# Homework Help: Exam corrections

1. Nov 9, 2008

### voidofidea

Hey everybody, nice to be a part of the forum. I needed some help understanding the concepts behind these questions. By correcting the questions answered incorrectly, we get some of the points back. Thanks for any help that I receive.

1. The problem statement, all variables and given/known data

Why is it that a fish displaces its own volume AND water? I thought that it only displaces its volume, making it larger to rise and smaller to sink.

Why is it that when a large block of wood and a smaller block of iron on weighing scales both register the same weight of 1 ton, the wood has a greater mass, taking buoyancy into account?

Why is it that two pennies of the same mass, when one of them is touched with an electrically charged rod, it has slightly less mass than the other one? I thought the proton count in an element had something to do with its atomic mass?

Why is it that when a system does work and no heat is added, its temperature decreases? I thought that it increased, and that heat arises when work is done, thus leading to a hotter temperature.

Why is it that 100% of of electric energy when sent to a light bulb is converted to heat?

Why is it that both the magnitudes and separation distance of two charges makes up the electrical force?

2. Relevant equations

Equations aren't really relevant to these questions, more the concepts behind them.

3. The attempt at a solution

The attempts are in number 1.

2. Nov 10, 2008

### voidofidea

3. Nov 10, 2008

### craigman

~~Just guessing~~
Fish-not sure
wood iron- the amount of air being displaced by the wood means it has more buoyancy. So there must be slightly more wood than iron.
pennies- the electrons are temporarly taken from the penny.
work- it must be loosing more heat energy to it's surroundings than what is put into the system, refer to the first law of thermo, http://en.wikipedia.org/wiki/First_law_of_thermodynamics
100%- not sure, so does all the light energy turn into heat energy?
charges- no idea

4. Nov 10, 2008

### HallsofIvy

All of these questions are much too general. Things that are tue in one situation may not be true in a slightly different situation.

I don't know what you mean by "displaces its own volume AND water". It displaces its own volume OF water because its in water and that's all there is to displace! "Displace" here means to push aside. Perhaps you are thinking that it changes its own volume. Yes, most (not all) fish have a swim bladder that they can make smaller or larger, thus displacing more or less of the water it is in.

[/quote]Why is it that when a large block of wood and a smaller block of iron on weighing scales both register the same weight of 1 ton, the wood has a greater mass, taking buoyancy into account?[/quote]
Again, please clarify the situation. If you are simply placing one ton of ANY thing on a scale they will have the same mass because weight is just the acceleration due to gravity times their mass. If you imagine the scale at the bottom of a large amount of water, because a mass of wood will have a larger volume than an equal mass of iron, it will displace more water, there will be more upward force on it and it will register slightly less "weight" on the scale. So if you have different masses that, in this situation, register the same "weight", the lighter wood must have the same mass.

My first reaction was that this simply isn't true! Protons are deep in the nucleus of the atom. Touching a penny with an electrically charged rod won't affect the protons at all. I don't know why you even mention protons. It will, however, affect the electrons. If the rod is positively charged, electrons will be attracted from the penny to the rod. I suppose you could say that the loss of the electrons will reduce the mass of the penny by the mass of those electrons. The change in mass would, of course, be much too small to measure.

What does it do work on? Again your question is not specific enough. In general though, both work and temperature are energy, temperature measuring heat or random energy. If a system does work on something outside the system, neglecting friction, the energy of the system decreases which MAY decrease temperature. On the other hand, if the system is having energy input, say as electricity or fuel, because there is always some friction, some of that added energy is converted to heat and the temperature will increase.

5. Nov 10, 2008

### tiny-tim

I can confirm that!

6. Nov 10, 2008

### voidofidea

Hey guys, thanks a lot for the responses. I'll include the question and the four choices to perhaps make it more clear what the professor was trying to ask.

3. A fish normally displaces its own
a. volume of water
b. weight of water
c. both of these.

(How is this for an explanation? : A fish normally displaces BOTH its volume of water and weight of water. The little sacs in fishes can be expanded or contracted, increasing or decreasing the volume of the fish. Increasing the volume makes density less, so the weight of water that is displaced is less, and the fish ascends. Conversely, when decreasing the volume, the density is more, so the weight of the water displaced is more, causing the fish to sink.)

20. The electrical force between two charges depends only on the charges'
a. magnitudes
b. separation distance
c. magnitudes and separation distance
d. none above

15. When you send electric energy to a light bulb, the amount that is eventually converted to heat is about
a. 15%
b. 30%
c. 45%
d. 60%
e 100%

6. A large block of wood and a smallerblock of iron on weighing scales both register the same weight of 1 ton. Takin buoyancy of air into account, which block has the greater mass?
a. Wood
b. Iron
c. Both have the same mass

Thanks again!

Last edited: Nov 10, 2008
7. Nov 10, 2008

### mgb_phys

3 - The swim bladder is getting a little over complicated!
An object submerged in water displaces it's volume of water.
Wether that volume weighs more or less than the object determines if it will float or sink.
It can't always displace it's own weight because then it would have to change size in liquids of different densities.

You have to post some attempt at the other questions.

8. Nov 10, 2008

### voidofidea

Okay well it may be over complicated but according to what you wrote what I said was right. Here is the rest of my corrections, hopefully I didn't leave anything out.

3. A fish normally displaces BOTH its volume of water and weight of water. The little sacs in fishes can be expanded or contracted, increasing or decreasing the volume of the fish. Increasing the volume makes density less, so the weight of water that is displaced is less, and the fish ascends. Conversely, when decreasing the volume, the density is more, so the weight of the water displaced is more, causing the fish to sink. So, both volume and weight are involved in this situation
6. Since buoyancy of air is taken into account, the block that has the greater mass is the wood, because there is more wood than iron, displacing more air. So the wood has a slightly greater mass.
7. Water would heat up the slowest because it has an unusually high specific heat capacity. It has more thermal inertia and therefore takes longer to heat up.
9. Convection is limited to gases and liquids, or, fluids. Conduction is limited to solids.
11. When an electrically charged rod touches a penny, electrons will be attracted from the penny to the rod. The loss of electrons will reduce the mass of the penny by the mass of those electrons. The loss is very minute in amount, though. So it has slightly less mass than the untouched penny.
13.According to the first law of thermodynamics, the system must be loosing more heat energy to its surroundings than what is put into the system, so the temperature decreases, due to this loss.
15. The point of a light bulb is to convert energy to light. Eventually, the light will be converted to heat, and eventually 100% of all energy is converted to heat.
16. Sound is not a transverse wave. Light, radio, and seismic S-waves are transverse waves.
20. Electrical attraction is like gravitational attraction in that it attracts, but can also repel. So, the further apart two charges are, the less their electrical force, and the closer, the more the electrical force.
Short answer : An electromagnetic wave has the longer wavelength. The velocity of electromagnetic waves are faster than sound waves, and therefore travel a greater distance, as can be seen in the example below, assuming both waves have a frequency of ten.

9. Nov 10, 2008

### mgb_phys

3, Correct if the fish is floating it is displacing it's own weigth and volume. An important distinction is that objects always displace their own volume but not always their own weight. If you put the fish in air it would still displace the same volume, but the weight of the air would be much less than the weigth of the water and so it wouldn't float in air.

6, Correct - the wood is displacing more air so is getting more 'floatation' from the air and so the weight measured will be reduced.

7, You haven't posted the question.

9, Convection is only in fluids but conduction isn't limited to solids - fluids do also conduct. I invite you to stand under a cold shower to test this!

15, Good answer. Eventually all the light emitted by the bulb will end up as heat - when it hits the walls for instance.

20, I think you mean Electrical attraction is like gravitational attraction EXCEPT that it attracts, but can also repel. Although this doesn't matter for the question.
The important point is that like gravity the force depends on the amount of charge and the distance.
Not sure if the 'short answer' is part of this question - it doesn't seem to fit?