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Date	May 2022	Marks available	2	Reference code	22M.2.SL.TZ2.4
Level	Standard level	Paper	Paper 2	Time zone	2
Command term	Identify	Question number	4	Adapted from	N/A

Question

A cell is connected to an ideal voltmeter, a switch S and a resistor R. The resistance of R is 4.0 Ω.

When S is open the reading on the voltmeter is 12 V. When S is closed the voltmeter reads 8.0 V.

Electricity can be generated using renewable resources.

Identify the laws of conservation that are represented by Kirchhoff’s circuit laws.

[2]

State the emf of the cell.

[1]

b.i.

Deduce the internal resistance of the cell.

[2]

b.ii.

The voltmeter is used in another circuit that contains two secondary cells.

Cell A has an emf of 10 V and an internal resistance of 1.0 Ω. Cell B has an emf of 4.0 V and an internal resistance of 2.0 Ω.

Calculate the reading on the voltmeter.

[3]

Outline why electricity is a secondary energy source.

[1]

d.i.

Some fuel sources are renewable. Outline what is meant by renewable.

[1]

d.ii.

A fully charged cell of emf 6.0 V delivers a constant current of 5.0 A for a time of 0.25 hour until it is completely discharged.

The cell is then re-charged by a rectangular solar panel of dimensions 0.40 m × 0.15 m at a place where the maximum intensity of sunlight is 380 W m⁻².

The overall efficiency of the re-charging process is 18 %.

Calculate the minimum time required to re-charge the cell fully.

[3]

e.i.

Outline why research into solar cell technology is important to society.

[1]

e.ii.

Markscheme

« conservation of » charge ✓

« conservation of » energy ✓

Allow [1] max if they explicitly refer to Kirchhoff’ laws linking them to the conservation laws incorrectly.

12 V ✓

b.i.

I = 2.0 A OR 12 = I (r +4) OR 4 = Ir OR 8 = 4I ✓

«Correct working to get » r = 2.0 «Ω» ✓

Allow ECF from (b)(i)

b.ii.

Loop equation showing EITHER correct voltages, i.e., 10 – 4 on one side or both emfs positive on different sides of the equation OR correct resistances, i.e. I (1 + 2) ✓

10−4 = I (1 + 2) OR I = 2.0 «A» seen✓

V = 8.0 «V» ✓

Allow any valid method

is generated from primary/other sources ✓

d.i.

«a fuel » that can be replenished/replaced within a reasonable time span

«a fuel» that can be replaced faster than the rate at which it is consumed

renewables are limitless/never run out

«a fuel» produced from renewable sources

gives an example of a renewable (biofuel, hydrogen, wood, wind, solar, tidal, hydro etc..) ✓

OWTTE

d.ii.

ALTERNATIVE 1

«energy output of the panel =» VIt OR 6 x 5 x 0.25 x 3600 OR 27000 «J» ✓

«available power =» 380 x 0.4 x 0.15 x 0.18 OR 4.1 «W» ✓

$t =$ « $\frac{27000}{4.1}$ =» 6600 «s» ✓

ALTERNATIVE 2

«energy needed from Sun =» $\frac{V l t}{e f f}$ OR $\frac{6 \times 5 \times 0.25 \times 3600}{0.18}$ OR 150000 «J» ✓

« incident power=» 380 x 0.4 x 0.15 OR 22.8 «W» ✓

$t =$ « $\frac{150000}{22.8}$ =» 6600 «s» ✓

Allow ECF for MP3

Accept final answer in minutes (110) or hours (1.8).

e.i.

coherent reason ✓

e.g., to improve efficiency, is non-polluting, is renewable, does not produce greenhouse gases, reduce use of fossil fuels

Do not allow economic reasons

e.ii.

Examiners report

a) Most just stated Kirchhoff's laws rather than the underlying laws of conservation of energy and charge, basically describing the equations from the data booklet. When it came to guesses, energy and momentum were often the two, although even a baryon and lepton number conservation was found. It cannot be emphasised enough the importance of correctly identifying the command verb used to introduce the question. In this case, identify, with the specific reference to conservation laws, seem to have been explicit tips not picked up by some candidates.

bi) This was probably the easiest question on the paper and almost everybody got it right. 12V. Some calculations were seen, though, that contradict the command verb used. State a value somehow implies that the value is right in front to be read or interpreted suitably.

bii) In the end a lot of the answers here were correct. Some obtained 2 ohms and were able to provide an explanation that worked. A very few negative answers were found, suggesting that some candidates work mechanically without properly reflecting in the nature of the value obtained.

ci) A lot of candidates figured out they had to do some sort of loop here but most had large currents in the voltmeter. Currents of 2 A and 10 A simultaneously were common. Some very good and concise work was seen though, leading to correct steps to show a reading of 8V.

cii) This question was cancelled due to an internal reference error. The paper total was adjusted in grade award. This is corrected for publication and future teaching use.

di) The vast majority of candidates could explain why electricity was a secondary energy source.

dii) An ideal answer was that renewable fuels can be replenished faster than they are consumed. However, many imaginative alternatives were accepted.

ei) This question was often very difficult to mark. Working was often scattered all over the answer box. Full marks were not that common, most candidates achieved partial marks. The commonest problem was determining the energy required to charge the battery. It was also common to see a final calculation involving a power divided by a power to calculate the time.

eii) Almost everybody could give a valid reason why research into solar cells was important. Most answers stated that solar is renewable. There were very few that didn't get a mark due to discussing economic reasons.

[N/A]

b.i.

[N/A]

b.ii.

[N/A]

d.i.

[N/A]

d.ii.

[N/A]

e.i.

[N/A]

e.ii.