Lesson summary
9.3.1 Resistance — AS PHYSICS 9702 notes: Define resistance, calculate it from V and I, and describe how to measure it.Resistance — the opposition to current.Effect — for a given p.d., the higher the resistance, the lower the current.Equation — R = V/I.R is resistance in ohms (Ω)…
- Conductors
- Higher resistance
- Lower resistance
- All components
- Charge and time
- Then resistance
Section 1 of 7Defining resistance
Resistance
- Resistance the opposition to current.
Effect
- Effect for a given p.d., the higher the resistance, the lower the current.
Equation
- R = V/I.
- R is resistance in ohms (Ω).
- V is potential difference in V.
- I is current in A.
The ohm
- 1 Ω = 1 V A−1.
Conductors
- Conductors materials with low resistance are good conductors.
Section 2 of 7Resistance and current
Resistance and current
Higher resistance
- Higher resistance a smaller current flows.
Lower resistance
- Lower resistance a larger current flows.
All components
- All components every electrical component, including wires, has some resistance.
Copper
- Copper wires are often made from copper because it has a low electrical resistance.
Section 3 of 7Measuring resistance
1
Connect the component in series with an ammeter and a power supply, with a voltmeter in parallel across the component.
2
Set the power supply to a low voltage, typically 1–2 V, to avoid heating the component.
3
Measure the potential difference and the current.
4
Calculate the resistance from R = V/I.
Section 4 of 7Combining equations
1Charge and time
- I = Q/t gives the current from charge passed in a time.
2Then resistance
- Then resistance substitute I into R = V/I.
Section 5 of 7More from the specification
More from the specification
Ohmic resistance verification
- Ohmic resistance verification plotting potential difference against current provides a direct graphical means to determine electrical resistance from line slope.
Resistivity and Temperature in Metals
- The electrical resistivity of a metallic conductor increases with rising temperature because increased thermal lattice vibrations cause more frequent collisions with conduction electrons.
Thermistor Characteristics (NTC)
- For a negative temperature coefficient (NTC) thermistor,
- increasing temperature releases additional charge carriers,
- so its resistance decreases sharply as temperature increases.
I-V Characteristics and Resistance Graphs
- On an I-V graph, the gradient represents the reciprocal of resistance (1 / R);
- a constant straight-line gradient passing through the origin signifies an ohmic component at constant temperature.
Resistivity and Temperature in Metals
- The electrical resistivity of a metallic conductor increases with rising temperature because increased thermal lattice vibrations cause more frequent collisions with conduction electrons.
Thermistor Characteristics (NTC)
- For a negative temperature coefficient (NTC) thermistor,
- increasing temperature releases additional charge carriers,
- so its resistance decreases sharply as temperature increases.
I-V Characteristics and Resistance Graphs
- On an I-V graph, the gradient represents the reciprocal of resistance (1 / R);
- a constant straight-line gradient passing through the origin signifies an ohmic component at constant temperature.
Proportional
- Changing by the same factor as another quantity.
Light intensity
- How bright the light falling on a surface is.
Semiconductor diode
- Component that lets current flow in one direction only.
Sensory resistor
- Resistor whose resistance changes with an environmental condition.
Temperature
- Measure of the average kinetic energy of the particles.
Resistivity
- Property of a material defined by ρ = RA/L.
Thermistor
- Resistor whose resistance falls as temperature rises.
Constant
- Not changing.
Gradient
- Slope of a graph line.
Characteristic
- Graph of current against p.d. for a component.
Filament
- Thin wire in a lamp that gets hot and glows.
Intensity
- Power per unit area perpendicular to the wave direction.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Resistance | Opposition to current, R = V/I. |
| Ohm | Resistance of a component when 1 V drives 1 A, 1 Ω = 1 V A⁻¹. |
| Conductor | Material with low electrical resistance. |
| Ammeter | Meter in series that measures current. |
| Voltmeter | Meter in parallel that measures p.d. |
Section 7 of 7Exam technique
1
Thinking resistance is the gradient of an I–V graph — it is V/I at each point.
2
Using a large p.d. when measuring resistance — the component heats up.
3
Saying a higher resistance gives a larger current — it gives a smaller current.
4
Forgetting that wires also have resistance.