Harder Physics Questions

[Edmund]

Aah thanks, I calculated the density with the timber floating horizontally =/

[Edmund]

GAMSAT over! Came across some interesting questions I might like to share

For 1 mark (1.5 minutes),

The amount of energy needed to raise 1 mL of water by 1°C is 4.2 J. One meal served in a restaurant is equivalent to 8000 kJ. If all the energy is converted to heat, how much ice water with a temperature of 0°C must a diner (body temperature =37°C) drink to maintain his body temperature (or counteract this increase in energy input)?

Feel free to clarify since I might not remember the question too well.

The options were 50L, ~70L, cant remember the other two options though...

[appianway]

Energy that the water gains = energy converted to heat.

Energy that water gains = volume of water * change in temperature * 4.2J
Change in temperature = 37 degrees

37 * 4.2 * V = 8 * 10^6
V = 8* 10^6 / (37*4.2)

And yeah, just plug it in.

[Edmund]

Well that looks right, I probably forgot the formula and guessed it wrong

[iamdan08]

From memory, I'm pretty sure i got 50L. Can't remember the exact question though. :S

[/0]

A nice problem back from my first quantum problem set:

Show that, if a photon collides with a stationary electron, they cannot both emerge
from the collision along the same trajectory.

Useful Formulae:
Relativistic momentum:
Rest energy:
Relativistic total energy:
Photon Energy:

Where

Hint: Reductio ad Absurdum


(I'll post up a really good question after I hand in my current thermo assignment xD)

[schnappy]

GAMSAT over! Came across some interesting questions I might like to share

For 1 mark (1.5 minutes),

The amount of energy needed to raise 1 mL of water by 1°C is 4.2 J. One meal served in a restaurant is equivalent to 8000 kJ. If all the energy is converted to heat, how much ice water with a temperature of 0°C must a diner (body temperature =37°C) drink to maintain his body temperature (or counteract this increase in energy input)?

Feel free to clarify since I might not remember the question too well.

The options were 50L, ~70L, cant remember the other two options though...

Trick question the water is in a solid state so it can't be readily consumed

[/0]

How long do black holes last?

1) The Schwarzchild radius of a Black Hole is the radius of the 'event horizon', past which even light cannot escape.
Find an expression for the Schwarzchild radius of a black hole using the notion of escape velocity (non-rigorous derivation :-o)

2) Over time, black holes lose energy and hence mass via "Hawking Radiation". The power emitted by a Black hole is:
, where , and is Planck's constant, is Boltzmann's constant, and is the speed of light, is the surface area, is emissivity, and is temperature.
Since we may assume a black hole to be a perfect blackbody (perfectly absorbs and emits radiation), .
Given that the temperature of a black hole is: , where is the Gravitational constant, and the energy of a black hole is , form a differential equation to find the lifetime of a black hole in terms of its initial mass.

3) How long would a black hole the mass of the sun last ()? How about the mass of a proton ()?