Heat is a form of energy in transit, flowing from hotter bodies to colder ones, while temperature tells us how hot or cold a body is. These notes move from the idea of heat and its units, through the temperature scales and the gas laws, to thermal expansion, the three modes of heat transfer, and finally the laws of thermodynamics and heat engines.
Heat (or thermal energy) is the total of the kinetic energies of all the molecules of a body, and it is best understood as energy on the move.
The SI unit of heat is the:
Heat always flows from:
One calorie is the heat needed to raise the temperature of 1 g of water from 14.5°C to °C.
Heat is the sum of all energies of all the molecules of a body.
The calorie is fixed by experiment, not by convenience, which is why the conversion factor is an awkward number rather than a round one.
| Quantity | Value |
|---|---|
| SI unit of heat | joule (J) |
| Practical unit of heat | calorie (cal) |
| 1 calorie | 4.186 joule |
| 1 joule | 0.239 calorie |
The exact conversion of 1 calorie into joules is:
How many calories is 1 joule equal to?
Exactly J of work heats 1 g of water through a one-degree rise.
Rounding 4.186 down to 4 gives the rough figure 1 cal ≈ J.
Heat can be classified by the effect it has, and it has a definite character as a physical quantity.
Heat that changes the state of a body without changing its temperature is called:
Heat is which kind of physical quantity?
Heat that changes the temperature of a body is called heat.
Heat is not stored in a body — it is from one body to another.
The heat needed to warm a body depends on how much of it there is, what it is made of, and how big a temperature change is wanted.
The heat equation is:
In the equation Q = m × s × ΔT, what does s represent?
In the heat equation, ΔT is the change in .
In Q = m × s × ΔT, m is the in kg.
There is a lowest possible temperature at which molecular motion theoretically ceases, and Lord Kelvin used it as the zero of a new temperature scale.
| Reference point | Kelvin | Celsius | Fahrenheit |
|---|---|---|---|
| Water boils | 373.15 K | 100°C | 212°F |
| Water freezes | 273.15 K | 0°C | 32°F |
| Absolute zero | 0 K | −273.15°C | −459.67°F |
Absolute zero is equal to:
The conversion from Celsius to Kelvin is:
The Kelvin scale (absolute scale) was begun by Lord .
At 0 K, a gas is supposed to have zero volume and zero .
To measure temperature we fix two reference points and divide the gap between them into equal parts; different choices of these divisions give us the different scales.
| Reference point | Fahrenheit | Celsius |
|---|---|---|
| Water boils | 212°F | 100°C |
| Water freezes | 32°F | 0°C |
| Dry ice (solid CO₂) | −108°F | −78°C |
| Liquid air / absolute zero | −459°F | −273°C |
On the Fahrenheit scale, the boiling point of water is:
The Celsius to Fahrenheit conversion is:
The Celsius scale was designed by Andre Celsius in the year .
The Kelvin scale is the absolute temperature scale and has no values.
There is one unique set of conditions at which ice, liquid water and water vapour can all coexist together in equilibrium.
At the triple point of water, which three coexist in equilibrium?
The triple point of water occurs at a temperature of:
The triple point of water occurs at a specific temperature of 273.16 K, equal to °C.
At the triple point, ice, liquid water and water vapour are all equally .
The air around us holds water vapour, and how much it holds can be described in two ways.
Absolute humidity is the amount of water vapour present in a unit:
Relative humidity is expressed as a:
Water vapour is ; more water vapour in the air means more humidity.
Relative humidity compares actual water vapour to the water vapour needed to the air.
The behaviour of an ideal gas links its pressure, volume, amount and temperature in a single equation that combines three classic gas laws.
| Law | Condition | Statement |
|---|---|---|
| Boyle's Law | temperature constant | P ∝ 1/V — pressure is inversely proportional to volume |
| Charles's Law | pressure constant | V ∝ T — volume is directly proportional to absolute temperature |
| Avogadro's Law | pressure & temperature constant | V ∝ n — volume is directly proportional to the number of moles |
The ideal-gas equation is:
Boyle's law (temperature constant) states that:
The universal gas constant R = J·mol⁻¹·K⁻¹.
Charles's law states that volume is directly proportional to absolute .
Almost every body expands when heated, and the expansion can show up in its length, its area or its volume.
Expansion of a body in its length is called:
Water has its maximum density at:
On heating, gases expand more than liquids, and liquids more than .
Expansion in volume of a body is called (volume) expansion.
Heat moves from a hotter body to a colder one in three distinct ways, each with its own medium requirement.
| Mode | Medium |
|---|---|
| Conduction | in solids |
| Convection | in liquids and gases |
| Radiation | no medium required |
Which mode of heat transfer needs no medium at all?
Conduction occurs mainly in:
Heat is transferred by conduction, convection and .
Convection occurs in liquids and .
In conduction, heat passes through a substance from molecule to molecule while the molecules themselves stay put.
In conduction, heat is transmitted:
Conduction is prominent in:
In conduction, the molecules stay in their mean positions of .
In a metal rod, heat flows from the hot end to the end.
In convection, the heated particles of a fluid actually move and carry their heat with them.
In convection, heat is transmitted by:
Which of the following is an example of convection?
In convection, warm fluid rises and cool fluid .
Convection is prominent in liquids and .
Radiation carries heat directly from one place to another, needing no medium at all — which is how the Sun's heat reaches us.
Radiation transmits heat:
The intensity of radiation follows which law?
We receive heat radiations straight from the , without affecting the medium in between.
Heat radiation is part of the spectrum.
Thermal conductivity tells us how readily a solid lets heat pass through it.
Which pair are good conductors of heat?
The unit of the coefficient of thermal conductivity K is:
Glass and wood are conductors (insulators) of heat.
Thermal conductivity measures a solid's ability to conduct .
A thermodynamic process happens whenever the state of a system changes — that is, when its pressure, volume, temperature or entropy changes with time.
| Process | Definition | Key condition |
|---|---|---|
| Isothermal | takes place at constant temperature | T = constant |
| Isobaric | takes place at constant pressure | P = constant |
| Isochoric | takes place at constant volume | V = constant |
| Adiabatic | no heat enters or leaves the system | Q = 0 (no heat exchange) |
A thermodynamic process at constant temperature is called:
An adiabatic process is one in which:
An isochoric process takes place at constant .
For a cyclic process, the change in internal energy ΔU is .
Thermodynamics rests on a small set of laws covering thermal equilibrium, energy conservation and the limits on converting heat into work.
The first law of thermodynamics is a restatement of the:
The second law of thermodynamics states that:
The first law of thermodynamics is written as ΔU = Q − .
The law of thermodynamics defines thermal equilibrium.
A heat engine turns heat into work, while a refrigerator and a heat pump run the same cycle in reverse.
A heat engine converts:
A refrigerator is essentially:
The efficiency of a heat engine is η = 1 − Q₂/.
A heat pump is essentially the same as a .
Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.