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Anderson Junior College
2013 JC2 H2 Physics PbI Worksheet
Topic: Electric Fields
1
Draw field lines around
(a) 1 positive charge
(b) 1 negative charge
(c) 2 positive charges
(of equal charges)
2 positive charges
(of unequal charges)
(d) 1 positive and 1negative charge
(of equal charges)
1 positive and 1negative charge
(of unequal charges)
Instructions:
Use the java applet to check your answers to question 1.
http://surendranath.tripod.com/Applets/Electricity/FieldLines/EFL.html
Thinking Question 1
The figure shows a negatively charged pear-shaped conductor.
1. Draw the distribution of the negative charges on the surface of the conductor.
2. Draw the electric field lines.
2
In the diagram below, two charges A and B are separated by a distance 0.72 nm. A has a
charge of +2 μC and B has a charge of -1 μC. Draw labeled arrows on Fig. 2 to represent
(a) The field EA at point X due to charge A only
(b) The field EB at point X due to charge B only
(c) The resultant field Eresultant at X due to both charges.
X
A
B
Fig. 2
(d) Sketch on Fig. 2 field lines representing the electric field caused by electrical charges A
and B. Include the field line passing through point X.
(e) Based on your answers in 2(c) and (d), comment on the direction of resultant field
Eresultant at X with respect to the field lines.
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(f) Based on the field pattern drawn in 2(d), identify regions of strong / weak / zero electric
fields. Explain your answers.
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Thinking Question 2
Rank in order, from largest to smallest, noting any ties, the electric field strengths E1 to
E4 at points 1 to 4 in the figure below.
3
Two positive charges +q and +Q (with Q > q) are held in place a distance s apart.
(a) Indicate the direction of the electric force exerted on each charge by the other.
(b) Is the force on the +q charge by the +Q charge greater than, less than or equal to the
force on the +Q charge by the +q charge? Explain.
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(c) By what factor would the magnitude of the electric force on the +q charge change if the
charges were instead separated by a distance 2s?
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4
Two more +Q charges are held in place the same distance s away from the +q charge as
shown. Consider the following student dialogue concerning the net force on the +q charge:
Student 1: “The net electric force on the +q charge is now three times as large as before,
since there are now three positive charges exerting forces on it.”
Student 2: “I don’t think so. The force from the +Q charge on the left will cancel the force
from the +Q charge on the right. The net electric force will be the same as in question 3.”
Do you agree with either student? Explain.
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5
Rank the four cases below according to the magnitude of the net electric force on the +q
charge. Explain how you determined your ranking.
6
A pith ball has a charge of the same sign as the rod.
Sketch vectors at each of the marked points to represent the electric force exerted on the
ball by the rod at the locations.
Side view
Top view
×
A
×
B
(a) Suppose that the charge +q on the pith ball were halved.
(i)
Would the electric force exerted on the ball at each location change? If so, how? If
not, explain why not?
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(ii)
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Would the ratio F/q change? If so, how? If not, explain why not?
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(b) Sketch vectors at each of the marked points to represent the electric field strength E at
that location.
Would the magnitude or the direction of the electric field strength at point A change if:
(i)
the charge on the rod were increased? Explain.
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(ii)
the charge on the pith ball were increased? Explain.
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(iii)
the sign of the charge on the pith ball were changed? Explain.
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Note: The charge on the pith ball functions as a test charge.