16-19 Sample test questions taken from a number of files

1. You are going to make a 50 W immersion heater that will run on a 12 V supply.
(a) what will be the current in the heater?
(b) what will be the resistance of the heater?
(c) what length of wire with a resistivity of 4x10^-8 W m and with a cross sectional area 2x10^-8 m² is needed to make a heater?

2. The potential divider shown in the diagram is set with the sliding contact at a point one third of the way from the negative end of the 12W resistor.
(a) What is the output voltage between A and B?
(b) What is the output voltage if a 4W resistor is connected across A and B?







3. Answer any FOUR of the following, giving full explanations in your answers:
(a) Comment on the advantages and disadvantages of the use energy from the wind
(b) Explain with the help of a graph what happens to the resistance of a light bulb filament as it gets hotter
(c) Estimate the mass and the volume of a normal sized empty china coffee mug
(d) Why is it less painful to put out a match flame, temperature 800 ^oC, than to put your hand into a bucket of water at 70 ^oC?
(e) You cannot jump up without bending your knees. Why?
(f) A firework rocket with a mass at launch of 500 grams and a constant thrust of 4 N. Describe carefully how the rocket moves when the fuel is first ignited and calculate the acceleration when its mass is 200 grams

4. The following results were obtained in an experiment to measure the loss of activity from a radioactive source that emits alpha particles:


(a) plot a graph of the activity (y axis) against time (x axis)
(b) use your graph to find the half life of the source
(c) use your graph to work out the background count
(d) predict the count rate at t = 90 s assuming a constant background count
(e) explain whether a sample of this material would be suitable for use in a domestic smoke detector.

5. Calculate the electric field intensity at the following places
(a) half way between two parallel plates separated by 0.8 cm with potential difference of 5000 V between them
(b) using the same figures as (a) but at a point 0.2 cm from the positive plate
(c) 0.30 m from a charge of 2.5x10^-6 C
(d) 0.1 nm from an electron
(e) between a thunder cloud and the ground if the potential difference between the cloud and the ground is 1500 MV and the cloud is 2.5 km from the ground

6. A lady lifts a shopping bag whose handles have a breaking force of 180N. If the mass of the bag is 12 kg what is the maximum possible vertical acceleration of the bag before the handles break?

7. Projectiles
A stone is thrown horizontally from a cliff at 25 ms^{- 1}.
(a) what is its horizontal velocity after 3 s?
(b) what is its vertical velocity after 3 s?
(c) what is its velocity (magnitude and direction) after 5s?
(d) if the cliff is 50 m high how long will it take for the stone to hit the ground?

8. Radiation with a wavelength of 450 nm falls on a metal surface that has a work function of 2.0 eV.
(a) what is the energy of a quantum of the incident radiation?
(b) what is the threshold frequency for the surface?
(c) what is the kinetic energy of the emitted electrons?
(d) what is the velocity of the emitted electrons?
(e) what is the stopping potential for the surface for this experiment?

9. Give approximate sizes of:
a) the diameter of a nucleus
b) the diameter of an atom
c) the diameter of a near spherical molecule

10. Calculate the activity of a radioactive source containing 4x10¹⁵ nuclei and with a decay constant of 3.0x10^-8 s^-1. Give your answer in Becquerel.