AP® Physics 2: Algebra-Based review sheet from Aim for Five (aimforfive.com/physics-2/units/9/9-3)
Unit 9 · Topic 9.3
9.3 Thermal Energy Transfer and Equilibrium
When two objects at different temperatures touch, energy flows on its own from the hotter one to the colder one until their temperatures match. That flow happens by conduction, convection or radiation. The end state, with no net energy flow, is called thermal equilibrium.
Key terms
- thermal contact
- heating and cooling
- conduction
- convection
- radiation
- thermal equilibrium
Heating, cooling and thermal contact
Two systems are in thermal contact when energy can pass between them because of a temperature difference. Heating means energy is transferred into a system this way; cooling means energy is transferred out. The symbol Q stands for the energy transferred by heating.
Keep three words straight. Temperature measures the average kinetic energy of the atoms. Internal energy is the total energy stored in all the atoms. Heating (Q) is energy on the move between systems because of a temperature difference. An object doesn't “contain heat”; it contains internal energy.
Three ways energy moves
Energy moves between systems at different temperatures in three ways.
- Conduction: atoms that touch pass energy along through collisions. A metal pan handle gets hot this way. No material moves from place to place.
- Convection: a fluid (liquid or gas) moves and carries its energy with it. Warm air rises from a heater and cooler air sinks to replace it.
- Radiation: objects give off electromagnetic waves, mostly infrared at everyday temperatures. It needs no material at all, which is how sunlight warms Earth across empty space. Hotter objects radiate much more.
Why energy flows from hot to cold
Picture the boundary between a hot block and a cold block. Atoms on the hot side are, on average, moving faster. When a fast atom collides with a slow one, the fast one usually loses energy and the slow one gains it. Now and then the reverse happens, but with so many collisions the net effect is a steady flow from the hotter system to the colder one.
After enough collisions, the most likely state by far is one where both systems have the same temperature. That's why the flow is spontaneous in one direction only. Cold coffee never gets hotter by pulling energy out of a cooler room.
Thermal equilibrium
Thermal equilibrium means no net energy is transferred between two systems in contact. Atoms still collide and trade energy, but the amounts going each way balance. The sign that two systems are in equilibrium is that they have the same temperature.
This is how a thermometer works: you wait until it reaches equilibrium with whatever it touches, so its reading equals that object's temperature.
Same temperature doesn't mean same internal energy. A swimming pool and a cup of water at 25 °C are in equilibrium with each other, but the pool stores vastly more internal energy because it has far more atoms.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Metal feels colder than wood
A metal spoon and a wooden spoon have been in the same room all day. When you grab them, the metal one feels colder. Are they at different temperatures? Explain.
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- Step 1: Both have been in contact with the room for a long time, so both are in thermal equilibrium with the air. They're at the same temperature.
- Step 2: Your hand is warmer than either spoon, so energy flows from your hand into each one by conduction.
- Step 3: Metal conducts much better than wood, so it pulls energy out of your skin faster. Your nerves sense your skin cooling quickly, not the spoon's temperature.
Answer: Same temperature; the metal feels colder because it conducts energy away from your hand faster.
- Example 2
Where does the final temperature land?
A small 80 °C metal block is dropped into a large insulated tank of water at 20 °C. Which way does energy flow, when does it stop, and will the final temperature be near 50 °C, near 80 °C or near 20 °C?
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- Step 1: Energy flows from the hotter block to the colder water, by conduction across the surfaces where they touch.
- Step 2: The flow continues until both have the same temperature (thermal equilibrium). With the tank insulated, energy lost by the block equals energy gained by the water.
- Step 3: The water has far more atoms to share the energy among, so its temperature barely rises, while the block's drops a lot. The final temperature will be just a little above 20 °C, not the average of 80 and 20.
Answer: Block to water, until both share one temperature, which ends up just above 20 °C.
Common mistakes
- Saying “cold flows into” something. Energy flows from hot to cold; cooling is energy leaving.
- Thinking equilibrium means the atoms stop exchanging energy. They keep colliding; only the net transfer is zero.
- Assuming the final temperature is the average of the starting temperatures. That only happens for equal amounts of the same material.
- Confusing temperature with internal energy. A big cool object can hold more internal energy than a small hot one.
On the exam
- Explanations that earn points describe the atom-level mechanism: faster atoms collide with slower ones and usually lose energy to them.
- Expect questions asking which process (conduction, convection or radiation) explains a scenario, such as warming by sunlight across space.
Connected topics
Videos
Check yourself
4 questions on 9.3 Thermal Energy Transfer and Equilibrium. Pick an answer to see if you got it, and why.
Energy from the Sun reaches Earth across the near-vacuum of space. By which process is this energy transferred?
A hot radiator sits near the floor on one side of a room. Air near the radiator warms, rises to the ceiling, spreads across the room and sinks as it cools, warming the whole room. Which process mainly carries energy across the room?
A hot metal block is placed in contact with a cold metal block. Which statement best explains, at the atomic level, why the hot block cools and the cold block warms?
Two blocks of different materials and different masses are placed in contact inside an insulated box. After a long time, which must be true?
0 of 4 answered