Electricity and magnetism · GCSE Physics
I–V characteristics
Teacher-written GCSE Physics revision on I–V characteristics: ohmic resistors, filament lamps, diodes, thermistors and LDRs, with the required practical wiring and what a curved graph says about resistance.
A straight I–V line through the origin means R is constant. A curve means R is changing. Lamps heat up; diodes only conduct one way; thermistors and LDRs respond to the surroundings.
The important bits
What you need to know
- 1
Plot current on the vertical axis and potential difference on the horizontal axis unless the paper tells you otherwise. Gradient of I against V is 1/R for a straight line through the origin.
- 2
A resistor at constant temperature: straight line through the origin, ohmic, R constant. Reverse the connections and the line continues through the origin into the opposite quadrant.
- 3
A filament lamp: the graph curves so that equal extra V buys less extra I as the lamp heats. Resistance rises with temperature because ions in the metal vibrate more and impede electrons.
- 4
A diode: almost no current in reverse; a sharp rise in forward current after a threshold (about 0.6 V for a typical silicon diode). It lets current flow mainly in one way.
- 5
A thermistor (NTC): resistance decreases as temperature rises. Used in thermostats and temperature sensors in potential dividers.
- 6
An LDR: resistance decreases as light intensity rises. Used in automatic lighting. Dark means high R; bright means low R.
- 7
Required practical: voltmeter in parallel with the component, ammeter in series, variable resistor or variable supply to change V. Do not leave a lamp on so long that temperature is uncontrolled if you are testing Ohm’s law.
- 8
R = V/I still holds at a single point on a curve. Quote that value as the resistance “at this potential difference”, not as a constant for the component.
Quotations worth analysing
Short evidence. Real method.
“The resistance of a filament lamp increases as the temperature of the filament increases.”
That sentence is the curve. Shallow gradient at high V means large R. Do not say “the lamp uses up current”.
“A diode has a very high resistance in the reverse direction.”
High resistance, not “no resistance the other way”. A tiny reverse current may exist; at GCSE it is treated as negligible.
Go deeper
I–V graphs are resistance stories
A straight line through the origin means R is constant: a resistor at steady temperature. A curve that gets shallower as V increases is a filament lamp: temperature up, resistance up, so extra V buys less extra I. A diode is almost no current one way, then a sharp rise the other way after a threshold. Thermistor in a potential divider: cold means high resistance, so a large share of the p.d.; hot means low resistance. LDR: dark, high R; light, low R. In the required practical, use a voltmeter in parallel with the component, an ammeter in series, a variable resistor or variable supply to change V, and reverse the supply to get both sides of the origin. Sketch the shape in revision until each curve has a name.
Go deeper
Potential dividers turn thermistors and LDRs into useful sensors
A thermistor in series with a fixed resistor shares the supply voltage. When the thermistor’s resistance falls, its share of the p.d. falls if it is the component you are measuring across — or rises if you measure across the fixed resistor, depending on the circuit. Read the diagram. The same idea lights a lamp at dusk: the LDR’s resistance rises in the dark, changing the p.d. that switches a transistor or a comparator in a real circuit; at GCSE you only need the resistance trend. Students lose marks by saying “the thermistor heats up the circuit” rather than “its resistance falls as temperature rises, so the current in the series loop rises”. Name R, then I or V.
See the idea in action
A filament lamp has V = 3.0 V and I = 0.20 A at one point on its curve, so R = V/I = 3.0 / 0.20 = 15 Ω. Later, V = 12 V and I = 0.50 A, so R = 12 / 0.50 = 24 Ω. Resistance has risen as the filament got hotter; the graph is not a straight line, so the lamp is non-ohmic. A diode in reverse at 12 V might show I ≈ 0, so R is very large. A 5.0 kΩ NTC thermistor at room temperature falls to 1.0 kΩ when heated; in series with 5.0 kΩ across 6.0 V, the current rises from 6.0 / 10 000 = 0.60 mA to 6.0 / 6000 = 1.0 mA.
Exam technique
Turn knowledge into marks
Name the component from the shape before you calculate. For a curve, find R = V/I at the point given, and say that R is not constant. For thermistors and LDRs, state how R changes with temperature or light.
Common mistakes
Do not give these marks away
- 01
Calling a lamp ohmic, or saying its resistance falls as it gets brighter and hotter.
- 02
Drawing a diode graph that conducts equally both ways, or putting the ammeter in parallel during the practical.
- 03
Saying an LDR’s resistance increases as light intensity increases — it decreases.
Why does the I–V graph for a filament lamp curve away from a straight line at higher potential difference?
ACurrent is used up in the filament
BThe filament’s temperature rises, so resistance rises and current does not increase in proportion to V
CThe lamp becomes a diode
DOhm’s law only works for a.c.
Show the answer
The filament’s temperature rises, so resistance rises and current does not increase in proportion to V. Ions in the hot metal vibrate more, impeding electrons. R = V/I at each point increases, so the graph gets shallower.
Quick questions
If this is the bit you searched
What does an LDR do?
A light-dependent resistor. Its resistance decreases as light intensity increases, so it can be used in potential dividers for lighting circuits.
What does a thermistor do?
An NTC thermistor’s resistance decreases as temperature increases. It is used in temperature sensors and thermostats.
How can you tell a component is ohmic from a graph?
I against V is a straight line through the origin, so R is constant. The same line reversed through the origin shows it works the same both ways.
Why include a variable resistor in the I–V practical?
It lets you change the potential difference across the test component in steps so you can plot a full graph, not a single point.