Physics Revision - I solved it but i need to make sure its right?

Question

1. Why is the cooling unit placed at the top of the refrigerator?
a)Cold air falls and warm air is displaced upwards.
b)Cold air is a bad conductor so heat is not conducted into the fridge.
c) Cold air is a good conductor so heat is conducted out of the fridge.
d) cold air stops at the top and so prevents convection.
2. Which substance is a liquid @ room temperature of 25*C?
a) Melting point = -218 boiling point = -193
b) Melting point = -39 boiling point = 357
c) melting point = 44 boiling point = 280
d) melting point = 119 boiling point = 444
3) a long bar copper is heated evenly along its length, what happens to the bar?
a) it becomes lighter
b) it becomes longer
c) it becomes shorter
d) it bends at the end.
4) four blocks, made of different materials, are each given the same quantity if internal (heat) energy. Which block has the greater thermal capacity?
a) Temperature rise = 2*C
b) Temperature rise = 4*C
c) Temperature rise = 6*C
d) Temperature rise = 8*C

Answer 1

1. a is the answer. Cold air is heavier and so falls, thus displacing warm air upwards and setting up a convection current inside without a fan.

2. b is the right answer. See its melting point has to be less than 25 and its boiling point has to be more than 25 for it to remain liquid at 25 C. Only the substance at b with a melting poin t of -39 C and a boiling point of 357 meets that condition.

3. b is the right answer. Whether it is heated uniformly or at one end, it gets heated and with heat comes elongation for most materials. It may bend at one end if some constraints are imposed and the heating is non-uniform and that is why the question confirms that the bar is heated evenly along its length.

4. a is the answer. The block with maximum thermal capacity will have the minimum temperature rise.

One assumption in this question is that the mass of the blocks and the initial temperature are equal. Remember the formula Q = m.s.(t2 - t1) where Q is the quantity of heat supplied, m is the mass, s the specific heat and t1, t2 are the initial and final temperatures respectively. Here Q, m and t1 are kept constant and only s changes for each material, so t2 changes inversely with s.

Knowing the right answer becomes fun when we know why our answers are right. Good luck.

Answer 2

Confirm: abba

Answer 3

a b b a