P2 - Energy Transfer By Heating✔️ Flashcards
Define thermal conductivity?
Thermal conductivity is a measure of how well a material conducts energy when it is heated.
Requirments for a material to be a good insulator?
Needs to have low thermal conductivity,so energy transfer through them is as low as possible
3 factors which the energy transfer per second, through a layer of insulating material depend on?
-the temperature difference across the material
-the thickness of the material
-the thermal conductivity of the material
2 ways to reduce energy transfer?
-thermal conductivity of the insulating material should be as low as possible
-thickness of the insulating layer should be as thick as practically possible
What is a ‘black body’ in regard to infared radiation?
Is an object that absorbs all the radiation that hits it and dosent reflect or transmit
any radiation - good absober
What happens when an object absorbs radiation faster then it emits radiation?
The tempertaure will increase
Why are light-coloured shiny blankets used to keep accident survivors warm?
A light,shiny outer surface emits a lot less raditaion than a dark,matt surface - so less infared radiation is emmited than an ordnary blanket
3 factors which the tempertaure of a heated object depends on?
-the amout of energy supplied to it
-the mass of the substance
-what teh substance is
Define specific heat capacity?
The specific heat capacity of a substance is the energy needed to raise the tempertaure of 1 kg of the substance by 1 degree
Equation for energy transferred?
Energy transferred = mass x specific heat capacity x tempertaure change
Unit for specific heat capacity?
J/kg°C
How to find specific heat capacity of a substance?
Use a joulemeter and a thermometer to measure energy transfered and temperature change
And then use c = ∆E / m ∆θ
4 ways to reduce rate of energy transfer at home?
-loft insulation
-cavity wall insulation
-aluminium foil between a radiator pannel and the wall
-double glazed windows
Explain how loft insulation reduces energy transfer?
Use fibreglass which is a good insulator becuase the air between the fibres help to reduce the rate of energy transfer by conduction
Explain how cavity wall insulation reduces energy transfer?
Cavity of an outer wall is the space between two layers of brick that make up the wall - insulation is pumped into the cavity and is a better insulator then air, which it replaces because it traps the air in small pockets
Explain how aluminium between a radiator pannel and the wall reduces energy transfer?
Reflects radiation away from the wall and so reduces the rate of energy transfer by radiation
Explain how double-galzed windows reduces energy transfer?
Has dry air or a vacuum between the panes and the thicker the glass and the lower its thermal conductivity is, the slower the rate of transfer of energy through it by conduction will be - the vacuum also prevents energy transfer by convection
Why are solar panels usefull?
Solar panels absorb infared radiation from the sun and are used to generate electricity directly or to heat water directly which reduces cost of heating your house
How do external walls with thicker bricks help reduce energy transfer?
If the external walls have thicker bricks and lower thermal conductivity,the rate of transfer of energy from the inside of the building to the outside will be lower
Describe the effect of an objects mass and the time it takes for the tempertaure to increase?
The greater the mass of an object, the more slowly its temperature increases when it is heated
Method - RP measuring the specific heat capacity of different materials
1)set up the apparatus with a power supply hooked up to a heater and ammeter to record the current and then get a block of a chosen material with two holes (one for the heater and the other for the thermometer to go into
2)measure the mass of the block and wrap in an inulating material to reduce energy transfer from the block to the surrounings insert the thermometer and heater
3)measuer the inital temperature and set the power supply to be 10V - turn on the power supply and start a stopwatch
4)as the block heast up take readings of the temperature and current every 10 min and record in a table
5)when you have enough reedings swicth of the power supply and repeat with different materials
Why do we wrap the block in an insulating material - RP measuring the specific heat capacity of different materials?
Wrap in a thick insulating material eg newspaper to reduce energy transfered from the block to the surroundings which would effect the results - also pour a drop of water into the hole that the thermometer goes into because it is a better conuctor of heat then air as it also gets ri of any air traped in the hole
Why does the metal block heat up - RP measuring the specific heat capacity of different materials?
The current in the circuit does work on the heater,transferring energy electrically from the power supply to the heaters thermal energy store - energy is then transferred to the materials thermal energy store by heating, causing the materials tempertaure to increase - this increase is then measure with the thermometer
How to caculate the power supplied to the heater and how much energy has been transffered to the heater - RP measuring the specific heat capacity of different materials?
1)caculate the power supplied to the heater, using power = voltage x current (we have the current from the ammeters reading and we know the voltage was set to 10 V)
2)work out how much energy has been transferred to the heater at the time of each tempertaure reading using the formula, energy = power x time (where, t is time in seconds since the experiment began)
