الأسبوع 4 · الدرس الثالث «الطاقة الداخلية ودرجة الحرارة»: النظم والطاقة الداخلية ودرجة الحرارة وعوامل تغير درجة الحرارة والحرارة النوعية · 3 دروس
3
Why some things warm faster: mass, material, state
Science Prep 2 Term 1, Unit 1: pp. 21–29
GoalExplain why equal heat warms different amounts and kinds of material by different amounts, using specific heat.
New words
specific heat · the heat needed to raise the temperature of 1 kg of a substance by 1 °Cvariable · something in an experiment that can changefair test · a test in which only one thing is changed
Explanation
Give the same heat to different things and they warm by different amounts. Three things matter. First, mass: the same heat raises the temperature of a small mass more than a large mass. Second, the material: the same heat warms 1 kg of oil more than 1 kg of water. Third, the state: ice warms more than the same mass of liquid water. We measure the difference with specific heat, the heat needed to warm 1 kg of a substance by 1 °C. Water has about 4180 J per kg per °C, ice about 2090, corn oil about 2000, aluminium about 900, iron about 450 and copper about 385 (the book’s rounded values). So, mass for mass, water needs about twice the heat of ice, and about nine times the heat of iron, for the same rise. This is why water cools car engines, why the sea changes temperature slowly, and why the body, mostly water, keeps a steady temperature. In a fair test we change one variable, measure another and keep the rest the same. Always use the Sun or warm tap water for tests at home, never a flame.
Worked examples
Two equal cups of cold tap water, one small (100 g) and one large (500 g), stand in the sun for the same time. The small cup ends warmer, because the same heat is shared among fewer particles.
Equal masses of water and cooking oil are left in the sun for the same time. The oil gets hotter, because its specific heat is lower: it needs less heat for each degree. Variables: independent, the liquid; dependent, the final temperature; controlled, the mass, the cups and the time.
A metal spoon and a wooden spoon from the freezer both feel cold, but the metal feels colder. Both are at the same temperature; the metal only takes heat from your hand faster. That idea comes back in the next lessons.
Class activity
Sun test, with a grown-up. Fill two equal cups: one with cold tap water, one with cooking oil, the same amount in each, and put them in a sunny spot for 30 minutes. Measure both with a digital kitchen thermometer. Describe what you see. Write the independent, dependent and controlled variables. Never use a flame, and do not taste the oil.
Practice
1
What is specific heat?
The heat needed to raise the temperature of 1 kg of a substance by 1 °C.
2
Equal heat is given to 1 kg of water and 1 kg of iron. Which warms more, and why?
The iron: its specific heat (about 450) is much lower than water’s (about 4180).
3
Why does the sea warm more slowly than the sand on a sunny day?
Water has a high specific heat, so it needs much more heat for each degree.
4
Is the specific heat of water the same as that of ice? Use the chart.
No: water about 4180, ice about 2090, so water needs about twice the heat for the same mass and rise.
