2.4 I-V characteristics
9 exam questions worth 18 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which component has an I-V graph that is a straight line through the origin?
Choose one answer.
Correct Not quite: the answer is Fixed resistor at constant temperature
Mark scheme
C: fixed resistor at constant temperature
Model answer
A fixed resistor at constant temperature (C).
Describe the shape of the I-V graph for a filament lamp and state what it shows about the lamp's resistance.
Mark scheme
a curve that gets less steep / flattens as potential difference increases (1); resistance increases as the current / potential difference increases (1)
Model answer
The graph is a curve that becomes flatter as the potential difference increases. This shows that the resistance of the lamp increases as the current increases.
Explain why the resistance of a filament lamp increases as the current through it increases.
Mark scheme
the current heats the filament / the temperature of the filament increases (1); ions in the metal vibrate more (1); electrons collide with the ions more often, so it is harder for charge to flow (1)
Model answer
The current heats the filament. The metal ions vibrate more, so the electrons collide with them more often. This makes it harder for the charge to flow, so the resistance increases.
Describe how a diode behaves when it is connected in the forward direction and in the reverse direction.
Mark scheme
forward direction: current flows (once the potential difference is large enough) (1); reverse direction: very high resistance / (almost) no current (1)
Model answer
In the forward direction the diode lets a current flow. In the reverse direction it has a very high resistance, so almost no current flows.
The current through a resistor is 0.20 A when the potential difference across it is 4.0 V. Calculate the resistance.
Mark scheme
R = V / I = 4.0 / 0.20 (1); 20 Ω (1)
Model answer
R = V / I = 4.0 / 0.20 = 20 Ω.
A fixed resistor is described as an ohmic conductor. State the condition needed for its resistance to stay constant and describe what the I-V graph looks like.
Mark scheme
temperature must be constant (1); straight line through the origin / current directly proportional to potential difference (1)
Model answer
Its temperature must stay constant. The graph is then a straight line through the origin, because the current is directly proportional to the potential difference.
Use the information in the table: calculate the resistance of the filament lamp when the p.d. is 3.0 V (current 1.20 A) and when the p.d. is 6.0 V (current 1.65 A). State what the results show.
Mark scheme
R = 3.0 ÷ 1.20 = 2.5 Ω (1); R = 6.0 ÷ 1.65 = 3.6 Ω (1); the resistance increases as the p.d. / current increases (1)
Model answer
At 3.0 V: R = 3.0 / 1.20 = 2.5 Ω. At 6.0 V: R = 6.0 / 1.65 = 3.6 Ω. The resistance of the lamp increases as the p.d. increases.
A student says that a steeper line on an I-V graph for a fixed resistor means that the resistor has a higher resistance. Explain whether the student is correct.
Mark scheme
no (1); the gradient is I ÷ V = 1 ÷ R, so a steeper line means a smaller resistance (1)
Model answer
The student is wrong. The gradient is I / V, which is 1 / R, so a steeper line means a lower resistance.
A diode is connected in a circuit with a lamp and a cell, the wrong way round. What happens?
Choose one answer.
Correct Not quite: the answer is The lamp does not light
Mark scheme
B: The lamp does not light
Model answer
The lamp does not light (B). In the reverse direction the diode has a very high resistance, so almost no current flows.
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