Week 4 · 1-3 Physical Properties of Water (B): heat and temperature, internal energy, the Kelvin scale, specific heat and Q = mcΔt with worked examples, sea breeze, latent heat of vaporisation, effects on marine organisms · 3 lessons
1
Heat, temperature and the kelvin
Integrated Science Sec 1 Term 1, Unit 1: pp. 20–27
GoalTell heat from temperature, give their units, and convert between °C and kelvin.
New words
heat · energy that moves from a hotter body to a colder one because of the difference in temperature; its unit is the joule (J)temperature · a measure of the average kinetic energy of the molecules of a body, showing how hot or cold it iskelvin (K) · the SI unit of temperature; 0 K is the lowest possible temperature
Explanation
Every body is made of molecules that are always moving. The sum of their potential and kinetic energies is the internal energy of the body. Heat is the energy that passes from a hotter body to a colder one because of a temperature difference, and it is measured in joules (J). Temperature tells how hot or cold a body is: it measures the average kinetic energy of its molecules. So heat and temperature are different things. A bath of warm water and a cup of warm water can have the same temperature, but the bath holds much more internal energy. In science the temperature unit is the kelvin (K), and T (K) = t (°C) + 273. A change of 1 °C is the same size as a change of 1 K, but the temperatures themselves differ by 273: 0 °C is 273 K, 100 °C is 373 K and 0 K is −273 °C. Kelvin is written without a degree sign.
T = t + 273
t in °C
T in K
−273
0
0
273
25
298
100
373
Worked examples
Convert 35 °C to kelvin: T = 35 + 273 = 308 K. Convert 300 K to °C: t = 300 − 273 = 27 °C. Convert −20 °C: T = −20 + 273 = 253 K.
A water sample warms from 15 °C to 40 °C. The change is 40 − 15 = 25 °C, which is also 25 K. Check in kelvin: 313 − 288 = 25 K. The change is the same on both scales, but the temperatures themselves are not.
A bath and a cup both hold tepid water at 35 °C. Which has more heat energy to give a cold hand? The bath, because it holds many more molecules, each with the same average kinetic energy. Same temperature, different amounts of internal energy.
Class activity
Bath or cup? Think and write. Two containers of tepid tap water stand in the same room for an hour: a big bowl and a small cup. Say whether (a) their temperatures and (b) the amounts of heat energy they hold are the same or different, and explain in a sentence. Then, with a grown-up, read the room temperature from a digital thermometer or weather app and write it in °C and K.
Practice
1
Convert 37 °C to kelvin.
37 + 273 = 310 K.
2
Convert 250 K to °C.
250 − 273 = −23 °C.
3
A liquid warms from 20 °C to 65 °C. What is the change in K?
65 − 20 = 45 °C, which is a change of 45 K.
4
Which is measured in joules, heat or temperature, and which tells the average kinetic energy of molecules?
Heat is measured in joules; temperature tells the average kinetic energy.
