Flashcards in Heat and Temperature Deck (35)
Created by chemical means by burning, or by mechanical means by friction, or by passing current through an electrical resistance also produces heat e.g. an electrical fire.
Can also be converted into other forms of energy e.g. into pressure energy in a steam boiler.
Temperature vs Heat
Amount of heat energy in a body cannot be measured.
Temperature is just a measure of how hot a body is, and not the amount of heat energy is contains.
Heat Energy Conversion
Heat energy can also be converted back to other forms e.g. chemical or electrical.
Heat Energy Flow
Always from areas of high temp to low temp.
Heat and Temperature Example
Fine copper wire over a match will heat up in seconds and be red hot meaning that it will be 800-900 degrees celcius.
The same match held under a kettle holding one litre would have no noticeable impact on the waters temperature, yet the amount of heat supplied to the water and to the wire will be the same.
Rise in temp in a body to which heat is applied, depends on:
Amount of heat transferred to body.
The mass of the body.
Specific heat capacity of the material from which the body is made.
Human body cannot measure temps, it can only make comparisons to other temps.
Temp can be measured by making use of one of the effects of heat on materials e.g. using the way that liquids expand as their temp rises, which is the principle behind a thermometer.
Contains usually mercury which has a high boiling point of 357 degrees Celsius, a uniform expansion coefficient, a low heat capacity and it is opaque.
Freezing point of -39 degrees Celsius meaning it is not suitable for measuring low temperatures below zero.
Alcohol has a lower boiling point but a lower freezing point so it is more suitable for measuring temps below zero.
Temperature scales in use e.g. Celsius, Fahrenheit, Kelvin (Has a specialist use).
Two fixed points are required to make a thermometric scale e.g. for Celsius, the freezing point of ice, and boiling point of water is used when at standard atmospheric pressure.
Thermometer made by holding stem in melting ice, and the upper fixed point is held in steam above boiling water (at standard atmospheric pressure-if pressure is different then adjustments must be made).
The level at which the liquid stands at each of the two fixed points is marked on the steam as the two ends of the scale.
The Celsius (or Centigrade) Scale
Lower fixed point is 0 and upper fixed point is 100.
The stem between these two points is divided into 100 equal divisions, or degrees. These divisions are called Celsius Degrees.
The Fahrenheit Scale
Uses freezing mixture to give lower point, and boiling point of water as upper fixed point, and the scale was divided into 212 equal divisions.
Therefore freezing point of water is 32 degrees Fahrenheit.
There are 180 Fahrenheit Degrees between freezing and boiling points.
The Kelvin or Absolute Scale
Starts at -273 degrees Celsius (lowest possible temperature).
The hotter a mass is, the faster the molecules are moving, but at -273 the molecules stop moving completely and it is impossible to cool the mass any further.
Kelvin scale has zero at -273. Degrees on this scale are the same size as Celsius Degrees and have the symbol K.
Absolute Temp (In Degrees Kelvin)= Celsius Temp + 273
0 Degrees Kelvin = -273 Degrees Celsius
273 Degrees Kelvin = 0 Degrees Celsius
373 Degrees Kelvin = 100 Degrees Celsius
When to use Kelvin Scale
When calculating how the volume of gas changes with temperature and pressure
For most purposes we can assume that the volume of gas is proportional to its temperature in degrees kelvin. Therefore if temp is doubled, the volume will double and if temp is halved, the volume will halve.
Air or Gas Thermometer
Instead of using a liquid, a bulb containing gas or air can be used. The expansion of gas causes a small thread of mercury to move along a scale.
These are very sensitive and may require correction to compensate for atmospheric pressure.
Using Solids to Measure Temperature
The way that a solid expands when its temp increases can be used for measuring temp.
The expansion can be used directly or the differing expansion of two dissimilar metals can be used (a comparison?).
When the junction of two different metal wires is heated, an electrical potential appears at the junction.
A calibration can be made between the potential and the temperature, so temp can be measured indirectly by measuring the potential with a voltmeter.
Electrical resistance in a wire increases with a rise in temp, and the change in resistance can be used to measure temp.
Platinum is normally used due to high melting point and high temperature coefficient of resistance, meaning a small rise in temp produces a relatively large rise in resistance.
Platinum resistance thermometers can measure between -200 and 1200 degrees Celsius.
But are large compared to thermocouples and do not follow rapid changes in temperature very easily.
These are semiconductor devices, which have a negative temperature coefficient of resistance, meaning that an increase in temp produces a decrease in resistance.
They are small and can follow rapid changes in temperature. Very robust too.
Can measure -70 to 300 degrees Celsius.
Less accurate than resistance thermometers.
Usually used in modern heat detectors.
Comparison by Brightness
Temps above 750 degrees Celsius, objects start to glow red, changing to yellow and brightening until the temp reaches about 1250 degrees Celsius.
Temp measurements are made by comparing the brightness of the object with the filament of an electric lamp whose brightness can be altered by changing the current through it.
If the current is too low, the filament appears darker than the object, and if the current is too large, the filament appears brighter.
When the filament 'disappears' against the object, they have the same temperature and brightness.
Temp can be found indirectly by measuring current through the filament.
Infra red cameras and sensors detect heat in the same way our eyes detect light.
Infra red radiation is given off by a body (and some gases) when they are hot.
Infra red sensors are sensitive to this type of radiation and can be designed to measure the temp of an object by analysing the strength and wavelength of the radiation.
Infra red radiation behaves in the same way as light, but it can pass through some things that light cannot and is blocked by some things that does not block light.
E.g. infra red radiation can pass through thick smoke which light cannot which is why infra red cameras can be useful in search and rescue ops...infra red radiation given off by a body can be detected.
Units of Heat
Units used to measure amount of energy in a body-called heat.
Conversion of one type of energy to another is never 100 per cent efficient and some of it will appear in another form, usually heat energy e.g. in braking a car is dissipated as heat generated by the friction of the brake pads.
The Joule (J)
Same as used to measure energy, also used to measure heat energy.
Defined from mechanics, where energy is the ability to do work- Therefore work and energy are measured in the same unit.
1J=The point at which 1N force is applied and moves through 1M in the direction of the force.
Quantity of heat required to raise temp of 1g of water through 1 degrees Celsius.
Often used to measure energy content of food, but the number of Calories in chocolate for example 200 is actually in kilocalories.
1 Kilocalorie= 1000 Calories
1 Calorie= 4.18 Joules.
The British Thermal Unit (Btu)
Quantity of heat required to raise temp of 1lb of water through 1 degrees Fahrenheit.
The word specific is generally used if discussing a unit of mass of a material is considered.
Lower the specific heat capacity, the quicker something heats up and vice versa.
Heat energy can only flow until both bodies are the same temperature or until the bodies are separated, until this point the an amount of heat energy would have been transferred and this is measured in J.
Rise in temp of a body depends on:
1. Amount of heat energy supplied to a body
2. Mass of the body
3. Specific heat capacity of the body
Specific heat capacity of an object is the heat required to raise the temp of 1kg of the material by 1 degrees Celsius. Measured in joules per kg per degrees centigrade (j/kg degrees Celsius).
Specific Heat Example
2 drums, one containing water and one containing same amount of oil, both fitted with thermometers and suspended over two identical heaters supplying the same amount of heat energy.
After a time, the oil has risen by 10 degrees Celsius but the water has only risen by 5 degrees Celsius in the same time.
The difference in temp rise is caused by a difference in specific heat capacity.
Therefore the water has a greater specific heat capacity as it showed a lesser rise in temp for the same amount of heat energy supplied.
The larger the specific heat capacity, the more energy it takes to raise the temperature by a given amount.
Water and Specific Heat
Water has high specific heat capacity (4200 kg j per degrees Celsius) meaning it is good for firefighting as it can absorb a large amount of heat energy.
In general, materials with low specific heat capacity are a greater fire risk.
Change of State and Latent Heat
Changes between states e.g. solids, liquids, gases etc.
Freezing, melting, boiling, condensation and sublimation all cause a change of state.
Latent heat of Vaporisation
When water is boiling, bubbles rise from bottom to top and burst and escape as steam. Once boiling temp remains at 100 and doesn't get any hotter, but the constant application of heat energy allows the water molecules to pull themselves apart and convert into steam.