In this chapter of AS/A-level Physics, you will learn about resistivity.

Resistivity

- Three things determine resistance:

1. Length (L) --> Longer wire = Harder for current

2. Cross sectional area (A) --> Wider wire = Easier for current to flow

3. Resistivity (𝜌) --> Depends on material, may make it easier or harder for current to pass . Usually environmental factors play a part such as temperature

--> Resistivity: Resistance of a 1m length with a 1m2 cross sectional area (ohm metres)

𝜌 = 𝑅𝐴/𝐿

- When it comes to AS/A-level Physics, to find the resistivity of a wire you need to find its resistance

• Need to know cross sectional area

Core practical

Refer to AS/A-level Physics, we use a micrometre to measure diameter in 3 different areas of the wire

1. Test wire should be clamped to a ruler with the circuit attached to the wire where the ruler reads zero

2. Attach to the flying lead --> lead with crocodile clips

3. Record the length of the test wire, current and voltmeter reading

4. Use readings to calculate the resistance 𝑅 = 𝑉/𝐼

5. Repeat measurements and take average

6. Repeat for several lengths --> 0.1 – 1m

7. Plot resistance against lengths

Gradient --> 𝑅/𝐼 = 𝑃 /𝐴 --> so multiply gradient cross-sectional area

Superconductors

Superconductors --> Zero Resistivity

- Below a transition temperature the metal becomes a super conductor

- This could be useful in power cables as there would be no loss of current or in really strong electromagnets that don’t need a constant power source + super fast electrical circuits

**Power**

- Power is the rate of transfer of energy in AS/A-level Physics

- Measured in watts (W), where 1 watt is equivalent to 1 joule per second --> 𝑃 = 𝐸/t

- Really simple formula for power in electrical circuits:

𝑃 = 𝑉𝐼

Combine P=VI & V=IR for more equations

**Energy**

- If you know power, you can easily work out energy

- Simply need to multiply power by time

- 𝐸 = 𝑉𝐼𝑇

You now can handle some concept on resistivity in AS/A-level Physics.