Physics 1a Flashcards
How does heat radiation occur ?
Transfer of energy through IR waves
What are the three ways heat is transferred ?
Conduction, convection and radiation
Does an object hotter than its surroundings emit more or less radiation ?
More
What is the best surface to absorb thermal radiation ?
Dark, Matt surfaces
What do light, shiny surfaces do with radiation ?
Reflect it
What is conduction ?
The process where vibrating particles pass on their extra kinetic energy to neighbouring particles
How do solar panels make the most of thermal radiation ?
The panels are painted black or the pipes painted black. This is to heat water inside them as black absorbs heat.
What state does conduction take place in ?
Solids
Give an example of a good conductor material ?
Metal
Why are metals good conductors ?
Metals have electrons that are free to move. When heated up they bounce off each other transferring energy to the cooler side
Where does convection occur ?
Liquids and gases
If there is water below the heat source, what does it do ?
Stays cool as relatively no convection occurs
How does convection work ?
Water/gas I heated so the particles spread out. Therefore that section of the fluid becomes less dense and rises. As it rises it begins to cool as the heat source is at the bottom. The particles move closer together and then they sink back down as that section of fluid is now more dense.
What are some characteristics of a solid ?
Strong forces of attraction hold them in a fixed arrangement / low energy so can only vibrate
What state has the most energy ? How can we see this ?
Gas which means they can freely move around us
True or false - liquids have weak forces of attraction
True
Describe condensation
When a gas turns to a liquid
What is evaporation ?
When liquid turns to gas
Why does a gas condense ?
When a gas cools net particles slow down and lose energy. The particles are pulled together and become a liquid.
What two things allow a liquid particle to evaporate ?
- If they are traveling in the right direction to escape
2. If they are traveling fast enough
How does evaporation occur ?
Particles with the most energy escape because gases have more energy than liquids. When the particles with the most energy have evaporated what is left behind is a liquid with a lower average energy per particle - this means the liquid will have a lower temperature. This is why evaporation can have a cooling effect (think of alcohol hand sanitizing gel).
The rate of evaporation will be fast if ……
Temperature is higher / density is lower / larger surface area / higher airflow
The rate of condensation is quicker if ……..
Temperature of gas is lower / lower the surface the gas touches / higher density / airflow is less
What is the formula for payback time ?
Initial cost
———————
Annual saving
How do vacuum flasks work ?
A vacuum (absence of particles) is contained inside the two walls, this stops convection and conduction. The walls are silvered to reflect IR radiation back into the liquid. The lid is made of foam to stop heat conduction
What is cost effectiveness ?
Rather than just looking at how well the energy saving techniques work, it evaluates how much it costs and payback time
What does cavity wall insulation prevent ?
Reduces radiation and convection
Loft insulation prevents what ? How does it do this ?
Conduction because the fibre glass wool has air gaps which would need convection
How do curtains prevent heat loss ?
They form air gaps between the room and glass so it reduces conduction
True or False - the lower the
U-Value the better the insulator
True
What Is a U-Value ?
It measures how fast heat transfers through materials
What is specific heat capacity ?
It tells you how much energy an object can store
What is the equation for specific heat capacity ?
Energy transfer =
Mass x specific heat capacity x temperature Change
What are the nine forms of energy ?
Electrical, light, nuclear, sound, kinetic, thermal, gravitational potential, elastic potential and chemical
How much energy is needed to heat 4kg of water from 0 degree c to10 degree c ?
4 x 4200 x 10 = 168,000
What can never happen to energy ?
It can never be created or destroyed
What are the elastic and gravitational energy examples of ?
Stored energy
What does a battery powered torch convert chemical energy into ?
Light and heat
What is the equation for efficiency ?
Useful energy/power out
—————————-
Total energy/power in
What happens to waste energy ?
Dissipates into surroundings or is transferred to another form (eg. Heat)
Why is it called wasted energy ?
Nothing useful can be done with it usually. It’s not what we want
In a Sankey Diagram what represents the amount of energy?
The width of the arrow.
In a Sankey Diagram what represents the wasted and useful energy transfers?
Useful energy transfers continue straight.
Wasted energy transfers go down.
What is power?
The amount of energy transferred in a second to an appliance.
What is the equation for power?
Power = energy / time
What is Power measured in?
Watts (W)
What is energy measured in?
Joules (J)
When paying for electricity we normally measure energy in…
KiloWatt Hours (kWh)
To work out the amount of energy transferred in kWh we use…
Energy = power x time
Where power has to be written in kW and time is in hours
How do you convert watts into kilowatts?
Divide it by 1000 (think metres and kilometres)
How do you convert seconds into hours?
Divide it by 3600 (its 60 x 60)
How do you convert minutes into hours?
Divide it by 60
- A microwave of 750 watts is on for 6 minutes how much energy does it transfer in that time?
- If the electricity costs 20p per kWh how much does it cost to be on?
- power x time = 0.75 * 0.1 = 0.075kWh
(put power into kW and then time into hours) - 0.075 x 20 = 1.5p
(number of kWh x cost per kWh)
In any energy transfer energy will be wasted as what type of energy? Why?
Heat due to friction.
Where does wasted energy go?
It spreads out into the surroundings in the form of heat energy.
