____________________________Physics 109    Test 2     Thursday, 15 April 2004

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1. What can we say about the resistance of the heating elements in a toaster compared to the resistance of the wires in your house?

  1. The resistance of the wires in your house is larger.
  2. The resistance of the heating elements is larger.
  3. They are about the same because they carry the same current.
  4. Impossible to tell.

 

Ans. = b.  (Power dissipated varies as I2R. ; we want the heating to take place in the toaster;.  In this example the same current flows through the supply wires into the heating element meaning that I is the same in each.  Thus R must be greater in the toaster.)

 

2. A common use of a diode in a circuit is to:

  1. Control the current flow to the transistor gate.
  2. Amplify small signals through transfer of resistance.

c.       Turn alternating current (AC) into direct current (DC). 

  1. Form a junction voltage source.
  2. Prevent unwanted 'shorts' in home electrical devices.

 

Ans. = c.  It will conduct for only one-half of each cysle.

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3. During the summer, 'brownouts' (reductions in the AC line voltage below the usual 120V) are experienced in some large cities when the electrical demand exceeds supply. During a brownout, the light emitted from an incandescent lamp would be:

  1. Less bright and more red.
  2. Less bright and more blue.
  3. Less bright but otherwise the same spectrum.
  4. Brighter and more red.
  5. Brighter and more blue.

 

Ans. = a.  (Both result from a reduction in filament temperature)

 

4. Consider two 120 Volt light bulbs, one a 75-watt bulb and the other a 100-watt bulb. The tungsten filaments in both bulbs are the same diameter. The 75-watt bulb is:

  1. Lower in resistance and therefore the filament is longer.
  2. Lower in resistance and therefore the filament is shorter.
  3. Higher in resistance and therefore the filament is longer.
  4. Higher in resistance and therefore the filament is shorter.

 

Ans. = c.  In this example, both bulbs see the same voltage.  Power varies as V2/R since V here is constant.  Thus the 75W bulb must have the greater R.   With the same diameter, the higher R filament must be longer.

 

 

5. Electromagnetic radiation includes infrared, visible, and ultraviolet light. Although light travels as a wave, it is absorbed as individual particles called photons, which carry an amount of energy that depends on the frequency of the light. Of the three types of light mentioned above, the type of light that carries that most energy per photon is:

  1. ultraviolet light
  2. infrared light
  3. visible light
  4. dependent on the velocity with which the light moves.

 

 

Ans. = a.  Higher frequency (shorter wavelength) photons carry more energy.

 

6. An AM radio wave has a wavelength as long as a few soccer fields. Compared to visible light, an AM wave:

  1. travels slower.
  2. has a longer period.
  3. is more energetic.
  4. none of the above.

 

Ans. = b.  Lower frequency (longer wavelength) implies longer period of oscillation.

 

 

7. If you hold a strong permanent magnet inside a metal ring and then quickly remove that magnet, you create an electric field in the metal ring and an electrical current flows through the metal ring. If you repeat this procedure with a glass ring, no current flows in the glass ring because:

  1. although removing the magnet creates an electric field in the glass ring, the glass ring has no mobile electric charges to carry an electrical current.
  2. removing the magnet from the glass ring does not create an electric field.
  3. the glass ring creates its own magnetic field in the opposite direction, passing the field of the permanent magnet and preventing any current flow.
  4. the glass ring is made of a non-magnetic material. No magnetic fields can be present inside the ring so removing the magnet has no effect on the ring.

 

Ans. a  The electrons in the glass feel a force but not great enough to get them unbound from their atoms.

 

8. The principal advantage of sending electrical power across the country on very high voltage transmission lines is that:

  1. the electrical power lost in the wires is greatly reduced.
  2. these transmission lines are less likely to get in the way than low voltage transmission lines (which are much closer to the ground)
  3. they carry much higher current than do low voltage transmission lines.
  4. they carry less energy per charge than low voltage transmission lines.

 

Ans. = a.  High voltage implies lower current (for the same power);  Power lost varies as I2R so the same length of wire dissipates less power at reduced current.

 

9.         The sunset appears quite red because:

a. Air molecules scatter red light but do not scatter blue light.

b. Air molecules radiate red light.

c. Air molecules scatter blue light.

d. Red light is unable to penetrate the depth of the atmosphere at sunset.

 

Ans. = c; The air scatters blue preferentially but the red wavelengths scatter much less and thus a long air path passes the red light while tending to scatter out most of the blue.

 

10. An electrical insulator can't carry an electrical current because

  1. its electrons are positively charged rather than negatively charged, as they are in an electrical conductor.
  2. it contains no electrons.
  3. its electrons are not free to respond to an electric field.
  4. it contains only molecules, not atoms.

 

Ans. = c. Electrons in an insulator are tightly bound to atoms. 

 

 

11. If you wanted to lower the pitch of a guitar string, so that it vibrated at a lower frequency than before, you could

  1. shorten the string, increase the string's tension, or decrease the string's mass.
  2. shorten the string, decrease the string's tension, or increase the string's mass.
  3. lengthen the string, increase the string's tension, or decrease the string's mass.
  4. lengthen the string, decrease the string's tension, or increase the string's mass.
  5. none of the choices shown above.

 

Ans. = d.  Less tension and/or greater mass per meter and/or longer string will all reduce the frequency

 

12. My comb attracts a stream of water because: 

a.   Water is charged positively and my comb is charged negatively.

b   Water is charged negatively and my comb is charged positively.

c.   The comb is uncharged but the water molecules are attracted to it because the water becomes charged when passing through the faucet.

d.  The water molecules, although neutral, are electric dipoles and are attracted to the charge on the comb.

 

Ans. = d.  The comb’s charge rotates the water molecules so that unlike charges in water and comb are closer and thus the water is attracted toward the comb.

 

13. In metallic conductors of electricity, 

 a.   all charges are mobile

 b.   a net charge excess of negative electrons causes currents to flow

 c.   the conduction band will repel electrons

 d.   only electrons can move

 e.   only positive charges can move

 

Ans.= d  Some, but not all, atomic electrons in a metal are able to move from one atom to the next.

 

 

14. How can visible light striking an insulating material turn it into a conductor as happens inside a Xerox machine?

 a. The light photons kick the electrons out of the insulating plate

b. The light photons separate the conduction electron pairs within the insulators.

c. the light photons move electrons from the conduction band to the fermi level.

d. the light photons move electrons from the valence level to the conduction band

 

 

Ans. = d;

 

15. Spinning a simple coil of wire for a few rotations in a permanent magnetic field (with no commutator connected to the coil) would give rise to:

a. a direct current in the coil

b. a pulsating direct current in the coil

c. an alternating current in the coil

d. a direct current induced in the permanent magnet

 

Ans. = c.  With no commutator, the induced current would reverse direction every half-turn.

 

16. If every high-voltage wire presently in use by power companies in the US were able to operate at twice the present voltage while transmitting the same power, the effect of such a change on the power lost in wires is such that power loss: 

a. would increase by x2

b. would decrease by x2

c. would increase by x4

d. would decrease by x4

e. would remain unchanged

 

Ans. = d.  Doubling the voltage would allow reducing the current by 2 with the same power being delivered. Reducing the current by 2 reduces the I2R losses in a wire by 4.

 

17. If you move the north pole of a permanent magnet toward a sheet of copper a current will flow in the copper. It will become magnetic while the magnet moves. If you stop the motion of the permanent magnet just before it touches the copper plate and them hold the permanent magnet stationary, the repulsive force will disappear between plate and magnet. That force disappears because:

a. once the permanent magnet stops moving, it (the magnet) becomes non-magnetic

b. current stops increasing and becomes steady once the permanent magnet stops moving.

c. current gradually slows to a stop once the permanent magnet stops moving.

d. it becomes an alternating current once the permanent magnet stops moving and the copper plate's magnetic poles then flip back and forth rapidly.

 

Ans. = c.  Resistance in the copper causes the eddy current to diminish and stop.

 

18. You and your roommate travel to Europe where the voltage is 240V rather than 120V as in the US. Your roommate suggests you bring your identical 120 Volt hair dryers. You would them to function normally if you use them:

 

a. Connected in parallel where the total current each would draw would be the same as at home.

b. Connected in parallel where the total current in each would be half that as at home.

c. Connected in series where they would each draw the current that they draw at home.

d. Connected in series where each would get hotter because of the 220V connected across the two dryers in series.

 

Ans. = c  That would double the resistance and thus each would see the same current through their heater resistance if the pair was then connected to the doubled voltage.

 

19.  What can we say about the resistance of the heating elements in a toaster compared to the resistance of a small night light in your home?  (Both are 120 V devices)

a. the resistance of the night light is less.

b. the resistance of the heating element is less.

c. they are about the same.

d. we are not given enough information to tell.

 

Ans. = b  Remember that a house electrical socket should supply a constant 120 Volts.  But power dissipated will then vary as V2/R.  Thus, the lower power night-light must have higher resistance.  Please look again at number 1.  In that case, the same current passes through both supply wires and appliance.  In this question, two appliances are assumed to be connected to the same voltage.  This question assumes constant voltage while question 1 assumes that the current is constant.

 

20. Which of the following electronic devices can allow a very small electrical signal to control a large electrical current:

a. a capacitor

b. a mosfet transistor

c. a diode

d. a resistor.

 

Ans. = b.  The transistor can amplify the effect of a small input signal.

 

21. Consider two carbon resistors, one is 100 Ohms and the other is 130 Ohms. Each can be connected in turn to the same10V battery. They are made of the same material and they have the same length. The 130 Ohm resistor is:

A. Thinner than the 100 Ohm resistor and will dissipate more power when connected to a 10-volt battery.

B. Thinner than the 100 Ohm resistor and will dissipate less power when connected to a 10-volt battery.

C. Thicker than the 100 Ohm resistor and will dissipate more power when connected to a 10-volt battery.

D. Thicker than the 100 Ohm resistor and will dissipate less power when connected to a 10-volt battery.

("Thicker or thinner" in this question refers to the resistor's diameter)

 

Ans. = B  The thinner conductor will have higher resistance (if they have identical length).  Higher resistance dissipates less power at the same voltage.

 

 

22.  Faraday's Law states: (circle all that are true)

A. Moving a bar magnet near a coil of wire will induce a current in the coil.

B. Moving the coil near a fixed bar magnet will induce a current in the coil.

C. A stationary electromagnet with a changing current in it will induce a current in a nearby coil.

D. A stationary electromagnet with a constant current in it will induce a current in a nearby coil.

E. A stationary permanent magnet will induce a current in a nearby coil in the absence of relative motion.

 

Ans.:  A, B and C are true.  A varying magnetic field induces a current in a nearby coil.

 

23.  . A certain guitar string is vibrating with the two halves of the string moving in opposite directions. This results in a pitch that is:

A. The fundamental mode of the string.

B. An octave above the fundamental mode of the string (twice the fundamental frequency).

C. An octave below the fundamental frequency.

D. This is an overtone which is completely unrelated to the fundamental frequency.

 

Ans. = B.  The fundamental note for this case is formed by a half-wavelength on the string.  This example has a whole wavelength on the string and thus twice the fundamental frequency.

 

24.  When you put a current from a single flashlight cell through a single straight piece of the wire in your take-home laboratory kit and place that wire about one-half inch from your magnetic compass you discovered:

 

a.  No effect on the compass when you had only 1.5Volts going through a single turn of wire.   Only a multi-turn coil could give such an effect at that distance

b.  The compass, if originally aligned at right angles to the wire, turned to become parallel to the wire when it carried current.

c.  The compass, if originally aligned parallel with the wire, turned to point at right angles to the wire.

d.  We should not expect any current-carrying wire to affect the direction of a magnetic compass.

 

Ans. = C. The current-carrying wire generates a magnetic field.  The field lines are circles surrounding the wire

and a compass needle will want to point along such a line.

 

25.  Upon opening one of the small motors in your take-home lab kit, you will find:

 

a.  Coils of wire, permanent magnets but no commutator as this motor runs on DC current.

b.  Coils of wire and a commutator but no permanent magnets are used in this small motor.

c.  Permanent magnets mounted on a rotating shaft and a commutator but no coils of wire are used in such a magnet.

d.  Coils of wire connected to a commutator and mounted on a shaft and surrounded by permanent magnets.

 

Ans.= d.  Take a look at such a motor.

 

Short Answer Questions

 

26. How does power move from one winding of the transformer (the primary circuit) to the other winding (the secondary circuit) 20 words or less?

 

Via an oscillating magnetic field generated by the primary.

 

 

 

27. If the gap in the head of your new tape recorder is 1/50,000 of a centimeter wide and if the tape runs past the head at 5 cm per second, what would you expect to be the limit of this recorder's high frequency response if all other components were of high quality? _________________(show work)

 

We note that 5 x 50,000 or 250,000 gap widths cross the head each second.  If two such widths are needed to form one wavelength, we can expect up to 125,000 cycles per second from this device.

 

 

28.. Your 120-volt stereo claims to be a 300 watt device. How many of these stereos can we safely connect simultaneously to a supply line rated up to 20 amperes? 

Answer: ______________________ Please show your work.

P = VI so each stereo draws a current I = P/V or, in this case, 300/120 = 2.5 Amps.  We can power up to eight of them at once.

 

 

29.   Most of the appliances in our homes these days are solid state devices which can operate on direct current. Most of our electric heating devices can also operate on direct current. Most of the motors in our appliances could be replaced easily by direct current motors. Is there any reason for the power companies not to change from alternating to direct current for the distribution of power? (Please be brief).

 

Yes; they can presently use transformers to make use of very high transmission voltages and then to reduce the voltage to a more modest 120V at homes.

 

 

30.  In your take-home lab kit there is an unmagnetized iron circular washer and a magnetic compass.  If that washer (unmagnetized) is brought near the compass, would the compass

be  affected?  Why or why not?   Reply in 17 words or fewer.

 

It would because it is iron and thus a magnetized compass needle is attracted to it.