7.1 Temperature Scales & Heat Definition

Temperature Scales & Heat Definition

Thermodynamics in EASA Part-66 Module 2 starts with two ideas that are often confused in everyday speech: temperature and heat. Temperature tells you how hot or cold something is. Heat is energy transferred because of a temperature difference. An engine oil sample at 90 °C has a high temperature; heat is what left the engine metal and entered that oil while the oil warmed. Clear separation of these terms underpins heat capacity, gas laws, engine cycles, and latent heat in the rest of this chapter.

Temperature: A Measure of Hotness

Temperature is a scalar property that indicates the thermal state of a body. At a microscopic level (Module 2 awareness), temperature relates to the average kinetic energy of the molecules or atoms. A hotter gas has faster-moving particles on average; a colder one has slower particles. When two bodies are placed in thermal contact and no further net energy flows, they are at the same temperature — thermal equilibrium.

Temperature does not tell you how much energy a body stores in total. A cup of coffee and a swimming pool of water can share the same temperature while the pool holds far more internal energy. That distinction reappears as heat capacity in the next section.

Temperature Scales

Three scales appear in aviation and Module 2 work: Celsius, Fahrenheit, and Kelvin.

Celsius (°C)

The Celsius scale is defined so that, under standard conditions:

• Ice point (pure ice–water mixture): 0 °C
• Steam point (pure water boiling at standard atmospheric pressure): 100 °C

The interval between ice and steam is divided into 100 equal degrees. Most European technical data, engine limits, ambient reporting in much of the world, and Module 2 calculations use Celsius for everyday temperatures.

Fahrenheit (°F)

The Fahrenheit scale places the ice point at 32 °F and the steam point at 212 °F, so there are 180 Fahrenheit degrees between ice and steam. Some older instruments and US-oriented documentation still use °F. Module 2 expects you to convert when a question mixes scales.

Kelvin (K) — the absolute thermodynamic scale

The Kelvin scale is an absolute temperature scale used in scientific gas laws and thermodynamics. Its zero is absolute zero, the theoretical lower limit of temperature at which molecular thermal motion reaches its minimum (classical ideal: zero kinetic energy of random motion). Important fixed relations:

• Absolute zero = 0 K = −273 °C (more precisely −273.15 °C; Module 2 almost always uses −273 °C)
• The size of one kelvin equals the size of one Celsius degree: a change of 10 K is the same temperature change as 10 °C
• Kelvin values are written without a degree symbol in modern SI: 273 K, not 273 °K

Because the kelvin starts at absolute zero, gas-law formulas that use absolute temperature must use kelvin (or another absolute scale). Using Celsius directly in PV = nRT-style work is a common exam trap.

Why absolute zero matters for technicians

You will rarely work near absolute zero in hangar practice, but the concept explains why absolute temperature appears in Charles’s and Gay-Lussac’s laws: volume and pressure of an ideal gas scale with distance above absolute zero, not with the Celsius zero at the ice point. Cold-soaked components at −40 °C are still 233 K above absolute zero — cold for humans and seals, but far from 0 K.

Conversions You Must Know

Celsius ↔ Kelvin

T (K) = t (°C) + 273

t (°C) = T (K) − 273

Examples:

• 20 °C → 20 + 273 = 293 K
• 0 °C → 273 K
• −40 °C → −40 + 273 = 233 K
• 100 °C → 373 K
• 300 K → 300 − 273 = 27 °C

Always convert before substituting into Boyle/Charles/Gay-Lussac combined problems. After the absolute-temperature arithmetic, you may convert back to °C if the question asks for Celsius.

Celsius ↔ Fahrenheit

t_°F = (9/5) t_°C + 32

t_°C = (5/9)(t_°F − 32)

Examples:

• 0 °C → (9/5)(0) + 32 = 32 °F
• 100 °C → (9/5)(100) + 32 = 212 °F
• 20 °C → 36 + 32 = 68 °F
• 68 °F → (5/9)(68 − 32) = (5/9)(36) = 20 °C
• −40 °C = −40 °F (the scales meet at −40)

A useful check: the Fahrenheit interval from ice to steam is 180 °F for 100 °C, so each Celsius degree is 1.8 Fahrenheit degrees. The offset of 32 accounts for the different zero points.

Worked conversion chain

An engine oil temperature gauge reads 194 °F. Convert to °C and to K.

1. t_°C = (5/9)(194 − 32) = (5/9)(162) = 90 °C
2. T_K = 90 + 273 = 363 K

If a gas-law problem later used this temperature as a hot-gas state, you would insert 363 K, not 90 or 194.

Thermometers: Measuring Temperature

A thermometer is any instrument that indicates temperature by measuring a physical property that varies reliably with temperature, then displaying a calibrated scale. Common principles:

1. Liquid-in-glass — mercury or coloured alcohol expands more than the glass when heated; the liquid column rises in a capillary. Simple, visual, limited range and fragility.
2. Bimetallic strip — two metals with different expansion coefficients bonded together; temperature change bends the strip. Used in thermostats, some dial gauges, and overheat indicators.
3. Resistance thermometer / thermistor — electrical resistance of a metal (e.g. platinum) or semiconductor changes with temperature; widely used for accurate electronic sensing.
4. Thermocouple — junction of two dissimilar metals produces a small voltage related to temperature difference between the measuring junction and a reference. Common for high temperatures (exhaust gas temperature, EGT, turbine inlet temperature sensing concepts).
5. Radiation / infrared — non-contact sensing of thermal radiation from a surface (more advanced; useful for hot components without contact).

Calibration historically uses fixed points (ice point, steam point, and other fixed points on the International Temperature Scale). Aircraft systems use sensors appropriate to range, response time, vibration, and installation (oil, cylinder head, outside air temperature, cabin, bleed air, etc.). For Module 2, know that thermometers measure temperature, not heat flow directly, via a calibrated property change.

Heat: Energy in Transit

Heat is energy transferred from one body or system to another solely because of a temperature difference. Key points:

• Heat is not “contained” as a substance; once transferred, it becomes part of the receiving body’s internal energy.
• Heat flows spontaneously from higher temperature to lower temperature. Spontaneous flow the other way requires a refrigeration process (work input).
• SI unit of heat is the joule (J), same as work and energy. Older aviation/engineering texts may still mention the calorie (1 cal ≈ 4.2 J) or British thermal unit (Btu); convert when needed, but Module 2 SI thinking prefers joules and kilojoules.
• The symbol Q is often used for heat transferred; ΔU for change in internal energy; W for work — linked by the first law in section 7.3.

Heat is not temperature

A large mass of warm water can transfer more heat to a cold object than a small mass of hotter water, depending on masses, specific heats, and final temperatures. Temperature alone does not fix the heat available.

Thermal equilibrium and direction of flow

If a hot brake disc contacts cooler air and structure, heat leaves the disc until temperatures equalise (or until forced cooling removes energy continuously in flight). Touching a cold structure with a warm hand: heat leaves your hand into the metal — the metal feels “cold” because your hand loses heat, not because coldness flows into you as a fluid.

Aviation Context

• Outside air temperature (OAT) and total air temperature sensors report temperature for performance, anti-ice logic, and crew information.
• Oil, cylinder head, EGT, and turbine temperature limits are temperature limits; exceeding them risks material damage even though the “heat load” history also matters.
• Cold-soak operations: low ambient temperature means components start cold; fluids are viscous; seals stiffen — temperature, not “lack of heat stored as a fluid,” is the stated condition.
• Cabin and avionics cooling manage heat removal so equipment temperatures stay within limits.

Formula and Concept Recap

• Temperature ≠ heat; heat = energy transfer due to temperature difference (unit J).
• Absolute zero: 0 K = −273 °C.
• T_K = t_°C + 273; t_°F = (9/5)t_°C + 32; t_°C = (5/9)(t_°F − 32).
• Thermometers use expansion, resistance, thermoelectric voltage, or radiation, calibrated to a scale.
• Spontaneous heat flow: hot → cold until equilibrium (unless a heat pump/refrigerator does work).

With temperature scales and the definition of heat fixed, the next section treats how much heat changes temperature (heat capacity) and how heat moves (conduction, convection, radiation), plus thermal expansion of solids and fluids.