QUESTIONS A2 November 2011 40 marks

A defibrillator is a machine that is used to correct irregular heartbeats by passing a large current through the heart for a short time. The machine uses a 6000 V supply to charge a capacitor of capacitance 20 μF. The capacitor is then discharged through the metal electrodes (defibrillator paddles) which have been placed on the chest of the patient.

Calculate the charge on the capacitor plates when charged to 6000 V. (2)

Calculate the energy stored in the capacitor. (2)

When the capacitor is discharged, there is an initial current of 40 A through the patient. Calculate the electrical resistance of the body tissue between the metal electrodes of the paddles.

(1)

Assuming a constant discharge rate of 40 A, calculate how long it would take to discharge the capacitor. (2)

In practice the time for discharge is longer than this calculated time. Suggest a reason for this, supported by a graph.

(2)

A 2.0 μF capacitor is charged to a potential difference (p.d.) of 50 V and a 3.0 μF capacitor is charged to a p.d. of 100 V.Calculate the charge on the plates of each capacitor and the energy stored by each.

(4)

The capacitors are then joined together in parallel with their positive plates connected together.

How has the charge distribution on each capacitor changed after they have been connected together? (2)

Suggest why there is a loss of stored energy when the capacitors are connected. (2)

The diagram is of a simplified cathode ray tube.

An electron beam is produced by the electron gun in the tube. A beam of electrons emerges from the hole in the anode and strikes the screen at point P. Explain why the electrons move from the filament towards the anode.

[2]

The potential difference between the filament and the anode is 10 kV. Calculate the energy in joules of an electron emerging from the hole in the anode.

[2]

The electron beam forms a current of 1.5 mA. Show that the number of electrons passing through the hole in the anode is about 9 x 1015 per second.

[2]

Hence calculate the rate at which energy is being delivered to the screen by the beam of electrons.

[2]

A uniform magnetic field is applied over the tube region from the anode to the screen, directed out of the page. Explain what would happen to the electron beam.

[2]The electrons are emitted from the anode at a constant speed, yet an observer looking at the screen sees a vertical line as if the beam appears to have fanned out. Explain why.

[2]The diagram shows the magnetic field due to a current in a long straight wire L, in a plane perpendicular to the wire.State the direction of the current in L.

[1] What feature of the magnetic field shown in the diagram indicates that there are no other magnetic fields close to the wire?

[1]

A second wire carrying a current of the same size is placed parallel and near to L. The magnetic field along a line joining the wires is investigated and it is found that at a certain distance from L, no magnetic field can be detected. Explain this observation.

[3]

Palaeontologists can find out a lot about dinosaur behaviour by studying their fossilised footprints.The tracks below show the path of a Tyrannosaurus - T rex as it attacks a stationary Triceratops

The time between footprints is 0.62s. Show that the maximum speed of the T rex is about 10ms-1

[2]

The mass of the T Rex is 7000kg and the Triceratops is 5000kg. Calculate the speed of their bodies after collision, assuming the T rex does not let go of the Triceratops after impact.

[2]State the law you have assumed.

[2]Is the collision elastic or inelastic? Show by calculation how you arrive at your answer.

[3]