Heat energy can be transferred by radiation, conduction or convection.
- Heat radiation by infrared radiation
- Conduction and convection by particles
- CONDUCTION by 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 objects can EMIT and ABSORb infrared radiation
The bigger the temperature difference, the faster energy is transferred by heating.
Object is hotter than surroundings - EMITS more radiation than it absorbs.
Object is cooler than surroundings - ABSORBS more radiation than it emits
The hotter an object is the more radiation it radiates in a given time
DARK, MATT surfaces ABSORB infrared radiation better than light, shiny surfaces. They also EMIT much more infrared radiation.
LIGHT, SHINY surfaces REFLECT infrared radiation.
SOLIDS - Strong forces of attraction hold the particles closer together. The particles don't have that much energy so they can only vibrate about their fixed positions.
LIQUIDS - Weaker forces of attraction, particles can move past each other. They have more energy than the particles in a solid.
GASES - No forces of attraction, free to move, travel at high speeds, more energy than solids and liquids.
When you heat a substance, you give its particles more kinetic energy - they vibrate or move faster.
Conduction and Convection
CONDUCTION of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles - conduction is faster in DENSER solids because the particles are closer together.
- 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 other free electrons, transferring energy.
CONVECTION occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them. Convection currents are about changes in density.
- Heat energy is transferring from the heater coils to the water by conduction
- Particles near the coil get more energy and move around faster
- The water expands and becomes less dense.
- The hotter water rises above the denser, cooler water and makes it sink towards the coil
- The cold water is heated by the coils and rises... so on.
Condensation and Evaporation
CONDENSATION is when a gas turns into a liquid. If the temperature gets cold enough and the gas particles get close enough condensation takes place.
RATE OF CONDENSATION WILL BE FASTER IF...
- Temperature of gas is lower
- Temperature of surface is lower
- Density is higher
- Airflow is less
EVAPORATION is when liquids turn to gas. Particles near the surface of a liquid can escape and become gas particles if particles are travelling in the right direction or particles are travelling fast enough.
RATE OF EVAPORATION WILL BE FASTER IF...
- Temperature is high
- Density is lower
- Surface area is larger
- Airflow over the liquid is greater
Rate of Heat Transfer
Heat energy is radiated from the surface of an object. The BIGGER the surface area the MORE infrared waves that can be EMITTED (or absorbed by) the surface.
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 the proportion of the object will be in contact with its surroundings.
Objects made from good CONDUCTORS transfer heat away more QUICKLY than insulating materials.
If an object is in contact with a conductor the heat will be conducted away much faster than if it's in contact with a good insulator.
Rate of Heat Transfer 2
- The glass bottle is double-walled with a VACUUM between to 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.
- The STOPPER is made out of plastic and filled with foam/cork to reduces any HEAT CONDUCTION.
The hairs on you skin stand up to trap a thicker layer of insulating air around the body. This limits the amount of heat loss by convection.
Arctic foxes have small ears,with a small surface area to minimise heat loss by radiation.
Desert foxes have huge ears with large surface area to allow them to lose heat by radiation.
Energy Efficiency in the home
The most effective methods of insulation are ones that give you the biggest annual savings. They are cost-effective if they have a short payback time.
PAYBACK TIME = INITIAL COST / ANNUAL SAVINGS
- Cavity wall insulation - reduces convection and radiation. Pockets of air in the foam reduce heat transfer by conduction.
- Loft insulation - reduces conduction and radiation
- Draught proofing - reduce heat loss due to convection
Heat transfers faster through materials with high U-value. So the better the insulator the lower the U-value.
Specific Heat Capacity
The measure of how much energy a substance can store is called its specific heat capacity.
- Water has a specific heat capacity of 4200J/kgC
ENERGY TRANSFERRED = MASS x SPECIFIC HEAT CAPACITY x TEMP CHANGE
The materials used in heaters usually have high specific heat capacities so that they can store large amounts of heat energy.
Some heaters are filled with oil, which has a specific heat capacity of around 2000J/kgC. Becuase 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 high boiling point, which usually means oil-filled heaters can safely reach higher temperatures than water based ones.
- Electrical energy - current
- Light energy - the sun, light bulbs, etc.
- Sound energy - loudspeaker
- Kinetic energy - anything moving
- Nuclear energy - nuclear reactions
- Thermal energy - hot objects
- Gravitational potential energy - anything which can fall
- Elastic potential energy - springs, elastic, etc.
- Chemical energy - food, fuels, batteries, etc.
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.
Efficiency of Machines
NO device is 100% efficient and the WASTED energy is usually spread out as HEAT.
The LESS energy that is 'wasted', the more EFFICIENT the device is.
EFFICIENCY = USEFUL ENERGY OUT / TOTAL ENERGY IN
EFFICIENCY = USEFUL POWER OUT / TOTAL POWER IN
USEFUL energy is concentrated energy.
- Heat exchangers reduce the amount of heat energy that is lost by pumping cool fluid through the escaping heat.
- The temperature of this fluid rises as it gains heat energy. The heat energy in the fluid can then be converted into a form of energy that's useful again.
The thicker the arrow, the more energy it represents - so you see a big thick arrow going in, then several smaller arrows going off it to show the different energy transformations taking place.
The Cost of Electricity
The amount of energy that is transferred by an appliance depends on its POWER and the AMOUNT OF TIME that appliance is switched on.
ENERGY = POWER x TIME
- Energy is usually measured in JOULES
- Power is usually measured in WATTS or KILOWATTS
- The standard units of electrical energy are KILOWATT-HOURS
Calculating the cost of electricity
UNITS = POWER x TIMES
COST = UNITS x PRICE
Energy Sources and Power Stations
Non-renewable energy resources/fossil fuels
- Coal, Oil, Natural gas
- Fossil fuels release CO2 into the atmosphere when they're burned. CO2 adds to the greenhouse effect and contributes to global warming. Burning coal and oil releases sulfur dioxide which causes acid rain.
Renewable energy resources
- Wind, Waves, Tides, Hydroelectric, Solar, Geothermal, Food, Biofuels
- The fossil fuel is burned to convert is stored chemical energy into heat
- Heat energy is used to heat water to produce steam
- Steam turns the turbine, converting heat energy into kinetic energy
- Turbine is connected to a generator, which transfers kinetic energy into electrical energy
Renewable Energy Sources
- The electricity is generated directly from the wind turning the blades, which turn the generator.
- There is no pollution but, they do spoil the view and they can be very noisy.
- They can be unreliable and initial costs are quite high
- There are no fuel costs and minimal running costs
- Generate electric currents directly from sunlight and they produce no pollution.
- Solar power is only reliable in sunny countries and the inital costs are high
- The energy is free and running costs almost nil.
- Solar cells are usually used to generate electricity on a relatively small scale.
Renewable Energy Sources 2
- Requires the flooding of a valley by building a big dam. Rainwater is collected and allowed out through turbines. There is no pollution
- Big impact on the environment due to the flooding of the valley and possible loss of habitat for some species.
- Can meet an increased demand for electricity.
- Initial costs are high, bit there's no fuel and minimal running costs
- Spare night-time electricity is used to pump water up a higher reservoir
- This can then be released quickly during times of peak demand.
- Pumped storage works similarly to hydroelectric power
Renewable Energy Sources 3
- Waves come into the shore and they provide an up and down motion which can be used to drive a generator. There is no pollution but they spoil the view and are hazards to boats.
- Fairy unreliable and initial costs are high. Provide energy on a large scale.
- Only possible in volcanic areas where hot rocks lie near to the surface. Steam and hot water rise to the surface and are used to drive a generator.
- No environmental problems but cost of building the power plant are high
- Generate electricity the same way as fossil fuels
- We can get biofuels from organisms or dead organic matter.
Energy Sources and the Environment
Non-renewable energy and environmental problems
- Coal mining makes a mess of the landscape, oil spillages can affect mammals and birds that live in and around the sea, nuclear waste is difficult to dispose of.
Disadvantages of biofuels
- When plants are burned they release CO2 into the atmosphere and biofuel production also creates methane emissions.
- Large areas of forest are cut down the make room to grow biofuels. The decay and burning of vegetation also increase CO2 and methane emissions.
- Used to reduce the amount of CO2 building up in the atmosphere by collecting the CO2 from power stations before it is released into the atmosphere. The captured CO2 can be pumped into empty gas fields and oil fields.
Electricity and the National Grid
Electricity is distributed via the national grid.
Voltages are increased using a STEP-UP TRANSFORMER and reduced with a STEP-DOWN TRANSFORMER.
The problem with a high current is that you lose loads of energy through HEAT in the cables. It is much CHEAPER to boost the voltage and keep the current very low
In order to meet demands in the future, the energy supplied to the National Grid will need to increase, or energy demands of consumers will need to decrease
- Supply can be increased by opening more power plants.
- Demand can be reduced by using more energy-efficient appliances
Frequency is the number of waves produced
by a source each second. Frequency is measured in HERTZ
Most waves are TRANSVERSE: EM waves, ripples, waves on a strings, slinky spring
In TRANSVERSE waves the vibrations are PERPENDICULAR (at 90o) to the direction of energy transfer of the wave
In LONGITDUDINAL WAVES the vibrations are PARALLEL to the direction of energy transder of the wave. E.g. sound waves, ultrasound, shockwaves
SPEED = FREQUENCY x WAVELENGTH
All waves can be REFLECTED, REFRACTED and DIFFRACTED.
- When light travelling in the same direction reflects from a UNEVEN surface the light reflects and different angles.
- When light travelling in the same direction reflects from an EVEN surface then it's all reflected at the SAME ANGLE and you get a clear reflection.
- ANGLE OF INCIDENCE = ANGLE OF REFLECTION
Refraction and Diffraction
- REFRACTION - The waves go through a material but change direction
- DIFFRACTED - The waves bend round obstacles, causing the waves to spread out.
- The amount of diffraction depends on the size of the gap relative to the wavelength of the wave. The narrower the gap, or the longer the wavelength, the more the waves spread out.
- Waves are only refracted if they meet a new medium at an angle. If they're travelling along the normal they will change speed, but are NOT refracted - they don't change direction.
EM Wave and Communication
- EM waves vary in wavelength from 10-15 m to more than 104m. EM waves travel at the same speed (3x108 m/s) in a vacuum. Higher frequencies have shorter wavelengths.
RADIOWAVES are used mainly for communication (TV and FM Radios)
- Long-waves diffract. This diffraction effect makes is possible for radio signals to be received. Short-wave radio signals are reflected from the ionosphere - an electrically charged layer.
MICROWAVES are used for satellites communication and mobile phones. Microwaves are picked up from the satellites and transmitted back to the earth. Some wavelengths of microwaves are absorbed by water molecules and heat up.
INFRARED WAVES are used for remote controls and optical fibres. They transmit information.
VISIBLE LIGHT is useful for photography
SOUND WAVES are caused by VIBRATING objects. These mechanical vibrations are passed through the surrounding medium as a series of compressions.
Sound waves travel faster in solids (denser materials) than in liquids etc. Sound can't travel through space because it's mostly a vacuum.
Sound waves can be reflected by hard flat surfaces. Echoes are just reflected sound waves. Sound waves will also refract as they enter different mediums.
- High frequency - high pitched - short wavelength
- Low frequency - low pitched
- FREQUENCY is the number of complete vibrations each second. It is measured in Hertz (Hz)
- AMPLITUDE is the loudness of the sound. The bigger amplitude the louder the sound.
Each element produces a specific pattern of dark lines at the frequencies that it absorbs in the visible spectrum.
When we look at light from distant galaxies we can see the same patterns but at slightly lower frequencies - they're shifted towards the red end of the spectrum. This is called RED-SHIFT.
Measurement of the red-shift suggests that all the galaxies are moving away from us.
Most distant galaxies have greater red-shifts than nearer ones. This means that more distant galaxies are moving away from us faster than nearer ones.
The Doppler Effect
When something that EMITS waves moves towards you or away from you, the WAVELENGTH and FREQUENCES of the waves seem different - compared to when the source of the waves is stationary.
The FREQUENCY of a source moving TOWARDS from you will seem HIGHER and its WAVELENGTH will seem SHORTER.
The FREQUENCY of a source moving AWAY from you will seem LOWER and its WAVELENGTH will seem LONGER.
The DOPPLER EFFECT happens to both LONGITUDINAL waves and TRANSVERSE waves.
The Origin of the Universe
BIG BANG - All the matter and energy in the universe must have been compressed into a very small space. Then it exploded from that single point and started expanding. We can use the current rate of expansion of the universe to estimate it's age.
- Scientists have detected COSMIC MICROWAVE BACKGROUND RADIATION coming from all parts of the universe. The Big Bang theory is the only theory that can explain this.
- After the Big Bang, everything in the universe emitted very HIGH frequency radiation. As the universe expanded it has cooled, and this radiation is now seen as microwave radiation.