Temperature Measurement and Physical Properties of Matter

Lessons

Senior 1 · Physics

Temperature Measurement and Physical Properties of Matter

5 lessons

Lesson 1 of 5

Heat and Temperature

22 min

Can you trust your hands?

On a cold morning in Musanze, a pupil sits on a metal bench and then on a wooden one. The metal feels much colder, yet both have been outside all night and are at the same temperature. A mother tests her baby's bath water with her elbow; a nurse would use a thermometer. Our senses tell us whether something feels hot or cold, but they are easily fooled. Physics needs two clear ideas, temperature and heat, and an instrument that measures one of them reliably.

Experiment: three bowls

You need: three bowls of water: cold (with a little ice if you have it), lukewarm, and hot but comfortable to touch (test it with a thermometer first: no more than 45 °C).

Put your left hand in the cold water and your right hand in the hot water for one minute.
Move both hands together into the lukewarm bowl.

Figure 1.1: The same water feels warm to one hand and cool to the other. Your skin senses whether heat is flowing into it or out of it, not the temperature itself.

Observation: the lukewarm water feels warm to the hand that was in cold water and cool to the hand that was in hot water.

Explanation: the cold hand is colder than the lukewarm water, so heat flows into it and it feels warmth; the hot hand is warmer, so heat flows out of it and it feels cold. The metal bench fools you the same way: metal conducts heat away from your skin quickly, so it feels colder than wood at the same temperature. To measure temperature we need a thermometer.

Important

Safety. Check the hot water with a thermometer before anyone puts a hand in it: water above about 50 °C can scald. Wipe up spills at once.

Definition

Temperature ( $\theta$ in °C, or $T$ in K) is a measure of how hot or cold a body is. It measures the average kinetic energy of the particles the body is made of. Its SI unit is the kelvin (K); in daily life we use the degree Celsius (°C). It is measured with a thermometer.

Heat ( $Q$ ) is energy that flows from a hotter body to a colder body because of the difference in their temperatures. Like all energy, it is measured in joules (J).

Heat always flows from hot to cold

Figure 1.2: Heat flows from the hot tea into the cooler air. The tea cools and the air warms slightly, until there is no temperature difference and the flow stops.

	Temperature	Heat
What it is	how hot a body is; the average kinetic energy of its particles	energy transferred because of a temperature difference
SI unit	kelvin (K); also °C	joule (J)
Measured with	a thermometer	calculated, e.g. from the power of a heater and the time
Depends on the amount of substance?	no: a cup and a pot of boiling water are both at 100 °C	yes: the pot needs much more heat to reach 100 °C
Direction	(a property, no direction)	always from a higher to a lower temperature

Worked example 1: a temperature change

A cup of tea cools from 85 °C to 60 °C. Find the fall in temperature in °C and in kelvin.

\Delta\theta = 85\,^\circ\mathrm{C} - 60\,^\circ\mathrm{C} = 25\,^\circ\mathrm{C}

In kelvin the two temperatures are 358 K and 333 K, so $\Delta T = 358\,\mathrm{K} - 333\,\mathrm{K} = 25\,\mathrm{K}$ . A change of 1 °C is the same size as a change of 1 K.

Worked example 2: heat from an electric heater

A 1000 W immersion heater is left in a bucket of water for 5.0 minutes. How much heat does it supply?

Power is energy per second (Unit 5), so $Q = P t$ .

Q = Pt = 1000\,\mathrm{W} \times (5.0 \times 60)\,\mathrm{s} = 300\,000\,\mathrm{J} = 300\,\mathrm{kJ}

Worked example 3: same temperature rise, different heat

About 4200 J of heat warms 1 kg of water by 1 °C. How much heat is needed to warm (a) 1.0 kg and (b) 2.0 kg of water from 20 °C to 100 °C?

\text{(a)}\quad Q = 1.0 \times 4200 \times 80 = 336\,000\,\mathrm{J}

\text{(b)}\quad Q = 2.0 \times 4200 \times 80 = 672\,000\,\mathrm{J}

Both end at the same temperature, 100 °C, but twice the mass needs twice the heat. This is the clearest difference between heat and temperature.

Warning

Heat is not temperature. Never write “the heat of the water is 60 °C”: 60 °C is its temperature. And “cold” is not something that flows: when you hold ice, heat flows OUT of your hand into the ice.

Temperature control in Rwanda

Milk collection centres cool fresh milk to about 4 °C within hours of milking, because bacteria multiply slowly at low temperature.
Tea factories in Nyungwe, Gisovu and elsewhere dry the green leaf in hot air at a carefully controlled temperature: too cool and it goes mouldy, too hot and the flavour is spoiled.
Health centres check a patient's temperature first: normal body temperature is about 37 °C, and a fever above about 38 °C is a sign of illness such as malaria.
Vaccines must be kept between 2 °C and 8 °C in fridges from the airport all the way to the village health post.

Tip

Memory hook: temperature is a level; heat is a flow. Just as water flows from a higher level to a lower one, heat flows from a higher temperature to a lower one.

Thermal energy, temperature and heat (Khan Academy). Watch for the particle picture of temperature, and for why heat flows only from a hotter object to a colder one.

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Practice

Define temperature and heat, and give the SI unit of each.
Why is your hand not a reliable way to measure temperature?
A metal spoon and a wooden spoon lie in the same room. Which feels colder, and are their temperatures different?
Water warms from 18 °C to 75 °C. Find the rise in temperature in °C and in K.
A 1500 W heater runs for 4.0 minutes. How much heat does it supply?
Using 4200 J to warm 1 kg of water by 1 °C, find the heat needed to warm 3.0 kg of water by 10 °C.
A bath of warm water at 40 °C and a cup of tea at 80 °C. Which has the higher temperature? Which could give out more heat as it cools to room temperature? Explain.

Show answersHide answers

Temperature is a measure of how hot a body is (the average kinetic energy of its particles), in kelvin. Heat is energy transferred from a hotter to a colder body because of a temperature difference, in joules.
It senses heat flowing in or out, which depends on its own temperature and on the material touched, so the same object can feel hot or cold.
The metal spoon feels colder because it conducts heat away from the hand faster; both are at the same (room) temperature.
57 °C, which is also 57 K.
$Q = 1500 \times 240 = 360\,000\,\mathrm{J} = 360\,\mathrm{kJ}$
$Q = 3.0 \times 4200 \times 10 = 126\,000\,\mathrm{J}$
The tea has the higher temperature. The bath can give out more heat, because it contains a far larger mass of water.

Key points

Temperature measures how hot a body is (the average kinetic energy of its particles); unit K or °C; measured with a thermometer.
Heat is energy transferred because of a temperature difference; unit J; it always flows from hot to cold.
A change of 1 °C equals a change of 1 K.
The heat needed depends on the mass as well as the temperature change; the temperature does not depend on the mass.

Sources and credits

13.1: Temperature, from College Physics (1st edition) by Paul Peter Urone and Roger Hinrichs, OpenStax, hosted by LibreTexts, licensed under CC BY 4.0. Consulted for the physics; no text was copied.
Video: Thermal energy, temperature, and heat | Khan Academy, Khan Academy, embedded from YouTube.

Written by Ganzaa: the lesson text, the three-bowls experiment, diagrams, table, worked examples, Rwandan applications and practice exercises.

1Can you trust your hands?

2Experiment: three bowls

3Heat always flows from hot to cold

4Temperature control in Rwanda

Can you trust your hands?

Experiment: three bowls

Heat always flows from hot to cold

Temperature control in Rwanda