Specific Struggles p3

Question 1

Figure 1 shows the apparatus used to investigate the vibration of a beam. A ruler is clamped to the bench with a length L projecting out from the bench. The upper end of the ruler tip undergoes a vertical deflection, s. The ruler is then made to perform small oscillations about its equilibrium position.
Describe a suitable procedure to determine an accurate value of $s$.

Answer 1

• Procedure to locate top edge of ruler tip e.g. set-square projecting from vertical ruler
• Procedure to locate horizontal from bench or upper surface of ruler on bench e.g. mark top edge of ruler on vertical scale / measure height of desk above floor then subtract distance to bottom of ruler.
• Procedure to ensure ruler is vertical e.g. set square located on floor / plumb line to provide reference vertical line.
• Procedure to minimise parallax error when measuring distances eg. vertical scale close to ruler / ensure eye is level with ruler tip and scale / secure graph paper behind ruler and close to ruler tip.

Question 2

Explain why multiple pendulum swings are counted, rather than just one. [1 mark]

Answer 2

Longer time measured, hence smaller percentage uncertainty.

Question 3

Suggest how the angle θ between the magnetic north and a compass needle can be measured accurately.

Answer 3

• Using paper / grid under the compass and mark ends of needle.
• Mark ends of needle, rule magnetic north line on bench when current is off.
• How marks are used to determine
• θ e.g. join marks, extend line and use protractor to measure θ.

Question 4

Explain why the resistance of a wire increases when a tensile stress is applied. [3 marks]

Answer 4

• Length increases.
• Cross-sectionial area / diameter / radius decreases.
• Link to resistivity equation.

Question 5

How to accurately measure a length of wire bounded by crocodile clips?

Answer 5

Always measure to the e.g. inside of the crocodile clips.

Question 6

Describe a disadvantage of using a thin test wire when measuring the current in a series circuit.

Answer 6

Thin wire gives smaller current (as small A means high R) hence larger uncertainty on current readings.

Question 7

Describe relevant procedures to limit the effect of random error in the result for a diameter of a rod. [2 marks]

Answer 7

ANY TWO FROM:
* repeat readings at different points along the rod and calculate a mean.
* repeat readings in different directions (perpendicular to the rod) and calculate a mean.
* reject / discard / ignore anomalous readings before calculating an average.

Question 8

How does one ensure a ruler is vertical?

Answer 8

Use a set square in contact with the bench/ground/floor