Paper 6 · Limitless Physics

About the paper

Paper 6 is the Alternative to Practical paper. It lasts 1 hour and has 40 marks. It is 20% of your IGCSE.
Every question is compulsory.
The questions describe or draw an experiment. You answer as if you had done it.
Paper 5 is the Practical Test, done in a laboratory. It tests the same skills. You take one of the two.
Expect experiments you know, such as springs, pendulums, cooling, circuits, lenses and glass blocks. Expect some you have not seen before. The same skills work for all of them.

One example runs through this page. A student asks: does a longer pendulum swing more slowly? The period T is the time for one complete swing, there and back.

Planning an experiment

A planning question asks you to describe an experiment in your own words. Answer these points, in this order:

What you will change, and what you will measure.
The apparatus, and why you chose it.
The method, as numbered steps.
What you will keep the same, and how.
How many values, over what range. Use at least 5 values, spread over a wide range. Repeat each reading and take the mean.
How you will record the results: a table with headings.
How you will use the results to reach a conclusion. This is usually a graph.
One risk, and the precaution that reduces it.

Question (6 marks): Plan an experiment to find out how the length of a pendulum affects its period.

Weak answer: Change the length of the string and time the swings with a stopwatch. Do it again to see if the time changes.

Why it is weak: it does not say how to measure the length, how many swings to time, how many lengths to use, or what to keep the same.

Full-mark answer:

Clamp the string firmly in a retort stand.

Measure the length l from the clamp to the centre of the bob with a metre rule.

Pull the bob a small distance to one side and let it go.

Time 10 complete swings with a stopwatch. Divide by 10 to find T.

Repeat the timing and find the mean.

Do this for at least 5 lengths, for example from 0.20 m to 1.20 m.

Keep the same bob and the same starting angle each time.

Record l and T in a table. Plot a graph of T² against l.

Variables

The independent variable is the one you change. Pendulum: the length l.
The dependent variable is the one you measure. Pendulum: the period T.
Control variables are the ones you keep the same. Pendulum: the mass of the bob and the starting angle.
Say how you keep a control variable the same, and why. If two things change at once, you cannot tell which one caused the result.

Question (2 marks): Name one variable the student should keep the same. Explain why.

Weak answer: Keep everything the same so it is fair.

Why it is weak: it does not name a variable.

Full-mark answer: The starting angle. Then only the length changes, so any change in the period is caused by the length.

Taking readings

Look at a scale at eye level, straight on. This avoids parallax error, the error from looking at a scale at an angle.
On an analogue scale (a pointer or a liquid level against marks), read to the nearest half of the smallest division when the marks are clear enough to judge it. A thermometer with 1 °C divisions can be read to 0.5 °C.
On a digital instrument, record exactly what the display shows. Do not add or drop digits.
Check each instrument for a zero error before you start. A zero error means the instrument does not read zero when it should. Correct every reading for it.
Measure multiples. Time 10 swings and divide by 10. Your reaction time affects the start and the stop once, so dividing by 10 makes its effect on one period 10 times smaller. In the same way, measure the thickness of 50 sheets of paper and divide by 50.
Time the pendulum as it passes the centre of its swing. Put a fixed marker there to judge it.
Read a measuring cylinder at the bottom of the meniscus, at eye level.
Choose apparatus that is precise enough, and say why. A stopwatch reads to 0.1 s or better. A metre rule reads to 1 mm. A light gate with an electronic timer removes reaction time.

Tables

Head each column with the quantity / unit, for example l / m or T / s.
Do not write units in the body of the table.
Write raw readings to the precision of the instrument. Use the same number of decimal places all the way down a column.
When you multiply or divide readings, give the calculated value to the same number of significant figures as the least precise reading you used. If the question asks for a number of significant figures, use that. Counted numbers, such as the 10 in “time for 10 swings”, do not limit it.
Leave room for repeat readings and the mean.

Question (3 marks): Record the student’s results in a table.

Weak answer:

Length Time

0.2 m 9

0.40 m 12.6

0.6 m 15.60

Why it is weak: the headings have no units, the body has units, and the number of decimal places changes down each column.

Length	Time
0.2 m	9
0.40 m	12.6
0.6 m	15.60

Full-mark answer:

l / m time for 10 swings / s T / s T² / s²

0.20 9.0 0.90 0.81

0.40 12.6 1.26 1.59

0.60 15.6 1.56 2.43

0.80 17.9 1.79 3.20

1.00 20.1 2.01 4.04

1.20 22.0 2.20 4.84

l / m	time for 10 swings / s	T / s	T² / s²
0.20	9.0	0.90	0.81
0.40	12.6	1.26	1.59
0.60	15.6	1.56	2.43
0.80	17.9	1.79	3.20
1.00	20.1	2.01	4.04
1.20	22.0	2.20	4.84

Graphs

Plot the graph the question asks for. “T² against l” means T² on the y-axis and l on the x-axis. If the question lets you choose, put the independent variable on the x-axis.
Label each axis with the quantity / unit, just like the table headings.
Choose scales that use more than half the grid in both directions. Use simple steps: each large square stands for 1, 2 or 5 units (or 10, 20, 50). Never use steps of 3.
Plot each point as a small cross (×) or plus sign (+), to within half a small square. Use a sharp pencil.
Draw one thin best-fit line, straight or a smooth curve. It does not have to pass through any point. Leave about the same number of points on each side. Do not join the dots.
If you have identified an anomalous point, leave it out when you draw the line.
A straight line through the origin means the two quantities are directly proportional.

T² against l for the results in the table. The gradient triangle covers most of the line.

Gradient. Draw a large triangle on the graph. Its long side should cover at least half of your line. Read both corners from the line, not from the table. Show the values, then divide. Give the answer to 2 or 3 significant figures, with a unit: the y-axis unit divided by the x-axis unit.

Question (3 marks): Find the gradient of the line. Show clearly how you got your values.

Weak answer: gradient = 4.84 ÷ 1.20 = 4.03

Why it is weak: it uses one point from the table instead of a triangle on the line, and it has no unit.

Full-mark answer: Triangle drawn from l = 0.10 m to l = 1.10 m.

gradient = (4.43 − 0.40) ÷ (1.10 − 0.10) = 4.03 ÷ 1.00 = 4.0 s²/m

A graph of T against l would be a curve. A graph of T² against l is a straight line through the origin. So T² is directly proportional to l.

Conclusions

State the relationship. Then back it up with the data or the shape of the graph.
Two values agree within the limits of experimental accuracy if they are within about 10% of each other. Show the working, then say whether they agree.

Question (2 marks): State a conclusion from the graph.

Weak answer: When l goes up, T goes up.

Why it is weak: it is true, but it does not use the graph to give the exact relationship.

Full-mark answer: T² is directly proportional to l, because the graph of T² against l is a straight line through the origin.

Question (2 marks): Another student finds a gradient of 4.3 s²/m. Do the two results agree? Explain your answer.

Weak answer: Yes, because they are close.

Why it is weak: “close” is not a reason. There is no working.

Full-mark answer: Difference = 4.3 − 4.0 = 0.3 s²/m. As a percentage: 0.3 ÷ 4.0 × 100 = 7.5%. This is less than 10%, so the results agree within the limits of experimental accuracy.

Sources of error and accuracy

Name a source of error for this experiment. “Human error” on its own scores nothing.
A random error makes readings scatter above and below the true value. Reduce its effect by repeating readings and taking the mean. Example: reaction time when you start and stop a stopwatch.
A systematic error makes every reading wrong in the same direction, because of the apparatus or the method. It can add the same amount to every reading, such as a zero error on a meter. It can also make every reading too big or too small by the same fraction, such as a scale that is marked wrongly. Repeating does not help. Find the cause and correct for it.
A reading is accurate if it is close to the true value. Readings are precise if they are close to each other.
An anomalous result does not fit the pattern of the others. Identify it, repeat that reading if you can, and leave it out of the best-fit line.

Sources of error in the pendulum experiment:

reaction time when starting and stopping the stopwatch
judging exactly when the bob passes the centre of the swing
parallax when reading the metre rule
measuring to the centre of the bob
the starting angle changing from one reading to the next

Question (1 mark): Suggest one source of error in measuring the period.

Weak answer: Human error.

Why it is weak: it does not say what went wrong.

Full-mark answer: The student’s reaction time when starting and stopping the stopwatch.

Improvements

An improvement must be specific and must deal with a named error. “Be more careful” or “use better apparatus” scores nothing.
Good improvements: use a light gate and an electronic timer; time 20 swings instead of 10; put a fixed marker at the centre of the swing; use more lengths over a wider range; repeat each reading and take the mean.

Question (2 marks): Suggest one improvement to the experiment. Explain how it helps.

Weak answer: Be more accurate with the timing.

Why it is weak: it does not say what to do differently.

Full-mark answer: Use a light gate at the centre of the swing, connected to an electronic timer. Then the student’s reaction time does not affect the readings.

Precautions that score marks, by experiment

Experiment	Precaution
Pendulum	Time from the centre of the swing, using a fixed marker. Use a small starting angle.
Cooling curve	Stir before each reading. Read the thermometer at eye level. Use the same starting temperature. Use a lid or insulation when comparing.
Springs	Read the pointer at eye level. Check that the spring returns to its original length.
Resistance of a wire	Switch off between readings so the wire does not heat up. Check the meters for zero error.
Optics pins and blocks	Put the pins at least 5 cm apart. Keep the pins vertical. Line up the bases of the pins.
Lenses	Move the screen both ways to find the sharpest image. Darken the room.
Density	Read the bottom of the meniscus at eye level. Remove air bubbles from the object.
Moments	Balance the rule before adding loads. Hang the masses exactly on the marks.

Safety

Name the specific hazard in this experiment, then a precaution that matches it. “Be careful” scores nothing.
Masses on a spring or a pendulum can fall. Keep your feet clear, or put a soft mat underneath.
A stretched spring or wire can snap. Wear eye protection.
A retort stand can tip over. Clamp it firmly to the bench.
A swinging bob can hit someone. Keep people clear of the swing.
Hot water can scald. Stand the beaker on a heat-proof mat and do not carry it while it is hot.
A resistance wire can get hot. Switch off between readings and do not touch the wire.

Question (2 marks): Suggest one safety precaution for an experiment that stretches a spring with masses.

Weak answer: Be careful with the masses.

Why it is weak: it does not say what the danger is or what to do.

Full-mark answer: Wear eye protection, because the stretched spring could snap and fly up.