| a. |
Define
potential V at a point in an electric field. Sketch a graph to show the
variation of potential V with distance r along a radius from the center
of a positively charged isolated conducting sphere to infinity. |
4
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Electric potential
V at a point is the work done per coulomb required to bring a positive
charge from infinity to that point. |
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| b. |
Describe
and explain how the potential of a positively charged conducting sphere
A is affected by the following nearby objects, separately:
i)
an earthed conductor B |
6
marks |
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Since B is earthed, the potential at B is always zero.
Negative charge is induced on B. Thus, the potential of A is less than
before. |
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ii)
an uncharged conductor C |
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The negative charges induced on C are more
close to A than the positive charges. Thus, the potential of A is less
than before. |
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iii)
a positively charged conductor D.
In each case, sketch
a graph to show the variation of potential between A and the object with
distance r from the center of A. |
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Since D carries positive charge, the potential
at A is higher than before (the algebraic sum of its own potential and
the potential due to D). |
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| c. |
Explain
why a conductor carrying a flame would not affect the potential of a nearby
charged object. Hence, suggest a possible experimental method for investigating
the variation in potential with distance outside a charged sphere. State
the result you would expect from the experiment and discuss how you would
analyze the data obtained. |
6
marks |
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A flame consists of both positive and negative
ions which would neutralize any excessive charge found on the conductor.
Even the flame-carrying conductor is placed near the charged object, there
is no influence on the potential of the object. |
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A flame probe is used. It consists of a needle
that carries a flame and links to a gold leaf electroscope. The deflection
of the electroscope indicates the potential of the point where the flame
probe is placed. |
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In this experiment, as the flame probe is moved
away from the sphere, the gold leaf is expected to fall gradually. |
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For an isolated sphere, the potential is |
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When a graph of V against 1/r is plotted, the
result should be a straight line. |
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