Thermal Physics 2 Flashcards
What is internal energy?
Internal energy is the sum of the randomly distributed kinetic and potential energies of the particles in a body.
What forms of energy are part of internal energy?
Kinetic energy, due to the speed of the molecules, and potential energy, due to the separation between the molecules.
What does the amount of kinetic and potential energy in a substance depend on?
It depends on the phase of matter (solid, liquid or gas).
What is the symbol and unit for internal energy?
The symbol is U and the unit is Joules (J).
Why are the particles’ energies called ‘randomly distributed’?
Because the particles have different speeds and separations, leading to varied kinetic and potential energies.
What four main factors determine the internal energy of a system?
- Temperature (higher temperature means higher kinetic energy), 2. Random motion of molecules, 3. Phase of matter (gases have the highest, solids the lowest internal energy), 4. Intermolecular forces (stronger forces increase potential energy).
How can the internal energy of a system increase?
By doing work on the system or adding heat to it.
How can the internal energy of a system decrease?
By losing heat to its surroundings or by changing state from solid to liquid or liquid to gas.
What should be included in the definition of internal energy in exams?
You must mention ‘random motion’ of the particles or ‘random distribution’ of energies to get full marks.
What does the First Law of Thermodynamics state?
When energy is transferred into a system by heating or doing work on it, its internal energy increases.
How can the First Law of Thermodynamics be expressed symbolically?
ΔU = q + W, where ΔU is change in internal energy, q is heat added, and W is work done on the system.
What happens to internal energy when a gas expands?
The gas does work on the surroundings, so its internal energy decreases.
What is the sign of work done when gas expands?
Work done is negative (–W).
What happens to internal energy when a gas is compressed?
Work is done on the gas, increasing its internal energy.
What is the sign of work done when gas is compressed?
Work done is positive (+W).
What does a pressure vs volume graph show about internal energy?
It shows the work done as the area under the curve. Constant pressure is a horizontal line, constant volume is a vertical line.
Draw and explain a graph of pressure vs. volume for constant pressure and volume.
See document for answer.
When is no work done in a thermodynamic process?
When the volume stays constant (vertical line on a p-V graph), area under the curve is zero.
What is specific heat capacity?
It is the amount of thermal energy required to raise the temperature of 1 kg of a substance by 1°C or 1 K.
What is the symbol and unit for specific heat capacity?
Symbol: c, Unit: J kg⁻¹ K⁻¹ or J kg⁻¹ °C⁻¹.
How does the mass and temperature change affect thermal energy required?
Larger mass and larger temperature change both require more thermal energy.
What is the formula for calculating change in thermal energy?
ΔQ = mcΔθ, where m is mass, c is specific heat capacity, and Δθ is temperature change.
What does a low specific heat capacity mean?
The substance heats up and cools down quickly.
What does a high specific heat capacity mean?
The substance heats up and cools down slowly.
What is a continuous-flow calorimeter used for?
To find the specific heat capacity of a fluid by flowing it over a heater and measuring energy transferred.
What is assumed in the continuous-flow experiment?
That heat lost to the surroundings remains constant in both trials.
What equation is used in the continuous-flow method?
I2V2t2 – I1V1t1 = (m2 – m1)cΔθ
What is latent heat?
It is the thermal energy required to change the state of 1 kg of a substance without any change of temperature.
What are the two types of latent heat?
Specific latent heat of fusion and specific latent heat of vaporisation.
What is the specific latent heat of fusion?
Thermal energy required to convert 1 kg of solid to liquid with no temperature change.
What is the specific latent heat of vaporisation?
Thermal energy required to convert 1 kg of liquid to gas with no temperature change.
What is the formula for energy needed to change state?
Q = mL, where Q is energy, m is mass, and L is latent heat.
What are the latent heat values for water?
Fusion: 330 kJ/kg, Vaporisation: 2.26 MJ/kg.
Why is vaporisation more energy intensive than fusion?
Because molecules must be fully separated, requiring more energy to overcome all intermolecular forces and to do work against atmospheric pressure.
During phase change, what type of energy changes?
Potential energy changes while kinetic energy remains constant.
Why is there no temperature change during phase changes?
Because all added energy goes into changing the potential energy (molecular separation), not kinetic energy.
Draw and explain a graph of temperature vs. heat supplied showing phase changes.
See document for answer.
