الأسبوع 4 · الدرس الثالث «الطاقة الداخلية ودرجة الحرارة»: النظم والطاقة الداخلية ودرجة الحرارة وعوامل تغير درجة الحرارة والحرارة النوعية · 3 دروس
2
Internal energy and temperature are not the same
Science Prep 2 Term 1, Unit 1: pp. 21–29
GoalExplain that temperature measures the average kinetic energy of particles while internal energy adds up all their energy, and say what heat is.
New words
internal energy · the sum of the kinetic and potential energy of all the particles in a systemtemperature · a measure of the average kinetic energy of the particlesheat · energy transferred because of a temperature difference
Explanation
The particles in a system have kinetic energy because they move, and potential energy because of the attractions between them. Internal energy is the sum of both kinds for all the particles. Temperature is different: it only measures the average kinetic energy of the particles, and so tells us how warm something is. Heat is the energy that moves from a hotter object to a colder one. Imagine two pans of water at the same temperature, 40 °C, one small and one large. The average kinetic energy of the particles is the same in both, so the temperature is the same. But the large pan has many more particles, so its internal energy is bigger. Think of coins: a big pile of 1-pound coins is worth more than a small pile, though every coin is worth the same. Temperature is like the value of each coin, internal energy is like the whole pile. So we should not say “this object has a lot of heat”: we say it has internal energy, or that heat flows into it.
Temperature
Internal energy
measures
average kinetic energy
total kinetic + potential energy
depends on the amount?
no
yes
unit
°C
joule (J)
Worked examples
A small cup (100 g) and a bucket (10 kg) of water are both at 25 °C. Which has the higher temperature? Neither: they are equal. Which has more internal energy? The bucket, because it has many more particles.
Water in a pan is heated by the Sun. Its particles move faster, so their average kinetic energy rises and the thermometer shows a higher temperature, and the internal energy rises too.
An ice cube at 0 °C is put into a cup of water at 20 °C. Heat flows from the water to the ice, never the other way round, until their temperatures meet.
Class activity
Same temperature, different amount. With a grown-up, fill a small cup and a large bowl with cold tap water from the same tap, and read both with a digital kitchen thermometer. Write the readings. Say which has the larger internal energy and why. Then write one sentence using the word heat correctly.
Practice
1
What does temperature measure?
How much kinetic energy the particles have on average.
2
Two jugs of water are both at 30 °C, one holds 1 L and the other 3 L. Compare their temperatures and their internal energies.
The temperatures are equal; the 3 L jug has more internal energy.
3
In which direction does heat flow between a hot cup and a cool table?
From the hot cup to the cool table.
4
Why is the sentence “this stone has a lot of heat” better said another way?
Heat is energy being transferred; the stone holds internal energy.
