Hello Everyone!. I have an exam coming up in a few days and I wanted to see how

Question

Hello Everyone!. I have an exam coming up in a few days and I wanted to see how someone else answers these questions. It is from a college physics 2 class. Electrical physics questions. If you guys want to answer these and show your work and any equations you use it would be helpful. I can get through most of them but it would be helpful to see how other people also get through them. The answer key is also here so that should help you know you get the right answer. Have fun.

Answer Key:

1: 5

2: 1

3: 4

4: 2

5: 5

6: 4

7: 1

8: 4

9: 4

10: 4

11: 3

12: 1

13: 4

14: 4

15: 5

Physics 131 Exam #3 October 26, 2010 USEFUL INFORMATION: mu = 4 pi times 10-7 Tm/A Speed of light = 3 times 108 m/sec A wire carries a current 1 into the page. Above it is a copper loop in the plane of the paper. If the current I is steadily increasing, then the current in the loop is: CW. CCW. CW at first, then CCW. CCW at first, then CW. Zero. A coil consists of 50 turns of wire wrapped around a square frame 0.3 m on a side. The coil is located in a region where there is a constant magnetic field B = 0.8 T. Initially, the field is perpendicular to the plane of the coil. What is the emf induced in the coil if in 1.5 seconds it is rotated until the magnetic field is in the plane of the coil? 2.4 Volts 3.6 Volts 4.8 Volts 7.2 Volts 8.0 Volts The two rectangular loops shown below lie in the plane of the paper. If the switch on the outer loop is closed, then a current is induced in the inner loop which: Flows clockwise constantly. Flows counterclockwise constantly. Flows clockwise for a brief time. Flows counterclockwise for a brief time. Alternates. A 10 cm long conducting bar in contact with two parallel conducting rails slides at constant speed of 5.0 m/s to the left. A 0.20 T magnetic field is present and points into the paper ( as shown). A 0.10 Ohm resistor connects the left ends of the rails. The induced current through the resistor is 2.0 A CW 1.0 A CW Zero 1.0 A CCW 2.0 A CCW The coil of a motor has a resistance of 4 ohms. When the motor is turning at its rated speed it draws a current 2 Amps from a 120 Volt source. Which statement is FALSE? At this speed the counter EmF is 112 Volts. At this speed the voltage across the motor 's resistor is 8 Volts. If the motor slows down, the counter EmF drops. If the motor slows down, the voltage across the resistor increases. If the motor slows down, the current in the circuit drops. The diagram below shows a transfer with 50 turns in the primary coil and 250 turns in the secondary coil. If the RMS Voltage in the primary coil is 8 Volts, find the RMS current through the 20 Ohm resistor in the secondary coil. 0.08 A 0.4 A 0.5 A 2 A 8 A A long time after the switch has been closed, what energy is stored in the inductor in the circuit below? 9 mJ 15 mJ 18 mJ 30 mJ 34.17 mJa An electric generator produces a peak voltage of 10.0 V when V when turning at 3000 RPM. What peck voltage would the same generator provide if it were turning at 9000 RPM? 1.1 V 3.3 V 10 V 30 V 90 V The voltage across a 40 ohm resistor in an AC circuit is V = 120sin(377t). What is the power dissipated in the resistor? 1.5 Watts 30 Watts 90 Watts 180 Watts 360 Watts Given that equation I = 28 sin 60 t describes an altering current, the peak current and frequency are: 20 A; 30 Hz 28 A; 30 Hz 20 A; 60 Hz 28 A; 9.55 Hz 28 A; 60 Hz What inductor has a reactance of 15 Ohm at a frequency of 60 Hz? 10 mH 34.17 mH 39.79 mH 392.7 mH 1.570 H A resistor R and a capacitor with a reactance of 40 Ohm are connected in a series with a 120 Volts RMS source. If the RMS current source. If the RMS current in this circuit is 2.4 Amps, what is R? 30 Ohm 40 Ohm 50 Ohm 90 Ohm 1600 Ohm An FM radio tuning circuit has a coil with an inductance of 8.0 times 10-7 H. What is the value of the capacitance if the ratio is tuned to resonate at a frequency of 96.26 MHz? 11.16 times 10-9 F 13.57 times 10-10 F 17.01 times 10-11 F 34.17 times 10-13 F 68.34 times 10-13 F In the circuit below XL = 5 Ohms, XC = 2 Ohms, R = 4 Ohms. What is the RMS current in the circuit? 4.8 A 15 A 17 A 24 A 30 A Which statement is false? Blue light with a wavelength of 500 nm has a frequency of 6 times 1014 Hz. (1 nm = 10-9 m) Electromagnetic waves are transverse. Electromagnetic waves carry energy. It takes light 500 seconds to travel the 93 million miles (1.5 times 1011 m) from the sun to the earth. Microwaves with a frequency of 3 times 101 Hz have a wavelength of 10 nm.

Explanation / Answer

1) since no field goes into the wire, the flux wont change so Zero. 5)

2) emf = dflux/dt = B N A/dt = 0.8*50*0.3^2/1.5= 2.4 v so 1)

3) current will flow clockwise in big loop
so field will be into page

it will want to oppose that so it will put its field out of page so it will have counterclockwise current

so 4)

4) emf = B L v
I = B L v/r = 0.2*.1*5/0.1= 1A

flux will be decreasing into page
so induced flux need to be into page
so current is CW
so 2)

5) resistance wont change so current will always be the same so 5)

6)

Vs/Vp = Ns/Np
Vs = 250/50*8 = 40 V

I = V/R = 40/20 = 2A

so 4)

7)
a long time after closed current I = V/R = 12/20
U = 1/2 L I^2 = 0.5*50*(12/20)^2= 9mJ so1)

8) emf = B A w

so triple w means triple emf
so 3*10 = 30 so 4)

9)
P = V^2/R = (120/sqrt(2))^2/ 40 = 120^2/40/2=180 W so 4)

10) we can see peak = 28
w = 60
so f = 60/(2*pi) =9.55
so 4)

11) XL = w l = 2*pi*60*L = 15
L = 39.79 mH so e)

12) V = IZ
120 = 2.4*sqrt( R^2 + (40)^2)
R =30 so 1)

13)
w = 1/sqrt( L C)
2*pi*96.26E6= 1/sqrt(8.0E-7 * C)

so C=34.17E-13 F so 4)

14) V = I Z
120 = I *sqrt( 4^2 + (5-2)^2)
I = 24 so 4)

15) lambda f = c
lambda = 3E8/3E10 = 1.0E-2 m

so 5) is false

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