Heat and Temperature

Question 1

Heat Energy

Answer 1

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.

Question 2

Temperature vs Heat

Answer 2

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.

Question 3

Heat Energy Conversion

Answer 3

Heat energy can also be converted back to other forms e.g. chemical or electrical.

Question 4

Heat Energy Flow

Answer 4

Always from areas of high temp to low temp.

Question 5

Heat and Temperature Example

Answer 5

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.

Question 6

Measuring Temperature

Answer 6

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.

Question 7

Thermometers

Answer 7

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.

Question 8

Thermometric Scales

Answer 8

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.

Question 9

The Celsius (or Centigrade) Scale

Answer 9

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.

Question 10

The Fahrenheit Scale

Answer 10

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.

Question 11

The Kelvin or Absolute Scale

Answer 11

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.

Question 12

Absolute Temperature

Answer 12

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

Question 13

When to use Kelvin Scale

Answer 13

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.

Question 14

Air or Gas Thermometer

Answer 14

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.

Question 15

Using Solids to Measure Temperature

Answer 15

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?).

Question 16

Thermocouples

Answer 16

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.

Question 17

Electrical Resistance

Answer 17

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.

Question 18

Thermistors

Answer 18

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.

Question 19

Comparison by Brightness

Answer 19

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.

Question 20

Infra-Red

Answer 20

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.

Question 21

Units of Heat

Answer 21

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.

Question 22

The Joule (J)

Answer 22

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.

1kJ=1000J
1MJ (Megajoule)=1000kJ=1000000J.

Question 23

The Calorie

Answer 23

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.

Question 24

The British Thermal Unit (Btu)

Answer 24

Quantity of heat required to raise temp of 1lb of water through 1 degrees Fahrenheit.

Question 25

Specific Heat

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.

Answer 25

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).

Question 26

Specific Heat Example

Answer 26

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.

Question 27

Water and Specific Heat

Answer 27

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.

Question 28

Change of State and Latent Heat

Answer 28

Changes between states e.g. solids, liquids, gases etc.

Freezing, melting, boiling, condensation and sublimation all cause a change of state.

Question 29

Latent heat of Vaporisation

Answer 29

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.

Question 30

Specific Latent (Hidden) Heat

Answer 30

2.26MJ are required to convert 1kg of water at its boiling point into steam at the same temperature. This is known as the Specific Latent Heat of Vaporisation for water.
The extra heat goes into the vapour but doesn’t cause a rise in temp of the water.

It’s this latent heat which makes water mist and sprays so effective at extinguishing flames-Vaporisation of the water droplets cool the flame and extinguish it.

Question 31

Specific Latent Heat of Vaporisation

Answer 31

Amount of heat energy required to change a unit mass of a substance from liquid to vapour without a temp change.

All liquids beside water absorb latent heat when turned into vapour.
E.g. 860,000J are required to convert 1kg of alcohol into vapour at its boiling point.

Question 32

Latent Heat Measurements

Answer 32

Measured in Joules per kilogram (J/KG) or KJ/KG.

Question 33

Effect of change of pressure on boiling point and latent heat

Answer 33

If external pressure is raised, then boiling point is raised,
and if external pressure is lowered, then boiling point is lowered.

This effect is used in pressure cookers and pressurised cooling systems in cars where increased pressure increases the boiling point of the liquid in the system.

Also used in the storage of liquified gases e.g. propane and butane. At increased pressures, these gases liquify at normal temps and allow large amounts of ‘gas’ to be stored.

Raising the boiling point increases heat energy needed to raise temp of the cold liquid to the new boiling point, but it decreases the latent heat of vaporisation.

Question 34

Latent Heat of Fusion

Answer 34

Just as latent heat is taken in when water changes to vapour at the same temperature, a similar thing happens when ice melts to form water.

Defined as:
Quantity of heat required to convert the unit mass of a substance from the solid state to a liquid state without a change in temperature. (J/KG or KJ/KG).

Question 35

Cooling by Evaporation

Answer 35

Some liquids have a low boiling point and change evaporate easily at ordinary temps. These are called Volatile Liquids.
Methylated spirits and ether are examples. If dropped onto your hand they evaporate rapidly and hand feels cold. Some anaesthetics work this way, ‘freezing’ the pain.
Done by the liquid absorbing heat energy from the hand to provide the latent heat of evaporation for the liquid.
This is why the hand then feels cold.