grade 9 Thermal Energy & Equilibrium

Thermal Energy and Thermal Equilibrium

Temperature measures how vigorously the particles that make up a substance move. When hot and cold water touch, heat flows from high temperature to low until they reach the same temperature, called thermal equilibrium. Heat Q is found from Q = mcΔt (mass times specific heat times temperature change), and at equilibrium heat lost equals heat gained. Change temperature and elapsed time and particle motion and the path to equilibrium appear.

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

Temperature50°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"

⑤This sketch starts at 90 ℃ and 20 ℃ and meets at 55 ℃

Time Step0step

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

Q_released = Q_absorbed

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)

Heat Q = mass (m) × specific heat (c) × temperature change (Δt).

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?

Heat always moves from the hotter object to the colder object.

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

Heat moves from hot to cold (① wrong), and the higher the temperature, the faster the particles move (② wrong).

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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Conservation of Mechanical Energy

Law of Conservation of Mass

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