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KS3 3.4.3 Wave effects

In this short section we will look at what waves can do.
There are a few things that they all do and there are things that are unique to types or groups of waves.

What is it that ALL waves do ?

1. All waves (sound waves, light waves, water waves etc) have one thing in common.
They all transfer energy from one place to another without any transfer of matter.
(We said this near the start of section 3.4.1 Sound)

So, for example a sound wave produced by a loudspeaker will transfer energy from the vibrating cone inside the loudspeaker to the ear of a person listening without any transfer/movement of the vibrating loudspeaker cone to the person and without any movement of the vibrating air particles to the person.

Or, light waves transfer energy from the Sun (93 million miles away) to the Earth which we see and feel, but without any movement of the Sun towards us.

2. In all cases, the energy is directed in the direction of the wave, obviously!

3. It is also obvious that waves of larger amplitude will transfer more energy than those of small amplitude (see 3.4.1 Sound for a reminder of "amplitude").
So a sound wave of large amplitude is louder than one of small amplitude because it transfers more energy.

4. It is less obvious that waves of higher frequency also transfer more energy than those of lower frequency (see 3.4.1 Sound for a reminder of "frequency").
Let's see if we can explain this fact.

The following wave has a frequency of 2 Hz (ie 2 complete waves in 1 second)

But this next wave has the same amplitude but a higher frequency; it is 4 Hz (4 complete waves in 1 second)

Can you see why the wave of higher frequency will transfer more energy?
There are simply more waves of energy in the same time compared to the first wave, so this wave will transfer more energy.
Its like hitting a nail with a hammer; if you hit it 4 times in 1 sec like the second wave, it will transfer more energy to the nail than if you hit it only 2 times in 1 second like the first wave above.

NB Max Planck and Albert Einstein were the first to make the connection between a wave's frequency and its energy.

To summarise:
1. All waves transfer energy from one place to another without any transfer of matter.
2. A wave's energy is directed in the direction of the wave.
3. Waves of larger amplitude transfer more energy than those of smaller amplitude.
4. Waves of higher frequency transfer more energy than those of lower frequency.

Now we will look at specific wave effects, not common to ALL waves.

Wave effects 1 - making particles move!

Making particles vibrate using very short wavelength radio waves (microwaves)

When you put a glass of water in a microwave oven and leave it in for about 30 seconds, when it is removed you find that the water is warmer.
The water has absorbed energy from the microwaves (very high frequency, or micro wavelength, radio waves) produced inside the microwave oven.

But what exactly are the microwaves doing to the water that causes it to get warmer?

Answer - the microwaves are causing the particles (molecules) of water to move/vibrate rapidly, generating heat within the water and raising its temperature.

The same thing happens when you put food into the microwave oven; the water molecules within the food are made to vibrate by the very high frequency micro-waves, heating the food and cooking it.

Micro-waves are used for this purpose because:
A. they are very high frequency radio waves and so transfer more energy than lower frequency radio waves.
B. they are of the exact frequency needed to vibrate water molecules.

Making particles vibrate using ultrasound sound waves

Ultrasound waves (very high frequency sound waves), beyond the audible range of humans, can also be used to make water particles vibrate.
Ultrasound is used to clean very small or very delicate items which are placed in an ultrasound bath which is agitated (shaken!) by the ultra high frequency sound waves; dirt is literally shaken off the items, cleaning them.

Making particles move using EXTREMELY high frequency electromagnetic waves

Ultraviolet waves, X - waves (or X-rays) and gamma waves are 3 types of electromagnetic wave which are very, very high frequency (compared with normal radio waves). So they transmit EXTREMELY high energy.
They can move an electron within an atom such that the atom loses the electron. The atom is then said to be "ionised" and if this occurs to atoms within living cells (especially within DNA) then the cells can become cancerous.

Ultraviolet waves emitted by the sun can cause skin cancer.
Excessive exposure to X-rays or gamma waves can damage the internal organs of a body.
Gamma waves and X-rays are higher frequency than Ultraviolet so they transfer more energy meaning that they can travel deeper into a body causing damage to vital internal organs.
The Ultraviolet waves only penetrate the outer layers of skin, but in doing so they are still very dangerous and can cause skin cancer.

Wave effects 2 - how to move a mechanical object!

A microphone can transform a sound wave into an electrical signal and a loudspeaker can transform the electrical signal back into a sound wave. How do they do this?

Explaining this in good detail will have to wait until you start GCSE Physics, but for now we will look at a simplified explanation:

When a person speaks into a microphone, a thin plastic membrane (known as the diaphragm) is made to move/vibrate by the sound wave.
The vibrating membrane is connected to a small coil of wire which then moves in and out of a circular magnet. When this happens a very small electrical signal is induced into the little coil of wire; the electrical signal carries the frequency and amplitude information of the original sound wave.

Once in the form of an electrical signal the information can be carried through wires to a loudspeaker which turns the electrical signal back into a sound wave. How does the loudspeaker do this?

See the diagram below.
The electrical signal goes through a coil of wire which sits within a circular magnet. The effect of the electrical signal plus the magnetism makes the coil move (this is the reverse of the induction which occured in the microphone). The coil is connected to a paper or plastic diaphragm (known as the loudspeaker cone) so it is made to move (vibrate) following the pattern of the electrical signal. Finally, the vibrating loudspeaker cone moves the air around it setting up a pressure wave or a sound wave!

Now it is time for you to have a go at a few questions:


Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.