thermal properties and temperature Flashcards
What is Thermal Expansion?
and why it happens
Note: When this happens,what remains the same and what doesn’t
When (most) materials are heated, they expand.
This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other apart.
it is the space taken up by the molecules that increases. The molecules themselves remain the same size.
Thermal Expansion in Solids, Liquids & Gase
The thermal expansion of materials can have some useful applications:
The expansion of a liquid in a thermometer can be used to measure temperature.
A bimetallic strip, consisting of two metals that expand at different rates, can be made to bend at a given temperature, forming a temperature-activated switch.
The thermal expansion of materials can have undesirable consequences:
Consequences:
The expansion of solid materials can cause them to buckle if they get too hot.
This could include:
Metal railway tracks.
Road surfaces.
Bridges.
Things that are prone to buckling in this way often have gaps built into them, providing some room for them to expand into.
When a substance is heated, some of its physical properties can change.
If these properties change in a well-defined way, by measuring the property you can
The volume (and density) of the substance.
The electrical resistance of the substance.
determine the temperature.
When a substance is heated, some of its physical properties can change.
If these properties change in a well-defined way, by measuring the property you can determine the temperature.
how?
In order to build a thermometer based on one of these properties, you need to start by measuring the property at some well-defined fixed points.
A fixed point is a temperature at which some easily identifiable change occurs, such as the melting of ice (at 0 °c) or the boiling of pure water (at 100 °c).
These fixed points allow you to know the temperature without having to measure it directly.
Usually two fixed points are used:
The lower fixed point: the melting temperature of ice.
The upper fixed point: the boiling temperature of pure water.
Once a property (such as electrical resistance) has been measured at these two fixed points, the values of that property at other temperatures can be worked out.
Measuring some property at two fixed points allows you to then work out the temperature for other values of that propertydraw gragp
about the way in which the property changes with temperature: linear
That it does so at a steady rate.
Properties that change in this way are said to be linear, and linearity is a very useful property for something to have as it makes it easy to work out the relationship between that thing and the temperature.
Sensitivity
– if the thing you are measuring changes significantly when the temperature changes, it makes it easier to detect small changes in temperature: Your thermometer is more sensitive.
Range
What are the lowest and highest temperatures the thermometer can measure? If a liquid-thermometer is too short, there may not be enough room for the liquid to expand into.
descirbe liquid-in-glass thermometer
a thin glass capillary tube containing a liquid that expands with temperature.
At one end of the tube is a glass bulb, containing a larger volume of the liquid which expands when heated, moving into the narrower tube.
A scale along the side of the tube allows the temperature to be measured based on the length of liquid within the tube.
The liquids used in liquid-in-glass thermometers are chosen for their thermal expansion properties:
thermal expansion properties:
They expand linearly with temperature.
They expand significantly, giving them greater sensitivity.
They have low freezing points and high boiling points, allowing them to be used over a greater range of temperatures.
The use of a large glass bulb coupled to a very narrow tube also results in…as
greater sensitivity
a small change in volume results in the liquid moving a significant distance along the tube.
describe A thermocouple
two different types of wire attached together.
When the joint between the two wires is heated, a potential difference (voltage) is created between the two wires.
The greater the temperature, the greater the potential difference.
Thermocouples are not as sensitive as liquid-in-glass thermometers, but because the metals have high melting points, they can be used to measure very high temperatures.
Thermocouples are also very responsive to rapidly changing temperatures.
When a substance is heated, its…increases.
internal energy increases.
The thermal capacity of an object is related to the specific heat capacity by the equation:
The amount of thermal energy needed is given by the equation:
describe an experiment to determine the specific heat capacity of a substance.
A method for carrying out such an experiment is given below.
The following apparatus will be needed:
A block of the substance (preferably 1kg in mass) or in the case of a fluid, a beaker containing a known mass of the fluid.
A thermometer.
An appropriate heater (e.g. an immersion heater).
A power source.
A joule meter or a voltmeter, ammeter and stopclock (I will assume we have the latter).
describe an experiment to determine the specific heat capacity of a substance.
A method for carrying out such an experiment is given below.
In this experiment you need to use the following equation to determine the specific heat capacity of the substance:
For an object of mass m, the total energy required to change its state is given by the equation:
A method for determining the specific latent heat for steam is given below.
In this experiment you need to use the following equation:
A method for determining the specific latent heat for steam is given below.
The following apparatus will be needed:
A vacuum flask containing pure water
A top pan balance
An appropriate heater (e.g. an immersion heater).
A power source.
A joule meter or a voltmeter, ammeter and stop-clock (I will assume we have the latter).
