The lines show the equipotential contours in the plane of three point charges, Q
ID: 1656468 • Letter: T
Question
The lines show the equipotential contours in the plane of three point charges, Q1, Q2, and Q3. The values of the potentials are in kV as indicated for the +5, 0, and -5 kV contours. The positions of the charges are indicated by the dots. Decide which of the following statement about electric field lines are true, and which are false.
A) The electric field at k is zero.
B) Charge Q3 has the smallest magnitude of all.
C) The force on an electron at g points to the top of the page.
D) Q2 is a negative charge.
E) Charge Q1 is the largest negative charge.
F) The electric field at j is stronger than at i.
Calculate the external work required to move a negative charge, q= -7e (where e is, as usual, the charge of a proton) from `a' to `b' (as shown in the figure of the previous problem) at constant speed.
(Note: The equipotential lines shown are separated by 1 kV, not 1 V!)
Explanation / Answer
From the contours of equipotential lines we can say that Q1, Q2 are -vely charged and Q3 is +vely charged
a. Now, Since Q1, Q2 are -ve charged and Q3 is +ve, electric field at K cannot be 0, as fields will add up due to three charges at k
Hence False
b. As the equipotential lines are most spaced around charge Q3, that means electric field is weakest around it, and hence it is the smallest of all in magnitude
Hence True
c. an electron will move from lower potential to higher potential , so forc eon electron at g points downwards and not upwards
hence false
d. Q2 is -ve charge as potential around it is -ve
hence true
e. charge Q1 is -ve but Q2 is greater in magnitude than Q1 as equipotential lines around Q2 are more tightly packed
hence false
f. at i equipotential lines are closer than that at j, so field is stronger at i than j
hence false
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