4.2.10Electricity and Magnetism
Resistance of a Wire — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 27 of 65 1 quiz
Lesson summary
4.2.10 Resistance of a Wire — IGCSE PHYSICS notes: State how the resistance of a metal wire depends on its length and cross-sectional area and describe an experiment to investigate this.Collisions — electrons collide with the metal ions in the wire, which transfers energy from th…
- More vibration
- Temperature
- Expected result
- To investigate area
- Contact resistance
- Zero errors
Part 1 of 6Why a wire has resistance
Collisions
- Collisions electrons collide with the metal ions in the wire, which transfers energy from their kinetic store to the vibrating ions.
Heating
- Heating the ions vibrate more, so the temperature of the wire rises.
More vibration
- More vibration greater vibration of the ions means more collisions with the electrons, which means more resistance.
Part 2 of 6Factors affecting resistance
Length
- Length a longer wire gives each electron more collisions, so the resistance is greater.
Cross-sectional area
- Cross-sectional area a thicker wire gives more space for electrons to flow, so the resistance is smaller.
Material
- Material different metals have different numbers of free electrons, so copper conducts better than nichrome or steel.
- Heating elements use nichrome (high resistance) and cables use copper (low resistance).
Temperature
- Temperature for a metal, the resistance increases as the temperature rises.
Part 3 of 6Proportionality (Extended)
Length
- R is directly proportional to length (R ∝ L), so doubling the length doubles R.
Area
- R is inversely proportional to area (R ∝ 1/A), so doubling the area halves R.
- For a round wire A = πd2/4, so doubling the diameter divides R by four.
Combined
- R ∝ L / A, giving R2 / R1 = (L2 / L1) × (A1 / A2).
Graphs
- R against L is a straight line through the origin, and R against 1/A is also a straight line through the origin.
Part 4 of 6Reducing errors
Heating
- Heating use a low current and switch off between readings, because heating raises the resistance.
Contact resistance
- Contact resistance poor contact at the clips gives a line that misses the origin.
Zero errors
- Zero errors check the meters read zero before starting.
Random errors
- Random errors repeat readings and draw a line of best fit.
Part 5 of 6Applications
Power lines
- Power lines made thick and from low-resistance aluminium or copper so little energy is wasted as heat.
Filament lamps
- Filament lamps use a very thin, long coiled wire so the resistance is high and the filament glows.
Fuse
- Fuse a thin wire that overheats and melts when the current is too large.
Part 6 of 6More from the specification
More from the specification
Electrostatic Charging by Friction
- Friction between insulating materials causes the transfer of mobile negative electrons;
- rubbing a polythene rod with a cloth transfers electrons onto the rod, leaving it negatively charged while the cloth acquires an equal positive charge.
Gold-Leaf Electroscope Operation
- A gold-leaf electroscope detects the presence of net electrical charge;
- bringing a charged rod near the brass cap induces charges,
- causing like charges to accumulate on the stem and gold leaf,
- which repel each other and deflect the leaf.
Resistance in an Electrical Circuit
- In an electrical circuit, the resistance of a conducting wire opposes the flow of electric charge (current);
- according to Ohm's law, resistance equals potential difference divided by current (R = V/I) and increases linearly with wire length.
Electrostatic Charging by Friction
- Friction between insulating materials causes the transfer of mobile negative electrons;
- rubbing a polythene rod with a cloth transfers electrons onto the rod, leaving it negatively charged while the cloth acquires an equal positive charge.
Gold-Leaf Electroscope Operation
- A gold-leaf electroscope detects the presence of net electrical charge;
- bringing a charged rod near the brass cap induces charges,
- causing like charges to accumulate on the stem and gold leaf,
- which repel each other and deflect the leaf.
Resistance in an Electrical Circuit
- In an electrical circuit, the resistance of a conducting wire opposes the flow of electric charge (current);
- according to Ohm's law, resistance equals potential difference divided by current (R = V/I) and increases linearly with wire length.
Electromotive force
- Energy transferred from other forms to electrical energy per unit charge.
Gold-leaf electroscope
- A device in which a charged gold leaf falls when photoelectrons discharge the plate.
Mains supply
- Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Charge
- Quantity of electricity in coulombs, Q = It.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Electroscope
- Instrument with a metal cap, stem and gold leaf used to detect charge.
Terminal
- Connection point of a cell, labelled + or −.
Insulator
- Material that does not allow current to flow.
Component
- Part of a vector along one axis.
Potential difference
- Work done by a unit charge passing through a component.
Electric field
- Region of space in which a charge experiences a force.
Alternating current
- Current that continually reverses direction.
Key terms
| Term | Meaning |
|---|---|
| Resistance | Opposition to current, R = V / I. |
| Length | Resistance is directly proportional to the length of a wire. |
| Cross-sectional area | Resistance is inversely proportional to the area of a wire. |
| Resistance wire | Wire such as constantan with a high, steady resistance. |
| Micrometer | Instrument used to measure the diameter of a thin wire. |
Avoid these mistakes
1
Saying resistance is proportional to diameter — it depends on area, so on diameter squared.
2
Leaving the current on between readings — the wire heats up and the results are not repeatable.
3
Mixing up direct and inverse proportion — R increases with length but decreases with area.
4
Forgetting that a different material has a different resistance even with the same dimensions.
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