P2 Heat & Radiation

Question 1

Infrared radiation

Answer 1

Can transfer thermal energy without a medium

An electromagnetic wave

Invisible

All bodies emit radiation

Longer wavelength than visible light

Shorter wavelength than radio waves

Question 2

How surfaces interact with infrared radiation

Answer 2

Can reflect, absorb, or emit infrared radiation

Good absorbers are usually bad reflectors

Question 3

How shiny surfaces interact with infrared radiation

Answer 3

Compared to dull surfaces of the same colour, shiny surfaces are:

Poorer absorbers
Poorer emitters
Better reflectors

Question 4

How perfect black bodies interact with infrared radiation

Answer 4

Absorbs all of the radiation that incidents on it

Doesn’t reflect or transmit radiation

As a good absorber is a good emitter, black body objects are the best possible emitters

Question 5

How white surfaces interact with infrared radiation

Answer 5

Good reflectors

Poor emitters

Poor absorbers

Question 6

How black surfaces interact with infrared radiation

Answer 6

Good emitters

Good absorbers

Poor reflectors

Question 7

Factors affecting the amount of radiation emitted

Answer 7

Surface area
Surface temperature

The higher the surface area or temperature, the higher the rate of emission

Question 8

Factors affecting temperature

Answer 8

Intensity of radiation emitted

Wavelength of radiation emitted

Question 9

When is temperature constant

Answer 9

radiation absorbed = radiation emitted

Question 10

When does the temperature change

Answer 10

radiation absorbed ≠ radiation emitted

Question 11

Diffusion of heat

Answer 11

Heat will always transmit from a hotter to a cooler area

Rate of emission is higher if an object is hotter than its environment

Question 12

Factors affecting absorption and emission

Answer 12

Internal temperature

Surface area

External temperature

Question 13

The greenhouse effect process

Answer 13

1. The sun emits short wavelength infrared radiation that enters the atmosphere and travels towards the Earth’s surface
2. The Earth absorbs some of this radiation, but long wavelength radiation is reflected back into the atmosphere
3. Greenhouse gases (e.g. carbon dioxide, methane, water vapour) can’t absorb the frequency of radiation emitted by the Sun. But they can absorb the longer wavelength reflected radiation
4. The gases then re-radiate this energy in all directions, including back towards Earth
5. This increases the temperature at the Earth’s surface

Question 14

Emission to balance energy on Earth

Answer 14

Absorbed sunlight is balanced by heat radiated from the Earth’s surface and atmosphere

Most heat escapes from areas just north and south of the equator, where the surface is warm, but there are few clouds

Along the equator, persistent clouds prevent heat from escaping

The poles radiate little heat

Question 15

Absorption to balance energy on Earth

Answer 15

340 Watts per square metre of solar energy falls on the Earth

29% is reflected back into space - primarily by clouds, but also by other bright surfaces and the atmosphere itself

~23% of incoming energy is absorbed in the atmosphere by atmospheric gases, dust, and other particles

The remaining 48% is absorbed at the surface

Question 16

Thermal conductors

Answer 16

Materials which allows the conduction of thermal energy

Question 17

Thermal insulators

Answer 17

Materials which prevent or do not readily conduct thermal energy

Question 18

The greenhouse effect

Answer 18

The process of gases trapping heat to keep the Earth warm enough for life

The greenhouse effect can cause global warming if it becomes too strong

Question 19

Examples of thermal insulators

Answer 19

Polystyrene foam

Water

Mineral wool

Plastic

Question 20

Examples of thermal conductors

Answer 20

Copper

Aluminium

Brass

Silver

Question 21

Formula for specific heat capacity

Answer 21

specific heat capacity(J/Kg°C) = (change in energy(J)) / (mass(Kg) * change in temperature(°C))

Question 22

Conductivity of metals and non-metals

Answer 22

Metal are better conductors than non-metals

Question 23

Conductivity of copper and steel

Answer 23

Copper is a better conductor than steel

Question 24

Conductivity of glass and wood

Answer 24

Glass is a better conductor than wood

Question 25

Specific heat capacity

Answer 25

The amount of energy needed to increase a kilogram of a substance by 1°C

Question 26

Relationship between mass and change in temperature

Answer 26

Rate of energy transfer decreases as mass increases

Question 27

Thermal conductivity

Answer 27

The rate at which a material conducts thermal energy

Question 28

Factors affecting rate of energy transfer

Answer 28

Temperature difference across material

Thickness of material

Thermal conductivity of material

Question 29

Relationship between thickness and thermal conductivity

Answer 29

Thermal conductivity decreases as thickness increases

Question 30

Relationship between temperature and radiation emitted

Answer 30

Amount of radiation emitted increases as temperature increases

Question 31

Radiation emitted from an object with a constant temperature

Answer 31

An object at a constant temperature emits radiation at a continuous range of wavelength

Question 32

Relationship between heat and wavelength

Answer 32

Wavelength decreases as heat increases

Question 33

Factors affecting change in temperature

Answer 33

Amount of energy supplied

Mass

Type of substance

Question 34

Methods to reduce rate of emission from homes

Answer 34

Loft insulation

Cavity wall insulation

Double-glazed windows

Thick bricks (with low thermal conductivity)

Aluminium foil behind radiators

Question 35

Methods to heat homes

Answer 35

Electric/gas heater

Oil/gas central heating

Solid fuel stove/fireplace

Question 36

Cavity insulation

Answer 36

Insulation, with a lower thermal conductivity than air, between two layers of brick of an external wall