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Test 3
PHY101
Name: _____________________________
Date: ______________
Total points: 50
Show detailed calculations to receive full points. Use proper units. If your work is not legible it
will NOT be graded.
1. Electrostatics
A. Particles of charges + 1C, 2C and -3C are placed in a line. The center one is 1 m from each of
the others.
+1 C
+2 C
1m
-3 C
X-axis
1m
I) (2 points) Calculate the force (magnitude + direction) of the +1C on the central charge (+2 C).
II) (2 points) Calculate the force (magnitude + direction) of the -3C on the central charge (+2 C).
III) (3 points) Calculate the net force (magnitude + direction) on the central charge (+2 C) due to the
other two.
B. (1 point) State one difference between electric forces and gravitational forces.
C.
(3 point) Define electric field. What is its unit?
Dr. Soma Dey
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Test 3
D. (2 points) Draw the electric field due to i) a positive charge and ii) a dipole. (Remember electric
field is a vector).
E. (3 points) How much kinetic energy will an electron gain if it accelerates through a potential
difference of 23,000 V in a TV picture tube. (charge on a electron = 1.6 x 10-19 C)
2. Electric Current
A. (1 point) State Ohm’s law.
B.
(1 point) Does current cause voltage or voltage cause current?
C. Three 1 ohm resistors are connected in series to a 6 V battery.
I) (1 point)) Draw a neatly labeled circuit diagram. (Show the direction of current flow.)
II) (1 point) Find the total resistance of the circuit.
III) (2 point) Calculate the current flowing through each resistor.
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IV) (2 point) Calculate the potential drop across each resistor.
D. Three 1 ohm resistors are connected in parallel to a 6 V battery.
i) (1 point) Draw a neatly labeled circuit diagram. (Show the direction of current flow.)
ii) (2 point) Find the total resistance of the circuit.
iii) (2 point) Calculate the current flowing through each resistor.
iv) (1 point) What is the potential drop across each resistor.
Dr. Soma Dey
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PHY101
E.
Test 3
(5 points) A car’s headlights dissipate 40 W on low beam and 50 W on high beam. If it is
connected to a 12 V battery, which has more resistance. (Do not just state, show calculation to get
points!!!) If it costs $ 0.15 per kWh, how much does it cost to run the headlights in low beam for
6 hours?
3. Magnetism:
A. (1 point) In what way are magnetic poles different from electric poles?
B.
(1 point) What is the direction of the magnetic field surrounding a current carrying wire?
C. (2 point) In what direction relative to a magnetic field does a charged particle move in order to
experience maximum deflecting force? Minimum deflecting force?
D. (1 point) What is a magnetic domain?
Dr. Soma Dey
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PHY101
Test 3
4. Electromagnetic induction:
A. (1 point) What is electromagnetic induction?
B.
(2 point) State Faraday’s law?
C.
(3 point) What are the 3 factors on which the induced voltage depends on during
electromagnetic induction?
D. (5 point) A transformer has 320 turns in the primary coil and 120 in the secondary coil. i) What
kind of transformer is this? ii) By what factor does it change the voltage? iii) By what factor does
it change the current? (For an ideal transformer power input is equal to power output.)
Formulae sheet:
F = k(Q1Q2)/d2
K = 9 x 10 9 N.m2/C2
Electric field = F/Q
Electric potential = electric potential energy /
charge
In series: R = R1 + R2 + R3 …
Dr. Soma Dey
In parallel: 1/R = 1/R1 + 1/R2 + 1/R3 …
Current = Voltage / resistance
Power = current X voltage
(Primary voltage/# of turns ) = (secondary voltage/
# of turns)
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