Practical exam

Question 1

Investigation of the force-extension relationship for rubber instructions?

Answer 1

-Hang a (cut) rubber band of (approximate) cross-section 1 mm by 2 mm vertically from a stand, boss and clamp. The base of the stand should be secured using a G-clamp.
-Hang a 50 gram mass holder from the band. Place a metre rule as close as possible to the mass holder.
-The length can be read using an optical pin attached to the base of the mass holder.
-Measure the length, width and thickness of the rubber when it is supporting the 50 gram holder.
-Increase the mass in 50 gram steps, measuring the extension each time.
-Continue until the band breaks.
-Plot the force - extension curve and determine the Young modulus from the linear section. (gradient x l/a) = Youngs modulus

Question 2

Risk assessment for force-extension relationship for rubber?

Answer 2

Hazard- breaking wire
Risk- Physical injury if the wire breaks
Control - wear eye protection and should provide safe landing for the load should sample break

Question 3

At the steeper gradient top side of a force extension graph for rubber why is the gradient steeper?

Answer 3

molecules fully stretched

Question 4

Features of force extension graph for elastic material like rubber?

Answer 4

-Hooke’s law only approximately obeyed
-low Young modulus and the extension due to straightening of chain molecules against thermal opposition

Question 5

Why is the force extension graph for rubber the shape it is?

Answer 5

-the polymeric structure of rubber
- Rubber consists of long chains of carbon atoms which are tangled around each other.
-Untangling requires a large force (because of cross-links), and so the gradient is steep to begin with.
-When further stress is applied, these chains begin to straighten.
- The rotation of the bonds between the atoms does not require much force, so a large extension occurs with a small force.
-Once they are straightened and continue to extend the rubber, the bonds must stretch. This requires more force, resulting in the increase in gradient of the graph.

Question 6

Investigation of the I-V characteristics of a filament of a lamp and a metal wire at constant temperature instructions?

Answer 6

-Starting with the output of the variable d.c. voltage supply set to its minimum value, slowly
-increase the value of the applied voltage. The current through the component and the potential difference across the component should be recorded for a range of values of the
applied voltage
- A graph of current against voltage should then be plotted.
(voltmeter in parallel to metal wire and ammeter in series)

Question 7

What does graph show for I-V graphs of filament of a lamp and a metal wire?

Answer 7

• directly proportional if resistance is constant therefore obeys Ohm’s law

Question 8

Risk assessment for I-V characteristics of a filament of a lamp and a metal wire at constant temperature?

Answer 8

Hazard- electrical
Heat - wires can get hot
Control measure- Students should wear eye protection and should only connect the power when taking measurements.