Black body radiation - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Black body radiation

Waves

Lesson 12 of 12

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Waves

Lesson 12 of 12

Answer each one, then check.

1

What is infrared?

2

What makes an object hotter?

3

What is absorption?

4

What is reflection?

5

What colour surface absorbs best?

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Waves

Lesson 12 of 12

1

What is infrared?

Electromagnetic radiation felt as heat

2

What makes an object hotter?

Absorbing energy faster than it emits it

3

What is absorption?

Taking in the energy of radiation

4

What is reflection?

A wave bouncing off a surface

5

What colour surface absorbs best?

Matt black

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Waves

Lesson 12 of 12

1

State that all bodies emit and absorb infrared radiation.

2

Describe a perfect black body and why it is the best emitter.

3

Explain that the intensity and wavelength distribution of emission depend on temperature.

4

Explain how the temperature of a body or of the Earth is related to absorption and emission (Higher tier).

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Waves

Lesson 12 of 12

BLACK BODY RADIATION

All bodies, whatever their temperature, emit and absorb infrared radiation. The hotter the body, the more infrared it radiates in a given time.

A perfect black body absorbs all the radiation that falls on it and is also the best possible emitter.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Required Practical 10: Infrared from Different Surfaces

Waves

Lesson 12 of 12

Keep the distance and the water temperature the same for each face.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Emission at Different Temperatures

Waves

Lesson 12 of 12

Higher temperature means greater intensity and shorter wavelengths.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Earth's Energy Balance (Higher)

Waves

Lesson 12 of 12

Constant temperature means absorption equals emission.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Required Practical 10: Method

Waves

Lesson 12 of 12

Compare how surfaces radiate infrared.

1

Fill the cube

Fill a Leslie cube with hot water and put in the lid.

2

Detector

Hold the infrared detector at the same distance from each face.

3

Record

Note the detector reading for each surface.

4

Control

Keep the distance, water temperature and time the same.

5

Result

The matt black surface emits most infrared; the shiny silver surface emits least.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

A Body at Constant Temperature (Higher)

Waves

Lesson 12 of 12

Explain why a body at a constant temperature is absorbing and emitting radiation at the same rate.

1

Absorbing more than emitting

The temperature rises

2

Emitting more than absorbing

The temperature falls

3

Constant temperature

Absorption equals emission

ANSWER

If the rates are equal the temperature does not change.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

The Earth's Temperature (Higher)

Waves

Lesson 12 of 12

Explain what happens to the temperature of the Earth's surface if the atmosphere absorbs more of the infrared radiation emitted by the surface.

1

More is absorbed

And re-radiated back to the surface

2

Emission to space

Decreases

3

Absorption exceeds emission

So the surface temperature rises until a new balance

ANSWER

The temperature of the surface increases.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Points

Waves

Lesson 12 of 12

Learn these.

All bodies

Emit infrared, whatever their temperature.

Hotter

More radiation in a given time and a shorter peak wavelength.

Good absorber

Also a good emitter (matt black surfaces).

Good reflector

A poor absorber and a poor emitter (shiny silver).

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Terms

Waves

Lesson 12 of 12

Black body

An object that absorbs all the radiation that hits it.

Emit

Give out radiation.

Absorb

Take in radiation.

Intensity

The amount of radiation per second per unit area.

Infrared

Electromagnetic radiation emitted by all objects.

Equilibrium

When absorption equals emission.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Waves

Lesson 12 of 12

YOUR TASK

Hot or Cold?

10 minutes

Two cans of hot water, one matt black and one shiny silver, cool for 10 minutes. Which cools faster? Explain.

1

Compare emission.

2

Use the black body idea.

WHAT A GOOD ANSWER SHOWS

The matt black can, because it is a better emitter of infrared radiation, so it loses energy faster.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Can I...?

Waves

Lesson 12 of 12

State that all bodies emit infrared.

Define a perfect black body.

Say a good absorber is a good emitter.

Describe Required Practical 10.

Explain the effect of temperature.

Describe the curves.

Explain constant temperature.

Explain the Earth's energy balance.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

All bodies emit and absorb infrared.

A perfect black body absorbs all radiation and is the best emitter.

Hotter bodies emit more with shorter wavelengths.

Temperature is steady when absorption equals emission.

EXAM FOCUS

Explain why a matt black surface is a better emitter of infrared radiation than a shiny silver surface. (2 marks)

Good absorbers are good emitters.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Black body radiation

Waves

Lesson 12 of 12

Answer all questions. Use the mark allocation as a guide to how much to write. · 17 minutes

Question 1 · 2 marks · State

State whether a matt black surface or a shiny silver surface is a better emitter of infrared radiation, and give the reason.

Question 2 · 4 marks · Describe

A student investigates how the surface of a Leslie cube affects the infrared radiation emitted. Describe how the investigation is carried…

Question 3 · 3 marks · Use the graph

The graph shows how the intensity of radiation emitted by a hot object depends on the wavelength at two different temperatures, A and B…

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Black body radiation (cont.)

Waves

Lesson 12 of 12

Question 4 · 3 marks · Explain

The temperature of a cup of tea stays constant at 50 °C when it is kept warm by a heater. Explain this in terms of radiation.

Question 5 · 4 marks · Explain

Explain how the Earth's temperature is related to the radiation it absorbs and emits. Explain what happens if the atmosphere contains more…

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · State

Waves

Lesson 12 of 12

“

State whether a matt black surface or a shiny silver surface is a better emitter of infrared radiation, and give the reason.

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · mark scheme

Waves

Lesson 12 of 12

2 marks available. Award a mark for each point made.

Matt black

1 mark

Good absorber is a good emitter

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 4 marks · Describe

Waves

Lesson 12 of 12

“

A student investigates how the surface of a Leslie cube affects the infrared radiation emitted. Describe how the investigation is carried out and how to keep it a fair test.

HOW TO ANSWER IT

Command word: Describe. Worth 4 marks, so plan before writing.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Waves

Lesson 12 of 12

4 marks available. Award a mark for each point made.

Hot water in a Leslie cube

1 mark

Detector at each face

1 mark

Control distance and temperature

1 mark

Compare readings

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 3 marks · Use the graph

Waves

Lesson 12 of 12

The graph shows how the intensity of radiation emitted by a hot object depends on the wavelength at two different temperatures, A and B. (a) Which curve is for the hotter object? (b) Give two reasons for your answer. (3 marks)

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · mark scheme

Waves

Lesson 12 of 12

3 marks available. Award a mark for each point made.

Curve B

1 mark

Higher intensity

1 mark

Peak at shorter wavelength

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 3 marks · Explain

Waves

Lesson 12 of 12

“

The temperature of a cup of tea stays constant at 50 °C when it is kept warm by a heater. Explain this in terms of radiation.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · mark scheme

Waves

Lesson 12 of 12

3 marks available. Award a mark for each point made.

Absorbs energy at some rate

1 mark

Emits at the same rate

1 mark

So constant temperature

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Explain

Waves

Lesson 12 of 12

“

Explain how the Earth's temperature is related to the radiation it absorbs and emits. Explain what happens if the atmosphere contains more greenhouse gases.

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · mark scheme

Waves

Lesson 12 of 12

4 marks available. Award a mark for each point made.

Temperature depends on absorption and emission

1 mark

Constant when equal

1 mark

Greenhouse gases absorb emitted infrared

1 mark

So the temperature increases

1 mark

openrevise.com