0.2 Tg physical science - chapter contexts  (Page 8/10)

The use of ultrasound to create images is based on the reflection and transmission of a wave at a boundary. When an ultrasound wave travels inside an object that is made up of different materials such as the human body, each time it encounters a boundary, e.g. between bone and muscle, or muscle and fat, part of the wave is reflected and part of it is transmitted. The reflected rays are detected and used to construct an image of the object. Ultrasound in medicine can visualise muscle and soft tissue, making them useful for scanning the organs, and is commonly used during pregnancy. Ultrasound is a safe, non-invasive method of looking inside the human body.

Electromagnetic radiation

In this section it is explained that some aspects of the behaviour of electromagnetic radiation can best be explained using a wave model, while other aspects can best be explained using a particle model. The educator can use the descriptions and diagrams in the text to explain:

• how electromagnetic waves are generated by accelerating charges;
• that an electric field oscillating in one plane produces a magnetic field oscillating in a plane at right angles to it, and so on; and
• how EM waves propagate through space, at a constant speed of $3x{10}^{8}m·s^{-1}$ .

The colourful visual representation of the electromagnetic spectrum as a function of the frequency and wavelength also showing the use of each type of EM radiation, will assist learners to connect physical concepts to real life experiences. The penetrating ability of the different kinds of EM radiation,the dangers of gamma rays, X-rays and the damaging effect of ultra-violet radiation on skin and radiation from cell-phones are discussed.

The particle nature of EM radiation is stated and the concept photon is defined. The energy of a photon is calculated using the equation: E = hf where $h=\mathrm{6,63}\times {10}^{-}34J\cdot s$ is Planck’s constant, f = frequency, and where $c=3\times {10}^{8}m\cdot s^{-}1$ is the speed of light in a vacuum. Learners can calculate the energy of an ultraviolet photon with a wavelength of 200 nm by using the equations given.

Electricity and magnetism

Magnetism

Magnetism is a force exerted by magnetic objects without touching each other. A magnetic object is surrounded by a magnetic field, a region in space where a magnet or object made of magnetic material will experience a non-contact force. Electrons inside any object have magnetic fields associated with them. The way the electrons' magnetic fields line up with each other explains magnetic fields in ferromagnetic materials (e.g. iron), magnetisation, permanent magnets and polarity of magnets. These concepts are explored with descriptions, diagrams and investigations.

Magnets have a pair of opposite poles, north and south. Like poles of a magnet repel; unlike poles attract. It is not possible to isolate north and south poles - even if you split a magnet, you only produce two new magnets. The magnetic field line around a bar magnet can be visualised using iron filings and compass needles. Learners need to be reminded that the field is three dimensional, although illustrations depict the fields in 2D. To show the shape, size and direction of the magnetic fields different arrangements of bar magnets are investigated and illustrated.