Thermal radiation and energy balance

What you'll learn

Why all objects emit and absorb infrared radiation.
How a body warms up, cools down, or stays at steady temperature.
How Earth’s temperature depends on the balance between incoming and outgoing radiation.
How to describe the core practical on different surfaces absorbing and radiating thermal energy.

The starting point: thermal energy and temperature

A body just means an object — for example a mug, a person, a planet, or a metal can in a practical.

Temperature tells you how hot something is. In particle terms, it is linked to the average kinetic energy of the particles in a substance.

Thermal energy is energy stored because of the motion and arrangement of particles. A large object can store more thermal energy than a small object at the same temperature.

Definition

Thermal radiation

Thermal radiation is energy transferred by electromagnetic waves, mainly infrared radiation, emitted by objects because of their temperature.

Infrared radiation is part of the electromagnetic spectrum. Unlike conduction and convection, radiation can travel through a vacuum, so it can transfer energy from the Sun to Earth through space.

Absorbing and emitting radiation

Objects can:

absorb radiation: take in energy from incoming waves
emit radiation: give out energy as infrared waves
reflect radiation: bounce incoming waves away

All objects emit some thermal radiation, but hotter objects emit more radiation per second than cooler objects.

Key Idea

Absorbers and emitters

A good absorber of infrared radiation is usually also a good emitter of infrared radiation. Dark, matt surfaces are good absorbers and emitters; shiny, light-coloured surfaces are poor absorbers and emitters.

That means:

A matt black surface absorbs infrared strongly and emits infrared strongly.
A shiny silver surface reflects infrared strongly, so it absorbs little and emits little.

Common Mistake

Dark means hot?

Do not say a black object is always hotter. The surface affects how quickly it absorbs and emits radiation; the actual temperature depends on the full energy balance.

Power: energy transferred per second

In this topic, you often compare the average power absorbed and radiated by a body.

Definition

Average power

Average power is the energy transferred per second over a time interval. It is measured in watts, W, where 1 watt means 1 joule per second.

A useful relationship from the energy topic is:

\frac{\Delta E}{t}

where $P$ is power in watts, $ΔE\Delta E$ is energy transferred in joules, and $t$ is time in seconds.

Energy balance for one body

At the same time, a body may be absorbing radiation and emitting radiation.

The important comparison is:

Pnet=Pabsorbed−PradiatedP_{\text{net}} = P_{\text{absorbed}} - P_{\text{radiated}}

If absorbed power is greater than radiated power, the body gains energy and warms up.
If radiated power is greater than absorbed power, the body loses energy and cools down.
If absorbed power equals radiated power, the body’s temperature stays constant.

This balance explanation is Separate Physics content in Edexcel 1PH0, and the detailed power-balance explanation is Higher Tier.

Key Idea

The balance decides the temperature change

A body warms up if it absorbs energy faster than it radiates energy away, and cools down if it radiates energy faster than it absorbs energy.

Example

Working out whether an object warms or cools

A metal block absorbs radiation with an average power of 30 W and radiates energy with an average power of 24 W.

Compare the two powers using the net power idea: $Pnet=Pabsorbed−PradiatedP_{\text{net}} = P_{\text{absorbed}} - P_{\text{radiated}}$ .
Substitute the values: $WP_{\text{net}} = 30\ \text{W} - 24\ \text{W} = 6\ \text{W}$ .
The net power is positive, so the block is gaining energy overall. Its temperature will increase.
In 60 s, the energy gained would be $J\Delta E = P t = 6\ \text{W} \times 60\ \text{s} = 360\ \text{J}$ .

Steady temperature and thermal equilibrium

A body can be receiving radiation and losing radiation but still stay at the same temperature.

Definition

Thermal equilibrium

A body is in thermal equilibrium with its surroundings when its temperature is constant because the average power it absorbs equals the average power it radiates.

This does not mean “no energy is being transferred”. It means the energy transfers are balanced.

Tip

Spotting equilibrium

If the temperature is constant, look for equal average powers: absorbed power equals radiated power.

Earth’s radiation balance

Earth receives energy from the Sun. Some of this incoming radiation is reflected back into space by clouds, ice, snow, and pale surfaces. The rest is absorbed by the land, oceans, and atmosphere, warming Earth.

Earth then emits infrared radiation back out towards space.

Schematic of Earth's radiation energy balance showing incoming solar radiation, reflection, absorption, emitted infrared radiation, and greenhouse gases

Earth’s average temperature depends on the balance between:

incoming radiation absorbed by Earth
outgoing infrared radiation emitted by Earth

If Earth absorbs more radiation than it emits, its average temperature increases. If it emits more radiation than it absorbs, its average temperature decreases. If the two are balanced, the average temperature stays steady.

Factors affecting Earth’s temperature

Several factors control Earth’s energy balance.

1. Reflection and albedo

Definition

Albedo

Albedo is a measure of how much incoming radiation a surface reflects. A high-albedo surface reflects a large fraction of incoming radiation.

Ice and snow have high albedo, so they reflect a lot of sunlight. Oceans and forests have lower albedo, so they absorb more.

If Earth’s average albedo decreases, more incoming radiation is absorbed, so Earth tends to warm.

2. Greenhouse gases

Definition

Greenhouse gases

Greenhouse gases are gases in the atmosphere, such as carbon dioxide, methane, and water vapour, that absorb and re-emit infrared radiation.

Greenhouse gases let much of the Sun’s incoming radiation reach Earth’s surface, but they absorb some of the infrared radiation emitted by Earth. They then re-emit infrared radiation in all directions, including back towards the surface.

This reduces the rate at which energy escapes to space, so it can make Earth’s surface and lower atmosphere warmer.

3. Earth’s own temperature

A warmer Earth emits more infrared radiation per second. So if Earth warms, the outgoing radiation increases until a new balance may be reached.

Example

Predicting a change in Earth’s average temperature

Suppose Earth absorbs radiation at an average rate of 240 W per square metre but emits infrared radiation to space at 235 W per square metre.

Compare the average incoming absorbed power with the average outgoing emitted power: absorbed is greater than emitted.
Calculate the imbalance: $W/m2240\ \text{W/m}^2 - 235\ \text{W/m}^2 = 5\ \text{W/m}^2$ .
Because there is a net gain of energy, Earth’s average temperature would increase.
As Earth warms, it emits more infrared radiation, so the emitted power rises towards a new balance.

Common Mistake

Clouds can do two things

Clouds can reflect incoming sunlight, but they can also absorb infrared radiation emitted by Earth. In exams, focus on the factor named in the question and explain its effect on the incoming or outgoing radiation.

Investigating different surfaces

For Edexcel Separate Physics, you must be able to describe the core practical investigating how the nature of a surface affects the amount of thermal energy radiated or absorbed.

One common version compares how quickly hot water cools in containers with different surfaces.

Core practical apparatus comparing matt black and shiny silver surfaces using identical containers, temperature probes, stopwatch, and optional infrared detectors

Method for radiation from surfaces

You could use two identical metal cans: one matt black and one shiny silver.

Fill both cans with the same volume of hot water at the same starting temperature.
Put a thermometer or temperature probe in each can.
Record the temperature of each can at regular time intervals.
Compare the temperature drop over the same time period.

If the matt black can cools faster, it is emitting thermal radiation at a greater rate.

Variables

Independent variable: the type of surface, such as matt black or shiny silver.
Dependent variable: temperature drop, rate of cooling, or infrared detector reading.
Control variables: starting temperature, volume of water, container size, time interval, distance from detector, and room conditions.

Absorption version

To investigate absorption, place different surfaces the same distance from an infrared lamp. Measure the temperature rise after the same time.

A matt black surface should show a larger temperature rise because it absorbs more infrared radiation. A shiny silver surface should show a smaller temperature rise because it reflects more radiation.

Example

Using cooling data to compare emitters

Two identical cans contain the same volume of water at 80 °C. After 10 minutes, the matt black can is at 62 °C and the shiny silver can is at 72 °C.

Calculate the temperature drop for the matt black can: 80 °C to 62 °C is a drop of 18 °C.
Calculate the temperature drop for the shiny silver can: 80 °C to 72 °C is a drop of 8 °C.
Compare the drops over the same time: 18 °C is greater than 8 °C, so the matt black can lost energy faster.
Conclude that the matt black surface is the better emitter of thermal radiation.

Common Mistake

Unfair comparison

Do not compare two cans if they started at different temperatures or contained different volumes of water. Those changes could affect the cooling rate, so the test would not be fair.

Practical improvements and safety

To make the results better:

Repeat readings and calculate a mean.
Use temperature probes connected to a data logger for more frequent readings.
Keep the cans away from draughts.
Use identical containers apart from surface finish.
Start timing as soon as the hot water is added.
Stir gently before taking temperature readings, if your teacher says it is safe.

For safety, take care with hot water and hot lamps. Use a heatproof mat and avoid touching hot metal containers.

Tip

Best GCSE conclusion

For this practical, link the result to the surface: “The matt black surface had the greatest temperature decrease, so it was the best emitter of infrared radiation.”

Exam technique

In the exam

Always compare absorbed power with radiated power before deciding whether temperature rises, falls, or stays constant.
For Earth questions, say whether the factor changes incoming absorbed radiation or outgoing emitted infrared radiation.
In practical questions, name the independent variable, dependent variable, and at least two control variables.

Self review

Check yourself

What happens to a body if its average radiated power is greater than its average absorbed power?
Why does a matt black surface cool faster than a shiny silver surface?
How can greenhouse gases affect Earth’s outgoing infrared radiation?

Thermal radiation and energy balance

What you'll learn

Why all objects emit and absorb infrared radiation.
How a body warms up, cools down, or stays at steady temperature.
How Earth’s temperature depends on the balance between incoming and outgoing radiation.
How to describe the core practical on different surfaces absorbing and radiating thermal energy.

The starting point: thermal energy and temperature

A body just means an object — for example a mug, a person, a planet, or a metal can in a practical.

Temperature tells you how hot something is. In particle terms, it is linked to the average kinetic energy of the particles in a substance.

Thermal energy is energy stored because of the motion and arrangement of particles. A large object can store more thermal energy than a small object at the same temperature.

Definition

Thermal radiation

Thermal radiation is energy transferred by electromagnetic waves, mainly infrared radiation, emitted by objects because of their temperature.

Infrared radiation is part of the electromagnetic spectrum. Unlike conduction and convection, radiation can travel through a vacuum, so it can transfer energy from the Sun to Earth through space.

Absorbing and emitting radiation

Objects can:

absorb radiation: take in energy from incoming waves
emit radiation: give out energy as infrared waves
reflect radiation: bounce incoming waves away

All objects emit some thermal radiation, but hotter objects emit more radiation per second than cooler objects.

Key Idea

Absorbers and emitters

That means:

A matt black surface absorbs infrared strongly and emits infrared strongly.
A shiny silver surface reflects infrared strongly, so it absorbs little and emits little.

Common Mistake

Dark means hot?

Do not say a black object is always hotter. The surface affects how quickly it absorbs and emits radiation; the actual temperature depends on the full energy balance.

Power: energy transferred per second

In this topic, you often compare the average power absorbed and radiated by a body.

Definition

Average power

Average power is the energy transferred per second over a time interval. It is measured in watts, W, where 1 watt means 1 joule per second.

A useful relationship from the energy topic is:

\frac{\Delta E}{t}

where $P$ is power in watts, $ΔE\Delta E$ is energy transferred in joules, and $t$ is time in seconds.

Energy balance for one body

At the same time, a body may be absorbing radiation and emitting radiation.

The important comparison is:

Pnet=Pabsorbed−PradiatedP_{\text{net}} = P_{\text{absorbed}} - P_{\text{radiated}}

If absorbed power is greater than radiated power, the body gains energy and warms up.
If radiated power is greater than absorbed power, the body loses energy and cools down.
If absorbed power equals radiated power, the body’s temperature stays constant.

This balance explanation is Separate Physics content in Edexcel 1PH0, and the detailed power-balance explanation is Higher Tier.

Key Idea

The balance decides the temperature change

A body warms up if it absorbs energy faster than it radiates energy away, and cools down if it radiates energy faster than it absorbs energy.

Example

Working out whether an object warms or cools

A metal block absorbs radiation with an average power of 30 W and radiates energy with an average power of 24 W.

Compare the two powers using the net power idea: $Pnet=Pabsorbed−PradiatedP_{\text{net}} = P_{\text{absorbed}} - P_{\text{radiated}}$ .
Substitute the values: $WP_{\text{net}} = 30\ \text{W} - 24\ \text{W} = 6\ \text{W}$ .
The net power is positive, so the block is gaining energy overall. Its temperature will increase.
In 60 s, the energy gained would be $J\Delta E = P t = 6\ \text{W} \times 60\ \text{s} = 360\ \text{J}$ .

Steady temperature and thermal equilibrium

A body can be receiving radiation and losing radiation but still stay at the same temperature.

Definition

Thermal equilibrium

A body is in thermal equilibrium with its surroundings when its temperature is constant because the average power it absorbs equals the average power it radiates.

This does not mean “no energy is being transferred”. It means the energy transfers are balanced.

Tip

Spotting equilibrium

If the temperature is constant, look for equal average powers: absorbed power equals radiated power.

Earth’s radiation balance

Earth then emits infrared radiation back out towards space.

Schematic of Earth's radiation energy balance showing incoming solar radiation, reflection, absorption, emitted infrared radiation, and greenhouse gases

Earth’s average temperature depends on the balance between:

incoming radiation absorbed by Earth
outgoing infrared radiation emitted by Earth