Unit B (1-5)

Question 1

What is the difference between an Insulator and a Conductor?

Answer 1

conductors: have lots of charge, free electrons, to allow current flow

insulators: do not have charge carriers so current cannot flow

Question 2

What is the difference between conventional current and electron flow?

Answer 2

conventional current: positive charge flows from the positive terminal of a voltage source, through the circuit, and into the negative terminal

electron flow: Electrons flow from the negative terminal of a voltage source, through the circuit, and into the positive terminal

Question 3

What is the difference between potential difference and EMF?

Answer 3

PD: the work done per unit charge in converting electrical energy into (fill in blank) energy in coulumbs

EMF: the work done per unit charge in converting chemical energy into electrical energy in volts

Question 4

Define the Electronvolt

Answer 4

If 1 electron travels through a potential difference of 1V then it will gain 1.6x10^-19 J of energy

Question 5

Summerize what a LDR, thermistor, stress sensor, potential divider circuit - what they are and what they do and for the first three, under what conditions does R increase or R decrease?

Answer 5

LDR: when light intensity increases, resistance decreases, used on iphone brightness

Thermistor: when the temperature increases, the resistance decreases, used to sensor an oven

Stress sensor: as the length increases, the area decreases and resistance increases, piece of metal placed under a bridge

Potential Divider: changes the amount of voltage that passes through different parts of the circuit, control of temperature in a fridge or as audio volume controls

Question 6

Ammeter and Voltmeter - what are they used for? What resistance properties do they have? Where do we put them in a circuit?

Answer 6

Ammeter: measures the current in a circuit, connected in series

Voltmeter: measures the voltage or potential difference between two places in a circuit, connected in parrallel

Question 7

What is Ohm’s Law?

Answer 7

If the potential difference across a resistor is proportional to the current flowing through it then the device is Ohmic, provided the physical conditions stay the same
If a conductor is Ohmic, current is proportional to V (you can’t get R from the instantaneous gradient of a V-I graph)

Question 8

What is the definition of internal resistance? How do we rearrange the formula E = I(R + r) to obtain the EMF and the internal resistance?

Answer 8

A cell consists of conductive components and a store of chemical energy. All these materials have resistance.
V = e-Ir

Question 9

Define heat and thermal energy

Answer 9

Heat - energy that is transferred from one body and into another as a result of a difference of temperature
Heat always flows from hot to cold

Thermal energy - the transfer of energy from a hotter object to a colder object

Question 10

Define thermal equilibrium

Answer 10

If two bodies are in thermal equilibrium – no heat flows from one body to another

Question 11

Define absolute zero

Answer 11

the lowest temperature that is theoretically possible, at which the motion of particles that constitutes heat would be minimal. It is zero on the Kelvin scale, equivalent to –273.15°C or –459.67°F.

Question 12

Define absolute temperature

Answer 12

measure of the average kinetic energy of the molecules of a substance. The average kinetic energy of the molecules is directly proportional to the absolute temperature in kelvin

Question 13

How do we convert from Kelvin to Celsius?

Answer 13

K = C + 273

Question 14

What is Internal energy?

Answer 14

the total random kinetic energy of the molecules of a substance plus any potential energy associated with forces between the molecules

Question 15

What is specific heat capacity?

Answer 15

The specific heat capacity is the amount of energy needed to raise 1kg of a substance by 1K
The units are J kg-1 K-1

Question 16

Define latent heat

Answer 16

Latent heat is the energy required to change the state of 1kg of as substance WITHOUT a change in temperature
The energy from the heat source goes into separating the molecules instead of heating the substance.

Question 17

What is the difference between latent heat of fusion and vaporization?

Answer 17

latent heat of fusion is about the transition from solid to liquid, like ice melting into water, while latent heat of vaporization is about the transition from liquid to gas, such as water boiling into steam

Question 18

What is a phase change?

Answer 18

A change of state of the matter (melting, freezing, boiling, condensation, sublimation)

Question 19

What do flat parts of a time – temperature graph mean?

Answer 19

The flat parts are when the phase changes are occurring and the slope lines are when the matter is in one state

Question 20

What are the three mechanisms that transfer heat? When is each one used?

Answer 20

Thermal Conduction - heat energy is transmitted through collisions between neighboring atoms or molecules

Convection - the movement of particles through a substance, transporting their heat energy from hotter areas to cooler areas. Mass movement of particles due to differences in density

Radiation - transfer of energy that travels in the form of waves or particles, EM radiation does not need a medium, can travel to us from the sun, Stefan – Boltzman

Question 21

What does it mean to be a blackbody?

Answer 21

an idealized object that absorbs all electromagnetic radiation that falls on it, regardless of the wavelength or angle of incidence. It is a perfect absorber and emitter of radiation

Question 22

What does thermal conduction depend on?

Answer 22

Atomic Vibration has larger amplitude at larger T so atoms collide until uniform T reached

Movement of free electrons diffuses the energy

Question 23

What are the two blackbody laws?

Answer 23

Wien’s Law - For blackbodies, the peak wavelength of radiation determines the T and vice versa, the wavelength at which a blackbody emits maximum radiation is inversely proportional to its absolute temperature (λ_max = b / T)

Stefan-Boltzmann Law - the total energy radiated per unit surface area of a blackbody per unit time is directly proportional to the fourth power of the blackbody’s absolute temperature
(E = σ T^4)

Question 24

Luminosity and Apparent brightness of stars?

Answer 24

Luminosity is defined as the amount of energy radiated by the star per second

apparent brightness – the amount of energy per second received per m2
Depends on the distance to the star

Question 25

What is the greenhouse effect?

Answer 25

when gases in the Earth’s atmosphere absorb and trap heat from the sun that is radiated back from the Earth’s surface

Question 26

What are the greenhouse gasses and why are they called this?

Answer 26

gases in the Earth’s atmosphere that absorb and trap heat radiated from the Earth’s surface, causing the greenhouse effect (carbon dioxide, methane, water vapor, nitrous oxide)
When the gases react with infrared radiation, Some gets reradiated back to earth and does not escape, heating the earth

Question 27

What is resonance?

Answer 27

when an object vibrates at the same natural frequency as another object, leading to an amplification of vibrations (the greenhouse gases resonate with the particles in the atmosphere)

Question 28

Define albedo and emissivity?

Answer 28

Albedo - amount of light reflected/amount of incident light
Emissivity - amount of light emitted/amount of light a black body at that T would emit (perfect black body = 1)

Question 29

What is the solar constant?

Answer 29

Energy from the sun that changes due to:
Orbital variations Jan – June
Solar 11 year cycle
Other longer period cycles

Question 30

Why is 1360W/m^2 incident but the total that converts to a T for earth only around 250W/m^2?

Answer 30

The energy has to be spread across the surface of the earth (4πd^2), from the perspective of the sun the earth is only a disk not a sphere and then some of this radiation is reflected back into the earths atmosphere

Question 31

What are the three gas laws?

Answer 31

Boyle’s Law: for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume
PV = constant
P proportional to 1/V

Charles’ Law: for a fixed mass of gas at constant pressure, the volume is directly proportional to the absolute temperature

Third Law (Guy-Lassac, Avagadro, Amontons): for a fixed mass of gas at constant volume, the pressure is directly proportional to the absolute temperature

Question 32

What is a mole?

Answer 32

One mole of any substance contains the same number of atoms as in 12g of carbon-12.

Question 33

What is Avogadro’s number?

Answer 33

One mole: The numerical value is NA – Avogadro’s number = 6.02x10^23

Question 34

How do we measure the gas laws?

Answer 34

PV = nRT
PV = NkT
R is the gas constant = 8.31 J K-1 mol-1
k is the Boltzman constant = 1.38x10-23 J K-1
where P is pressure in Pa or N/m^2
V is volume in m^3
T is temperature in K
n - number of moles
N number of atoms

Question 35

What is diffusion?

Answer 35

when molecules move from an area of high concentration to an area of low concentration

Question 36

What are the assumptions of the ideal gas law? (kinetic theory)

Answer 36

A gas consists of a large number of molecules N which is large enough that averages can be made

The volume of the molecules themselves is negligible compared to the volume of the gas

Collisions between molecules and the walls of the container are elastic

There are no intermolecular forces between the molecules, except during collisions

Question 37

What is the total energy of a gas?

Answer 37

Remember nR = NK so k = nR/N
And n = N/NA
So KE = 3/2 (R/NA) T
This gives the average KE of a particle of a gas of temperature T

Question 38

When is an idea gas ideal? I.e. what are the conditions on temperature and pressure?

Answer 38

If T too low or P too high then a gas is not ideal
Under those conditions, gases liquefy and then cannot be compressed
Ideal gases cannot, therefore, be liquified
So a gas is ideal when T is high, P is low and density is low