Energy

Question 1

Specific energy

Answer 1

Total energy released from 1kg of fuel or power source

Question 2

Specific energy

Answer 2

Total energy released from 1kg of fuel or power source

Question 3

Bond enthalpy

Answer 3

The amount of energy required to break a chemical bond or the amount of energy released when forming a chemical bond. In kj/mole

Question 4

mole

Answer 4

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

Question 5

energy released is with a minus sign

Answer 5

as released from system so system has less

Question 6

Calculating energy released in reaction, eg methane burning in air

Answer 6

using bond enthalpies of the relevant bonds. Calculate energy needed to break bonds (+) and energy released (-). If minus sign overall, then energy released

Question 7

Calculate specific energy eg of methane

Answer 7

Convert energy released per mole to a value per kg. Mass/molar mass gives mol/kg. Multiply by energy produced per mole

Question 8

Nuclear fission

Answer 8

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

Question 9

nuclear fission reaction notation

Answer 9

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

Question 10

Einstein’s equation. Used to find energy release, unlike in chemical reactions where bond enthalpies used to find energy release

Answer 10

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

Question 11

Energies in nuclear fission

Answer 11

no of protons and neutrons constant but total energy changes due to PE and KE of constituents.

Question 12

Calculate energy release in nuclear fission

Answer 12

calculate mass on lhs and rhs of equation to give total mass change, then multiply by c^2

Question 13

Heat

Answer 13

Transfer of energy that results from temp diff

Question 14

Thermal energy transfer rate from hotter to colder area, heat conduction equation

Answer 14

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

Question 15

Thermal conductivity(k)

Answer 15

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

Question 16

Heat conduction equation equation rearranged to calculate thickness a wall needs to be for a particular heat loss.

Answer 16

l= kA((triangle)T/P)

Question 17

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

Answer 17

Triangle T= -P/A (l(a)/k(a)+l(b)/k(b)) etc. So heat losses added up = total heat loss

Question 18

Convection

Answer 18

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

Question 19

Radiation

Answer 19

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.

Question 20

Power emitted as radiation equation

Answer 20

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

Question 21

Rolling resistance, friction, air resistance

Answer 21

forces acting on a car

Question 22

Rolling resistance

Answer 22

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

Question 23

Rolling resistance

Answer 23

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

Question 24

Power and energy

Answer 24

energy is joules , power j/s (w). Rate of energy transfer = power

Question 25

Friction

Answer 25

approx 15% of power lost to transmission in a car

Question 26

Slowing down

Answer 26

Kinetic energy (1/2mv^2) is dissipated as heat when car comes to a standstill