Clicker Questions Flashcards
Clickers from Midterm 1
a mercury thermometer sits in a glass of water. if the thermometer reads 20 degrees C, we can conclude that:
the temp of the mercury in the thermometer is 20 degrees C
What happens if we put a hot block in contact with a room temperature block?
heat flows until they are the same temp
two objects (initially in equilibrium) are put into thermal contact and the pair is thermally insulated from its environment. If the heat is observed to flow from object A to object B we can say that:
object A initially had a higher temperature than B
- this is one of the defining properties of temperature and we cannot say object A has more energy because energy does not necessarily relate to temperature
the air pressure in the room is about 100kPa. The force of the air on top of your head (10cm by 10cm) is similar to the downward force from (hint: force is equal to pressure * area):
100kg mass
box 2 is twice the height of box 1, with twice the number of molecules, moving at the same average speed. Compared to the pressure on the left wall of box 1, the pressure on the left wall in box 2 will be:
The same;
P= F/A; Force is doubled but so is area so it cancels out
an ideal gas thermometer is calibrated by placing it in equilibrium with water at its triple point. The pressure reads 50kPa at 273.15K. The same thermometer is placed in equilibrium with another container in water. If the pressure reads 100kPa,we can say that the temperature of the water is:
546.32K;
T is directly proportional to P. The pressure went up by a factor of two so the temperature goes up by a factor of two
a 0.010m^3 rigid container of gas has a pressure of 1.0kPa at 20.0 degrees C. The pressure at 120.0 degrees C is closest to:
1.3kPa;
(P_2)/(P_1) = (T_K2)/(T_K1) = 390.15/ 290.15 = 4/3 so P_2 = 4/3P_1
a steel ball does not quite fit through the hole in a copper plate. if α_steel < α_copper we could help the ball fit through the hole by:
Heating the system; ΔL= αL_oΔT; Copper expands quicker due to bigger α
The piston’s α = 2.410^-5. The cylinder’s α = 1.210^-5. If the engine needs to operate between 0C- 20 C, is it better to have a piston that barely fits in the cylinder at 120 or a piston that barely fits at 0
A piston that barely fits at 120 is better since when it shrinks at zero degrees it does not effect it because the piston will shrink quicker than the cylinder. If it had barely fit at 0 C, then the piston will not fit at 120 because it expands faster than the cylinder.
when heated, each side of a cube of material expands by 0.1%. As a percentage of the original volume of the cube, the extra volume after expansion is:
0.3% ;
volume of a cube is L^3. volume of each part increase by 0.1% so 0.1 times 3 is 0.3%L^3
a copper wire of varying thickness is pulled at each end with a force of 40N. What is the tension and the stress?
the tension is a constant 40N all along the wire but the stress varies along the wire
suppose you compare the Y of a mini-marshmallow to a reg marshmallow. We would expect that the Y of the mini marshmallow to be:
about the same; Y is a property of the material and doesn’t depend on the size
suppose you compare the Y of a mini-marshmallow to a reg marshmallow. how would the stress of the mini compare?
much larger (same F, smaller A); stress = F/A
suppose you compare the Y of a mini-marshmallow to a reg marshmallow. how would the strain of the mini compare?
much larger (larger stress, same Y); Y= stress/ strain
the force on the right brick from the left brick has a magnitude of _______ if F is being applied to both sides
F;
bricks not moving so net force is zero. Force of left brick on right exactly opposes force from right
do you expect that the young’s modulus you measure for a marshmallow is higher or lower than for steel?
Lower;
Y is lower if it takes less stress to give the same change in L
if 0.2 mm diameter nylon fishing line is good for catching fish up to 2kg, what thickness of line would you need to catch a 50kg fish?
1mm;
F is 25 times greater. To get equivalent “safe” stretching, needs A to be 25times bigger and the diameter must be 5 times larger.
if 0.2 mm diameter nylon fishing line is good for catching fish up to 2kg, by roughly how much would 1 m of 0.2 mm diameter line be stretched by a 2 kg fish? (Y_nylon = 3 Gpa)
0.2m ; ΔL= L_oF/(YA) = ((1m)(20N)/(3*10^8pi(10^-4)^2))
a steel rod of length L_o is heated by a temperature ΔT. How much stress (force per unit area) is required to compress the rod back to its original length? Write an answer for the magnitude of F/A in terms of Y, α , L_o, and ΔT
|F/A| = YαΔT
Total change in length is the sum of the change due to temp and change due to applied stress:
ΔL = ΔL_thermal + ΔL_stress
change in length under heating is:
ΔL_thermal = αL_oΔT
Find ΔL_stress = (F/A)(L_o/Y)
10m long steel train rails are laid end to end on a winter day (0°C) If the engineer forgot to leave gaps for thermal expansion, roughly how much force is generated at the ends of each rail due to thermal stress when the temperature reaches 30°C?
Cross section area of rail: 0.01m^2
Y_steel = 2010^10 Pa
α_steel = 1.210^-5K^-1
F/A = -Yα
700 000N;
ΔF= Yα ΔTA = 700 00N;
10m long steel train rails are laid end to end on a winter day (0°C) If the engineer forgot to leave gaps for thermal expansion, roughly how much force is generated at the ends of each rail due to thermal stress when the temperature reaches 30°C?
Cross section area of rail: 0.01m^2
Y_steel = 2010^10 Pa
α_steel = 1.210^-5K^-1
F/A = -Yα
How much extra gap should be left over>
3.2mm;
ΔL_thermal = αL_oΔT = 3.6mm
Two objects with the same mass are put in thermal contact but
insulated from their environment. If the initial temperatures are
100º C and 0º C, the final equilibrium temperature will be?
Somewhere between 100º C and 0º C but not necessarily 50°C;
some materials require more energy for a given temperature.
A mass M of ice at temperature T1 < 0 is heated until we have water at
temperature T2 > 0. How much heat has been added?
Mc_ice (- T1) + ML_f + Mc_water (T2)
