Ohmic vs non-ohmic conductor
Ohmic conductor: current is directly proportional to the p.d. at constant temperature — the I-V graph is a straight line through the origin and resistance is constant. Two elements: (1) I directly proportional to V, (2) at constant temperature. Non-ohmic conductor: resistance changes (with temperature, current or direction) — the I-V graph is not a straight line through the origin. Do not just say "follows Ohm's law" — describe the resistance behaviour.
Three I-V shapes and their reasons
Ohmic (metal at constant T): straight line through the origin; gradient . Filament lamp: a curve that flattens as current rises (curves away from the I-axis) — the filament heats up, lattice vibrations increase, more electron collisions, so resistance rises and each extra volt adds less current. Diode (silicon): almost no current in reverse or below ≈ 0.6 V forward, then a steep rise above the ≈ 0.6 V threshold; it conducts one way only.
I-V practical: vary the current
Method: put the component in series with an ammeter and a variable resistor (or variable supply) — essential to change the current; connect a voltmeter in parallel across the component only. Vary the resistance in steps, record I and V each time, and plot current (y) against voltage (x). Repeat and average to cut random error. For a diode, reverse the connections for the reverse-bias part. Omitting the variable resistor is the classic error.
Drawn from real examiner reports.
Filament lamp curve bends wrong way
The filament lamp curve gets LESS steep as current rises — it curves AWAY from the current (y) axis and flattens. Reason: resistance increases with temperature, so each extra volt adds less current. A curve steepening at high current (towards the I-axis) is wrong — it would mean the lamp gets easier to drive when hot. Correct: steep near the origin, flattening at high V.
June 2019 Paper 1P Q6: candidates who drew the filament-lamp I-V curve with the curve bending upward (towards the current axis) rather than flattening lost the mark for the correct shape.
Ohmic definition must be graph-based
Saying "an ohmic conductor obeys Ohm's law" is circular and scores zero. The mark scheme wants two elements: (1) current is directly proportional to p.d. (constant resistance), AND (2) at constant temperature. Likewise, "non-ohmic does not obey Ohm's law" gives no credit — say the resistance CHANGES (with temperature, current or direction). Describe behaviour, not the label.
No variable resistor = one data point
An I-V investigation must give a RANGE of (V, I) pairs to plot a graph. With no variable element the operating point is fixed and no graph is possible. Include a variable resistor (rectangle with a diagonal arrow) or a variable supply. Do not draw a fixed resistor instead. Also, the voltmeter goes across the component under test, not across the variable resistor.
June 2019 Paper 1P Q6(a)(i): the most common response scoring 3 out of 4 marks had all components present and correctly connected but omitted any means of varying the current.
Diode threshold is about 0.6 V
A silicon diode does not conduct appreciably until the forward voltage reaches ≈ 0.6 V; below that (and in reverse) the current is effectively zero. Writing "0 V" or leaving out the threshold loses the mark. In reverse bias the diode does not conduct at all for practical voltages. Show the near-zero region AND the steep rise above 0.6 V.
Read the voltmeter, not the supply
In the I-V practical, read the component voltage from the voltmeter across it, not from the power supply dial. The supply voltage is shared between the variable resistor and the component, so the dial over-reads the component's p.d. Using the dial value gives the wrong resistance. Always take V from the voltmeter and I from the ammeter.
Filament lamp R is not constant
A filament lamp's resistance is NOT fixed — it rises as the filament heats up. Computing gives the resistance only at that operating point; at a lower voltage the filament is cooler and its resistance is smaller. Do not treat a lamp as ohmic or quote a single resistance as if it held everywhere. Only a metal at constant temperature has one fixed R.
Record current in amps, not mA
Convert current to amperes before any calculation: 250 mA = 0.25 A. Substituting mA straight into gives a resistance 1000× too large. The report states "current should be in amps, rather than milliamps". Write A, not mA, in the final substitution.
Explain I-V shapes, do not just describe
To "explain" an I-V graph, link the shape to the physics: filament lamp — heating raises resistance, so it flattens; diode — conducts one way past ≈ 0.6 V; ohmic — constant resistance at constant temperature. A description without the reason misses the explanation marks.
Circuit: A series, V across component
Draw the I-V circuit with the ammeter in series, the voltmeter in parallel across the component (not the variable resistor), and a variable resistor to vary the current. Label the axes (/A, /V). Omitting the variable resistor is the classic lost mark.
Resistance from an I-V point: R = V/I
To find resistance at a point on an I-V graph, read its and and use — do NOT use the gradient except for an ohmic straight line (where gradient ). For a lamp or diode R differs at every point, so state the operating point. Keep current in amps.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Resistance | ohm, Ω | ||
| Current | ampere, A | ||
| Potential difference | volt, V |
Where = potential difference (p.d., V), = current (A), = resistance (Ω).
An I-V graph plots current on the y-axis and voltage on the x-axis. The gradient of an I-V graph equals . A steeper gradient means a lower resistance.
Define an ohmic conductor.
A metal wire is connected to a variable power supply. At one setting, the potential difference across the wire is 4.5 V and the current through it is 0.30 A.
(a) Calculate the resistance of the wire at this setting. Give the unit.
(b) The power supply is adjusted so that the potential difference doubles to 9.0 V. State what happens to the current through the wire, assuming its temperature remains constant. Explain your reasoning.
