Topic 6.1: Experimental Skills Questions

In an experiment to find the resistivity of a metal rod, a student measures the rod's diameter with a micrometer at several different positions along its length and takes the mean.

Outline why the diameter is measured at several positions rather than just once.

An experiment requires the length of a straight resistance wire, which is about 85 cm long.

Suggest a suitable instrument for measuring this length and justify your choice.

A wire obeys Ohm's law, so the potential difference V across it is directly proportional to the current I through it (V = R·I).

A student plots V on the vertical axis against I on the horizontal axis.

Sketch the expected graph, and state what its gradient represents.

To find the spring constant of a spring, a student plots the applied force F (in newtons) on the y-axis against the extension x (in metres) on the x-axis and obtains a straight line through the origin.

State the SI units of the gradient of this graph.

A nail is driven into wood.

The total length of the nail is measured with a ruler as 32.4 ± 0.1 cm and the length sticking out above the wood is 8.7 ± 0.1 cm.

The embedded depth is the difference of these two readings.

State the embedded depth and its absolute uncertainty.

Two power supplies are connected in series.

Their voltages are V₁ = 6.0 ± 0.3 V and V₂ = 9.0 ± 0.2 V.

State the total voltage of the combination and its absolute uncertainty.

A class is collecting data on how the pressure of a fixed gas sample depends on its volume, by pushing in a sealed syringe and reading a pressure gauge.

State one variable that must be controlled during the experiment.

Before plotting a sin(i) against sin(r) graph for a refraction experiment, a student needs to finish processing the data.

One row records the angle of refraction as r = 42.0°.

Calculate the value of sin r to be entered in that row, giving your answer to three significant figures.

A ray of light passes from air into a glass block.

A student uses a protractor with a resolution of 1° to measure the angle between the refracted ray and the normal.

State this measured angle and its absolute uncertainty if the refracted ray crosses the protractor scale at the 33° mark.

A liquid's density is found to be ρ = 1.03 ± 0.04 g cm⁻³.

State the density and its absolute uncertainty in kg m⁻³, with appropriate precision.

A marker sinks to a depth d under a water pressure P, and theory predicts d = k√P.

To linearize the data the student plots depth d (vertical) against root-pressure √P (horizontal).

One table row reads P = 16.0 kPa and d = 5.2 cm.

Determine the coordinates of the point this row should be plotted at on the graph.

In a double-slit experiment the fringe separation s is measured for several screen distances D.

Theory predicts s = (λ/a)·D, where λ is the wavelength and a is the slit separation.

The student plots s on the vertical axis and wants a straight line through the origin so the gradient gives λ.

State the quantity that should be plotted on the horizontal axis.

In a diffraction experiment the fringe separation is measured with the same absolute uncertainty (±0.1 mm) each time.

When a smaller slit separation is used, the fringe separation increases.

Discuss how the fractional uncertainty in the fringe separation changes as a result.

A rectangular metal bar has cross-sectional sides measured with a vernier caliper as a = 2.50 ± 0.05 mm and b = 4.00 ± 0.05 mm.

The cross-sectional area is A = a × b.

Calculate the percentage uncertainty in the cross-sectional area.

A student measures how the deflection of a light ball on a long thread changes when a horizontal electric force is applied.

They are told to make the thread much longer than the gap between the charged plates.

Explain why a much longer thread gives a more reliable measurement of the electric force.

A student times the swings of a pendulum with a stopwatch whose smallest division is 0.01 s, but their reaction time is much larger than this.

Describe how the student can obtain the period with an uncertainty far smaller than their reaction time would allow.

A nail is partly hammered into a block of wood.

A student rests a metre rule against the wood to measure the length of nail sticking out.

Describe two considerations when positioning and reading the rule so that the length is measured accurately.

A student measures the same deflection five times and records: 12.1, 11.8, 12.3, 12.0 and 12.3 (all in mm).

Determine the mean deflection and a reasonable estimate of its absolute uncertainty (take the uncertainty as half the range of the readings), and state both to an appropriate precision.

A student plots the count rate from a radioactive source against time and the points clearly follow a smooth curve that falls steeply at first and then levels off, rather than a straight line.

Outline why the relationship between count rate and time is unlikely to be linear.

A set of processed points with vertical error bars has been plotted, and a best-fit straight line has already been drawn.

On the same axes you are asked to draw the line of MAXIMUM gradient.

Describe how to draw this line correctly using the error bars.