Fechar
Infrared Radiation(heat)
-Electromagnetic spectrum, beyond visible red light
-Emitted by all objects
-Can travel through a vacuum
-The hotter an object, the more radiation it emits in a given time
-Dark, matt surfaces are good absorbers and emitters of IR. Transfer energy and cool down quickly
-Light, Shiny surfaces are good reflectors of IR
Solid
-Fixed Shape
-Transfers energy by vibrating
-Good conductors(metals)
Gas
-Particles are far apart, move at random, much faster
-No fixed shape and can flow
-Less dense
Conduction
-One end of a solid is heated, the particles at the end gain kinetic energy and vibrate more
-Energy is passed to neighbouring particles helped by free electrons colliding amongst the atoms
-Occurs in metals
Convection
-occurs in fluids
-When a fluid is heated it expands
-Becomes less dense and rises
-Warm fluid replaced by cooler, denser fluid
-The resulting convection current transfers energy throughout the fluid
Evapouration
-Liquid turns into a gas
-The most energetic liquid moleculesescpae from the surface and enter the air; the average kinetic energy of the remaining molecules is less so the temp. of the liquid decreases
-So evapouration causes cooling
Condensation
-Gas turns into a liquid
-Takes place on cold surfaces(mirrors)
The greater the temp. difference between an object and its surroundings, the greater the rate at which energy is transferred
Sometimes we want to maximise the rate of energy transfer to keep things cool
-we use things that are good conductors
-dull black
-the air flow around them maximised
Motorcycle Engine Fins
-Has many small fins to provide a large surface area so it keeps the engine cool and does not overheat
- Releases heat quickly
Vacuum flask
-stucture minimises energy transfer by conduction, convection and radiation
-reduces the rate of energy transferto keep hot things hot and cold things cold
Specific Heat Capacity
-amount of energy required to raise the temp of 1 kilogram of the substance by 1 degree c
-The greater the shc, the more energy required for each degree temp. change
Energy transferred=Mass x shc x temp. change
E=m x c x 0
U-Value
-how much energy per second passes through different materials; allows us to compare them
-The lower the u-value the better the material is as an insulator
Forms of energy
-Light, sound, kinetic(movement), nuclear, electrical, gravitational potential, elastic potential and chemical
-last three are forms of stored energy
Conservation of energy
-law stating it is not possible to create or destroy energy. It is only possible to transfer it from one form to another, or from one place to another
-Total amount of energy is always the same. Applies to all energy transfers
-A machine is something that transfers energy from one place to the other or from one from to another. The energy produced by the machine consists of:
useful energy- energy in the place we want it and the form we need it
wasted energy- energy not usefully transferred
Energy(J) and Efficiency
-Energy supplied is the input energy. From the conservation of energy we know:
input energy= useful energy transferred+energy wasted
No appliance can be 100% efficient, except an electric heater, which usefully transfers all of the electrical energy supplied to it by heating its surroundings
-Energy transfer through an appliance can be represented with a Sankey diagram
Electrical appliances
-transfer electrical energy into whatever form of energy we need at the flick of a switch
-lamps(light), speakers(sound),tv(light and sound)
Electrical Power
-the power of an appliance is the rate at which it transfers energy
-1 watt transfers 1 joule every second
1kilowatt=1000 watts
Using electrical energy
-A kwh is the amount of energy transferred by a one-kw appliance when used for one hour
The cost of the electrical energy
total cost=number of kWh x cost per kWh
-the cost per kWh is given on the electricity bill
To reduce energy bills many may buy newer, more efficient appliances, or install materials designed to reduce energy wastage
-the payback time is the time it takes for an appliance or installation to pay for itself in terms of energy savings
