph.
- The line passes through the origin.
Test yourself on Resistance
Why are tasers non-lethal?
Wire A is longer than wire B. Both wires are made of the same material and have the same thickness. Which wire has more resistance?
Select the true statement from each pair