Energy Transfers And Particle Motion
Übersicht
Walk out of a warm house on a frosty morning and take hold of the metal gate latch, then rest your other hand on the wooden gatepost beside it. The latch is painful. The post is merely chilly. Both have stood in the same air all night, so a thermometer pressed against either one would give you the same reading to within a fraction of a degree. Your hand is not measuring their temperature at all. It is measuring how quickly each of them is emptying your fingers of energy, and on that measure the two are nowhere near each other.
This lesson is about rate: not how much energy an object holds, but how fast it moves and what governs the speed. You will trace energy through a solid by vibration and by free electrons, watch a fluid carry its own energy upwards because heating has made it thinner, work out why a film of sweat leaves your skin colder than it found it, calculate a surface area to volume ratio, and see why most substances grow a little when they are warmed. It is the best examined topic on the whole physics paper: on the Core Tier specimen it carries a ten mark question, an eight mark question and the paper's only six mark question, and the Extension Tier sets two of those three word for word.
Ziele
- Energy may be transferred by conduction and convection. Students should be able to explain, in terms of particles, how these energy transfers take place. They should understand in simple terms how the arrangement and movement of particles determine whether a material is a conductor or an insulator and understand the role of free electrons in conduction through a metal. They should be able to use the idea of particles moving apart to make a fluid less dense, to explain and apply the concept of convection.
- Energy may be transferred by evaporation and condensation. Students should be able to explain evaporation, and the cooling effect this causes, using kinetic theory. Students should be able to discuss the factors that affect the rate of evaporation.
- The rate at which an object transfers energy by heating depends on: its surface area and volume; the material from which the object is made; the nature of the surface with which the object is in contact. Students should be able to explain the design of devices in terms of energy transfer, for example cooling fins, and 63 should be able to explain animal adaptations in terms of energy transfer, for example relative ear size of animals in cold and warm climates.
- The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating.
- Most substances expand when heated. Students should understand that the expansion of substances on heating may be a hazard (for example, the expansion of roofs and bridges) or useful (for example, the bi-metallic strip thermostat).
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- Why is copper a much better thermal conductor than plastic? A. The particles in copper are further apart B. Copper contains free electrons that can move through it and transfer energy by collision C. Copper particles are heavier, so they vibrate more slowly D. Copper allows convection currents to form inside it Answer: B
- A solid metal cube has sides of 3.0 cm. What is its surface area to volume ratio? A. 0.50 per cm B. 2.0 per cm C. 27 per cm D. 54 per cm Answer: B
- Warm water at the bottom of a beaker rises. Why? A. Its mass decreases when it is heated B. Its particles expand when they are heated C. It expands, so its density becomes lower than that of the water around it D. Free electrons carry it upwards Answer: C
- Why does a liquid cool down as it evaporates? A. All the particles in the liquid slow down as some escape B. The particles that escape are the fastest ones, so the average kinetic energy of those remaining falls C. Energy is destroyed when a particle leaves the liquid D. The liquid loses mass, and less mass always means a lower temperature Answer: B
- A mug of tea is left in a room. Why does it cool more slowly after twenty minutes than it did in the first minute? A. The tea has less mass than it started with B. The temperature difference between the tea and the room has become smaller C. The specific heat capacity of the tea has increased D. Energy transfer stops once the tea is below 50 degC Answer: B
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