P2 - Energy Transfer by Heating Flashcards
What is a Conductor?
A conductor allows the flow of charge.
What type of material is the best conductor of energy?
Metals
Why are metals such good conductors?
Their bonding enables a ‘sea’ of delocalised electrons to carry charge through the metal.
What is an insulator?
An insulator resists the flow of charge.
How are insulators bonded?
Their electrons are held much more tightly together and will resist the flow of charge (no electrons can move through them).
What makes a good insulator?
- Thick
- Low thermal conductivity
What is Conduction?
The process by which heat energy is transferred by collisions with neighbouring molecules.
- Conduction occurs mostly in solids.
- BUT CAN ALSO HAPPEN IN SOME LIQUIDS AND GASES.
What are some examples of Conduction?
1) Melting chocolate - heat transferred from the hand to the chocolate.
2) Ironing a shirt - heat transferred from the iron to the shirt.
What is thermal conductivity a measure of?
A measure of how well a material can conduct heat.
What does having a higher thermal conductivity mean?
It means more energy can be transferred (better conductor).
Which is a better conductor - copper or steel?
Copper - whence why it’s used in wires
How does thickness impact conductivity?
The thicker a material, the slower it will allow the flow of charge.
Thin = Good conductor
What is Convection?
The movement within a fluid caused by hotter (less dense) material to rise, and colder (more dense) material to sink under the influence of gravity.
What state does convection take place?
All fluids - liquids and gases.
How does convection work?
1) Fluid heated up.
2) Hotter, less dense material rises.
3) Cooler, more dense material sinks, replacing the hot material.
4) A ‘convection current’ is set up.
What does ABSORB mean?
To take in.
What does REFLECT mean?
To bounce back or mirror.
What does RADIATE mean?
To give out waves or rays.
What does TRANSMIT mean?
To let waves or rays through.
What does EMIT mean?
To give off.
What is black like as an absorber and emitter of heat?
Black is an excellent absorber and emitter of heat - the best.
Why is black a good absorber and emitter of heat?
Black absorbs all incoming light.
What is white like as an aborber and emitter of heat?
White is a poor absorber and emitter of heat - the worst.
Why is white a poor absorber and emitter of heat?
White reflects all incoming light.
Why can people with darker skin cool themselves down more effectively?
Darker skin absorbs more heat - but this does not increase the body’s internal heat temperature.
Darker skin can emit the heat absorbed by the skin and from the body.
How do shiny surfaces and matt surfaces compare in terms of absorption and emission?
Matt surfaces are the best absorbers and emitters.
Shiny surfaces are the worst absorbers and emitters.
What is radiation?
Radiation is waves of energy.
What is infrared radiation?
A type of electromagnetic radiation which we feel as heat.
How do infrared rays/waves differ from light rays in terms of wavelength?
Infrared radiation is before visible light on the EM spectrum - so infrared rays have longer wavelengths.
What speed do infrared rays travel at?
300,000,000 m/s - speed of light
Why are infrared cameras used for security and in the police?
All objects emit and absorb infrared radiation.
Things that respire (i.e people) are heat sources and can easily be identified.
How does the temperature of something impact the infrared radiation it emits?
The hotter an object, the more infrared it emits.
How does the sun transfer heat energy to the earth?
The Sun heats the earth by infrared radiation which travels through the vaccum of space.
As space is a vaccum - no heat transfer can take place by conduction or convection.
What is a perfect black body?
An object that absorbs and then re-emits ALL of the radiation that hits it.
-NO reflection or transmission (nothing passes through it).
What is blackbody radiation?
The radiation emitted by a body that absorbs all radiation that hits it.
How does the intensity of blackbody radiation vary with wavelength?
Shorter wavelength = Greater heat intensity
Hotter object = shorter wavelength
What does the temperature of the earth’s surface depend on?
The balance between the absorbtion and emission of infrared radiation.
Absorption > Emssion —> °C Increases
Absorption < Emission —> °C Decreases
How can a system stay at a constant temperature - theoretically?
A system will remain at a constant temperature if:
Absorption = Emission
How are emergency/space blankets used to keep someone warm?
By keeping the heat that is generated by the body from emitting into the surrounding air.
How do barns use infrared lamps to keep the animals warm?
You can’t have fire as there is a lot of wood and hay. Therefore infrared lamps give out heat to the barn.
What is Specific Heat Capacity?
The amount of thermal energy needed to raise the temperature of 1kg of a substance by 1°C or 1K.
Units: J/kg
High Specific heat capacity = more energy needed.
Low Specific heat capacity = less energy needed.
What is the equation used to calculate energy that involves specific heat capacity?
Energy Transferred = mass x specific heat capacity x temperature change
E = m x c x Δt
(J) = (kg) x (J/kg) x (°C or K)
By re-arranging this equation, how do you calculate specific heat capacity?
c = E / m x Δt
What is the Specific Heat Capacity Required Practical?
Aim: To calculate the specific heat capacity of aluminium, using a circuit.
1) Measure the current, voltage and temperature of the materials at certain intervals.
2) Use E = IVt (Energy = Current x Voltage x time) to work out E.
3) Then do c = E / m x Δt (temperature)
How are homes heated?
Gas or oil-fired central heating.
How can the rate of energy transfer out of a home be reduced?
(Saves money and warms house)
1) Loft Insulation - Loft insulation reduces the rate of energy transfer through the roof. Fibreglass is used.
2) Cavity Wall Insulation - The Cavity of an outer wall is the space between the 2 layers of brick that make up the wall. The insulation is pumped into the cavity and reduces the rate of energy transfer by conduction.
3) Double Glazed Windows - Double glazed windows have two glass panes with dry air or a vacuum between the panes. The dry air reduces the rate of energy transfer by conduction and a vacuum prevents convection.
4) Alluminium Foil behind radiators - The foil reflects radiation away from the wall and reduces energy transfer by radiation.
