Electricity (PPQs & Answers)

Question 1

Using band theory, explain why conduction can take place in a
semiconductor at room temperature.

Answer 1

The energy gap/band gap (between 
the valence and conduction bands) is 
small. (1)
Some electrons have enough energy 
to move from the valence to the 
conduction band. (1)

Question 2

State what is meant by the term electromotive force (e.m.f.).

Answer 2

The number of joules/energy gained
by/supplied to 1 coulomb (of charge
passing through the cell).

Question 3

The student connects a red LED and a blue LED, in turn, to the battery.
The LEDs are forward biased when connected.
The student observes that the battery will operate the red LED but not
the blue LED.
LEDs emit light when electrons fall from the conduction band into the
valence band of the p-type semiconductor.
Explain, using band theory, why the blue LED will not operate with this
battery.

Answer 3

The electrons do not gain enough energy to
move into/towards the conduction band of
the p-type.

Question 4

The student observes that the red LED is only lit when the
ammeter gives a positive reading and the green LED is only lit
when the ammeter gives a negative reading.
Explain these observations.

Answer 4

The LEDs will light when they are forward
biased. (1)
The change in polarity of voltage changes
the biasing. (1)

Question 5

State what is meant by an e.m.f. of 1·5 V.

Answer 5

1·5 J (of energy) is supplied
to/gained by each coulomb (of
charge passing through the cell).

Question 6

Solar cells are made by joining n-type and p-type semiconductor materials. A
layer is formed at the junction between the materials.
A potential difference is produced when photons enter the layer
between the p-type and n-type materials.
State the name of this effect.

Answer 6

Photovoltaic (effect)

Question 7

The lamp is now moved closer to the solar cell.
Explain, in terms of photons, why the maximum output power from
the solar cell increases.

Answer 7

Greater number of photons (strike

the solar cell) per second

Question 8

A voltage is applied across an LED so that it is forward biased and
emits light.
Using band theory, explain how the LED emits light.

Answer 8

(Voltage applied causes) electrons
to move towards conduction band of 
p-type/ away from n-type (towards 
the junction) (1)
Electrons move/ drop from 
conduction band to valence band
(1)
Photon emitted (when electron 
drops) (1)

Question 9

Suggest why the connecting wires used in this circuit have a large
diameter.

Answer 9

(Wire of large diameter) has a low 
resistance (1) 
or
to prevent overheating (1)
or
to prevent wires melting (1)

Question 10

Explain why the charging current decreases as the battery

charges.

Answer 10

The e.m.f. of the battery
increases (1)
Difference between the two
e.m.f.s decreases (1)

Question 11

A sample of pure semiconductor is heated. Use band theory to explain
what happens to the resistance of the sample as it is heated.

Answer 11

resistance decreases (1)
electron jumps (from valence band) to 
conduction band (1)