■ Questions on experimental work SL

🧪 Common experimental questions 

Questions about experimental work may concentrate on ten key experiments:
■ Determining the acceleration of free-fall.
■ Applying the calorimetric techniques of specific heat capacity or specific latent heat.
■ Investigating at least one gas law.
■ Investigating the speed of sound.
■ Determining refractive index.
■ Investigating one or more of the factors that affect resistance.
■ Determining internal resistance.
■ Investigating half-life experimentally or by simulation.
■ Investigating Young’s double-slit.
■ Investigating a diode bridge rectification circuit.

🔬 Question 1: Double-slit Interference

A student uses observations of a double-slit interference pattern in a darkened room to determine the wavelength of the light used in the experiment.
a) Explain why light from a laser is a good choice for this experiment.
b) Discuss if it is a good idea to move the screen as far as possible from the slits.
c) Explain what effect the diffraction pattern of light through the individual slits has on the observations.

a) Why is laser light a good choice?

✅ Answer: Laser light is monochromatic (single wavelength) and coherent (constant phase difference), making it ideal for producing clear, stable interference patterns.

b) Should the screen be moved far from the slits?

✅ Answer: Yes, moving the screen farther increases the fringe spacing, making the pattern easier to measure and improving accuracy.

c) Effect of diffraction through individual slits?

✅ Answer: Each slit produces a diffraction envelope; this modulates the brightness of the interference fringes. The central fringe is the brightest, and outer fringes may fade.

📏 Question 2: Resistivity of a Wire

a) A student is investigating the resistances of wires of different cross-sectional areas of the same metal alloy (of the same length). She measured the radius of one wire to be 0.48 ± 0.01 mm and the length
of the same wire to be 86.4 ± 0.5 mm. The resistance of this wire was found to be 1.1 ± 0.1 Ω. She can calculate the resistivity of the wire from the equation ρ = RA/L. Use the data to determine the resistivity
of the material of the wire and the uncertainty in the result.
b) Suggest how the uncertainty in this experiment could be reduced.

a) Calculate resistivity and uncertainty

🧠 Given:

• Radius
• Length
• Resistance

Cross-sectional area:

Resistivity:

✅ Answer:

b) How to reduce uncertainty?

• Use a digital micrometer for more precise radius measurements.
• Take multiple readings and average them.
• Use longer wires to reduce percentage uncertainty in length.

✅ Answer: Improve precision in measurement tools and use longer wires.

☢️ Question 3: Half-life of a Radioisotope

The half-life of a radioisotope was measured in a school laboratory. Before the experiment was carried out the background count was recorded on three occasions to be , and .
a) Explain why these readings are different from each other.
b) List two safety precautions that should be taken when doing this investigation.
c) The experiment had to be completed within 10 minutes. Suggest a suitable half-life for the chosen radioisotope.

a) Why are background readings different?

✅ Answer: Radioactive decay is a random process, and background radiation fluctuates naturally due to cosmic rays and environmental sources.

b) Two safety precautions for radioisotope use

• Use tongs to handle sources to maintain distance.
• Minimize exposure time and use shielding (e.g., lead blocks).

✅ Answer: Keep distance and minimize exposure time using tools and shielding.

c) Suitable half-life for experiment completed in 10 min

• Choose a half-life that allows measurable change within 10 minutes.
• A half-life of around 2–5 minutes would allow multiple half-lives within the experiment time.

✅ Answer: A suitable half-life is 2–5 minutes.