Electricity

Question 1

State what is meant by a superconductor (2)

Answer 1

Zero resistance

At or below 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

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 3

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

Question 4

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 4

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

Question 5

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

Answer 5

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 6

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

Answer 6

At low V, I increases proportionally
As V increases, temperature of filament increases
Resistance increases with temperature due to atoms/ions vibrating and taking up more space.
Rate of increase of I with V decreases
Same form of the curve in negative quadrant

Question 7

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 7

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

Question 8

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

Answer 8

Internal resistance limits current

Hence can provide higher current

Question 9

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

Answer 9

Energy changed to electrical energy per unit charge passing through

Question 10

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

Answer 10

The resistance of the materials within the battery

Question 11

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

Answer 11

Car will probably not start

Battery will not be able to provide enough current

Question 12

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

Answer 12

Energy loss in internal resistance

Pd across internal resistance increases with current

Question 13

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

Answer 13

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

Question 14

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

Answer 14

Emf is the y-intercept

Gradient is negative internal resistance

Question 15

State Ohm’s Law (1)

Answer 15

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 16

Explain why resistivity of wire changes with temperature (3)

Answer 16

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

Question 17

Define resistance (1)

Answer 17

Ratio of voltage across component to current through component

Question 18

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

Answer 18

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

Question 19

Define current

Answer 19

Rate of flow of charge

Question 20

Define potential difference

Answer 20

Work done per unit charge

Question 21

What is the resistance of an ideal ammeter?

Answer 21

zero

Question 22

What is the resistance of an ideal voltmeter

Answer 22

infinite

Question 23

Describe and explain how the resistance of a NTC thermistor varies with temperature

Answer 23

As temperature increases, resistance decreases.
The increase in temperature causes atoms to vibrate and take up more space. This also causes more electrons to be released for conduction. the extra electrons released over compensates for the atoms vibrating more and taking up more space.

Question 24

What are the rules for current, potential difference and resistance for series circuits

Answer 24

Current - same at all points
p.d. from battery shared amongst components
Resistance increases as more components are added.

Question 25

What are the rules for current, potential difference and resistance for parallel circuits

Answer 25

Current - splits up but total current must stay the same
p.d. from battery same across each branch
Resistance decreases as electrons have more branches/paths to take

Question 26

What is the relationship between resistance and light intensity for an LDR

Answer 26

As light intensity increases resistance decreases.