Module 5: C14 - Thermal Physics Flashcards
What is the triple point of a substance?
A triple point of a substance is one specific temperature and pressure where 3 phases of matter (solid, liquid, and gas) of that substance can exist in thermal equilibrium (no net transfer of thermal energy between the phases)
What is temperature a measure of?
Temperature is a measure of the hotness of an object on a chosen scale. The hotter an object is, the higher its temperature.
What happens when one object is hotter than another (where does the net flow of thermal energy go)
If one object is hotter than another, there is a net flow of thermal energy from the hotter object into the colder one. This increases the temperature of the colder object and lowers the temperature of the hotter one. For example, when the outside air temperature is lower than your body temperature, there is a net flow of energy from you to your surroundings.
What happens/what does it mean when 2 objects are in thermal equilibrium
When two objects are in thermal equilibrium, there is no net flow of thermal energy between them. This means any objects in thermal equilibrium must be at the same temperature.
What is the Zeroth Law
The zeroth law states that if two objects are each in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.
If both A and C are in thermal equilibrium with B, then A is in thermal equilibrium with C. This means that all three objects are at the same temperature.
What is the relationship between Kelvin and Celsius
T (K) = Θ (°C) + 273
Temperatures in K are always positive, and the lowest temperature on the absolute scale is 0K
What is the Kinetic Model
The kinetic model describes how all substances are made up of atoms or molecules, which are arranged differently depending on the phase of the substance.
How are atoms or molecules arranged in a solid?
In solids the atoms or molecules are regularly arranged packed closely together, with strong electrostatic forces of attraction between them, holding them in fixed positions, but they can vibrate and so have kinetic energy.
How are atoms or molecules arranged in a liquid?
In liquids the atoms or molecules are still very close together, but they have more kinetic energy than in solids, and - unlike in solids - they can change position and flow past each other.
How are atoms or molecules arranged in a gas?
In gases, the atoms or molecules have more kinetic energy than those’d in liquids, and they are much further apart. They are free to move past each other, as there are negligible electrostatic forces between them, unless they collide with each other on the container walls. They move randomly with different speeds in different directions.
In general how does the state of matter of something affect how dense it is?
The spacing between the particles (atoms or molecules) in a substance in different phases affects to the density of the substance. In general, a substance is most dense in its so,I’d phase and lest dense in it’s gaseous phase.
Why is solid water less dense than liquid water?
Water freezes into a regular crystalline pattern held together by strong electrostatic forces between the molecules. In this structure, the molecules are held slightly further apart than in their random arrangement in liquid water, so ice is slightly less dense.
How does a thermometer detect a change in temperature?
Some physical property of a system changes as the system’s temperature changes, such as
- the volume of a liquid
- the dimensions of a solid
- the pressure of a gas at constant volume
- the volume of a gas at constant pressure
- the electric resistance of a conductor
What does it mean 2 objects are in thermal contact?
Two objects are in thermal contact with each other if energy can be exchanged between them due to a temperature difference.
Thermal contact and physical contact are not necessarily the same.
Example:
Warm your hands near a fire
What does it mean 2 objects are in thermal equilibrium?
A situation in which two objects would not exchange energy if they were placed in thermal contact.
Example:
Mixing hot water and cold water. Hot water cools off and cold water warms up. Eventually, a final intermediate temperature is reached, which is equal throughout the mixture. At this point, energy exchange ceases, reaching thermal equilibrium.
What is the internal energy of substance defined as
The sum of the randomly distributed kinetic and potential energies of the atoms or molecules within the substance.
Heat definition
Heat is the amount of energy stored in a substance and can be regarded as the sum of the random kinetic potential energies of all the molecules in that substance. This is sometimes called the Internal Energy of the substance.
Specific Heat Capacity Definition
The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1K (or 1ºC). It’s unit are J kg^-1 °C^-1 or J kg^-1 K^-1
Equation for Specific Heat Capacity
ΔE = mcΔθ
or
c = E/mΔθ
where
∆E is the energy change in J
m is the mass in kg
c is the specific heat capacity in J K-1 kg-1 (or J°C^-1 kg^-1 )
∆θ is the temperature change in K (or °C)
Worked Example:
A bucket containing 11.5 litres of cold water at 10°C is
taken into a house at a warmer temperature and left
inside until it has reached thermal equilibrium with it
new surroundings. The specific heat capacity of water
is 4200 J kg^-1°C^-1.
If 504 kJ of energy is absorbed from the surroundings
to heat the water, what is the temperature of the
room?
1 litre = 1kg
ΔE = 504kJ = 504x10^3J
m = 11.5kg
c = 4200J kg^-1 °C^-1
ΔΘ = ?
ΔE = mcΔΘ
ΔΘ = ΔE/mc
ΔΘ = 504x10^3 / 11.5x4200
ΔΘ = 10.4°C
Water was 10°C
Now it is 20.4°C
Worked Example:
A 60 W immersion heater takes 2.5 minutes to heat 0.5 kg of water from 21to 25°C, what is the specific heat capacity of the water. Why is it higher than the accepted value of c = 4200 J kg^-1 °C^-1?
ΔΘ = 4°C
m = 0.5kg
t = 150s
P = 60W
E = mcΔΘ
P = E/t
60x150 = E
E = 9000J
c = E/mΔΘ
c = 9000/0.5 x 4
c = 4500J kg^-1 °C^-1
It is higher than the accepted value of c = 4200 Jkg^-1°C^-1, because heat is being lost from the object.
Equation showing relationship between Energy gained by cooler object and Energy lost by hotter object
𝐸𝑛𝑒𝑟𝑔𝑦 𝑔𝑎𝑖𝑛𝑒𝑑 𝑏𝑦 𝑐𝑜𝑜𝑙𝑒𝑟 𝑜𝑏𝑗𝑒𝑐𝑡 = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑙𝑜𝑠𝑡 𝑏𝑦 ℎ𝑜𝑡𝑡𝑒𝑟 𝑜𝑏𝑗𝑒𝑐𝑡
or
𝑚1𝑐1∆𝜃1= 𝑚2𝑐2∆𝜃2
Worked Example:
If 500ml of water is in thermal equilibrium with a copper object at 25 degrees Celsius and the specific heat capacity of water is 4200 J kg^-1 °C^-1 ,the water was initially at 20 °C, the copper at 100 °C and the copper has a mass of 0.155 kg, what is the specific heat capacity of copper? (Hint: What is the density of water?)
Water:
m = 0.5kg
c = 4200 J kg^-1 °C^-1
ΔΘ = 5°C
Copper:
m = 0.155kg
c = ?
ΔΘ = 75°C
𝑚1𝑐1∆𝜃1= 𝑚2𝑐2∆𝜃2
c2 = 𝑚1𝑐1∆𝜃1/𝑚2∆𝜃2
c2 = 0.5x4200x5/0.155x75
c2 = 903 J kg^-1 C^-1
Specific Latent Heat Definition
The specific latent heat (L) of fusion or vaporisation is the quantity of thermal energy required to change the phase of 1kg of a substance.
• Fusion (liquid → solid)
• Vaporisation (liquid → gas)
• …or the other way (Melting/Condensing)
Equation for Energy in Specific Latent Heat
E = mL
(E in Jkg^-1)
Worked Example:
The specific latent heat of fusion (melting) of ice is 330,000 J kg-1
.
What is the energy needed to melt 0.65 kg of ice?
E = mL
0.65 x 330000 = 210,000J
Worked Example:
The power of the immersion heater in the diagram is 60 W. In 5 minutes, the top pan balance reading falls from 282g to 274g. What is the specific latent heat of vaporisation of water?
P = 60W
t = 300s
Δm = 8g = 0.008kg
P = E/t
E = Pt
E = 60x300
E = 18000 J
E = mL
L = E/m
L = 18,000/0.008
L = 2.25x10^6 Jkg^-1
What equation for change in energy should be used when a question includes heating and then changing the phase of a substance
ΔE = mcΔΘ + mL
Worked Example:
A container of water at 18°C containing 1500g of water is heated until it boils dry. How much energy is required to make this happen?
(Lv = 2.25x10^6 Jkg^-1
c = 4200 J kg^-1 °C^-1)
m = 1.5kg
Lv = 2.25x10^6 J kg^-1
Θ = 18°C
c = 4200 J kg^-1 °C
ΔE = mcΔΘ + mL
ΔE = 1.5 x 4200 x 82 + 1.5 x 2.25x10^6
ΔE = 3.8916x10^6 J
ΔE = 3.89MJ
What happens in Specific Latent Heat of Fusion?
When the substance changes from solid to liquid phase, we refer to the specific latent heat of fusion, Lf.
What happens in Specific Latent Heat of Vaporisation?
When the substance changes from solid to liquid phase, we refer to the specific latent heat of vaporisation, Lv.
How can you find the energy transferred from a heater to a substance (specific heat capacity)
E = IVt
c = ItV/mΔΘ
How can you determine the specific latent heat of fusion using current and voltage
Lf = IVt / m
What happens when a substance reaches its melting or boiling point
When a substance reaches its melting or boiling point, while it is changing phase the energy transferred to the substance does not increase its temperature. Instead, the electrostatic potential energy of the substance increases as the electrical forces between the atoms or molecules change. Only once the phase change is complete does the kinetic energy of the atoms or molecules increase further, and so the temperature rises again.
What is the electrostatic potential energy of atoms in a gas
The electrostatic potential energy is zero because there negligible electrical forces between atoms or molecules
What is the electrostatic potential energy of atoms in a liquid
The electrostatic forces between atoms or molecules give the electrostatic potential energy a negative value.
The negative simple means that energy must be supplied to break the atomic or molecular bonds.
What is the electrostatic potential energy of atoms in a solid
The electrostatic forces between atoms or molecules are very large, so the electrostatic potential energy has a large negative value.
The negative simple means that energy must be supplied to break the atomic or molecular bonds.
