P1/2 - Energy conservation/dissipation/transfer Flashcards
What is a system? How can the energy of a system be transferred?
-an object or a group of objects
-heating
-work done by forces
-work done when a current flows
State the equation for:
-kinetic
-elastic potential
-gravitational potential
-change in thermal energy
Eₖ = ½ m v²
Eₑ = ½ k e²
Eₚ = m g h
∆Eₜ = m c ∆T
Eₑ assumes the limit of proportionality has not been exceeded, and Eₚ assumes that the force of gravity is constant regardless of height
RP1 - How can you find the specific heat capacity of a copper block?
-record mass
-wrap in insulation + put on heatproof mat
-put heater in larger hole, and thermometer in other
-start heater, record temp. of block regularly
-plot graph of T vs work done in joules (time x power of heater)
-take inverse of gradient, divide by mass
The thermometer hole needs some water to ensure a better reading, as it increases thermal contact
RP1 - If the power of the heater isn’t given, how might you find it out?
-connect heater with a voltmeter in parallel around it, and an ammeter in series
-use P = IV
What is an example of how an increased power would be useful?
-2 motors lifting an equal mass individually
-the one with a higher power will lift it faster
-it has a higher rate of energy transfer to increasing the GPE store of the object
What 3 things can happen to energy within a system?
-transferred usefully
-stored
-dissipated
How can unwanted energy transfers in a system be reduced?
-lubrication (reduces friction + heat dissipation)
-thermal insulation
What affects the rate of cooling of a building?
-thickness of the walls
-thermal conductivity of the walls
The thickness of the material affects thermal conductivity, because heat travels through solids by conduction which is just particles vibrating - thicker materials have more particles, and must vibrate more to get to the other side.
What does it mean to have a higher thermal conductivity?
the higher the thermal conductivity of a material, the higher the rate of energy transfer (by conduction) across the material
RP2 - How can you investigate how the type of material affects their rate of cooling?
Describe the entire practical
-wrap at least 5 beakers with different materials of the same thickness (use rubber bands to secure insulation so no air gets in)
-use another without an insulating material as a control
-cut circles of card (larger than the beaker) as a lid, and cut a small hole to fit a thermometer
-add boiling water to each ensuring they start at the same temperature by checking it with a thermometer
-use stopwatch to record temp. at regular intervals
RP2 - How would you modify the previous experiment to see the how the thickness of a certain material affects rate of cooling?
Don’t explain the full experiment
-wrap each beaker in varying thicknesses of the same material
-use elastic band to secure the materials and stop air entering between it
-keep the control beaker
What is black body radiation?
the radiation that is emitted by any object (above 0K)
The key word is emitted - any light that is reflected off it does not count towards its total black body radiation. Objects that are very hot (eg iron being melted) glow because they have a very high temperature, and their peak emission is in the visible portion of the EM spectrum.
Explanation
What is a perfect black body?
-an object that absorbs all the radiation incident on it and would emit all of it again, making it the best possible emitter
-it cannot reflect/transmit any radiation
Absorption is taking the incident radiation and converting it to heat, while transmission is how much light passes through it and into another medium
How does the radiation emitted by a body change with temperature?
-all bodies emit some amount of radiation across all wavelengths
-the intensity and wavelength distribution depends on the temperature
-peak intensity shifts further into the shorter wavelengths as temp. increases
A body is at constant temperature. Describe the processes happening to it in terms of radiation, and what happens if its temperature increases:
-absorbing radiation at the same rate it is emitting radiation
-if T increases, it absorbs radiation faster than it emits it
