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astarmathsandphysics:
Answers to second batch. Sorry it took long. Working on test centre.

ashwinkandel:
Capactiance

Capacitance:

Q1.
a. State one use of a capacitor in a simple electric circuit. [1]
b. The diagram shows the change in current with time when the capacitor is discharged.
i. Deduce the resistance R of the resistor.[2]
ii. Explain why the current decreases as the capacitor discharges. [2]
iii. The charge on the capacitor is equal to the area under the graph. Estimate the charge on the capacitor when the potential difference across it is 9.0 V. [2]
iv. Calculate the capacitance of the capacitor. [2]

Q2. The spherical dome on a Van de Graff generator is placed near an earthed metal plate. The dome has a diameter of 40 cm and the potential at its surface is 54 kV.
a
i.Calculate the charge on the dome.[2]
ii. Calculate the capacitance of the dome. [2]
The metal plate is moved slowly towards the dome and it discharges through the plate. The graph in the diagram shows how the potential at the surface of the sphere changes during the discharge.
b.Calculate the energy that is dissipated during the discharge.[4]
c.Suggest why the discharge ceases while there is still some charge on the dome.[2]

Q3.
a.Show that the capacitance C of an isolated conducting sphere is given by the formula:
C=4*pi*EpsilonNought*r
The diagram shows two identical conducting brass spheres of radius 10 cm mounted on insulating stands. Sphere A has a charge of +5.0*10^-8 C and sphere B is uncharged.
b
i.Calculate the potential at the surface of sphere A.[2]
ii. Calculate the energy stored on sphere A.[2]
Sphere B is brought up to sphere A and is touched on it so that the charge is shared between the two spheres, before being removed to its original position.
c
i.Calculate the energy stored on each sphere.[3]
ii.Suggest why there is a charge in the total energy of the system. [1]

Q4.
a. Define the term capacitance of a capacitor. [2]
b. The diagram shows a circuit which can be used to measure the capacitance of a capacitor.
The reed switch vibrates back and forth at a frequency of 50Hz. Each time it makes contact with A the capacitor is charged by the battery so that there is p.d of 12V accross it. Each time it makes contact with B it is fully discharged through the resistor.
i. Calculate the charge that is stored on the capacitor when there is a p.d of 12V accross it.[2]
ii.Calculate the current in the resistor. [2]
iii. Calculate the power in milliammeter. [3]
c. A second capacitor of the same value is connected in series with the first capacitor.Discuss the effect on both the current recorded and the power dissipated in the resistor. [4]

ashwinkandel:
Electromagnetic Induction

1
a. Explain what is meant by a magnetic flux linkage of 1Wb.[2]
b. The diagram shows how the magnetic flux density through a 240 turn coil with a cross-sectional area 1.2*10^-4 m^2 varies with time.
i.Determine the maximum rate of change of flux in the coil. [2]
ii. Determine the induced e.m.f in the coil. [2]
iii. Sketch a diagram to show the induced e.m.f. varies with time. Mark values on both the e.m.f and time axes. [2]

2
The diagram shows a square coil about to enter a region of uniform magnetic field of magnetic flux density 0.30T. The magnetic field is at right-angles to the plane of the coil.The coil has 150 turns and each side is 2.0 cm in length. The coil moves at a constant speed of 0.50 m/s
a
i. Calculate the time taken for the coil to enter completely the region of magnetic field. [1]
ii. Determine the magnetic flux linkage through the coil when it is all within the region of magnetic field. [2]
b. Explain why the induced e.m.f. is constant while the coil is entering the magnetic field. [1]
c. Use your answer in a to determine the induced e.m.f across the ends of the coil. [4]
d. What is induced e.m.f across the ends of the coil when it is completely within the magnetic field? explain your answer. [2]
e. Sketch the graph to show the variation of the induced e.m.f with the time from the instant that the coil enters the magnetic field. Your time axis should go from 0 to 0.08 s. [2]

3.
a. State Faraday's law of electromagnetic induction. [2]
b. A circular coil of diameter 200mm has 600 turns. It is placed with its plane perpendicular toa horizontal magnetic field of uniform flux density 50 mT. The coil is then rotated through 90 degree about a vertical axis in a time of 120 ms.
Calculate:
i. the magnetic flux passing through the coil through the coil before the rotation. [2]
ii. the change of magnetic flux linkage produced by the rotation. [2]
iii. the average e.m.f. induced in teh coil during the rotation. [2]

4.
a. State Len'z law and explain how you would use a coil and a magnet to demostrate the law. Make clear any other appratus that you use.[4]
b. A vehicle brake consists of aluminium disc attached to a car axle. Electromagnets cause an e.m.f to be induced in the disc.
i. Explain how the induction of an e.m.f causes the vehicle to slow down. [3]
ii. Explain why the braking effect increases when the speed of car increases. [2]

5. A bicycle wheel is mounted vertically on a metal axle in a horizontal magnetic field. Sliding connections are made to the metal edge of the wheel and to the metal axle.
a
i.Explain why an e.m.f. is induced when the wheel rotates.[2]
ii. state and explain two ways in which the e.m.f. can be increased. [2]
b. The wheel rotates five times per second and has a radius of 15 cm. The magnetic flux density may be assumed to be uniform and of value 5.0*10^-3 T.
Calculate:
i. the area swept out each second by one spoke. [2]
ii. the induced e.m.f between the contacts. [2]

astarmathsandphysics:
first batch on capacitance

astarmathsandphysics:
At last those magnetic field questions answered

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