All subjects · Physics · WPH16 Practical Skills in Physics II

Practical skills in core practicals 15 and 16

Edexcel IAL Physics · WPH16 Practical Skills in Physics II

Applying Unit 6 skills to gamma absorption by lead and finding a mass from resonant frequencies. Revise the Unit 5 lessons on these core practicals first.

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Core practical 15: gamma absorption

Measure the background count rate first (several minutes, no source) and subtract it from every reading.
Measure lead thickness with a micrometer (correcting zero error), at several places.
Decay is random, so count for a long time and repeat: larger counts have smaller percentage uncertainty.
Keep the source-to-detector distance fixed (intensity also falls with distance).
Plot ln C against x: gradient = -μ; half-thickness = ln 2 / μ.
Safety: handle the sealed source with long tongs, keep it pointed away from people, minimise exposure time, keep your distance, return it to its lead-lined box. Do not suggest PPE or shielding already shown.

Core practical 16: mass from resonant frequencies

For each known mass, increase the driving frequency slowly and find where the amplitude is greatest; approach the peak from both sides with small steps and average.
At least five or six masses.
From f = (1/2π) sqrt(k/m): 1/f² = (4π²/k) m. Plot 1/f² against m: gradient 4π²/k.
Find the unknown mass from its resonant frequency, using the graph (which allows for the spring's own mass if the line has an intercept) or the gradient.

Worked example

A GM tube records 1260 counts in 5.0 minutes with a source; background is 24 counts per minute. Find the corrected count rate.

Total count rate = 1260 / 5.0 = 252 counts per minute
Corrected = 252 - 24 = 228 counts per minute

Answer: 228 counts per minute

Key idea: CP15: subtract background; long counts for random decay; fixed distance; ln C against x, gradient -μ, x_1/2 = ln 2 / μ; tongs and distance. CP16: find resonance from both sides; 1/f² against m, gradient 4π²/k.

Check yourself

A GM tube records 2850 counts in 10.0 minutes; background is 18 counts per minute. Find the corrected count rate in counts per minute.
Show answer
267 counts per minute. 2850 / 10.0 = 285; 285 - 18 = 267 counts per minute.
A graph of ln C against x has gradient -0.0525 mm^-1. Find μ in m^-1.
Show answer
52.5 m^-1. μ = 0.0525 mm^-1 = 52.5 m^-1.
For μ = 52.5 m^-1, find the half-thickness of lead in mm, to 3 s.f.
Show answer
13.2 mm. x_1/2 = ln 2 / 52.5 = 0.01320 m = 13.2 mm.
A graph of 1/f² against m has gradient 1.58 s²/kg. Find k in N/m, to 3 s.f.
Show answer
25.0 N/m. k = 4π² / 1.58 = 25.0 N/m.
The unknown mass resonates at 1.62 Hz. Using the gradient 1.58 s²/kg (line through the origin), find the unknown mass in kg, to 3 s.f.
Show answer
0.241 kg. 1/f² = 1 / 1.62² = 0.3810 s²; m = 0.3810 / 1.58 = 0.241 kg.
Why is the background count measured in core practical 15?
- It must be subtracted from each reading to give the count rate due to the source
- To check the GM tube works
- To find the half-life
- To calibrate the micrometer
Show answer
It must be subtracted from each reading to give the count rate due to the source. Background radiation adds to every count; it must be removed.
Which safety answer for a sealed gamma source would examiners credit?
- Wear rubber gloves
- Wear a lab coat and goggles
- Handle it with long tongs and keep it pointed away from people
- Put it in a lead box during the experiment
Show answer
Handle it with long tongs and keep it pointed away from people. Distance and minimal exposure time protect you; PPE does not stop gamma, and the source cannot be measured while boxed.

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Last updated 8 November 2026. Spotted a mistake? Tell us.