**This one is mainly definitions & measurements:**

- What is Temperature?
- Thermometers
- Thermometric Substances
- Thermometric Properties

- Empirical Calibration of a Thermometer
- Absolute Scale / Thermodynamic Scale
- Types of Thermometers

__What is Temperature?__

A measure of the degree of ‘hotness’ of a body. It is NOT the amount of thermal energy a body possesses.*

*Might revisit this, I’m not satisfied with this definition

__Why is Temperature important?__

t tells us how thermal energy will flow between 2 objects.

- Thermal energy will flow from an object at a higher temperature to an object at a lower temperature, until they both reach the same temperature.
- 2 objects at the SAME temperature will NOT have a net flow of thermal energy between them. They are in
**THERMAL EQUILIBRIUM.**

__How do you measure temperature?__

Using a **thermometer**, an instrument for measuring temperature.

A thermometer uses **thermometric substances** which changes their **thermometric properties** according to their temperature.

Thermometric Substance | The working material of a thermometer, which has varying properties according to temperature. Examples:Mercury Silver Alcohol |

Thermometric Property | The property of a substance which varies according to temperature.Examples:Volume Length of liquid thread in a tube Electrical resistivity/conductivity |

__How do you empirically determine a temperature scale?__**Empirical scale of temperature:** a temperature scale which is determined EXPERIMENTALLY for a specific thermometer.

Compare the reading of the thermometer at FIXED POINTS | Fixed points: known reference temperatures where physical processes (such as state changes) occur. The most common fixed points are the ICE POINT (0°C) and the STEAM POINT (100°C). Thus, we can take a reading on my new thermometer at these 2 points. |

Divide the difference between fixed points into equal intervals (degrees) | If you divide the difference between the ice & steam points into 100 points, you get a CENTIGRADE scale (aka the Celsius scale).However, I can just as easily divide it into 200 points or 50 or 2 or 1 – it’s completely arbitrary (though not always useful)! |

**In a general formula:**

### θ = k(P_{θ} – P_{lower})/(P_{upper} – P_{lower})

where P is the value of any thermometric property (pressure, volume, length, resistance, emf, etc.)

**For the Celsius scale, we use the ice point (P _{i}) & steam point (P_{s}):**

#### θ = 100(P_{θ} – P_{i})/(P_{s} – P_{i})

To recap: an EMPIRICAL temperature scale depends on the properties of thermometric substance. The value of 0 here is completely arbitrary. However, scientists require a universal scale: an ABSOLUTE SCALE or THERMODYNAMIC SCALE.

__What is an absolute temperature scale?__

A scale independent of a thermometric property. It defines 0 as the same for ALL thermometers. Thus, 0 must be the ABSOLUTE coldest temperature possible in the universe.

We call this scale the KELVIN SCALE (with units of K), & 0K is known as ABSOLUTE ZERO.

0K = -273.15°C

Now for some standardisations. This is all arbitrary (as long as 0K is absolute zero, an absolute scale is valid), but it’s useful for scientists to agree on a scale.

__How is the Kelvin internationally defined?__

Using GAS PRESSURE, interestingly.

For an ideal gas,

pV/T = constant

Here T is the ABSOLUTE or thermodynamic temperature. If we can keep V constant & measure p across different temperatures, we have a reliable method of defining T!

A CONSTANT-VOLUME GAS THERMOMETER is used to do exactly this.

**Now we must identify our fixed points:**

**Lower Fixed Point:**absolute 0 (0K, -273.15°C)**Upper Fixed Point:**the triple point of water, 0.01°C

We don’t use the ice point (0°C) or the steam point (100°C) since these actually vary depending on atmospheric pressure. The triple point only occurs at a very specific pressure at 0.01 C, so it’s useful for defining the absolute temperature scale. Click here to find out more about the triple point.

**Using our previous equation to define any temperature scale,**

θ = k(P_{θ} – P_{lower})/(P_{upper} – P_{lower})

T = 273.16(p_{θ} – 0)/(p_{tr} – 0)

### T = 273.16p/p_{tr}

where T = temperature in Kelvin

**This gives us the definition of a Kelvin:**

One kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

__Converting between the Kelvin & Celsius scales__

θ/°C = T/K – 273.15

__Types of thermometers__

Type | Working Principle | Advantages | Disadvantages |

Gas | Expansion of gas with temperature (pV = nRT) | Most accurate to the thermodynamic scale | Large/bulky Unsuitable to measure small objects Not suitable for rapidly varying temperatures |

Liquid-in-glass | Expansion of a liquid with temperature | Convenient, sensitive, moderately quick-acting Accurate to thermodynamic temperature within range | Only operates within specific range (MP and BP of liquid) Cannot be read from a distance through a display |

Metal resistor | Resistance of a metal increases with temperature (not always linear) | Wide range Can be read from a distance through a display | Inaccurate over wide range since variation is non-linear Requires multiple calibration over different ranges |

Thermistors | Semiconductor which has decreasing resistance with increasing temperature AKA negative temperature coefficient (NTC) | Wide range Small size Fast response time Can be read from a distance through a display App: Used in car radiators | Not always linear, requires calibration Power supply needed |

Thermocouple | Thermoelectric effect: 2 different metals release different number of electrons at a temperature, creating an emf between them. | Wide range Very small size Very fast response time Can be read from a distance through a display No power supply needed Can measure temperature difference between 2 points |

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