IGCSE Physics Review Syllabus - Thermal Physics - Theory

#复习提纲 #学校/Y10/物理 #IGCSE #笔记 By: Steven Zhou, visit Ste-z.com for more

2.1 Simple kinetic molecular model of matter

2.1.1 States of matter properties of solids, liquids and gases

2.1.2 Molecular Model molecular structure of solids, liquids and gases

relation of properties of solids, liquids and gases to the forces and distances between molecules and to the motion of the molecules

solid shape fixed: strong force between each particle - prevent the molecules from moving cannot compressed: already very close to each other - repel forces cannot flow: molecules only vibrate in fixed positions high density: very close distance between molecules

liquid take shape of container: molecules have enough energy to weaken the force between them (so it can no longer hold the particles in place) cannot compressed: already very close to each other - repel forces

can flow: molecules can move by sliding past each other high density: molecules are close to each pther

gas can move quicly: forces broken completely can compress: molecules are far apart can flow: molecules can move around low density: molecules are far apart

temperature of a gas ↑ temperature ↑ mean speed of motion of molecules (some molecules can move faster than others)

pressure of a gas pressure is caused by the particles striking the walls creating a force

force exerted because of the change of momentum of particles when striked at the wall pressure exerted by the gas = the force exerted by the particles striking the walls / area of the wall

evidence for the kinetic molecular model of matter - Brownian motion (random motion of particles in a suspension)

result of random molecular bombardment massive particles may be moved by light, fast-moving molecules

light and fast-moving = high momentum which can transfer to massive particles therefore, large molecule’s speed and directions change randomly each time when they are stroked by a light, fast-moving molecules

e.g. tiny fast moving gas molecule can collide with visible smoke particles and make them move

2.1.3 Evaporation evaporation: escape of more-energetic molecules from the surface of a liquid

(molecules in a liquid have a range of different energies) factors influencing evaporation

↑temperature - ↑more molecules have enough energy to escape - ↑rate of evaporation ↑surface area - ↑more area from which the molecules can escape - ↑rate of evaporation draught over a surface - removal of less energetic molecules (which might not have quite enough energy to escape) from the liquid - ↑rate of evaporation

when liquid evaporates, average energy of the remaining molecules decreases - temperature of the remaining liquid ↓, thus:

evaporation results cooling of the liquid a body will cool when in contact with an evaporating liquid

2.1.4 Pressure changes factors affecting pressure of a gas when

↑ temperature at constant volume —— ↑ pressure ↑ temperature ↑ motion ↑ motion - ↑ probability of collision - ↑ rate of collision

↓ volume at constant temperature —— ↑ pressure ↓volume - gas is compressed - ↑density of the gas ↑density of the gas - ↑collisions against surfaces

pressure × volume = constant pV= constant only for a fixed mass of gas at constant temperature

2.2 Thermal properties and temperature

2.2.1 Thermal expansion of solids, liquids and gases thermal expansion of solids, liquids, and gases at constant pressure

Heating a material will increase the movement of the particles The particles then strike into each other and push each other apart the distance between particles increase

Relative order of the magnitude of the expansion of solids, liquids and gases

Everyday applications and consequences of thermal expansion Application

Liquid-in-gas thermometer (described afterwards in Section 2.2.2) temperature-activated switch - two metals that expand at different rates, can be

made to bend at a given temperature Consequences - can cause solid to buckle if they get too hot

e.g. at Metal railway tracks, Road surfaces, Bridges Therefore, gaps are often built into these things

2.2.2 Measurement of temperature by measuring a physical property that varies with temperature - we can measure temperature; properties include:

volume (and density) of the substance (Thermal expansion) electrical resistance of the substance

Fixed point - a temperature at which some easily identifiable change occur Once a property has been measured at two fixed points (upper and lower point), the values of that property at other temperatures can be worked out need for fixed points - allow us to know the temperature without having to measure it directly Usual fixed point

melting temperature of ice boiling temperature of pure water

sensitivity, range and linearity sensitivity - how significant was changes of a property when the temperature changes?

a sensitive thermometer makes it easier to detect small changes in temperature range - what are the lowest and highest temperatures the thermometer can measure? linearity - Is the property change with temperature at a steady rate?

linearity makes it easy to work out the relationship liquid-in-glass thermometers

utilize thermal expansion structure & action

liquid contained in a small glass bulb with a capillary tube above it As the bulb is heated, the liquid will expand and the level of liquid in the capillary will rise upwards the level of the liquid risen will be measured (higher the level, higher the temperature)

structure’s relation with sensitivity, range and linearity the liquid expand linearly with temperature high sensitivity

liquid expand significantly when temperature changes narrow tube - small change in volume results in the liquid moving a significant distance along the tube

the liquid have low freezing points and high boiling points - great range thermocouple

structure

two different types of wire attached together When the joint between the two wires is heated, a potential difference (voltage) is created between the two wires greater the temperature, the greater the potential difference.s

it is used as a thermometer for measuring high temperatures and those that vary rapidly

Wide range - Any temperature below the melting point of the metals used to make the wires can e measured Respond very rapidly - glass thermometers take more time to respond as the liquid inside takes time to expand

2.2.3 Thermal capacity (heat capacity) internal energy consist of

Kinetic energy - due to molecule’s motion/vibration Potential energy - due to bonds between the molecules

rise in the temperature of a body relates to the rise of Kinetic energy, therefore relates to the increase in its internal energy thermal capacity of a body - the amount of heat energy required to raise the temperature of the body by 1 °C

↑thermal capacity of an object, ↑ heat energy it takes to raise its temperature Thermal capacity (J/°C) = mass (kg) * specific heat capacity (J/kg/°C)

Thermal capacity = mc specific heat capacity - The amount of energy required to heat 1 kg of a material by 1 °C

change in energy (J) = mass (kg) * specific heat capacity (c) * change in temperature (°C)

ΔE = mc∆T PRA: Describe an experiment to measure the specific heat capacity of a substance

2.2.4 Melting and boiling melting point and boiling point

melting point - temperature at which a material changes from a solid to a liquid boiling point - temperature at which a material changes from a liquid to a gas

when a substance undergo melting and boiling - energy is inputted without a change in temperature

energy is being used to break the bonds between the molecules instead of increasing the kinetic energy of the molecules

Distinguishing between boiling and evaporation

condensation and solidification condensation - a substance turns from gas to liquid solidification - a substance turns from liquid to solid temperature also remains constant - bonds reforming (giving out energy) and losing internal energy

latent heat - energy required to change the state of a substance This energy is required to break the bonds holding molecules together (also given out again once the bonds reform) types

latent heat of vaporisation - energy required to turn a liquid into a gas latent heat of fusion - energy required to turn a solid into a liquid

specific latent heat - energy required to change the state of 1 kg of that substance total energy required to change the state (J) = mass (kg) * Specific Latent Heat

E = ml PRA: Describe an experiment to measure specific latent heats for steam and for ice

2.3 Thermal processes

2.3.1 Conduction PRA: Describe experiments to demonstrate the properties of good and bad thermal conductors properties of good and bad thermal conductors

good thermal conductors - heat spreads rapidly bad thermal conductors (insulator) - heat travels slowly

molecular account of conduction in solids lattice vibration - vibrating particles strike into each other - transferring energy from atom to atom transfer by electrons - delocalised electrons can collide with the atoms, helping to transfer the vibration through the material

2.3.2 Convection

convection - an important method of thermal transfer in fluids (liquid and gas) convection in fluids

when fluid heated - thermal expansion - distance between particles increase - density decrease - became less dense than surroundings - rises above - cooler take its place the hot fluid then cools - distance between particles decrease - density increase - sinks back down

PRA: describe experiments to illustrate convection

2.3.3 Radiation PRA: Describe experiments to show the properties of good and bad emitters and good and bad absorbers of infrared radiation thermal energy transfer by infra-red (part of the electromagnetic spectrum) radiation

does not require a medium Factors affecting rate radiation

Factors affecting the amount of radiation emitted ↑surface temperature ↑amount of radiation ↑surface area of a body ↑amount of radiation

2.3.4 Consequence of energy transfer Examples of consequences of conduction, convection and radiation

We may use a lot of energy (and money) to heat our homes during cold weather, and the energy may escape We eat food to supply the energy we need to keep our bodies warm, but energy escapes from us

Everyday applications of conduction, convection and radiation home insulation

temperature retaining flasks and bottles

Glass is generally used - good insulator gap between the double walls is evacuated to reduce losses by conduction and

convection Silvering reduces losses by radiation by reflecting back any infrared radiation stopper prevents losses by convection and evaporation