Condensation and Evaporation
Condensation is when gas turns to liquid.- When a gas cools, the particles in the gas slow down and lose kinetic energy. The attractive forces between the particles pull them closer together. If the temperature gets cold enough and the gas particles get close enough together that the condensation can take place, the gas becomes a liquid. Water vapour in the air condenses when it comes into contact with cold surfaces e.g. drinks glasses. The steam you see rising from a boiling kettle is actually invisible water vapour condensing to form tiny water droplets as it spreads into cooler air.
Evaporation is when liquid turns to gas
1. Evaporation is when particles escape from a liquid. Particles can evaporate from a liquid at temperature that is much lower than the liquids boiling point. Particles near the surface of the liquid can escape and become gas particles if: The particles are travelling in the right direction to escape the liquid. The particles are travelling fast enough (they have enough kinetic energy) to overcome the attractive forces of the other particles in the liquid. The fastest particles (with the most kinetic energy) are most likely to evaporate from the liquid –so when they do, the average speed and kinetic energy of the remaining particles decreases. This decrease in the average particle energy means the temperature of the remaining liquid falls-the liquid cools. This cooling effect can be really useful. For example, you sweat when you exercise or get hot. As the water from the sweat on your skin evaporates, it cools you down.
Properties of solids, liquids and gases
Solids- Strong forces of attraction hold the particles close together in a fixed, regular arrangement.The particles don’t have much energy so they can only vibrate about their fixed position.
Liquids- There are weaker forces of attraction between the particles. The Particles are close together, but can only move past each other, and form irregular arrangements.They have more energy than the particles in a solid. They move in random directions at low speeds
Gases-There are almost no forces of attraction between the particles.The particles have more energy than those in a liquid and solid. They are free to move and travel in random directions and at high speeds.
page 20 conduction of heat
Conduction of heat-occurs mainly in solids. Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles.This process continues throughout the solid and gradually some of the extra kinetic energy (or heat) is passed all the way through the solid, causing a rise in temperature at the other side of the solid. And hence an increase in the heat radiating from its’ surface.Usually conduction is faster in denser solids, because the particles are closer together and so collide more often and pass energy between them. Materials that have larger spaces between their particles conduct heat energy much more slowly-these materials are insulators.Delocalised electrons are transferring their heat energy through collisions with ions.Metals are good conductors because of their free electrons. Metals “conduct” so well because the electrons are free to move inside the metal.At the hot end the electrons move faster and collide with the other free electrons, transferring energy. These other electrons then pass on their extra energy to other electrons, etc.Because the electrons can move freely, this is obviously a much faster way of transferring the energy through the metal than slowly passing it between jostling neighbouring atoms.This is why heat energy travels so fast through metals.Conductors: metal. Insulators: wool, polystyrene, air, wood,
Page 136 Life cycle of a star
The life cycle of stars
1. Stars initially form from clouds of dust and gas. The force of gravity makes the gas and dust spiral together to form a protostar.2.Gravitational energy is converted into heat energy so the temperature rises. When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of energy in the form of heat and light. A star is born. Smaller masses of gas and dust may also pull together to make planets that orbit the star.3.The star immediately enters a long stable period, where the heat created by the nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards. The star maintains its energy output for millions of years due to the massive amount of hydrogen it consumes. In this stable period it’s called a main sequence star and it lasts several billion years.4.Eventually the hydrogen begins to run out. Heavier elements such as iron forms which are made in the nuclear fission of helium. The star then swells into a red giant, if it’s a small star or a red super giant if it’s a big star. It becomes red because the surface cools.5.A small-to-medium-sized star like the sun then becomes unstable and ejects its outer layer or dust and gas as a planetary nebula.6.This leaves behind hot, dense solid core-a white dwarf, which cools down to form a black dwarf and eventually disappears.7.Big stars, however start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. Eventually they explode in a supernova, forming elements heavier than iron and ejecting them into the universe to form new planets and stars.8.The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. If the star is big enough this will become a black hole.
Convection page 21
Convection of heat-liquids and gases only. Convection occurs when the more energetic particles move from the hotter region to the cooler region-and take their heat energy with them.This is how immersion heaters in kettles and hot water tanks and convection heaters work. Convection simply can’t happen in solids because the particles can’t move.
The immersion heater example- 1.Heat energy is transferred from the heater coils to the water by conduction (particle collisions)2.The particles near the coils get more energy so they start moving around faster.3.This means there’s more distance between them i.e. the water expands and becomes less dense.4.This reduction in density means that the hotter water tends to rise above the denser, cooler water.5.As the hot water rises it displaces (moves) the coolers water out of the way, making it sink towards the heater coils.6.This cold water is then heated by the coils and rises0and so it goes on.7.You end up with convection currents going up, round and down, circulating the heat energy around the water.
Note that convection is most efficient in round or squarish containers because they allow the convection currents to work best. Shallow wide containers just don’t work as well.Also note that because the hot water rises (because the lower density) you only get convection currents in the water above the heater. The water below the heater stays cold because there is almost no conduction.
Convection currents are all about changes in density.
The radiator example- Heating a room with a radiator relies on convection currents too. Hot less dense air by the radiator rises and denser, cooler air flows to replace it.Water heats, Hot water less dense, less dense water rises, fast moving particles collide with slow moving particles and transfer heat, water cools and becomes more dense, denser water sinks again.
Page 24 condensation and evaporation
The rate of evaporation will be faster if the...-Temperature is higher At higher temperatures, the average particle energy will be higher, so more particles will have enough energy to escape.Density is lower-The forces between the particles will usually be weaker, so more particles will have enough energy to overcome these forces and escape the liquid.Surface area is larger- More particles will be near enough to the surface to escape the liquid. Airflow over the liquid is greater. The lower the concentration of an evaporating substance in the air its evaporationg into, the higher the rate of evaporation.A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower.
The rate of condensation will be faster if...
Temperature of the gas is lower. The average particle energy in the gas is lower. So more particles will slow down enough to clump together and form liquid droplets.
Airflow is less The concentration of the substance in the air will eb higher, and so the rate of condensation will be greater.
Temperature of the surface the gas touches is lower
Density is higher The forces between the particles will be stronger. Fewer particles will have enough energy to overcome these forces and will instead clump together and form a liquid