Thermal Physics (unit 3) Flashcards
What is internal energy?
the sum of the potential and kinetic energies of the particles in that system.
U = KE + PE
What is the internal energy of an ideal gas?
total TRANSITIONAL kinetic energy of the gas particles.
3/2 nRT or 3/2 NKt
Why for an ideal gas its internal energy is just it’s kinetic energy?
the attractive forces between the particles are negligible. This means that the amount of potential energy that they have is negligible.
When is the internal energy of a system at it’s minimum?
absolute zero
How does heat enter or leave a system?
heat enters or leaves a system through its boundary or container
wall
How does heat react to in the context of it’s temperature and it’s surroundings?
Heat enters a system if the temperature of the system is lower than the temperature of its surroundings.
Heat leaves a system if the temperature of the system is higher than the temperature of its surroundings.
What do we conclude as heat enters and leaves a system?
heat is energy in transit and not contained within the system
What happens if temp is the same at both sides of the BOUNDARY?
said to be in thermal equilibrium and there is no heat flow between them
What happens when to systems are placed into contact?
heat flows from one to the other until thermal equilibrium is reached and both systems are at the same temperature.
What is energy also?
-Energy can also enter or leave a system by means of work.
-work is also energy in transit.
How to calculate work done by a gas or on a gas?
W= p∆V (constant pressure)
How can you also work out work done?
Area under P V graph
Q context in Q=∆U+W ?
Q is positive (+ve) when heat enters a gas and negative (-ve) when heat leaves a gas.
W context in Q=∆U+W ?
W is positive (+ve) when work is done by the gas (i.e. it expands) and negative (-ve) when work is done on the gas (i.e. it is compressed).
ΔU Context in Q=∆U+W ?
ΔU is positive (+ve) when the internal energy of the gas increases and negative (-ve) when the internal energy of the gas decreases.
If volume is constant in graph?
Work done is zero
If temp is constant in graph?
ΔU =0
How to work out NET WORK DONE?
-area inside loop
If volume increases?
Work done by gas
Volume decreases?
Work done on gas
Amount of external work on a solid or liquid?
for a solid (or liquid), W is usually negligible, so Q = ΔU
Equation for heat capacity?
Energy = mass x heat capacity x rise in temp
How to use heat capacity?
If a substance has a specific heat capacity of 2000 J kg -1°C-1, then:
2000 J will raise the temperature of 1 kg by 1°C
2 x 2000 J will raise the temperature of 2 kg by 1°C
3 x 2 x 2000 J will raise the temperature of 2 kg by 3°C
How to calculate heat flow?
Work done
-Negative when FLOW OUT OF GAS
-Positive when FLOWN INTO SYSTEM
Estimation of absolute zero by gas laws? (Charles Law)
-Heat the water using a Bunsen burner and stir regularly. Measure the length of the trapped air every 10 oC up to 80 oC
-Plot a graph of the length of trapped air (y-axis) against temperature (x-axis). The temperature scale should cover the range -400 oC to 100 oC
-The length scale should start at zero
- Draw a line of best fit extended back until it cuts the x-axis, this is absolute zero.
What does estimation of absolute zero using gas laws practical look like?
- sulfuric acid with length of trapped air with thermometer in a container of water
Measurement of the specific heat capacity for a solid practical?
-Use a cylindrical block of the metal to be tested (such as copper or aluminium)
- The block should be well lagged using an insulator such as polystyrene and it needs two pre-drilled holes, one for a heater and one for a thermometer. -Measure the mass, m, of the block and
record its initial temperature, θ1.
-Switch the heater on and start the stopwatch. Record the voltmeter and ammeter readings. When the temperature has risen by 30 oC switch the heater
off and record the time taken, t.
-The formula can then be used to determine a value for c.
What does measurement of specific heat capacity for a solid look like?
- circuit with power supply with ammeter in series to voltmeter and heater parallel to voltmeter
- heater is in a metal block with square insulation alongside a thermometer
What equations for measurement of specific heat capacity?
-Assuming no energy losses:
-Electrical energy supplied by the heater = heat received by the block
-ItV = mc(θ2 – θ1)
- c = specific heat capacity
Specific heat capacity units?
- J kg^-1 C^-1
What is the energy at absolute zero?
- minimum internal energy
