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Grade 9 / Middle 3 (age 14-15)

Thermal Energy & Equilibrium

Thermal Energy and Thermal Equilibrium

Temperature and Particle Motion
🔥 Intuition — Hotter Means More Active
①Matter is made of tiny particles (molecules)
②They move constantly
③Higher temperature → faster particles
④Temperature = how vigorous particle motion is
50°C
Heat Transfer & Equilibrium
↔️ Heat Flows From Hot to Cold
①When hot and cold water touch
②Heat flows from high → low temperature
③Eventually both reach the same temperature
④This state is called "thermal equilibrium"
0step
Thermal Energy & Heat (Q)
🌡️ What is Heat (Q)?
①The amount of heat absorbed or released
②Units: cal or J
③1 cal = heat to raise 1 g water by 1 °C
④1 cal ≈ 4.2 J
Heat Equation
Q = mcΔt
Heat = mass × specific heat × temperature change
Summary
Heat (Q)
Q = mcΔt
Q (cal) = m (g) × c (cal/g·°C) × Δt (°C)
Equilibrium Condition
Qreleased = Qabsorbed
Heat lost = heat gained (at equilibrium)
Unit Conversion
1 cal ≈ 4.2 J
Calorie ↔ joule
🎯 Exam Points
①Temperature = vigor of particle motion
②Heat flows hot → cold
③Equilibrium: both reach the same temperature
④Q = mcΔt (larger c → smaller ΔT)
⑤Water has high specific heat = 1 cal/g·°C
Examples and Unit Test
Example 1
How much heat, in calories, is needed to raise 100 g of water from 20°C to 30°C? (specific heat of water = 1 cal/g·°C)
1
Heat Q = mass (m) × specific heat (c) × temperature change (Δt).
2
Q = 100 g × 1 cal/g·°C × (30 − 20)°C = 100 × 1 × 10 = 1000 cal.
▸ 1000 cal (100 × 1 × 10)
The larger the specific heat, the more heat is needed to raise the temperature by the same amount.
Example 2
When hot water and cold water are kept in contact, after a while the two reach the same temperature. In which direction does heat move, and what is the state that is reached called?
1
Heat always moves from the hotter object to the colder object.
2
The state in which the two objects reach the same temperature and heat no longer flows is called thermal equilibrium.
▸ Heat moves from hot to cold / the equal-temperature state = thermal equilibrium
At thermal equilibrium the heat lost equals the heat gained (Q released = Q absorbed).
Unit Test
Which statement about thermal energy and thermal equilibrium is correct?
①Heat moves on its own from cold to hot
②The higher the temperature, the slower the particles move
③At thermal equilibrium the two objects have the same temperature
④The unit of heat is °C
⑤The larger the specific heat, the faster the temperature changes
▸ ③ At thermal equilibrium the two objects have the same temperature
1
Heat moves from hot to cold (① wrong), and the higher the temperature, the faster the particles move (② wrong).
2
The unit of heat is the calorie or joule (④ wrong), and the larger the specific heat, the more slowly the temperature changes (⑤ wrong); at thermal equilibrium the two objects have the same temperature (③ correct).
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