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How are current, voltage and resistance related?
Define electric current, potential difference and resistance, and apply Ohm's law ($V = IR$) to ohmic and non-ohmic conductors, including filament lamps and diodes
A focused answer to the VCE Physics Unit 1 dot point on electrical quantities and Ohm's law. Defines $I = Q/t$, $V = W/Q$ and $R = V/I$, distinguishes ohmic and non-ohmic conductors, and works the VCAA SAC-style I-V characteristic problem.
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What this dot point is asking
VCAA wants you to define current, voltage and resistance, apply Ohm's law to ohmic devices, and distinguish non-ohmic devices (filament lamps, diodes, thermistors) where resistance varies with operating conditions.
Electric current
The rate of flow of charge:
SI unit: ampere (A) = coulomb per second. By convention, current flows in the direction of positive charge motion (opposite to electron flow in a metal).
Potential difference
Work done per unit charge:
SI unit: volt (V) = joule per coulomb. Drives current through circuit elements.
Resistance
Opposition to current:
SI unit: ohm () = volt per ampere. Depends on material, geometry and temperature.
Ohm's law
For an ohmic conductor (constant temperature, most metals at low currents):
with constant. The I-V graph is a straight line through the origin.
For non-ohmic devices, still gives the instantaneous resistance at the operating point, but varies with or .
Non-ohmic devices
Filament lamp. As current flows, the filament heats up and its resistance rises. I-V graph curves so that the slope decreases at higher .
Semiconductor diode. Zero current below the threshold voltage (about V for silicon). Near-vertical above. Strongly direction-dependent (only conducts one way).
Thermistor. A semiconductor whose resistance falls steeply with temperature. Used in temperature sensors. The I-V curve is concave up.
VCAA exam style
Year 11 SAC tasks typically include:
- Calculating at two operating points and identifying whether the device is ohmic.
- Reading an I-V graph to find at a stated voltage.
- Distinguishing ohmic and non-ohmic shapes.
Common traps
Calling drift velocity "current". Electron drift velocity in a wire is typically fractions of a millimetre per second. Current is the rate of charge flow.
Treating as a definition that proves Ohm's law. defines instantaneous resistance for any device. Ohm's law is the additional claim that is constant for ohmic devices.
Forgetting direction in diodes. A diode conducts only in one direction. Reversing the voltage usually drops the current to near zero.
Mixing units of charge and current. Ampere-second is coulomb; ampere-hour is C.
In one sentence
Electric current () is the rate of charge flow in amperes, potential difference () is the energy delivered per coulomb in volts, resistance () is the opposition to current flow in ohms, and ohmic conductors obey with constant , while non-ohmic devices (lamps, diodes, thermistors) have that varies with the operating point.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
Year 11 SAC4 marksA circuit element draws $0.20$ A at $4.0$ V. When the voltage is doubled to $8.0$ V, the current rises to $0.50$ A. (a) Find the resistance at each operating point. (b) Is the element ohmic? Justify.Show worked answer →
(a) Resistance at each point.
At V: .
At V: .
(b) Ohmic test. An ohmic conductor has constant . Here changed from to when doubled. So the element is non-ohmic.
An ohmic prediction would have been A at V (proportional); the observed A exceeds this, suggesting a device where falls with increasing or (a thermistor with negative temperature coefficient, for example).
Markers reward in ohms at each point, the ohmic test against the proportional prediction, and named non-ohmic examples.
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