EXAM 2
NAME____________________________
IUPUI Physics 219
Part I – Multiple Choice Questions – 2 points each
Directions: Circle for the one alternative that best completes the statement or answers the question. Unless
otherwise stated, assume ideal conditions (no air resistance, uniform gravity, etc.)
1) A charge is initially at rest next to a wire which has current running through it. In what direction
will the charge accelerate?
(A) towards the wire
(B) away from the wire
(C) nowhere F=qvB so if v=0…
(D) perpendicularly away from the wire
(E) parallel to the wire
2) A proton is shot down a solenoid with current going counterclockwise.
What is the direction of the magnetic field inside the solenoid?
(A) directed 
(B) directed 
(C) directed 
(D) directed 
RHR 2b
(E) equal to zero.
3) A proton is shot down a solenoid with current going counterclockwise.
What is the direction of the force on the proton (hint what is the direction of the
magnetic field?).
(A) no force the velocity is in the same direction as the B field, so no force
(C) 
(D) 
(E) 
(B) 
4) Which of the following is NOT a way to generate electricity in a circular loop of wire exposed to a
magnetic field?
(A) vary the magnitude of the field
(C) vary the resistance of the loop
(B) vary the diameter of the loop
(D) vary the orientation of the loop in the field
5) This question requires a 1 sentence answer instead of being a multiple choice question. How is
most of the electricity in the United States produced (focus on the electricity and not the energy that
becomes the energy)?
We vary the orientation of the loop in the field – that is we rotate a wire inside a magnetic field.
6) An ideal step-up transformer has MORE
(A) voltage across its primary coil.
(B) current through its secondary coil.
(E) power in its primary coil.
(C) turns in its secondary coil.
(D) power transferred to its secondary coil.
7) An ac voltage source is connected across an ideal inductor. The phase angle between the voltage
and the current is
(A) +180
(B) 45
(C) 0
(D) 45
(E) 90
8) A 100-Vmax source is connected to a load. The rms voltage applied to the load is
(A) 70.7 V
(B) 50 V
(C) 100 V
(D) 141 V
(E) 200 V
9) Resonance occurs in an RLC series ac circuit whenever
(A) the phase angle equals +90.
(C) the resistance equals zero.
(B) the rms current equals zero.
(D) the total impedance equals the resistance.
(E) the rms voltage equals zero.
10) A proton is traveling in a magnetic field with a velocity which is 45 degrees from perpendicular to
the magnetic field. The path of the proton will be a:
(A) ellipse
(B) no motion
(C) straight line
(D) spiral
(E) circle
Part II – Word Problems – 20 points each
Directions: Solve each problem using the formulas listed on the last page. Show your work to receive full
credit. Box your final answer and record its appropriate unit.
1)
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B
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Some stoves heat pans through induction. To use these stoves it is best to use a specially made iron
pan. On the bottom of the pan are ridges which act like circular wires. For simplicity lets assume that
the average wire has a radius of 0.1 m sits in an oscillating magnetic field which goes from 0.0-T to
3.0-T in 4*10-3 seconds. There are 20 of these wires. The field is directed perpendicular to the bottom
of the pan. The pan has an effective resistance of 170 Ohms. Determine:
(A) the magnitude of the induced emf for the entire pan (that is all the wires combined).
(B) the magnitude of the induced current for the entire pan (all the wires together)
(C) the direction of the induced current when the magnetic field is falling.
(D) the magnitude of the power that the induced current generates
(E) Note that a conventional stove supplies 200 W of power to the food but consumes 500 W. The
induction stove supplies 800 W of power to the food while an induction stove will consume 1200 W
while supplying the food with the power you found in D. Which stove cooks faster and which does a
better job of applying its energy to its food?
A) E   N
ΔΦB
= - 20 * (Φf – Φi) / 4*10-3 sec
Δt
Φf = BAcos(theta) = 3T * pi * (0.1m * 0.1m) * 1 = 0.094 Wb
Φi = 0 Wb (no mag field)
E = -20 * (0.094 Wb – 0 Wb) / 4*10-3 sec = -4.7 * 102 V (magnitude = +4.7 * 102 V)
B) I = V / R = 4.7 * 102 V / 170 Ohms = 2.76 A
C) Use RHR 2b. Falling mag field will induce a positive mag field which gives a counter clockwise
current.
D) P = IV = 470 V * 2.76 A = 1300 W
E) The induction stove has higher power and therefore cooks faster. The induction stove is about 75%
efficient in this example while the conventional is only 40% so the induction stove is also more
efficient.
2) A 120-Hz (linear), 70-V(rms) source is connected in series with a 30-Ω resistor, a 90-μF capacitor,
and a 0.070-H inductor. Calculate
(A) the total impedance of the circuit.
(B) the total maximum current in the circuit.
(C) the phase angle between the source voltage and the current.
(D) the resonance frequency of the circuit
(E) if you removed the resistor from the circuit when it was in resonance, what would happen to the
circuit and why?
A) Z = (R*R + (Xl – Xc)2)1/2
Xl = 2pi f L = 2 pi * 120 Hz * 0.07 H = 53 Ohms
Xc = 1 / (2 pi f C) = 1 / (2 pi 120 Hz * 9 * 10-5 F) = 15 Ohms
Z = (900 + 1444) 1/2 = 48 Ohms
B) Imax = Vmax / Z and Vmax = Vrms * 1.4 (1.4 is the square root of 2)
so, Imax = Vrms * 1.4 / Z = 70 V * 1.4 / 48 Ohms = 2 A
 X  XC 
C) φ  tan 1  L
 = tan-1[(53 Ohms – 15 Ohms) / 30 Ohms)] = 52 degrees
R


D) f = 1/[2 pi * (LC)1/2] = 1/[2 pi * (0.07 H * 9 * 10-5 F )1/2] = 63 Hz
E) The circuit would burn up because you would have no resistance and an infinite current.
3) Two wires along the ground.
There are two wires running along the ground parallel to each other. Each has a current going in the
+x direction. Wire 2 is a distance 0.2 m from wire 1 in the +y direction. Wire 1 has a current of 5 A
and Wire 2 has a current of 9 A. The wires are both of length 3 m. Find:
(A) the magnitude of the magnetic field caused by Wire 1 on Wire 2 (solve in terms of r)
(B) the direction of the magnetic field caused by Wire 1 on Wire 2 (↑, →, ↓, ←, , ).
(C) the magnitude of the magnetic force on Wire 2 (solve in terms of r and L)
(D) the direction of the magnetic force on Wire 2 (↑, →, ↓, ←, , ).
A) B 
μ0 I
= (2 * 10-7 * 5 /0.2 m) T*m = 5 * 10-6 T
2πr
B) RHR 2a gives 
C) F = IL X B = 9A * 3m X 5 * 10-6 N = 1.35 * 10-4 N
D) RHR1 here gives ↓ as the force direction
4) Transformers! They are exactly what meets the eye…
A transformer has blown close to your business. Every hour your power is out as a result means that
you loose money! The power company is a bit slow on this day and so you are going to fix it yourself.
A) The 30 loops attached to the 1200 V power line are still in tact. However, the loops leading to your
business have short circuited and fused together. They must be replaced. How many loops do you
want to replace them with (remember that your business uses a 120 V line)?
B) Your business uses an average power of 15,000 Watts. How much power goes from the power line
to the transformer?
C) What is the current used by your business?
D) What is the current that flows into your transformer from the power line (no you will not have the
same answer as part C, think about it).
E) Is this a step-up or step-down transformer? Explain how you could tell.
A) V1/V2 = N1/N2 so, solve for N2 gets: N2 = N1 * V2/V1 = 30 loop * 120V/1200V = 3 loop
B) P1 = P2 as energy and therefore power are conserved, so P2 = 15000 W
C) P = IV, so I = P/V = 15,000 W / 120 V = 125 A
If you had I1 you could use I2/I1 / N1/N2 also
D) I1 / I2 = N2 / N1 so, I1 = I2 * N2/N1 = 125A * 3 loops/30 loops = 12.5 A
E) Step down as the voltage steps down.
5) EVIL QUESTION – bank heist
Having been foiled on his last attempt by some smart P219 students the evil scientist has a bolder plan.
He is going to rob a very large bank and take millions, err billions of dollars. Oh but how to do it? He
will use a single proton to disable the underground alarm system! How? well he is going to accelerate
a proton up to 90% of the speed of light (we will ignore relativity – and if you don’t know what that is,
don’t worry, we will learn about it later). Once he does that he will shut off the electromagnetic field
and let the proton zoom towards it target.
The only chance you will have to stop him is after he turns off his magnetic field. You will need to
activate a field so that the proton misses the building completely. The building is very long. If the
proton is moving in the +y direction and is initially at a distance of 10 meters from the building find:
(A) After drawing a dot for the initial position of the proton, find the initial direction of the force on
the proton and draw it onto the initial position.
(B) What will the path of the proton be like if you make the magnetic field go in the +z direction (draw
it and make sure that your answer to question A matches this drawing. If not either you have made a
mistake in question A or a mistake in this question).
(C) The minimum magnetic field required for the proton to miss the building – if you get stuck, look at
your drawing that might help you to figure it out.
(D) The magnitude of the force on the proton (using your answer to C for the magnetic field).
A) B is +Z and V is +y so the force using RHR 1 is therefore in the +X direction.
B) The path will be a circle that goes in the clockwise direction.
C) Since the path is a circle that circle has a cyclotron radius of r = mv / qB
So, B = mv / qr = 1.67×10–27 kg * 2.7 * 108 m/s / (1.60×10–19 C * 10 m)
B = 0.28 T
D) F = qvB = 1.60×10–19 C * 2.7 * 108 m/s * 0.28 T = 1.2 * 10-11 N