LOADS of Final Review Questions

[physicszman]

Hi, these are off of my final review sheet. Theres a lot of them but I think most my answers are correct

* = my answer.

1. Kinetic Theory accounts for the increase in pressure as a confined gas is heated by what?

*An increase in particle motion, more violent and frequent particle collison.

2. To protect flowers in very cold weather, florists in northern states may wrap wet newspaper around the flowers before loading them in their vans for delivery. Explain why this helps.

*Heat is released as water freezes, and this takes place before the solutions in the plants freeze.

3. Aluminum heat sinks are attached to computer chips that have to dissipate a lot of heat. Why are these chips usually painted black?

*The color black absorbs heat beter, radiates heat better, the emissivitity will be smaller, and the chips thermal conuctivity will be greater

4. Rapidly approaching a stationary sound source produces a change in pitch. Explain a similar situation.

*When a person is at rest and the sound source rapidly moves away.

5. What type of waves does Ultrasound refer to?

*It refers to waves of high frequencies.

6. Briefly summarize Electrical charge interaction.

*Negative charges repel each other. Positive charges repel each other. Positive and negative charges attract.


7. Two Styrofoam packing beads suspended from threads show a slight attraction toward each other. What do you infer from this?

*Both are charged with opposite charges.

8. Consider two "point" charges qA and qB that are separated by a distance r. As far as the effect upon the magnitude of the force between them is concerned, increasing one of the charges by a factor of 9 produces the same effect as doing what?

*Increasing the other charge by a factor of nine or decreasing the distance between them by a factor of 3.

9. As one moves away from a positive "point" charge, what happens to the electric potential?

*It is independed from the distance since the field is constant.

10. On an equipotential line, what is true about V?

*V = 0.

11. The symbol for a variable resistance shows a contact which can be moved along a resistor so that varying amounts of it are included in the circuit. On such a device, if the wires at M and N are connected to the + and - terminals of a battery, sliding the connection at N to the right will produce what type of current flow?

*A current which is constant?

12. A bird is perched on a high voltage (10,000 volt) power line. Why is the bird not electrecuted?

*The electrical resistance between the bird's feet on a path through its body is a lot less than the resistance through the wire between these two points.

13. The Earth's magnetic field in this room right now points toward which wall of this room?

*The wall with the greates negative charge.

14. An bar magnet is mounted so that it is precisely balanced in a horizontal position in a region where the magnetic field is zero. If this balanced magnet is placed in this room, what will its position be?

*It will be horizontal in this room.



15. Small ceramic magnets can be in many different shapes. Some are shaped like washers or "Life Savers" candies and are called face magnets because their poles are on their top and bottom faces. One of these is strung over a vertical pencil with N down and S up. In order to get the next one strung over this pencil to "levitate" or be supported by the interaction with the magnetic field of the first magnet, what does the second magnet need?


*The next magnet logically needs to have its N downs and S up.

16. What is the source of magnetic fields?

*Electric dipoles.

17. A positive charge moving horizontally to the right enters a region where there is a strong magnetic field directed to the right. The force acting upon this charge due to its interaction with this field will be in what direction?

*The charge will be directed towards me.

18. A physics teacher wants to demonstrate the Hall effect in a solid during lecture. What can be done to increase the measured voltafe or Hall emf?

*If the teacher uses ions with a large charge.

19. Imagine that you have three iron rods that appear to be identical. However, two of these rods are magnetized and one is not. How could you identify the non-magnetized rod without using any additional equipment?

*See which rod is repelled or attracted?

20. If the water level in a dish remains constant over a period of time,does this mean that no condensation or evaporation is taking place?

*Not neccessarily, it could mean that the evaporation and condensation is equal throughout time.

21. The kind decreed that there be made two massive (but equal mass) coffee mugs, one of gold for him, and the other aluminum for the gold jester. The king always complains that his coffee was too hot, while the court jester complains his was too cold. What is the explanation for this?

*Assuming both hold equal volume, this has to do with the thermal conductivity of each metal. Gold has a very small conductivity rate and therefore keeps the coffee very hot. Aluminum on the other hand has a large condcuctivity rate and outside temperature has a great impact on the coffee.

22. What would happen to the temperatur in a room if u put an Air conditioner in the room and turned it on.

*Well assuming the ac is functional (unlike mine) the temperature would stay the same. Since the air conditioner takes air from inside the room and circulates it out the back.

Any corrections are MUCH appreaciated, please also make suggestions that would help me get to the right answer if anythign is incorrect.

thanks, now i have to go do the math portion

 

[Janitor]

I’d say you pretty well nailed 1, 2, 3, 4, 5, 6, 7, 8, 20. As far as the others go...

9. Potential drops as one moves away from a positive charge

10. V = constant, not necessarily zero.

11. An increasing current--increasing as long as you keep sliding away from N. Without seeing a diagram of the setup, I am taking it that N is connected to the - terminal, M is connected to the + terminal, and the sliding contact is connected to the - terminal. If I am not picturing the connections properly, then my answer may be wrong.

12. My intuition says that your claim that the bird-path resistance is less than the copper-path resistance is wrong, and even if it were right, I don't think that it really nails down the answer. (An important point that you have already probably figured is that you are supposed to presume the wire is not insulated.) When the voltage in a wire varies at 60 cycles per second, the difference in potential across a pair of points only an inch or so apart (like the bird’s feet would be) at any given fixed moment in time is a tiny fraction of the peak voltage of 10,000 volts. That is why the bird can get away with perching there. If the bird stretched one of its feet out to another wire that is connected to the same generator back at the power station as the wire that the bird first landed on, there would be a substantial flow of current through the bird. I’ve seen video of birds in such situations literally being enveloped in a ball of flame.

13. I don’t believe your answer is correct. I think the answer is either going to be the north wall or the south wall. I am not sure what the convention is for assigning a vector to a magnetic field. Do you point the vector toward the north pole of the field? If so, then the answer is presumably the north wall of the room. Maybe somebody more knowledgeable than I will chime in on this one.

14. Maybe the point of the question is to note that the magnet will twist to (anti) align itself with the local magnetic field of the Earth. More than likely, in your part of the world there is a nonzero vertical component, so that the magnet will not be horizontal. Possibly that is the sort of answer that is wanted, but I am not sure.

15. I believe you have it backward. If the second magnet has its S side down, it will be repelled by the upward-facing S pole of the first magnet, and thus will levitate if the field is strong enough.

16. A stationary electric dipole would not have a magnetic field associated with it. On the other hand, a moving charge (be it simply a single monopole of electric charge, or some more complicated distribution of electric charge) will generate a magnetic field.

17. The magnetic field causes the electric charge to feel a force according to a cross product rule of vector multiplication, with the two vectors in question being the velocity of the charge and the magnetic field. Remember that the cross product is proportional to the sine of the angle between the vectors. What can you say about the sine of zero?

18. The electric current and the magnetic field intensity are the two variables that can be most easily controlled in the Hall effect demonstration. Your answer about using ions of large charge is a little iffy. The charge carriers in the solid material are typically either an electron or an electron hole, and it is not clear to me that the experimenter could manipulate the material so as to change the type of ion flowing in it. You are probably safer answering either that the current could be increased, or the applied magnetic field could be increased.

19. You are on the right track. The critical thing is that either end of the non-magnetized rod will be attracted--not repelled--to each end of each of the other two rods. That will not be true of the magnetized rods.

21. Gold is actually a 27% better conductor than aluminum according to my lookup of parameters. The gold cup is thinner than the aluminum cup, since gold is more dense and the same mass of each metal is used. Both of those factors are working in the same direction: the gold cup’s contents should cool down to room temperature faster than the aluminum cup’s contents. Could it be sort of a trick question? It seems to me that the jester has received the gold cup, since he complains his coffee gets cold. Maybe there is some sort of intrigue going on in the castle, and all is not what is appears to be. Hmmmm.

22. You probably will get credit for saying the temperature will remain the same. To get really precise about it, you could say that the room’s temperature will actually rise. The air conditioner is receiving energy from outside the room, via the power cables to it, and is using that energy to do work. The work degenerates into sound and heat, which will warm up the room.

 

[Doc Al]

Earth's magnetic field

13. I don’t believe your answer is correct. I think the answer is either going to be the north wall or the south wall. I am not sure what the convention is for assigning a vector to a magnetic field. Do you point the vector toward the north pole of the field? If so, then the answer is presumably the north wall of the room.

The Earth's magnetic field is roughly like that of a huge bar magnet. What we call the geographic North pole is where the South magnetic pole is (actually it's not aligned with the axis of rotation). The magnetic field outside a bar magnet points from the north pole to the south pole (out of the north pole and into the south pole). Thus the Earth's magnetic field points (very roughly) towards geographic north, and thus the north wall.

(It certainly doesn't point towards the wall with the greatest charge. )

 

[physicszman]

Thanks for the clarification on #13 doc.

Thanks for checking my answers Janitor I really appreciate your time and for some reasons understand your explanatiosn better than my professors.

 

[Janitor]

Glad to hear that I was of some help. As you have probably figured out by now, there is no force on the charge in Q 17, so that is a bit of a trick question. If you have time after you get your graded paper back, let us know what the story is on the King/Jester question.