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Static Electricity and Capacitance
2011 Question 9 (a) (i) State Coulomb’s law. (ii) Two identical spherical conductors on insulated stands are placed a certain distance apart. One conductor is given a charge Q while the other conductor is given a charge 3 Q and they experience a force of repulsion F. The two conductors are then touched off each other and returned to their original positions. What is the new force, in terms of F , between the spherical conductors?
(b) (i) Draw a labelled diagram of an electroscope. (ii) Why should the frame of an electroscope be earthed? (iii)Describe how to charge an electroscope by induction.
(c) (i) How does a full-body metal-foil suit protect an operator when working on high voltage power lines? (ii) Describe an experiment to investigate the principle by which the operator is protected.
2010 Question 12 (d) (i) Define electric field strength and give its unit of measurement. (ii) Copy the diagram into your answerbook and show on it the direction of the electric field at point P.
(iii)Calculate the electric field strength at P. (iv) Under what circumstances will point discharge occur? (permittivity of free space = 8.9 × 10–12 F m–1)
2009 Question 9 (i) Define potential difference. (ii) Define capacitance. (iii) A capacitor stores energy. Describe an experiment to demonstrate that a capacitor stores energy.
The ability of a capacitor to store energy is the basis of a defibrillator. During a heart attack the chambers of the heart fail to pump blood because their muscle fibres contract and relax
randomly. To save the victim, the heart muscle must be shocked to re-establish its normal rhythm. A defibrillator is used to shock the heart muscle. A 64 μF capacitor in a defibrillator is charged to a potential difference of 2500 V. The capacitor is discharged through electrodes attached to the chest of a heart attack victim. (iv) Calculate the charge stored on each plate of the capacitor. (v) Calculate the energy stored in the capacitor. (vi) Calculate the average current that flows through the victim when the capacitor discharges in a time of 10 ms. (vii) Calculate the average power generated as the capacitor discharges.
2004 Question 8 (i) Define potential difference. (ii) Define capacitance. (iii)Describe an experiment to demonstrate that a capacitor can store energy. (iv) The circuit diagram shows a 50 μF capacitor connected in series with a 47 kΩ resistor, a 6 V battery and a switch. When the switch is closed the capacitor starts to charge and the current flowing at a particular instant in the circuit is 80 μA. Calculate the potential difference across the resistor and hence the potential difference across the capacitor when the current is 80 μA. (v) Calculate the charge on the capacitor at this instant. (vi) Calculate the energy stored in the capacitor when it is fully charged. (vii) Describe what happens in the circuit when the 6 V d.c. supply is replaced with a 6 V a.c. supply.
2003 Question 12 (c) (i) State Coulomb’s law of force between electric charges. (ii) Define electric field strength and give its unit. (iii)How would you demonstrate an electric field pattern? (iv) The diagram shows a negative charge – Q at a point X. Copy the diagram and show on it the direction of the electric field strength at Y.
2002 Question 11 Read the following passage and answer the accompanying questions. Benjamin Franklin designed the lightning conductor. This is a thick copper strip running up the outside of a tall building. The upper end of the strip terminates in one or more sharp spikes above the highest point of the building. The lower end is connected to a metal plate buried in moist earth. The lightning conductor protects a building from being damaged by lightning in a number of ways. During a thunderstorm, the value of the electric field strength in the air can be very high near a pointed lightning conductor. If the value is high enough, ions, which are drawn towards the conductor, will receive such large accelerations that, by collision with air molecules, they will produce vast additional numbers of ions. Therefore the air is made much more conducting and this facilitates a flow of current between the air and the ground. Thus, charged clouds become neutralised and lightning strikes are prevented. Alternatively, in the event of the cloud suddenly discharging, the lightning strike will be conducted through the copper strip, thus protecting the building from possible catastrophic consequences. Raised umbrellas and golf clubs are not to be recommended during thunderstorms for obvious reasons. On high voltage electrical equipment, pointed or roughly-cut surfaces should be avoided. (Adapted from “Physics – a teacher’s handbook”, Dept. of Education and Science.)
(a) Why is a lightning conductor made of copper?
