# A few conceptual MCQs

1. Jul 27, 2006

### al_201314

Hi guys I've got a few questions here but I'm unsure whether my answers are correct, although these are the few that I don't have much confidence in. Maybe you guys can help me correct the wrongs.

1) 2 large plates are placed a distance of x apart. The plates are charged to a potential difference V and hold charges of +Q and -Q respectively. P.D now increased to 2 V. Capacitance of system would be:

a) halved
b) unchaged
c) doubled
d) undetermined, because we do not know length of plates
e) undetermined, because we do not know charge on plates

My answer is a. (only one option)

2)A light bulb with a tungsten filament is attached to a source of variable voltage. As the voltage is increased on bulb, some of the following would happen:

a) bulb's resistance increases
b) current in bulb increases
c) power dissipated remains constant

My answer is b and c. (More than one option allowed for this)

3) Which of the following "bullet" particles can be used to cause fission in the target nucleus of uranium-235?

a) low velocity alpha particle
b) high velocity neutron
c) low velocity neutron

My answer is c. (More than one option allowed for this)

4) We commonly observe light travels in a straight line but sound does not. Which of the following reasons accounts for this?

a) Freq of light is much higher than sound
b) Wavelength of light much shorter than sound
c) Speed of light much higher than sound

(More than one option allowed for this) I'm unsure for this, can anyone help?

5) For an ideal gas of a given mass, if volume increases while pressure remains the same:

a) Average KE of molecules decreases because work is done by the gas
b) Average KE of molecules remains the same because work is done by the gas
c) Average KE of molecules remains the same because work done by the gas equals heat supplied
d) Average KE of molecules increases because heat is supplied to gas
e) Nothing can be determined about the molecular energy.

My answer is a. (only one option)

2. Jul 27, 2006

### Hootenanny

Staff Emeritus
Sounds good to me.
I'm not sure with this one. If the temperature was kept constant, then (a )would be true. (b) is certainly true. However, power is given by P = IV thus is the voltage increases and the current also increases it must follow that the power dissapated is not constant.
I agree.
Here, you will want to consider diffraction. What is the magnitude of the diffraction dependant on?
You may wish to reconsider your answer here.

3. Jul 27, 2006

### al_201314

3) What is the proper explanation for this? I admit this is just a guess. A slower moving neutron will be better since too fast a neutron could result in it not 'hitting the target"?

4) For this I only know that the smaller the aperture in relation to wavelength, the greater is the spreading of the waves. So light waves are very short therefore for example we can't see round corners? So answer is a and b?

5) dU = dQ + dW. where dW = p x dV? How do I know whether there is heat suppied?

Another question - for p x dV, for the system after the change in volume, would the pressure be the same as before doing work?

4. Jul 27, 2006

### Hootenanny

Staff Emeritus
In essence, yes. Thermal neutrons (lower velocity) have a larger 'cross section' than fast neutrons. Cross sections (of particles) represent the probability of an interaction between two particles. Thus, the greater the cross section of the neutron, the more probable it is that it will be absorbed by a nucleus. Do you follow?
Almost, the angle of diffraction is greatest when the aperture is of the same size as the wavelength of the wave.
Sorry, I messed up there, your are correct. Work is done by the gas, therefore the internal energy and hence the kinetic energy of the gas decreases.
Yes, $W = p\Delta V$ only holds when the process is isobaric. If the process occurs when the pressure is non-constant we must find the work thus;

$$W = \int^{V_{f}}_{V_{i}} P\; dV$$

5. Jul 27, 2006

### al_201314

3) I understand.

4) I got it after some reading up as well. Just to confirm the options I chose were correct?

5) How would one know whether there is a value for dQ, ie, if heat was supplied? Would they mention in the question in anyway or it is up for deduction?

However, why is the answer not consistent with when I use the equation pV = nRT? Volume increase, temp increase by charles law hence internal energy of the system increases?

I've got another here.

A small wooden boat that contains a man floats in a swimming pool, and the level of the water is marked at the edge of the pool. The man drops gently into the pool as a result, the level of the water:

a) will rise
b) will fall
c) will stay the same
d) cant be determined without relative mass of man and boat
e) canot be determined without mass and volume of the man

My answer is c, because Archimedes' principle states that any floating or submerged object displaces an amount of water = to the object's weight. There is no change is weight here, so the weight of water hence volume displaced is constant. Am I right here?

Many thanks once again.

6. Jul 27, 2006

### Hootenanny

Staff Emeritus
I'm not sure, perhaps someone else could offer their opinion?
The kinetic temperature;

$$E_{k} = \frac{3}{2}kT$$

Assumes constant volume, which we do not have in this case. Do not forget that in this case heat is being added to the system (see above link)
Sounds good to me.

Last edited: Jul 27, 2006
7. Jul 27, 2006

### al_201314

Thanks Hootenanny for all the help.

If anyone do have an alternative opinion for 4) do post!

Thanks for any inputs!