Current-voltage characteristicsAQA A-Level Physics: Subtopic test

1. 1
   A student investigates a fixed resistor kept at a constant temperature, using an ideal ammeter and an ideal voltmeter. She records a pd of 2.0 V with a current of 4.0 mA, a pd of 4.0 V with a current of 8.0 mA, and a pd of 6.0 V with a current of 12.0 mA. She plots a graph with pd on the vertical axis and current on the horizontal axis.

   (a)

   What does the gradient of her graph represent?

   [1 mark]

   • AThe conductance of the resistor
   • BThe charge flowing per second
   • CThe energy transferred per coulomb
   • DThe resistance of the resistor

   (b)

   Which of the following conductors obeys Ohm's law?

   [1 mark]

   • AA filament lamp as the pd increases from 0 to 12 V
   • BA metal wire kept at a constant temperature
   • CA semiconductor diode in the forward direction
   • DA metal wire whose temperature rises as the current increases

   (c)

   Calculate the resistance of the resistor at 2.0 V and at 6.0 V, and state what the results show about the resistor.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two components, X and Y, are connected in turn to a variable power supply, with an ideal ammeter and an ideal voltmeter. For each component a graph is plotted of pd (vertical axis) against current (horizontal axis). The graph for X is a straight line through the origin with a gradient of 40 Ω. The graph for Y is a curve through the origin whose gradient increases as the current increases.

   (a)

   Which component could Y be?

   [1 mark]

   • AA filament lamp, because its resistance increases as it gets hotter
   • BA semiconductor diode, because its resistance decreases as the pd increases
   • CA second ohmic conductor with a larger resistance
   • DA metal wire at constant temperature

   (b)

   What is the pd across X when the current through it is 0.50 A?

   [1 mark]

   • A80 V
   • B0.0125 V
   • C20 V
   • D40.5 V

   (c)

   X is a metal wire. State the condition needed for X to obey Ohm's law, and explain what would happen to the graph for X if the current became large enough to make the wire very hot.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician tests a semiconductor diode with an ideal ammeter and an ideal voltmeter. In the forward direction the current is negligible at 0.50 V, 2.0 mA at 0.70 V and 20 mA at 0.75 V. In the reverse direction, with 5.0 V across the diode, the current is 0.10 μA.

   (a)

   Describe how the current in the diode varies with pd in the forward and reverse directions.

   [3 marks]

   (b)

   Calculate the resistance of the diode at 0.70 V forward, at 0.75 V forward and at 5.0 V reverse, and explain whether the diode obeys Ohm's law.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates three components using an ideal ammeter and an ideal voltmeter: a fixed resistor, a filament lamp and a semiconductor diode. She increases the pd across each component from zero in the forward direction and then reverses the supply to repeat the readings in the reverse direction.

   (a)

   The student records these results for the lamp: a current of 0.80 A at 2.0 V, 1.20 A at 4.0 V, 1.80 A at 8.0 V and 2.20 A at 12.0 V. Calculate the resistance of the lamp at each pd and explain, using the results, why the lamp does not obey Ohm's law.

   [6 marks]

   (b)

   Compare the I–V characteristics of the three components in the forward and reverse directions, and explain which of them obey Ohm's law.

   [6 marks]

   Total for question 4: 12 marks

End of questions