Section 4 -Electricity Exam Questions

Question 1

State what is meant by a superconductor (2)

Answer 1

Zero resistance

At critical temperature

Question 2

What are the required conditions for a material to become superconducting? (2)

Answer 2

Resistivity decreases with temperature

Becomes superconducting when you reach the critical temperature

Question 3

A copper cylinder cable contains several filaments. State and explain what happens to the cable’s resistance when the filaments are made superconducting (5)

Answer 3

Resistance decreases to zero
Copper still has resistance
But this is in parallel with filaments
So total resistance is zero
Current goes through filaments

Question 4

Conducting putty is rolled into a cylinder. It is reshaped into a cylinder with half the radius and four times the length, determine the change to the resistance (2)

Answer 4

Halving diameter will increase resistance by 4
Increasing length by 4, increases resistance by 4
Resistance will be 16 times greater

Question 5

How would a voltmeter, ammeter and standard lab equipment be used to determine a value for the resistivity of the putty? (length and radius known) (6)

Answer 5

Measure length with a ruler
Measure diameter with micrometer
Use diameter to calculate cross-sectional area
Measure voltage
Measure current
Calculate resistance using V against I graph (gradient)
Flat metal electrodes at each end to improve connection

Question 6

Describe a procedure that could obtain an I-V curve for a semiconductor diode (6)

Answer 6

Connect circuit up
Measure current and voltage
Vary resistance
Obtain a range of results
Reverse connections to power supply and repeat
Plot a graph of V against I
Disconnect between readings

Question 7

Explain the shape of the I against V graph of a filament lamp (4)

Answer 7

At low V, I increases proportionally
As V increases, temperature of filament increases
Resistance increases
Rate of increase of I with V decreases
Same form of the curve in negative quadrant

Question 8

In a circuit of two resistors in series, the second is replaced by a thermistor. State and explain what happens to the reading on the voltmeter on the resistor as the thermistor’s temperature increases (3)

Answer 8

Reading will increase
Resistance of thermistor decreases
Current in circuit increases so pd across resistor increases

Question 9

Describe the procedure of obtaining accurate and reliable measurements of the resistance of a thermistor at different temperatures (6)

Answer 9

Thermistor is connected in a suitable circuit with voltmeter
Thermistor is heated in a beaker of water and a thermometer measures the temperature at small regular intervals
Resistance is found at various temperatures by diving voltage by current
Water must be stirred.
Temperature in range 20-90

Question 10

State and explain why it is an advantage for a rechargeable battery to have a low internal resistance (2)

Answer 10

Internal resistance limits current

Hence can provide higher current

Question 11

State what is meant by the emf of a battery (1)

Answer 11

Energy changed to electrical energy per unit charge passing through

Question 12

Explain what is meant by the internal resistance of a battery (1)

Answer 12

The resistance of the materials within the battery

Question 13

State and explain the effect of attempting to use a battery with a high internal resistance to start a car (2)

Answer 13

Car will probably not start

Battery will not be able to provide enough current

Question 14

A cell is connected to a variable resistor. Why does the terminal pd (across the cell) decrease as current increases? (2)

Answer 14

Energy loss in internal resistance

Pd across internal resistance increases with current

Question 15

Why does a voltmeter have a reading lower than the emf of a battery in a circuit? (3)

Answer 15

Battery has internal resistance
Current passes through this resistance
Voltage lost, which reduces value of emf

Question 16

How is emf and internal resistance found from the V against I graph for a cell? (2)

Answer 16

Emf is the y-intercept

Gradient is negative internal resistance

Question 17

State Ohm’s Law (1)

Answer 17

The current through a conductor between two points is directly proportional to the potential difference across the two points,provided the temperature remains constant

Question 18

A conducting putty of 20Ω is set up in series with a cell, ammeter and 10Ω resistor. The voltmeter is across the cell. Explain the problems for showing the putty obeys Ohm’s law (4)

Answer 18

Voltmeter position should not be across the cell
Voltmeter should be connected across the putty
10Ω resistor is not suitable to control the current
Because its resistance is only half of the putty

Question 19

With reference to two uses for superconductors, explain the advantage of their use compared with copper conductors (3)

Answer 19

Electromagnet
Produces very high magnetic field strength
Transmission
Less energy wasted

Question 20

Explain why resistivity of wire changes with temperature (3)

Answer 20

Increasing temperature, increases resistivity
Lattice ions vibrate with greater amplitude
Rate of movement of charge carriers reduced

Question 21

Define resistance (1)

Answer 21

Ratio of voltage across component to current through component

Question 22

State and explain, in terms of electron flow, how the resistance of the filament lamp changes as the current in the lamp increases (3)

Answer 22

Resistance increases
Temperature rise increases lattic vibration
More frequent collisions

Question 23

Explain in terms of electron motion, why the I-V characteristic for the filament lamp is a curve (4)

Answer 23

Increase in voltage causes temp increases
This causes an increase in the movement of the atoms
Increase in rate of collisions with electrons
So resistance increases as shown by V/I changing