Energy Flashcards
Specific energy
Total energy released from 1kg of fuel or power source
Specific energy
Total energy released from 1kg of fuel or power source
Bond enthalpy
The amount of energy required to break a chemical bond or the amount of energy released when forming a chemical bond. In kj/mole
mole
A mole (abbreviated by mol) is the amount of a substance that contains a number of atoms or molecules equal to Avogadro’s constant which is NA= 6.022 × 10^23. The molar mass of a substance is the mass of 1 mole
energy released is with a minus sign
as released from system so system has less
Calculating energy released in reaction, eg methane burning in air
using bond enthalpies of the relevant bonds. Calculate energy needed to break bonds (+) and energy released (-). If minus sign overall, then energy released
Calculate specific energy eg of methane
Convert energy released per mole to a value per kg. Mass/molar mass gives mol/kg. Multiply by energy produced per mole
Nuclear fission
neutron collides with heavy nucleus and causes nucleus to split into 2 smaller ones and an additional 2 or 3 neutrons. 2 nuclei always produced rather than 3 or more. Total no of protons and neutrons doesn’t change
nuclear fission reaction notation
eg n+^235U (right arrow) X+Y+(2 or 3)n. x and y are the 2 produced nuclei. No of protons and neutrons must be same on both sides of equation - dont forget to include the original single neutron that collides with the nucleus
Einstein’s equation. Used to find energy release, unlike in chemical reactions where bond enthalpies used to find energy release
e=mc^2. c is speed of light. Equation says a mass has energy associated with it. So E is associated with m, so a nucleus of mass m has energy E=mc^2. Quite similar in looks to kinetic energy eq (E=1/2mv^2). e=mc^2 is total energy of whatever it describes - ie energy due to mass of protons and neutrons as well as PE and KE of protons and neutrons
Energies in nuclear fission
no of protons and neutrons constant but total energy changes due to PE and KE of constituents.
Calculate energy release in nuclear fission
calculate mass on lhs and rhs of equation to give total mass change, then multiply by c^2
Heat
Transfer of energy that results from temp diff
Thermal energy transfer rate from hotter to colder area, heat conduction equation
proportional to temp diff (triangle)T=T(in)-T(out). In is hotter inside, out cooler outside, so P=E/t. Proportional to surface area of boundary (A). Thickness (l) and rate of heat transfer smaller when material thicker, so inversely proportional to thickness of boundary material.
So P= -kA((triangle)T/l). k is heat conduction equation. Minus because system is losing energy
Thermal conductivity(k)
Larger with a poor insulator, so larger energy transfer and higher value of k. Air is best insulator. W m^-1 K^-1 (watts per meter Kelvin). So units for insulation thickness need to be in metres for calculation
Heat conduction equation equation rearranged to calculate thickness a wall needs to be for a particular heat loss.
l= kA((triangle)T/P)
When calculating heat loss for cavity wall, 2 different materials plus air in middle so 3 different heat losses so equation reworked to give total heat loss
Triangle T= -P/A (l(a)/k(a)+l(b)/k(b)) etc. So heat losses added up = total heat loss
Convection
Transfer of heat by air moving around. Air warms by heat coming through wall and into air gap, so air warms and its volume increases, density decreases. Lower density warm air rises and is replaced by cooler air. Good for heating rooms but not in cavity wall as increases area for heat loss. Convection restricted by materials inside cavity walls, rather than them just being air
Radiation
Transfer of energy by electromagnetic waves. All objects above 0 (zero)K radiate. Warmer objects radiate more and wavelength of radiation gets shorter with increasing temperature.
Power emitted as radiation equation
Surface with area A and temp T (in kelvin) emits radiation with power P. P=A x (emissivity) x (stefan Boltzmann constant) x T^4
Rolling resistance, friction, air resistance
forces acting on a car
Rolling resistance
Squashing of tyre from weight of vehicle. Force applied is weight of car, so heavier car more force (speed doesn’t matter). Energy dissipated (ie no longer available) more with more mass (heavier car). Power dissipated does depend on speed as well
Rolling resistance
Squashing of tyre from weight of vehicle. Force applied is weight of car, so heavier car more force (speed doesn’t matter). Energy dissipated (ie no longer available) more with more mass (heavier car). Power dissipated does depend on speed as well. Energy dissipated depends on distance
Power and energy
energy is joules , power j/s (w). Rate of energy transfer = power
Friction
approx 15% of power lost to transmission in a car
Slowing down
Kinetic energy (1/2mv^2) is dissipated as heat when car comes to a standstill
