Physics GCSE (Unit 1b)

Electromagnetic waves:

• Electromagnetic radiation travels as a wave and moves energy from one place to another.
• All electromagnetic waves travel at the same speed through space but have different wavelengths and frequencies.
• The waves together are known as the 'electromagnetic spectrum' and within this they are divided into groups; gamme rays, X-rays, ultraviolet rays, visible light, infra-red rays, microwaves and radio waves.
• When electromagnetic waves move through substances they may be reflected, abosorbed or transmitted.
• Different wavelengths or electromagnetic radiation have different effects on living cells.
• There are different uses and hazards for each part of the spectrum.
• Radio waves, microwaves, infra-red and visible light can be used for communication.

Electromagentic waves (cont.):

• Communication signals can be analogue (continuously varying) or digital (certain values only, usually on or off).
• Electromagnetic waves obey the wave formula: wave speed = frequency x wavelength.

Radioactivity:

• Radioactive substances give out radiation from their nuclei all the time whatever is done to them.
• The three main types of nuclear radiation are alpha particles, beta particles and gamma radiaiton.
• Properties of alpha particles, beta particles and gamma radiation.
• Half life is the time it takes for the number of parent atoms in a sample to halve.

The origins of the universe:

• Red shift provides evidence that the universe is expanding and began with a 'Big Bang'.
• Observations of the Solar System and the galaxies in the universe can be carried out on the Earth or from space.
• Telescopes used to make observations may detect visible light or other electromagnetic radiations.

1) Which part of the electromagnetic spectrum has the longest wavelength?

2) What is the unit of wavelength?

3) How are X-rays used in hospitals?

4) How are gamma rays used in hopsitals?

5) How can ultraviolet radiation affect the body?

6) What is visible light?

7) State two uses of microwaves.

8) Which parts of the electromagnetic spectrum is detected by night-vision equipment?

9) What is an optical fibre?

10) Which parts of the electromagnetic spectrum are used to carry signals in optical fibres?

11) What is digital signal?

12) What is meant by 'amplification'?

Key points- 

• The electromagnetic spectrum (in order of increasing wavelength): gamm rays, X-rays, ultraviolet, visible, infra-red, microwaves, radio waves.
• All electromagnetic waves travel through space at a speed of 300 million m/s.
• Wave speed (metres) = frequency (hertz, Hz) x wave length (metres)

Electromagnetic radiations are electric and magnetic disturbances. They travel as waves and move energy from place to place.

All electromagnetic waves travel through space (a vacuum) at the same speed but they have different wavelengths and frequencies.

All of the waves together are called the 'electromagnetic spectrum'. We group the waves according to their wavelength and frequency.

• Gamma rays have the shortest wavelength and highest frequency.
• Radio waves have the longest wavelength and the lowest frequency.
• Different wavelengths of electromagnetic radiation are reflected, absorbed or transmitted differently by different substances and types of surface.

KEY WORDS: electromagnetic spectrum, wave speed, wavelength, frequency.

CHECK YOURSELF QUESTIONS:

1) What is the unit of frequency?

2) Which part of the electromagnetic spectrum has the highest frequency?

3) Which part of the electromagnetic spectrum has the shortest wavelength?

N.B.- Electromagnetic waves transfer energy, not matter.

Key points- 

• X-rays and gamma radiation are abosorbed by dense materials such as bone and metal.
• X-rays and gamma radiation damage living tissue when they pass through it.
• X-rays are used in hospitals to take radiographs.
• Gamma rays are used to kill harmful bacteria in food, to sterilise equipment and to kill cancer cells.

Gamma rays are used to keep food fresh for longer by killing the bacteria on it.
X-rays are used to produce shadow pictures of bones (radiographs).
Gamma rays and X-rays mostly pass through soft body tissues, but some is abosorbed and will damage the cells.
In lower doses, both radiations can cause changes in the cells that make them cancerous.

In higher doses, they can kill the cells.

Gamma radiation is used in hospitals, under carefully controlled conditions, to kill cancer cells.

Working with these radiations is hazardous. Gamma sources are kept in thick lead containers. Staff should wear lead aprons and stand behind lead screens when using X-rays.

They monitor their exposure to the radiation with film badges.

KEY WORDS: Gamma rays, X-rays, dose, cancer

CHECK YOURSELF QUESTIONS:

1) How does gamma radiation sterilise surgical instruments?

2) Why do workers in X-ray departments wear lead aprons?

3) Why is a radiograph a 'shadow' picture?

Key points- 

• Ultraviolet radiation is in the electromagnetic spectrum between violet light X-radiation.
• Ultraviolet radiation has a shorter wavelength than visible light.
• Ultraviolet radiation can harm the skin and the eyes.

Ultraviolet radiation has a longer wavelength than X-rays. It has a shorter wavelength than the light at the violet end of the visible spectrum.

Ultraviolet radiation from the Sun causes damage to skin cells - tanning, sunburn, skin ageing and skin cancer. Over-exposure can also damage the eyes. Sun beds work by giving out UV rays.

Fluorescent tubes are coated with substances that absorb the ultraviolet radiation produced inside the tube.

Then they emit the energy as visible light.

The same substances are used to make hidden security marks that can only be seen with UV light.

Visible light is the part of the electromagnetic spectrum that is detected by our eyes.

We see the different wavelengths within it as different colours.

Visible light can be transferred along optical fibres.

KEY WORDS: ultraviolet, fluorescent, visible, optical fibres. 

CHECK YOURSELF QUESTIONS:

1) How can the skin be protected from damage by ultraviolet radiation?

2) How do hidden security markings work?

3) Is the frequency of ultraviolet radiation higher or lower than the frequency of X-rays?

Key points-

• Infra-red: Heaters, communications (remote handsets, optical fibres)
• Microwaves: Microwave oven, communications.
• Radio waves: Communications

Infra-red (IR) radiation: given out by all objects. The hotter the object, the more IR it emits. Night-vision equipment works by detecting this radiation. IR is absorbed by the skin, we sense it as heat and it can burn. It is used as the heat source in toasters, grills and radiant heaters. TV, video and other remote controls use IR. It can be transmitted along optical fibres.

Microwaves: used for cooking and in communications. Microwave ovens produce frequencies that are absorbed by water molecules. They heat the water in food, cooking it from the inside out.The water in living cells will absorb microwaves and they may be damaged or killed by the heat released. Microwave transmitters produce wavelengths that are able to pass through the atmosphere. They are used to send signals to and from satellites and within mobile phone networks.

Radio waves: used to transmit radio and TV programmes. When an alternating voltage is applied to an aerial, it emits radio waves with the same frequency as the alternating voltage. When the waves are received they produce an alternating current with the same frequency as the radiation.

KEY WORDS: infra-red cameras radiation, microwaves, radio waves, communications, optical fibres, transmitters.

CHECK YOURSELF QUESTIONS:

1) How can infra-red cameras be used to find survivors after accidents?

2) Why do microwaves cook food from the inside out?

3) How are radio waves produced?

Key points-

• The use we make of radio waves depends on the frequency of the waves.
• Visible light and infra-red radiation are used to carry signals in optical fibres.

The microwave and radio wave part of the electromagnetic spectrum is used for communications.
This includes terrestrial TV, satellite TV, mobile phones, emergency services radio, amateur radio transmissions, local, national and international radio.
Different frequencies are used for different applications.
Optical fibres are very thing glass fibres. They are flexible and can be bent around curves.
Light or infra-red radiation is transmitted along the fibre by continuous reflections.

KEY WORDS: communications, optical fibres, reflections

CHECK YOURSELF QUESTIONS:

1) Which electromagnetic waves are used for transmissions along optical fibres?

2) How are electromagnetic waves transmitted along optical fibres?

3) Which type of electromagnetic wave is used for satellite communications?

Key points-

• Analogue signals vary in continuously in amplitude.
• Digital signals are either high ('1') or low ('0').
• Digital transmission, when compared with analogue transmission, is free of noise and distortion. It can also carry much more information.

Analogue signal varies continuously in amplitude.
Digital signals only have certain values. Usually they are either high (on/'1') or low (off/'0'). They can be processed by computers.
When signals are transmitted they become less strong over distance and have to be amplified.
They also pick up noise. When amplification takes place the noise is also amplified.

With analogue this can make the signal very distorted.

1) What are electromagnetic waves?

2) What do all electromagnetic waves have in common?

3) Which part of the electromagnetic spectrum has the lowest frequency?

4) What are gamma rays used for?

5) What is infra-red radiation used for in the home?

6) What is an analogue signal?

7) How are workers in radiography departments in hospitals protected from exposure to X-rays?

8) How does the wavelength of ultraviolet radiation compare with the wavelength of visible light?

9) Why are microwaves dangerous to the body?

10) How does the frequency of radio waves compare with the frequency of visible light?

11) How are optical fibres used in medicine?

12) What is the formula that relates frequency, speed and wavelength?

1) Describe the basic structure of a nucleus.

2) What is the effect of pressure on the rate of radioactive decay?

3) What is the structure of an alpha particle?

4) What is the range of alpha particles in air?

5) What happens to the radioactivity of a sample of a radioactive material over time?

6) What happens to the count rate from a sample during one half life?

7) Where are alpha sources commonly found in the home?

8) Which is the most dangerous type of nuclear radiation if the source is inside the body?

Key points-

• A radioactive substance contains unstable nuclei.
• An unstable nucleus becomes stable by emitting radiation.
• There are three types of radiation from radioactive substances- alpha, beta and gamma radiation.
• Radioactive decay is a random event- we cannot predict or influence when it will happen.

The atoms of an element always have the same number of protons. However, diff. isotopes of the element will have different number of neutrons.

Nuclei of radioactive substances= unstable.

They become stable by radioactive decay.

In this process they emit radiation and turn into other elements.

The three types of radiation emitted are :

• alpha particles
• beta particles
• gamma rays
  Radioactive decay is a random process and is not affected by external conditions.

KEY WORDS: nuclei, proton, neutron, radioactive decay, alpha particles, beta particles, gamma rays.
CHECK YOURSELF QUESTIONS:

1) Which part of an atom might emit alpha particles?

2) What is meant by 'isotopes of an element'?

3) What happens to the rate of radioactive decay if the temperature is doubled?

Key points-

• Alpha radiation is stopped by paper or a few centimetres of air.
• Beta radiation is stopped by thin metal or about a metre of air.
• Gamma radiation is stopped by thick lead and has an unlimited range in air.
  An alpha particle is a helium nucleus. It is made up of 2 protons and 2 neutrons.

A beta particle is a high-speed electron from the nucleus. It is emitted when a neutron change to a proton of an electron. The proton remains in the nucleus.
Gamma radiation is very short wavelength electromagnetic radiation that is emitted from the nucleus.
When nuclear radiation travels through a material it will collide with the atoms of the material and known electrons off them, creating ions. This is called ionisation.

Alpha particles are large. Which means they have lots of collisions with atoms. They are strongly ionising. Because of these collisions, the alpha particles do not penetrate far into a material. They can be stopped by a thin sheet of paper., human skin or a few centimetres of air. Alpha particles have a positive charge, and are deflected by electric and magnetic fields.
Beta particles are much smaller and faster than alpha particles, so they are less ionising and penetrate further. They are blocked by a few metres of air, or a thin sheet of aluminium. Beta particles have negative charge, and are deflected by electric and magnetic fields  in the opposite sense to alpha particles.
Gamma rays are electromagnetic waves so they will travel a long way through a material before colliding with an atom. They are weakly ionising and very penetrating. Several centimetres of lead or several metres of concrete are needed to absorb most of the radiation. Gamma rays are not deflected by electric and magnetic fields.

KEY WORDS: ionisation, electric and magnetic fields, charge.

CHECK YOURSELF QUESTIONS:

1) Which type of nuclear radiation is the most penetrating?

2) Which type of nuclear radiation is the most ionising?

3) Why is gamma radiation not deflected by electric and magnetic fields?

Key points-

• The half-life of a radioactive substance is the time it takes: 1) for the number (and therefore the mass) of parent atoms in a sample to halve 2) for the count rate from the original substance to fall to half its initial level.

We can measure the radioactivity of a sample of a radioactive material by measuring the count rate from it.
The radioactivity of a sample decreases over time. How quickly the count rate falls to nearly zero depends on the material.
Some take a few minutes- others take millions of years.
We use the idea of half-life to measure how quickly the radioactivity decreases.

It is the time take for the count rate from the original substance to fall to half its initial value.

Or we can define it as the the time it takes for the number of unstable nuclei in a sample to halve.

The half-life is the same for any sample of a particular material.

KEY WORDS: half-life, count rate.

CHECK YOURSELF QUESTIONS:

1) What happens to the count rate from a sample over  time?

2) What has happened to the original count rate of a sample after two half-lives have passed?

3) What has happened to the number of atoms in a sample after two half-lives have passed?

Key points- 

• The use we can make of a radioactive substance depends on: 1) its half-life, and 2) the type of radiation it gives out.

Alpha sources- used in smoke alarms. Alpha particles= not dangerous, because not penetrating. The source need s a half life of several years.

Beta sources- used for thickness control in the manufacture of things like paper. Alpha particles would be stopped by a thin sheet of paper and all gamma rays would pass through it. The source needs a half life of many years- so that decreases in count rate are due to changes in the thickness of the paper.
Gamma and beta sources- used as 'tracers' in medicine. The source= injected or swallowed by patient. Its progress around the body is monitored by a detector outside the patient. Source needs a half life of a few hours so that the patient is not exposed to unnecessary radioactivity.

Gamma sources-  are also used to sterilise medical equipment and prevent food spoilage.

If nuclear radiation enters living cells, it causes ionisation which damages cells and may cause cancer. 

If the source of radiation is outside the body, alpha particles will be stopped by clothing or skin. Gamma and beta radiation are more dangerous because they may pass through the skin and damage cells.

If the source of radiation is inside the body (e.g. it is inhaled), alpha radiation is the most dangerous because it is very strongly ionising.

KEY WORDS: tracers

CHECK YOURSELF QUESTIONS:

1) Why is a beta source less dangerous inside the body than an alpha source?

2) Why do medical tracers have half-lives of just a few hours?

3) Why isn't an alpha source used as a tracer in medicine?

1) What is a beta particle?

2) What is the range of a beta particle in air?

3) What is meant by 'half-life'?

4) What is gamma radiation?

5) Which is the most dangerous typed of nuclear radiation is the source is outside the body?

6) What happens to the count rate from a a radioactive sample during three half-lives?

7) Which typed of nuclear radiation is the least ionising?

8) Which type of nuclear radiation is used for the thickness control in the manufacture of paper?

1) What happens to the sound waves we hear if the source of the waves is moving?

2) Which galaxy is our Sun part of?

3) What happens to light from distant galaxies before it reaches the Earth?

4) How do most scientists think the Universe started?

5) What is a telescope?

6) Where are telescopes used?

Key points- 

• Light from a distant galaxy is red-shifted to longer wavelengths.
• The further away the galaxy the bigger the red-shift.

RED SHIFT- If a source of waves is moving relative to an observer, the wavelength and frequency ' seen' by the observer will have changed (shifted) from the original produced by the source.

This effect can be heard with sound waves. E.G. The sound of an ambulance siren will sound different depending on whether it is moving towards you (pitch is higher) or away from you (pitch is lower).

The effect occurs with light waves. Light observed from distant galaxies has been 'shifted' towards the red end of the spectrum. This means the frequency has decreased. The further away the galaxy, the bigger the red-shift.

This suggests that distant galaxies are moving away from us, and the most distant galaxies are moving the fastest. This is true of galaxies no matter which direction you look in.

• Galaxies are collections of billions of stars.
• Our Sun is one of the starts in the Milky Way galaxy.
• The Universe is made up of billions of galaxies.

KEY WORDS: red shift, galaxies.

CHECK YOURSELF QUESTIONS:

1) What is a 'galaxy'?
2) What is 'red shift'?
3) Which galaxies are moving away from us fastest? 

Key points-

• Red shift provides evidence evidence that the Universe is expanding.
• The Universe started with the Big Bang, a massive explosion from a very small point.

Red shift shows us that distant galaxies are moving away from us and the furthest ones are moving the fastest. This gives us evidence that the Universe is expanding outwards in all directions.

We can try and imagine backwards in time to see where the Universe came from.

If it is not expanding outwards, this suggests it started with a massive explosion at a very small initial point. This is known as the 'Big Bang' theory.

KEY WORDS: expanding, Big Bang

CHECK YOURSELF QUESTIONS:

1) How does red shift show us that the Universe is expanding?

2) What is meant by the 'Big Bang'?

3) How is an expanding Universe evidence for the Big Bang?

Key points-

• Observations are made with telescopes that may detect visible light or other electromagnetic radiations.
• Observations of the Solar System and galaxies can be carried out from the Earth of from space.

Scientists use telescopes to collect the visible light coming from stars, and so see them.

They can also use telescopes that collect radiation from other parts of the electromagnetic spectrum such as X-rays, or radio waves.

This also allows them to 'see' distant stars.

The atmosphere is a layer of gases surrounding the earth.

Telescopes on satellites are able to receive all types of electromagnetic radiation from space, without it being distorted or absorbed by the Earth's atmosphere.

Because there is no distortion, the pictures produced by these telescopes are clearer and have more detail. So it is possible for us to observe stars that are further away.

KEY WORDS: telescope, atmosphere, satellite

CHECK YOURSELF QUESTIONS:

1) What is the Earth's atmosphere?

2) What does the Earth's atmosphere do to electromagnetic radiation from space?
3) Why are telescopes on satellites able to produce clearer pictures than those on earth

1) Roughly how many galaxies is the Universe thought to contain?

2) What would a 'blue shift' of light from distant galaxies show?

3) What evidence is there that the Universe is expanding?

4) What has happened to the frequency of light that reaches the Earth from distant galaxies?

5) What is the advantage of using a telescope on a satellite rather than on Earth?

6) What is a radio telescope?