Download Optional Extra Credit Exercise

Optional Extra Credit Exercise
1, True/False questions: State, in complete sentences, why your answer is wrong.
2, Multiple choice concept questions: Determine the correct answer and state, in
complete sentences, why your answer was wrong and why the correct answer is correct.
3, Multiple choice problem questions: Determine the correct answer by working the
problem correctly. Show your work. No credit will be given for answers not supported
by your work.
Test Questions
2, Which of the following is not true about the electric potential, V?
a, V at a point is the electric potential energy per unit charge at that point.
b, We are normally only interested in difference in potential.
c, V is a vector so it s direction must be considered.
d, The units of V may be expressed as Joule/Coulomb.
My answer was D and the other students got C.
Discussion: The electric potential at a point in an electric field is some times defined
as the energy required to bring unit test charge from infinity (out of the field
practically) to the point, and hence the electric potential is the energy per unit
charge at that point.
As the energy is a scalar quantity, the potential is also a scalar quantity and the
direction cannot be assigned with it.
As the potential is zero at infinite distance from the charged objects, it is not possible
to measure absolute potential at a point in the field, hence we only measure
difference of potential between two points in the field and this multiplied with the
charge, gives the electrostatic potential energy difference of the charge at the two
points, like the gravitational potential energy difference is mgh.
As the potential is energy per unit charge (V = U/Q) the units will be unit of energy
divided by the unit of charge, which gives the unit Joule per Coulomb of
joule/Coulomb. This unit is called volts as well.
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4, If you bring a negatively charged insulator near two metallic spheres that are in contact
as shown and then separate the spheres (with an insulator), the sphere on the right will
have:
A. no net charge.
B. a positive charge.
C. a negative charge
D. either a positive or negative charge.
E. none of the above
My answer was C the other students got B.
You have not shown the figure. It will depend on which side you put the charged
insulator. I guess the charged is placed on the right.
This is the phenomenon of electrostatic induction. The negative charge of the
charged insulator repels negative free electrons and
as the two conducting spheres are in contact, some
+
----of them will transfer to the sphere on the other
+
----side. Resulting a negative charge on the sphere on
+
the other side and the lack on electron on the
nearer sphere makes it positively charged.
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5. Three charges +q, +Q, and –Q are placed at the corners of an equilateral triangle. The
net force on charge +q due to the other charges (the magnitudes of +Q and –Q are the
same is.
A. vertically up
B. vertically down
C. zero
D.horizontal to the right
E. horizontal to the left.
My answer was A the other students got D.
F+
Again the figure is not there.
R
As the distance of +q is same from the other two charges,
the magnitude of the electric force on it due the two will be
the same but the direction is along the line joining. Due to
+Q it is repulsive and due to –Q it is attractive.
The resultant of the two forces will bisect the angle between
them.
F-
+Q
-Q
-------------------------7. If non-electric forces are negligible, the electric potential energy of a charged particle
released from rest in an electric field.
A. increases
B. decreases
C. is conserved
D. increases or decreases depending on the sign of the charge
My answer was C and the other students got B.
When the charge is released in an electric field it is accelerated due to the electric
force (may be +ve or –ve). The direction will be different but the magnitude of
velocity increases. Thus its kinetic energy will increase. This increase in kinetic
energy is due to reduction in its electrostatic potential energy. Hence the potential
energy will decrease.
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9. A point charge q sets up an electric field E at the position of another charge q; the
force exerted by E on q’ is independent of the magnitude of q’
My answer was True the other students marked False.
No, the field is the force experienced per unit charge and hence cannot be
independent of the charge q’. the force is given by
F = Eq’
Moreover Coulomb’s law gives the force between the two charges as
F = Kqq’/4r2. which depends on q’.
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15. If the resistance in a constant voltage circuit is doubled, the power dissipated by
that circuit will
A. double
B. increase by a factor of four
C. decrease by one-half
D. decrease by a factor of four
My answer was B the other students got C.
The power dissipated by the circuit is given by
P = IV = I2R = V2/R
The voltage is constant so we should use the third formula and hence if
the resistance is doubled the power dissipated is one halfed.
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16. When two or more resistors ( of different resistances) are connected in series with
a battery.
A. the voltage drop across each resistor is the same
B. the current through each resistor is the same
C. the power dissipated by each resistor is the same
D. all of the above
E. none of the above
My answer was D the other students got B.
Reading this question again out of a stressful environment, I know the answer
was B.
[Don’t get stressed during exams. Physics is the easiest subject in the world of
knowledge. Once you understand basic concepts, nothing is to be crammed or
remembered. Everything can be derived in a systematic way. I am having a very
little memory even then I am managing this subject while in chemistry I was the
most duffer in my class.]
In electric circuit the charge is not getting accumulated anywhere and hence the
current will be the same in all component if they are in series.
21. A battery is connected to a series combination of a switch, a resistor, and an
initially uncharged capacitor. The switch is closed at t=0. As the charge on the
capacitor increases.
A. the current increases
B. the electric field inside the capacitor decreases
C. the current remains constant
D. the voltage drop across the capacitor decreases
E. the voltage drop across the resistor decreases
My answer was C the other students got E.
As the switch is closed there is potential difference across the resistance and the
capacitor. As there is no charge on the capacitor the potential drop across it is
zero and the whole potential is dropped across the resistor. This flows a large
current (V/R) in the resistance, which charges the capacitor. As the charge on
jthe capacitor increases the potential drop across it increases and decreases
across the resistance. Resulting a decrease in the current through the circuit. As
the charge in the capacitor increases the electric field in it increases.
Hence the only correct answer is E.
22. The capacitance of a charged capacitor is directly proportional to the charge.
My answer was True the other students got False.
No, the capacitance of a capacitor depends on its construction. On the size and
the gap. It does not depend on the charge given to it. This is evident by the
formula of the capacitance of a parallel plate capacitor, which is
C=
0 A
d
Where A is the area of each plate and d is the distance between them.
----------------------------
23. A 200-Micro-Farad capacitor is charged to 200V and is then discharged across a
100ohm resistor. The time constant for this circuit is approximately
a. 20ms
b. 40ms
c. 10ms
d. 30ms
e. 15m
My answer was B the other students got A.
The time constant of a CR circuit is given by
  CR  200 *10 6 *100  2 *10 2  20 *10 3 Seconds = 20 milliseconds.
24. A charged parallel-plate capacitor is disconnected from a battery, and the plates
are pulled a small distance further apart. For the following quantities, indicate
where they increase, decrease, or stay the same.
A. Q stay the same
B. E Between the plates stays the same ( This one was wrong)
C. Energy stored in the capacitor increase
A. Yes as the capacitor is disconnected the current becomes zero and the
charge on the capacitor plates remains the same.
B. As the fringing effects are negligible initially, when the distance is very
small, as the charge Q on the plates remains the flux per unit area
Q
remains the same and hence the field between the plates remains
0 A
the same.
When the gap between the plates increases to large values such that
the fringing effects cannot be neglected then the field between the
plates decreases.
C. As the opposite charges on the plates of the capacitor attracts each
other, we have to do work against the electric forces between them,
this increases the energy of the capacitor.
The energy of the capacitor is given by (1/2) CV 2 and the increasing
the separation between the plates decreases the capacitance, the
energy of the capacitor decreases.
25. A current reading is obtained by properly placing an ammeter ( a real one, not an
ideal one) in a circuit consisting of one resistor and a battery. As a result,
A. the voltage drop across the resistor increases
B. the current flowing in the circuit increases.
C. The current flowing in the circuit decreases
D. The current flowing in the circuit does not change
My answer was D the other students got C.
As a real ammeter is having some small resistance and is connected in series, the
total resistance of the circuit increases which decreases the current in the circuit
and decreases voltage drop (RI) across the resistance.
28. An ideal voltmeter has a very large internal resistance.
My answer was False the other students put True.
Yes, the ideal voltmeter is having infinitely large resistance. Because of this only
when it is connected in parallel of some component in the circuit to measure P.D.
the current and the voltage across the component does not change.
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