physics 1a - 1

physics 1a - 1

1. Heat Radiation

   • heat energy tends to flow away from a hotter object to its cooler surroundings
   • Heat is transferred in 3 ways
     • heat energy can be transferred by radiation, conduction or convection
     • heat radiation is the transfer of heat energy by infrared (IR) radiation
     • conduction and convection involve the transfer of energy by particles
     • conduction is the main form of heat transfer in solids
     • convection is the main form of heat transfer in liquids and gases
     • infrared radiation can be emitted by solids, liquids and gases
     • any object can both absorb and emit infrared radiation, wether or not conduction or convection are also taking place
     • the bigger the temperature difference between a body and its surroundings, the faster energy is transferred by heating
   • Infrared Radiation - emission of electromagnetic waves
     • all objects are continually emitting and absorbing infrared radiation. infrared radiation is emitted from the surface of an object
     • an object that's hotter than its surroundings emits more radiation than it absorbs (as it cools down).
     • an object that's cooler than its surroundings absorbs more radiation than it emits (as it warms up)
     • the hotter an object is, the more radiation it radiates in a given time
     • you can feel this infrared radiation if you stand near something hot like a fire or if you put your hand just above the bonnet of a recently parked car
   • Radiation depends an awful lot on surface colour and texture
     • dark, matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces, such as gloss white or silver. they also emit much more infrared radiation (at any given time)
     • light, shiny surfaces reflect a lot of the infrared radiation falling on them
     • solar hot water panels contain water piped under a black surface (or black painted pipes under the glass)
     • radiation from the sun is absorbed by the black surface to heat the water in the pipes
     • this water can be used for washing or pumped to radiators to heat the building

2. Kinetic Theory and Conduction

   • Kinetic Theory can explain the 3 states of matter
     • three states of matter are solid (e.g ice), liquid (e.g water) and gas (e.g water vapour)
     • the particles of a particular substance in each state are the same - only the arrangement and energy of the particles are different
     • SOLIDS
       • strong forces of attraction hold the particles close together in a fixed, regular arrangement
       • particles don't have much energy so they can only vibrate so they can only vibrate about their fixed position
     • LIQUIDS
       • weaker forces of attraction between the particles
       • particles are close together, but can 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
       • almost no force of attraction between the particles
       • particles have more energy than those in liquids and solids - they are free to move, and travel in random directions and at high speed
     • when you heat a substance, you give its particle more kinetic energy - they vibrate or move faster
       • this is what eventually causes solids to melt and liquids to boil
   • 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 passes 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 will 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
   • Metals are good conductors because of their free electrons
     • metals 'conduct' so well because the electrons are free to move inside the metals
     • at the hot end the electrons move faster and collide with 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
     • conduction is more efficient through a short, fat rod than through a long, thin rod
       • it all comes down to how far the electrons have to transfer the energy
   • kinetic theory sounds complicated but it's actually pretty simple
     • it just describes how particles move in solids, liquids and gases
       • the energy an object (an particle) has because of its movement is called its kinetic energy

3. Convection

   • Convection of heat - liquids and gases only
     • convections 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 kettle and hot water tanks and (unsurprisingly) convector heaters work
       • convection simply can't happen in solids because the particles can't move
   • 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 colder water out of the way, making it sink towards the heater coils
     • 6. this cold water is then heated by the coils and rises - and so it goes on
       • you end up with convection currents going up, round and down, circulating the heat energy through the water
     • convection is most efficient in roundish or squarish containers, because they allow the convection currents to work best. shallow, wide containers or tall, thin ones just don't work quite so well
       • because the hot water rises (because of the lower density) you only get convection currents in the water above the heater. the water below it stays cold because there's almost no conduction
   • Radiator Example
     • 1. heating a room with a radiator relies on convection currents too
     • 2. hot, less dense air by the radiator rises and denser, cooler air flows to replace it

4. Condensation and Evaportation

   • 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 condensation can take place, the gas becomes a liquid
     • water vapour in the air condenses when it comes into contact with cold surfaces - eg. 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
   • Evaportation is when liquid turns to gas
     • evaporation is when particles escape from a liquid
     • particles can evaporate from a liquid at temperatures that are much lower than the liquid's boiling point
     • particles near the surface of a 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 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 excercise or get hot. as the water from the sweat on your skin evaporates, it cools you down
   • Rates of evaporation and condensation can vary
     • rate of evaporation will be faster if :
       • temperature is higher - 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 be usually 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 it's evaporating 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
     • 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 evaporation
       • temperature of the surface are 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
       • airflow is less - the concentration of the substance in the air will be higher, and so the rate of condensation will be greater

5. Rate of Heat Transfer

   • The rate of heat energy transfer depends on many things
     • heat energy is radiated from the surface of an object
     • the bigger the surface area, the more infrared waves that can be emitted from (or absorbed by ) the surface - so the quicker the transfer of heat
     • this is why car and motorbikes engines often have 'fins' - they increase the surface area so head is radiated away quicker. so the engine cools quicker
     • heat sinks are devices designed to transfer heat away from objects they're in contact with, e.g computer components. they have fins and a larger surface area so they can emit heat as quickly as possible
     • if two objects at the same temperature have the same surface area but different volumes, the object with the smaller volume will cool more quickly - as a higher proportion of the object will be in contact with its surroundings
     • other factors, like the type of material, affects the rate too. objects made from good conductors transfer heat away more quickly than insulating materials, e.g plastic. it also matters wether the materials in contact wiith it are insulators or conductors. if an object is in contact with a conuductor, the heat will be conducted away much faster if it is in contact with a good insulator
   • Some devices are designed to limit heat transfer
     • the glass bottle is double-walled with a vacuum between the two walls. this stops all conduction and convection through the sides
     • the walls either side of the vacuum are silvered to keep heat loss by radiation to a minimum
     • the bottle is supported using insulating foam. this minimises heat conduction to or from the outer glass bottle
     • the stopper is made of plastic and filled with cork or foam to reduce any heat conduction through it
   • Humans and Animals have ways of controlling heat transfer too
     • in the cold, the hairs on your skin 'stand up' to trap a thicker layer of insulating air around the body. this limits the amount of heat loss by convection. some animals do the same using fur
     • when you're too warm, the body diverts more blood to flow near the surface of your skin so that more heat can be lost by radiation - that's why some people go pink when they get hot
     • generally, animals in warm climate have larger ears than those in cold climates to help control heat transfer
       • artic foxes have evolved small ears, with a small surface area to minimise heat loss by radiation and conserve body heat
         • desert foxes on the other hand have huge ears with a larger surface area to allow them to lose heat by radiation easily and keep cool

6. Energy Efficiency in the home

   • Effectiveness and cost-effectiveness are not the same
     • payback time = initial cost / annual saving
     • the most effective methods of insulation are ones that give you the biggest annual saving (they save you the most money each year on your heating bill)
     • eventually, the money you've saved on heating bills will equal the initial cost of putting in the insulation (the amount it cost it buy) the time it takes is called the payback time
     • the most cost-effective methods tend to be the cheapest
     • they are cost-effective because they have a short payback time - this means the money you save covers the amount you paid really quickly
   • Know which types of heat transfer are involved
     • cavity wall insulation - foam squirted into the gap between the bricks stops convection currents being set up in the gap and radiation across the gap. the insulating foam and the air pockets trapped in it (air is an insulator) also helps reduce heat loss by conduction
     • loft insulation - a thick layer of fibreglass wool laid out across the loft floor and ceiling reduces heat loss from the house by conduction and convection
     • draught-proofing - strips of foam and plastic around doors and windows stop draughts of cold air blowing in, i.e they reduce heat loss due to convection
     • hot water tank jacket - lagging such as fibreglass wool reduces conduction and radiation
     • thick curtains - prevents hot air reaching the window by convection, and reduces conduction
   • U-Values show how fast heat can transfer through a material
     • heat transfers faster through materials with higher u0values than through metals with low u-values
     • so the better the insulator the lower the u-value

7. Specific Heat Capacity

   • Specific heart capacity tells how much energy stuff can store
     • it takes more heat energy to increase the temperature of some materials than others
       • you need 4200J to warm 1 kg of water by 1 degree, but only 139J to warm 1 kg of mercurry by 1 degree
     • materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. they can't store' a lot of heat
     • the measure of how much energy a substance can store is called its specific heat capacity
     • specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 degree. water has a specific heat capacity of 4200 J
   • Handy formula for specific heat capacity
     • E = m x c x 0
       • E - energy transferred (J)
       • m - mass (kg)
       • c - specific heat capacity (J/kg)
       • 0 - temperature change
     • if you're not working out the energy, you'll have to rearrange the equation, using the formula triangle
     • remember - you need to convert the mass to kilograms first
   • Heaters have high heat capacities to store lots of energy
     • the materials used in heaters usually have high specific heat capacities so that they can store larger amounts of heat energy
     • water has a really high specific heat capacity. it's also a liquid, so it can easily be pumped around pipes - ideal for central heating systems in buildings
     • electric storage heaters are designed to store heat energy at night (when electricity is cheaper), and then release it during the day. they store the heat using concrete or bricks, which have a high specific heat capacity
     • some heaters are filled with oil, which has a specific heat capacity of around 2000 J. because this is lower than water's specific heat capacity, oil heating systems are often not as good as water-based systems
       • oil does have a higher boiling point through, which usually means oil-filled heaters can safely reach higher temperatures than water-based ones

8. Energy Transfer

   • 9 Types of Energy
     • Electrical
       • whenever a current flows
     • Light
       • from the sun, light bulbs, etc
     • Sound
       • from loudspeakers or anything noisy
     • Kinetic / Movement
       • anything that's moving has it
     • Nuclear
       • released only from nuclear reactions
     • Thermal / Heat
       • flows from hot objects to colder ones
     • Gravitational Potential
       • possessed by anything which can fall
     • Chemical
       • possessed by foods, fuels, batteries, etc
     • Elastic Potential
       • stretched springs, elastic, rubber bands, etc
   • Potential and Chemical-Energy are forms of stored energy
     • gravitational + elastic potential and chemical energy are forms of stored energy because the energy is not obviously doing anything , its kind of waiting to happen
   • Conservation of energy principle
     • energy can be transferred usefully from one form to another, stored or dissipated - but it can never be created or destroyed
     • energy is only useful when it can be converted from one form to another

9. Efficiency of Machines

   • Most energy transfers involve some losses, often as heat
     • useful devices are only useful because they can transform energy from one form to another
     • in doing so, some of the useful input energy is always lost or wanted, often as heat
     • the less energy that is 'wasted', the more efficient the device is said to be
     • the energy flow diagram is pretty much the same for all devices
   • Really Simple to Calculate the Efficiency
     • a machine is a device which turns one type of energy into another. the efficient of any device is defined as :
       • efficiency = useful energy out / total energy in
     • you might not know the energy inputs and outputs of a machine, but you van still calculate the machine's efficiency as long as you know the power input and output :
       • efficiency = useful power out / total power in
     • you can give efficiency as a decimal or you can multiply your answer by 100 to get a percentage
   • Useful Energy input isn't usually equal to total energy output
     • no device is 100% efficient and the wasted energy is usually spread out as heat
     • electric heaters are the exception to this. they're usually 100% efficient because all the electricity is converted to 'useful' heat. ultimately, all energy ends up as heat energy. if you use an electric drill, it goes out various types of energy but they quickly end up as heat
   • We call it wasted heat because we can't do anything useful with it
     • useful energy is concentrated energy. as you know, the entire energy output by a machine, both useful and wasted, eventually ends up as heat
     • this heat is transferred to cooler surroundings, which then become warmer. as the heat us transferred to cooler surroundings, the energy becomes less concentrated - it dissipates
     • the total amount of energy stays the same. the energy is still there, but as it comes increasingly spread out, it can't be easily used or collected back in again
   • Think About Cost-Effectiveness and Efficiency when choosing appliances
     • light bulbs
       • a low-energy bulb is about 4 times as efficient as an ordinary light bulb
       • energy-efficient light bulbs are more expensive to buy but they last much longer
       • if an energy-saving light bulb costs £3 and save £12 of energy a year, its payback time would be 3 months
       • energy-saving light bulbs are normally more cost-effective than ordinary bulbs
       • LED light bulbs are even more efficient than low-energy bulbs, and can last even longer
         • but they are more expensive to buy and don't give out as much light as the other two types of bulb
     • replacing old appliances with newer energy-efficient ones
       • new, efficient appliances are cheaper to run than older, less efficient appliances. but new appliances can be expensive to buy
       • you've got to work out if it's cost-effective to buy a new appliance
       • to work out how cost- effective a new appliance will be you need to work out its payback time
   • Sometimes 'waste' energy can actually be useful
     • heat exchangers can be used to reduce the amount of energy wasted by a machine
     • they do this by pumping a cool fluid through the escaping heat and using it for other purposes
     • the temperature of this fluid rises as it gains heat energy
     • the heat energy in the fluid can then be transferred to somewhere it's useful again
       • for example, the heat from a car's engine can be transferred to the air that's used to warm the passenger compartment