Question

"During the night, the air temperature drops to -5 Celcius. The ice that melted during the day freezes again. Outline one mechanics by which thermal energy is lost by the ice." I only got 2 marks out of 9 when I answered: "By decreasing temperature of liquid water below 0C liquid water changes phase to solid by reverting the fusion reaction. This change of phase requires the liquid water to lose thermal energy which results in radical decrease in kinetic energy of the molecules in the resulting ice. - So I guess I was looking for the wrong thing here. Any ideas?

Answer 1

"Additional question: A body of mass is in a gravitational field of strength g. The body is moved through a distance h at a constant speed v in the opposite direction to the field. Derive an expression in terms of m, g and h for the work done on the body." - What I responded was just: Work = mgh. What should I have responded instead?

Answer 2

There are many questions to ask so I'll keep them coming. Next: A sample of an ideal gas is contained in a cylinder fitted with a piston. The piston is suddenly moved inwards, decreasing the volume of the gas. By considering the speeds of molecules, suggest why the temperature of the gas changes. - My answer: Temperature in Kelvins is inversely proportional to the volume. When volume is decreased the pressure increases resulting in more *FREQUENT COLLISIONS* due to more kinetic energy. Since temperature changes result from changes in heat energy per unit volume, we can conclude that a decrease in volume, in this case, results in increase in temperature. I got 4 out of 5 marks with *Frequent collisions* underlined. What might have been wrong with that?

Answer 3

The previous question was part ii) to a same entity of questions. So, "the gas now expands at constant pressure p so that the volume increases by an amount [delta]V. Derive an expression for the work done by the gas."

I guess you should use [delta]U, U_final and U_initial and the equation W = Q-[delta]U

But I'm not sure what to actually do with those. What are the tools to use here?

Answer 4

1) heat flows from hotter body to colder body -> heat from water to surrounding to it freezes.

Answer 5

2) right.

Answer 6

for the ice question i would have answered that
a heat energy of (latent heat of fusion ) is required by ice to change its state to just water(still at temperature 0 degrees)
so that much energy should be relesed by the body to turn into ice

2 whenever we do work from lower potential to higher potential against the field that is we are tiring/exhausting ourselves by doing a work of +mgh(non-negotiable!)
otherwise if i were to move it via the graviational field a slightest force would be enough

Answer 7

experimentX what do you mean by "2) right"?

salini: okay the correct answer was simply that F = mg -> W = mgh.

I'll create a new question for the other problems I had.