P1 Heat And Energy Transfer Flashcards
Explain the emission of inferred waves.
All objects continually emit and absorb inferred radiation.
If an object is hotter than the surroundings, it will cool by emitting more radiation then absorbed. If it is cooler than the surroundings it will absorb more radiation and heat up.
The hotter an object, the more radiation emitted.
What are the three ways in which energy can be transfered?
Conduction, radiation, convection.
What can affect the radiation of heat?
Dark, matt surfaces absorb more and emit more energy than light shiny surfaces. This is because shiny surfaces reflect a lot of energy back. Vacuum flasks have a silver lining.
Solar hot water panels are water pipes under a black sheet. The suns heat is absorbed by the sheet and heats the water.
Explain the molecular structure of solids.
Strong forces of attraction hold the particles together in a regula arrangement. The particles have little energy so only vibrate about their fixed position.
Explain the molecular structure of liquids.
The forces of attraction are weaker. The particles are close together but can move past each other, forming irregular arrangements. They have more energy than in a solid.
Explain the molecular structure of gases.
Almost no forces of attraction between the particles. These particles have the most energy. They are free to move and travel in random directions at high speed.
Explain conduction in solids.
Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles.
This happens faster in denser solids as the particles are closer together and will collide more often, thus meaning a larger energy transfer.
Why is metal a good thermal conductor?
Electrons are free to move in them.
Electrons are heated and vibrate more, hitting other electrons.
This is faster than in ordinary solids as the electrons are free to move around.
Explain what convection is.
Convection occurs when the more energetic particles move from a hotter region to a cooler region and take their heat energy with them.
Explain convection currents in an immersion heater.
Conduction transfers heat from the coils to the water.
Particles near the coils have more energy and so move faster.
This causes the density of that area to decreases as there is more space between particles.
This reduction in density causes that water to rise through the denser water.
As the warm water rises it displaces the cooler water out of the way towards the heater coils.
Also the risen water warms the water at the top and will raise the temperature slightly when it matches the temperature.
This is heated and rises and the convection currents continue.
Note that convection currents only occur in water above the heater coils and that these currents work best in roundish, squarish containers, not thin tall ones.
Explain condensation.
When a gas cools, the particles lose kinetic energy and slow down. The attractive forces pull them closer.
When it gets cool enough, the particles become so close together the gas becomes a liquid.
Explain evaporation.
This is when particles escape from a liquid and it can happen at temperatures lower than the boiling point.
Particles near the surface evaporate when they are traveling in the right direction and when they have enough kinetic energy to leave.
When the fast particles evaporate off, the average speed and kinetic energy of the remaining particles decreases meaning the average particle energy falls. The liquid cools.
What can increase the rate of evaporation?
The temperature is higher. This will increase average particle energy so more particles can escape.
The density is lower. The inter-molecular forces are weaker meaning less energy is needed to escape.
Surface area is larger. More particles will be near enough to the surface to escape.
The airflow over the liquid is great. This means the air above the liquid will have less of the evaporated substance in it and the lower the concentration the higher the rate of evaporation.
What can increase the rate of condensation?
The temperature of the gas is lower. This means the particles have less energy so they join together.
The temperature of the surface the gas touches is lower.
The density is higher. This means stronger intermolecular forces meaning less particles will have the energy needed to escape.
Airflow is less. The concentration of the substance in the air will be higher.
What can affect the rate of heat transfer?
An increase in surface area. More inferred waves can be emitted or absorbed.
Heats sinks transfer heat away from the thing they touch. They have fin which increase the surface area.
A higher proportion of the object in contact with the surroundings.
A larger difference between the surroundings and the object.
Is the object made of an insulator or conductor.
Is it shiney or matt?
How are vacuum flasks designed to minimise heat transfer?
It has two walls with a vacuum in the middle which stops, conduction and convection.
The ways are silvered. These reflect back heat and minimise radiation.
The bottle is supported with insulating foam which minimises heat conduction from the inner to outer bottle.
The stoppers made out of cork or foam which is an insulator.
Explain how animals and humans control heat transfer.
In the cold, your hairs stand up and trap a layer of insulating hair around you. Fur also does this and it stops convection.
Vasculodilation occurs when hot meaning heat is lost through radiation.
Animals in warmer climates have larger ears to lose heat.
Arctic foxes have small ears and a small surface area which conserves heat.
Desert foxes have big ears and a large surface area meaning heat is lost.
What does cavity wall insulation do?
Foam is squirted in the gaps between wall reducing convection and radiation across the gap.
What does loft insulation do?
A thick layer of fibreglass will is laid out, reducing conduction and radiation of heat into the attic.
What does draft proofing do?
Foam and plastic is put around doors and floors which stops draughts of cold air, thus reducing convection.
