Topic B Definitions

Definitions for Topics B1-B5 (only 4,5 ratings), parts in bold most important for each definition, sometimes missing extra notes

Internal energy

The total random kinetic energy and intermolecular potential energy of the molecules of a substance

Temperature difference

Determines the direction of the resultant thermal energy transfer between bodies

Thermal radiation

Emission of (infrared) electromagentic/infrared energy/waves/radiation

Absolute zero

The temperature at which all random motion of molecules stops (OR temperature at which a gas would exert no pressure)

Mole

Quantity of a substance containing as many particles as atoms in 12g of carbon-12

Pressure

The normal force on an area per unit area

Specific heat capacity, c

The energy required per unit mass to raise the temperature of a substance by 1K

Specific latent heat, l

Energy per unit mass absorbed or released during a phase change

Specific latent heat of fusion

The energy needed to change a unit mass from the solid to the liquid phase at constant temperature

Specific latent heat of vaporisation

The energy needed to change a unit mass from the liquid to the vapour phase at constant temperature

Temperature

1. The property that determines the direction of thermal energy transfer between two objects

2. (as Kelvin temperature) measure of the average random kinetic energy of the particles of a substance

Conduction

The transfer of heat through electron and intermolecular collisions

Convection

The transfer of heat in fluids through differences in fluid density

Convection current

Motion of a fluid as a result of differences in fluid density

Stefan - Boltzmann law

The power radiated by a black body is proportional to the body’s surface area and the fourth power of its Kelvin temperature; P = σAT^4

Boiling

A phase change of a liquid into a gas that occurs at a fixed temperature

Evaporation

When faster moving molecules have enough energy to escape from the surface of a liquid that is at a temperature less than its boiling point (leaving slower moving molecules behind which results in a cooling of the liquid)

Kelvin temperature / absolute temperature

A measure of the average random kinetic energy of the particles of a substance

Luminosity

(same as power)) total radiant energy emitted by stars per unit time

Apparent brightness

(same as intensity) intensity of a star’s radiation received on earth

Greenhouse gases

CO₂, N₂O, H₂O, CH₄ (Carbon dioxide, nitrous oxide, water vapour, methane)

Greenhouse effect explanation

Short wavelength (visible light and near IR) radiation received from Sun

Atmosphere (mainly) transparent* to this radiation

Absorbed by Earth’s surface (ignoring the bit reflected due to albedo)

Earth warms up

Earth radiates the same power, but in infrared (i.e. longer wavelengths)

Absorbed* by greenhouse gases in atmosphere

Reemitted in all directions, but some of those directions are down

*These parts may need expansionw ith the particular absorption mechanism explanation

1. Molecular energy levels: the available molecular energy level transitions in greenhouse gases are equal to infrared photon energy (so absorbed) but not visible photon energy (so not absorbed)

2. Resonance: the natural frequency of greenhouse gases is the same as infrared frequency, but not visible light frequency. SO resonance and energy absorption with IR not visible

Solar constant

The average intensity of the Sun’s radiation at the position of the Earth’s mean orbit.

Albedo (α)

Fraction of the total incoming solar radiation received by a planet that is reflection back out into space (OR: ratio of total solar radiation power scattered by a planet to total solar radiation received by a planet)

Greenhouse gas

A gas in the atmosphere that absorbs infrared radiation

Enhanced greenhouse effect

Augmentation of the greenhouse effect due to human activities (e.g. by greenhouse gas emissions) (also called anthropogenic greenhouse effect)

Emmisivity

Ratio of power emitted by an object to the power emitted by an ideal black surface at the same temperature

Ideal gas

A gas in which there are no intermolecular forces (except during collisions) OR a gas that obeys pV = nRT at all pressures, volumes and temperatures OR molecules have zero PE / only KE

Assumptions of kinetic model of ideal gases

1. Particles are small compared to their separation

2. There are no forces between particles except during collisions

3. Collisions with the walls and other particles are elastic

4. Duration of collisions is small compared to time between collisions

5. Particles have the same mass

6. Motion & distribution of particles is random

7. Gravity is ignored

8. Particles obey Newton’s laws of motion

When does real gas approximate ideal gas?

Low pressure, moderate temperature, low density

How does pressure arise in a gas

Due to change in momentum of particles due to collisions with surfaces

by N3, force also on surface

rate of change in momentum = force

p = F/A

Pressure - temperature explanation (constant volume)

temp increases, so ke increases, so collisions more frequent AND have more force. Both of these increase F; A same, so p= F/A increases

Pressure - volume explanation (constant temp)

Volume decreases, so distance between collisions decreases, so more frequent collisions, increases total F, AND area of impact decreases. By p=F/A both of these changes increase pressure

Avogadro constant

The number of particles in one mole

Adiabatic process

«a process in which there is» no thermal energy transferred between the system and the surroundings

First Law of Thermodynamics (Q = deltaU + W)

The thermal energy transferred to a system from its surroundings is equal to the work done by the system plus the change in internal energy of the system (an application of the principle of conservation of energy)

Isobaric

A process that occurs at constant pressure (deltaP = 0)

Isovolumetric

A process that occurs at constant Volume (deltaV = 0)

Isothermal

A process that occurs at constant temperature (deltaT = 0)

Carnot cycle

Sets a limit for the efficiency of a heat engine at the temperatures of its heat reservoirs

Entropy

A system property that expresses the degree of disorder in the system

Local changes in entropy

entropy of a non-isolated system can decrease locally, but this is compensated by an equal or great increase of the entropy of the surroundings

Second Law of Thermodynamics

The overall entropy of the universe is increasing (OR - All natural processes increase the entropy of the universe)

(Note: The second law implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature)

Electric resistance

The ratio of the voltage across a device to the current through it (R=V/I)

Ideal Ammeter

Has zero resistance

Ideal Voltmeter

Has infinite resistance (OR draws no current from the circuit)

Ohm’s law

(if obeyed, as for most metallic conductors) at constant temperature the current through the conductor is proportional to the voltage across it; i.e. resistance is constant (for ohmic conductors)

(Electric)) potential difference

The work done per unit charge in moving a small point posiitive charge between two points

Electromotive force (emf)

The work done per unit charge in moving charge across the terminals of a battery (OR “… all the way around the circuit” OR “Energy supplied per unit charge when making a complete circuit”)

Ohmic device

One whose resistance remains constant over a wide range of potential differences

Chemical cell

a source of emf

an energy source in circuits

(remember a ‘battery’ is strictly a word for several cells)