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Physics Form 4

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Background knowledge

In the topic electromagnetic spectrum, X-rays were named as part of the spectrum. In this topic we shall narrow down

our discussion into the production, properties, dangers and uses of X-rays.

Background knowledge

In the topic electromagnetic spectrum, X-rays were named as part of the spectrum. In this topic we shall narrow down

our discussion into the production, properties, dangers and uses of X-rays.

Objectives

By the end of the lesson you should be able to:

-Explain the production of X-rays

-State the properties of X-rays

-State the dangers of X-rays

-Explain the uses of X-rays

-Solve numerical problems involving X-rays

Introduction

X-rays are important in our day to day life. In medicine, they aid in diagnosis of ailments and in industry they

assist in detecting cracks and flaws in finished products.

Production of X-rays

X-rays are produced when fast moving electrons/cathode rays hit a target which is normally a heavy metal e.g.

tungsten. The animation below shows the production of X-rays.

Observation

When the heater circuit is switched on, the cathode glows red hot and a stream of electrons is produced. These

electrons hit the target producing a stream of X-rays.

Explanation

These electrons are produced by thermionic emission and attracted to the tungsten anode (target). Upon hitting the

target, most of their kinetic energy is converted into heat and a small percentage into X- rays.

Parts of the X-ray tube

The following animation shows the working of the parts of the X-ray tube. Place the mouse over the various parts to

get the corresponding explanations.

Energy changes in the X-ray tube

At the heater circuit; electric current is used to heat the cathode transforming the electrical energy into heat

energy. The electrons gain enough energy to leave the cathode. At this point the heat energy is transformed into

kinetic energy of the electrons. The electrons are then made to move faster by the high accelerating voltage between

the anode and the cathode. On hitting the anode, the kinetic energy of the electrons is transformed into heat and

X-rays.

Types of X-rays

There are two types of X-rays namely:

* Hard X-rays and

* Soft X-rays

When high accelerating voltage is used, the electrons move with a very high velocity producing X-rays with a very

short wavelength. These are referred to as hard X-rays. They have high velocity and hence high penetrating power. They

penetrate flesh and are stopped by bones and other materials of high density. When a low accelerating voltage is used,

the electrons move with a low velocity producing X-rays with long wavelengths. These are referred to as soft X-rays.

They have low velocity hence low penetrating power. They can penetrate flesh.Click “next” to view a visualisation on

this.

Click on the play button to make your observations.

Properties of X-rays.

The following are the properties of X-rays:

1. They are not deflected by either magnetic or electric field. Click on the switch on the animation below to generate

X-rays. Then click on the words ELECTRIC FIELD and later on the word MAGNETIC FIELD button and observe what happens on

the X-rays as they move through the said fields.

Observation

The X-rays are observed to pass through both magnetic and electric field without being deflected.

Explanation

X-rays are not charged hence they are not deflected by either a magnetic or an electric field.

2. They affect a photographic film. Click on the play button to observe what happens on the photographic film.

Observation

The film appears foggy on the spot hit by the X-rays.

3. They cause fluorescence on certain materials like zinc sulphide. Click on the play button and observe what happens

on the screen which has a zinc sulphide coating.

Observation

The X-rays are observed to cause fluorescence on the spot where they hit the zinc sulphide coating.

4.They penetrate matter

Click on the respective switches for each given material and observe what happens

Observation

The X-rays are able to penetrate the paper, the aluminum plate but are stopped by the thick lead sheet

Explanation

X-rays have a short wavelength and a very high speed hence have a high penetrating power.

5. They can ionize air.

Click on the air molecules button. Observe what happens to the air molecules.

Observation

As the X-rays pass through air they knock off electrons from the air molecules leaving them charged.

Explanation

This is because the X-rays ionise the air molecules leaving them positively charged

6. X- rays can cause photoelectric emission

When X-rays fall on a metal surface, part of their energy is used to eject the electrons (work function, hfo) and the

rest is converted into kinetic energy, 1/2mv2. Hf = hfo +1/2mv2 Where hf = eV = energy of the X-rays. Play the

animation below to observe how this takes place.

7. X-rays show diffraction when they pass through regularly spaced layers of atoms of a crystal. Click on the play

button to observe what happens on the photographic plate.

Observation

A regular arrangement of the atoms in the crystal is seen.

Other properties of X-rays include:

They travel with the speed of light, have no mass and obey the equation c = f x l where c= 3.0 x 108 m/s. l (lambda)

is the wavelength of the X-rays and f is the frequency

Dangers of X-rays

Over exposure to X-rays can lead to:

1. Killing of body cells

2. Gene mutations. This is the alteration of the gene structure which may lead to disorder or deformities like skin

cancer in the body.

The X-ray tube is covered with lead shield to protect the operator from exposure to the rays. The machines are

operated in rooms that have concrete walls to absorb any leakages.

Uses of X-rays

The following are some of the uses of X-rays:

1. X-rays are used to detect fractures in bones

2. Hard X-rays can be carefully directed to destroy cancerous cells in the body. Click on the play button to make your

observations.

3. X-rays are used for detecting flaws or cracks on metal castings. Click on the play button of the following

animation and observe how this is done.

4. They can be used for sterilizing surgical equipment. To observe this click on the play button.

5. X-rays are used to study the crystal structure of substances. The following animation shows this.Play it to observe

how this takes place.

6. X-rays are used to inspect luggage and people in airports. Play following video and see how this is done.

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