4.2.4Electricity and Magnetism
Resistance — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 8 parts Lesson 18 of 65 1 quiz
Lesson summary
4.2.4 Resistance — IGCSE PHYSICS notes: Define resistance and use R = V/I, describe how to find resistance, interpret I–V graphs and relate the resistance of a wire to length and area.Resistance = the opposition to current.Cause — free electrons collide with the metal ions in the…
- Consequences
- Thermistor
- LDR (light-dependent resistor)
- Heating effect
- Cross-sectional area
- Ohmic conductor
Part 1 of 6Resistance and Ohm's law
1Resistance
Resistance the opposition to current.
2Cause
- Cause free electrons collide with the metal ions in the wire, which slows them down.
3Effect
- Effect the higher the resistance of a circuit, the lower the current.
4R
V / I
- R is resistance in ohms (Ω), V is p.d. in volts and I is current in amperes.
- Ohm's law — the current in a metallic conductor is directly proportional to the p.d. across it at constant temperature.
- Rearranged, V = I R and I = V / R.
5Consequences
- Consequences for a constant p.d., current falls as resistance rises;
- for a constant current, the p.d. across a conductor rises as resistance rises.
Part 2 of 6Finding resistance experimentally
1
Connect the component in series with an ammeter, a variable resistor and the supply.
2
Connect a voltmeter in parallel across the component.
3
Adjust the variable resistor and record V and I for at least five values.
4
Switch off between readings so the component does not heat up.
5
Calculate R = V / I for each pair or plot I against V and interpret the graph.
Part 3 of 6I–V graphs
| Component | Graph shape | Resistance | Reason |
|---|---|---|---|
| Fixed resistor or wire (constant temperature) | Straight line through the origin | Constant (ohmic) | Current is directly proportional to p.d. |
| Filament lamp | Curve that flattens as p.d. rises | Increases | Filament gets hotter, the ions vibrate more and the electrons collide more often |
| Diode | Current only in the forward direction | Very low forward, very high reverse | Allows current one way only (starts to conduct above about 0.6 V) |
Part 4 of 6Other resistors
Other resistors
Thermistor
- Thermistor its resistance decreases as temperature increases.
LDR (light-dependent resistor)
- LDR (light-dependent resistor) its resistance decreases as light intensity increases.
Heating effect
- Heating effect higher-resistance components get hotter for the same current because electrons collide with ions and transfer energy.
Part 5 of 6Resistance of a wire (Extended)
Length
- Length resistance is directly proportional to length (R ∝ L).
- Doubling the length doubles the resistance.
Cross-sectional area
- Cross-sectional area resistance is inversely proportional to area (R ∝ 1/A).
- Doubling the area halves the resistance.
- A wire with twice the length and twice the diameter has four times the area, so its resistance is R × 2 ÷ 4 = R/2.
Part 6 of 6More from the specification
More from the specification
Resistance
- Resistance potential difference divided by current, R = V / I, measured in ohms
Potential difference
- Potential difference energy transferred per unit charge, with a voltmeter connected across the component
Static charge
- Static charge rubbing a polythene rod gives it negative charge by gaining electrons, and like charges repel
Electrical power
- P = I x V, the rate of energy transfer, so higher resistance in a wire means more energy dissipated as heat for the same current
Mains supply
- Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Resistance
- Resistance potential difference divided by current, R = V / I, measured in ohms
Potential difference
- Potential difference energy transferred per unit charge, with a voltmeter connected across the component
Static charge
- Static charge rubbing a polythene rod gives it negative charge by gaining electrons, and like charges repel
Electrical power
- P = I x V, the rate of energy transfer, so higher resistance in a wire means more energy dissipated as heat for the same current
Terminal
- Connection point of a cell, labelled + or −.
Insulator
- Material that does not allow current to flow.
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.
Field lines
- Lines showing the direction and strength of a magnetic field.
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.
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.
Key terms
| Term | Meaning |
|---|---|
| Resistance | Opposition to current, R = V / I, in ohms (Ω). |
| Ohm's law | Current is directly proportional to p.d. at constant temperature. |
| Ohmic conductor | Component whose I–V graph is a straight line through the origin. |
| Diode | Component that allows current in one direction only. |
| LDR | Resistor whose resistance falls as light intensity rises. |
| Thermistor | Resistor whose resistance falls as temperature rises. |
Avoid these mistakes
1
Saying resistance is proportional to diameter — it depends on cross-sectional area, so on diameter squared.
2
Saying a filament lamp is ohmic — its resistance rises as it gets hot, so its I–V graph curves.
3
Connecting the voltmeter in series — it must be in parallel across the component.
4
Mixing up direct and inverse proportion — R increases with length but decreases with area.
Marked practiceQuizzes for this lesson
Last updated:
\n