understand that a physical property that varies with temperature may be used for the measurement of temperature and state examples of such properties, including the density of a liquid, volume of a gas at constant pressure, resistance of a metal, e.m

Temperature Scales & Measurement

Quick Overview: Temperature tells us how hot or cold something is. Scientists use different scales (Celsius, Kelvin, Rankine) to give a consistent number. But to actually read a temperature, we need a device that changes something we can measure, like length, volume, resistance, or voltage.

1. Physical Properties that Vary with Temperature

  • Density of a liquid – e.g., water is densest at 4 °C.

  • Volume of a gas at constant pressure – follows Charles’ Law: \$V \propto T\$.

  • Electrical resistance of a metal – resistance increases with temperature.

  • EMF of a thermocouple – voltage produced is proportional to temperature difference.

2. Example: Density of Water

Water’s density changes with temperature. At 0 °C it’s 0.9998 g cm⁻³, peaks at 4 °C (1.0000 g cm⁻³), then drops again. This property lets us design a thermometer that uses a liquid column whose height changes with density.

Temperature (°C)Density (g cm⁻³)
00.9998
41.0000
200.9982
1000.9584

3. Example: Volume of a Gas (Charles’ Law)

At constant pressure, the volume of a gas is directly proportional to its absolute temperature:

\$V \propto T\$

Imagine a balloon that inflates as the air inside warms up – that’s exactly what happens in a gas thermometer.

4. Example: Resistance of a Metal (Ohm’s Law & Temperature Coefficient)

For most metals, resistance increases with temperature:

\$R = R0[1 + \alpha (T - T0)]\$

A thermistor uses this property – a tiny resistor that changes resistance sharply with temperature, allowing precise readings.

5. Example: EMF of a Thermocouple

A thermocouple consists of two different metals joined at one end. When the junction is heated, a small voltage (EMF) is produced:

\$E = S \Delta T\$

Here, \$S\$ is the Seebeck coefficient. Thermocouples are great for measuring high temperatures because they’re rugged and fast.

6. Exam Tips & Quick Checks

Tip 1: Remember that density of water peaks at 4 °C – useful for questions about water’s expansion.

Tip 2: For Charles’ Law, keep the proportionality constant in mind: \$V1/T1 = V2/T2\$ (use Kelvin!).

Tip 3: When asked about resistance change, write the formula \$R = R0[1 + \alpha (T - T0)]\$ and explain each term.

Tip 4: For thermocouples, note that the EMF is proportional to the temperature difference between the hot junction and the reference junction.

7. Analogy Corner 🚀

Think of temperature as the “speed” of molecules. Just like a car’s speedometer tells you how fast the car is going, a thermometer tells you how fast the molecules are moving. Different devices (liquid column, gas balloon, metal wire, metal junction) are like different kinds of speedometers – each uses a property that changes with speed (temperature) to give a readable number.