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.