Define resistance
Resistance is a measure of how much a material opposes the flow of electric current.
It is defined as the ratio of potential difference (V) across a component to the current (I) flowing
through it:
R=V/I
2. Ohm’s law is both a relationship and a condition.
(a) Explain why these are incorrect statements:
(i) "The potential difference across a component is proportional to its resistance."
Incorrect because p.d. is proportional to current (if resistance is constant), not resistance itself.
(ii) "The potential difference across a component is proportional to the current."
This is sometimes correct, but it only applies to Ohmic conductors under constant temperature —
not a universal truth.
(iii) "The potential difference across a component equals the current multiplied by the
resistance."
This is a correct mathematical expression (V = IR), but it’s not the definition of Ohm’s Law,
which also requires that resistance remains constant with varying voltage and current.
(iv) "The resistance of a wire is constant, provided the temperature is constant."
This is a condition for Ohmic behavior, not Ohm's Law itself.
(b) Best correct version:
Ohm’s Law states that the current through a conductor is directly proportional to the
potential difference across it, provided the temperature and other physical conditions
remain constant.
3. Graph: Current vs. p.d.
(a) Component A obeys Ohm’s law — it has a straight-line graph passing through the origin,
showing proportionality between I and V.
(b) Component C shows increasing resistance — the curve flattens at higher currents, indicating
less increase in current for each volt (i.e., resistance increases).
(c) Constant resistance ranges:
•
A: All voltages
•
•
B: At low voltages only
C: No range of constant resistance
4. Light-emitting diode (LED)
(a) The graph is nonlinear; current does not increase proportionally with voltage. Hence, Ohm’s
law is not obeyed.
(b) At 1.5 V, estimate the current from the graph (e.g., assume 20 mA):
R = V/I = 1.5 / 0.020 = 75 Ω
(c) As voltage increases from 0 to 1.6 V, resistance decreases rapidly. At first, current is nearly
zero (high resistance), then increases steeply.
(d) If voltage is reversed, the current is approximately zero — resistance is extremely high
(effectively infinite).
(e) The current–voltage graph will show no current for negative voltages until breakdown (if
applicable); the graph lies flat along the voltage axis for negative V.
5. I-V Characteristics: P and Q
Statement
T/F
(i) P is a resistor and Q is a filament lamp. T
(ii) The resistance of Q increases as the
T
current in it increases.
(iii) For a current of 1.9 A, the resistance of
F
Q is half that of P.
(iv) For a current of 0.5 A, the power
F
dissipated in Q is double that in P.
6. Identify components (graph X, Y, Z)
Correct answer: D
•
•
•
Z = metallic conductor (linear I–V)
Y = semiconductor (nonlinear, steep rise)
X = filament lamp (curve flattens)
Explanation
P has a straight line graph (Ohmic); Q has a
curve typical of a filament lamp.
Graph flattens — less current increase per volt.
At that current, Q has a higher voltage drop —
higher resistance.
Need to compare VxI for both; power is not
necessarily double.
7. Component with greatest resistance at V1
Choose the component with the smallest current at V1 (since R = V/I).
The graph with the shallowest slope at V1 represents the greatest resistance.
8. Graph showing resistance vs. p.d. for filament lamp
The correct graph is the one where resistance increases non-linearly with voltage.
(Choose the graph that curves upwards as V increases.)
9. Filament lamp — resistance vs. temperature
Correct answer: D
•
Resistance increases because temperature increases with current.
10. I-V characteristics (graph given)
(a)
(i) R = V/I = 2.0 / 0.4 = 5 Ω
(ii) R = V/I = 5.0 / 0.6 = 8.33 Ω
(b) The component is likely a filament lamp, as resistance increases with voltage — a signature
of temperature-dependent resistance.