P7.2 Flashcards
what is the equation to calculate efficiency?
efficiency = useful output energy transfer (J) / total energy (J) x 100
define power (and what is the main equation)
the rate of energy transfer
p = e/t
energy / time
what is the relation between 1 watt and 1 joule?
1 watt is 1 joule of energy transferred in 1 second
what unit do electrical companies measure energy in?
kWh
(kilowatt-hour)
as e = p x t
(energy = power x time)
is kilowatt-hour a measure of energy or power?
measure of energy
as e = p x t
1kWh is the amount of ……… transferred by a ….. applicance in …… hour
energy transferred, 1kW, 1 hour
3 advantages of electrical energy?
1 - easily transferred into other forms
2- easily transferred over long distances
3 - is portable
downside about an electrical current flowing?
some energy will always be transferred as heat to the surroundings
when will thermal energy transfer always occur?
when there exists a temperature difference (or energy difference) between two points/media
define a system
a group of bodies interacting
when would the energy of a system stay constant?
when the system is closed (no external forces acting on it)
two ways to reduce unwanted energy transfer
lubrication, thermal insulation
how does insulation reduce unwanted energy transfer?
traps air and stops it moving
what are the 4 factors which affect the rate at which heat energy is transferred through a wall?
- temperature difference
- area of wall
- thickness of wall
- material of wall (thermal conductivity)
how does the thermal conductivity of a wall affect the rate of heat energy transfer?
lower the thermal conductivity = lower rate of heat energy transfer
define thermal conductivity
the heat energy transferred, per second, through a 1m thick wall (of area 1m^2) when a temperature difference of 1C exists across it
what is the unit for thermal conductivity?
W/mK
what most common form of energy waste happens in mechanical devices?
through form of heat
(force of friction) which acts between parts/surfaces
what do Sankey diagrams do?
show energy transfer through a device (by separating useful and wasted energy)
for the following methods of wasting energy suggest how to reduce the problem
a) friction in moving parts = heating
b) resistance of wire = wire gets hot
c) air resistance = energy to surroundings
d) sound created by machinery
a) lubricate
b) wires with little resistance
c) streamline shape of moving objects
d) cut out noise (tighten loose parts to reduce vibration)
how does lubrication work?
a layer of fluid between two solids
- surfaces no longer in direct contact, friction reduced
