Thermal Energy Transfer
is transferred from a region of higher temperature to a region of lower temperature
- When a thermometer is placed in a beaker of boiling water, the thermometer reading increases
- This is because the thermometer is a lot cooler than the water
- The thermometer gradually becomes hotter from the thermal energy (or heat) transferring from the water to the thermometer
The conservation of energy
states that energy is never created or destroyed, only transferred from one form to another
direction of energy flow
This means temperature tells us the direction of energy flow when two regions are in contact (from hotter to cooler)
-The mechanism by which the thermal energy is transferred is by either conduction, convection or radiation
Defining Thermal Equilibrium
-When two substances in physical contact with each other no longer exchange any heat energy and both reach an equal temperature
-There is no longer thermal energy transfer between the regions
The two regions need to be in contact for this to occur
-The hotter region will cool down and the cooler region will heat up until they reach the same temperature
-The final temperature when two regions are in thermal equilibrium depends on the initial temperature difference between them
Thermal energy is always transferred from a
hotter region to lower region
Measurement of Temperature
- A thermometer is any device that is used to measure temperature
- Each type of thermometer uses a physical property of a material that varies with temperature – examples of such properties include:
- —-The density of a liquid
- —-The volume of a gas at constant pressure
- —-Resistance of a metal
e. m.f. of a thermocouple
-In each case, the thermometer must be calibrated at two or more known temperatures (commonly the boiling and melting points of water, 0oC and 100oC respectively) and the scale divided into equal divisions
The Density of a Liquid
- A liquid-in-glass thermometer depends on the density change of a liquid (commonly mercury)
- It consists of a thin glass capillary tube containing a liquid that expands with temperature
- A scale along the side of the tube allows the temperature to be measured based on the length of liquid within the tube
Volume of a Gas at Constant Pressure
-The volume of an ideal gas is directly proportional to its temperature when at constant pressure (Charles’s law)
V ∝ T
- As the temperature of the gas increases, its volume increases and vice versa
- A gas thermometer must be calibrated – by knowing the temperature of the gas at a certain volume, a temperature scale can be determined depending on how quickly the gas expands with temperature
Resistance of a Metal
- electrical resistance changes with temperature e.g. the resistance of a filament lamp increases when current increases through it
- —-For metals: resistance increases with temperature at a steady rate
- —-For thermistors: resistance changes rapidly over a narrow range of temperatures
- As a thermistor gets hotter, its resistance decreases
- This means a thermometer based on a thermistor can be used to measure a range of temperatures
- The relationship between the resistance and temperature is non-linear
- —This means the graph of temperature against resistance will be a curved line and the thermistor will have to be calibrated
E.M.F. of a Thermocouple
- A thermocouple is an electrical device used as the sensor of a thermometer
- It consists of two wires of different, or dissimilar, metals attached to each other, producing a junction on one end
- —-The opposite ends are connected to a voltmeter
- When this junction is heated, an e.m.f. is produced between the two wires which is measured on the voltmeter
- The greater the difference in temperature between the wires, the greater the e.m.f
- However, a thermocouple requires calibration since the e.m.f. does not vary linearly with temperature
- The graph against e.m.f. and temperature is a positive, curved line
The Kelvin scale
is known as the thermodynamic scale and was designed to overcome the problem with scales of temperature
- The thermodynamic scale is said to be an absolute scale that is not defined in terms of a property of any particular substance
- This is because thermodynamic temperatures do not depend on the property of any particular substance
Scale of Thermodynamic Temperature
- As an everyday scale of temperature, Celsius (oC) is the most familiar
- This scale is based on the properties of water – the freezing point of water was taken as taken as 0 oC and the boiling point as 100 oC
- —However, there is nothing special about these two temperatures
- —The freezing and boiling point of water will actually change as its pressure changes
The Celsius scale
is used to measure the temperature in a liquid-in-glass thermometer
- —However, the expansion of the liquid might be non-linear
- Other temperature scales include:
- —Fahrenheit, commonly used in the US
- —Kelvin, used in thermodynamics
Absolute Zero
- On the thermodynamic (Kelvin) temperature scale, absolute zero is defined as:
- The lowest temperature possible. Equal to 0 K or -273.15 °C
- It is not possible to have a temperature lower than 0 K
- —This means a temperature in Kelvin will never be a negative value
- Absolute zero is defined in kinetic terms as:
The temperature at which the atoms and molecules in all substances have zero kinetic and potential energy
- This means for a system at 0 K, it is not possible to remove any more energy from it
- Even in space, the temperature is roughly 2.7 K, just above absolute zero
Using the Kelvin Scale
-To convert between temperatures θ in the Celsius scale, and T in the Kelvin scale, use the following conversion:
θ / oC = T / K − 273.15
T / K = θ / oC + 273.15
- The divisions on both scales are equal. This means:
- A change in a temperature of 1 K is equal to a change in temperature of 1 oC
Defining Specific Heat Capacity
The amount of thermal energy required to raise the temperature of 1 kg of a substance by 1 °C
This quantity determines the amount of energy needed to change the temperature of a substance
- The specific heat capacity is measured in units of Joules per kilogram per Kelvin (J kg-1 K-1) or Joules per kilogram per Celsius (J kg-1 oC-1) and has the symbol c
- —-Different substances have different specific heat capacities
- —-Specific heat capacity is mainly used in liquids and solids
From the definition of specific heat capacity, it follows that:
- The heavier the material, the more thermal energy that will be required to raise its temperature
- The larger the change in temperature, the higher the thermal energy will be required to achieve this change
Calculating Specific Heat Capacity
-The amount of thermal energy Q needed to raise the temperature by Δθ for a mass m with specific heat capacity c is equal to:
ΔQ = mcΔθ
- Where:
- ΔQ = change in thermal energy (J)
- m = mass of the substance you are heating up (kg)
- c = specific heat capacity of the substance (J kg-1K-1or J kg-1 oC-1)
-Δθ = change in temperature (K or oC)
If a substance has a low specific heat capacity
it heats up and cools down quickly
If a substance has a high specific heat capacity
it heats up and cools down slowly
The specific heat capacity of different substances determines
- how useful they would be for a specific purpose eg. choosing the best material for kitchen appliances
- Good electrical conductors, such as copper and lead, are also excellent conductors of heat due to their low specific heat capacity
Defining Latent Heat Capacity
- Energy is required to change the state of substance
- Examples of changes of state are:
- Melting = solid to liquid
- Evaporation/vaporisation/boiling = liquid to gas
- Sublimation = solid to gas
- Freezing = liquid to solid
- Condensation = gas to liquid
Specific latent heat of fusion (melting)
- The thermal energy required to convert 1 kg of solid to liquid with no change in temperature
- This is used when melting a solid or freezing a liquid
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Chapter 1 Motion in a Circle17
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Chapter 2 Gravitational Fields33
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Chapter 3 Temperature27
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Chapter 4 Ideal Gases33
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Chapter 5 Thermodynamics26
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Chapter 6 Oscillations48
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Chapter 7 Electric Fields47
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Chapter 8 Capacitance27
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Chapter 9 Magnetic Fields82
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Chapter 10 Alternating Currents30
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Chapter 11 Quantum Physics37
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Chapter 12 Nuclear Physics44
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Chapter 13 Medical Physics66
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Chapter 14 Astronomy & Cosmology36
