Download Concept Questions with Answers

W03D2
Work, Potential Energy
and Electric Potential
Concept Questions with
Answers
8.02
W03D2
2
Concept Question: Sign of W
Concept Question Ans.: Sign of W
Suppose a fixed positively
charged object (charge qs > 0) is
at the origin and we move a
negatively charged object (charge
q1 < 0) from A to B with rA < rB ,
where r is the distance from the
origin. The work done on the
negatively charged object is
1. 
2. 
3. 
4. 
Answer 3: W is negative and we do a positive amount
of work
W is the work done by the electrical force. This is the
opposite of the work that we must do in order to move
a charged object in an electric field due to source.
The electrical force is attractive and we are moving
the positively charged object away from the source
(opposite the direction of the electric field).
is positive and we do a positive amount of work
is positive and the electrical force does a positive amount
of work
is negative and we do a positive amount of work
is negative and electrical force does a positive amount of
work
3
4
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Concept Question: Motion of Charged Objects
Two oppositely charged are released from rest in an electric
field.
1. 
2. 
3. 
4. 
Concept Q. Ans.: Motion of Charged Objects
2. Both charged objects will move from higher to lower
potential energy so that
Both charged objects will move from lower to higher
potential energy.
Both charged objects will move from higher to lower
potential energy.
The positively charged object will move from higher
to lower potential energy; the negatively charged
object will move from lower to higher potential
energy.
The positively charged object will move from higher
to lower potential energy; the negatively charged
object will move from lower to higher potential
energy.
ΔU < 0
5
Concept Question: Motion of Charged Objects
Two oppositely charged are released from rest in an electric
field.
1. 
2. 
3. 
4. 
6
Concept Q. Ans.: Motion of Charged Objects
Two oppositely charged are released from rest in an electric
field.
Both charged objects will move from lower to higher
electric potential.
Both charged objects will move from higher to lower
electric potential.
The positively charged object will move from higher
to lower electric potential; the negatively charged
object will move from lower to higher electric
potential.
The positively charged object will move from higher
to lower electric potential; the negatively charged
object will move from lower to higher electric
potential.
3. The positively charged object will move from
higher to lower electric potential; the negatively
charged object will move from lower to higher electric
potential.
For the positively charged object:
ΔV < 0 ⇒ ΔU = qΔV < 0
For the negatively charged object:
ΔV > 0 ⇒ ΔU = qΔV < 0
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Concept Question: Two Point
Charges
Concept Question Answer: Two
Point Charges
The work done in moving a
positively charged object that
starts from rest at infinity and
ends at rest at the point P
midway between two charges
of magnitude +Q and –Q
1. 
2. 
3. 
4. 
3. Work from ∞ to P is
zero.
is positive.
is negative.
is zero.
can not be determined – not enough info is given.
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The potential at ∞ is zero.
The potential at P is zero because equal and
opposite potentials are superimposed from the
two point charges (remember: V is a scalar,
not a vector)
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3