Physics topic 1

  • Created by: ca123
  • Created on: 27-05-19 11:03

Types of Waves

Transverse Waves

  • (light, water, strings)
  • Vibrations are at 90° to the direction of wave travel

Longitudinal Waves

  • (sound, slinky spring)
  • Vibrations are along the same direction as wave travel

Characteristics of Waves

Waves carry energy from one place to another

Amplitude, a - The distance from the middle to either the trough or peak

Wavelength,  (metres) - The distance of one full cycle of a wave

Frequency, f (Hertz, Hz) - The number of waves that pass a point in 1 second

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Wave Properties

Amplitude is the maximum displacement (change in position) from the undisturbed position.

Wavelength (λ) is the distance in metres from any point on the wave to an exactly similar point.

Frequency is the number of waves that pass a point in one second. This depends on how fast the source of the waves is vibrating. The
frequency is usually expressed in hertz (Hz) where one Hz is one cycle (wave) per second.

Wave speed depends on the medium that the wave is travelling through:

distance wave travels = wave speed × time (d = v × t )

The wave equation relates the wavelength and frequency to the wave speed. For all waves: wave speed = frequency × wavelength (v = f λ where f is in Hz, λ is in m, v is in m/s).

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Describing Longitudinal and Transverse Waves

wave is a vibration or disturbance transmitted through a material (a medium) or through space. Waves transfer energy and information from one place to another, but they do not transfer material.

transverse wave has vibrations at right angles (perpendicular) to the direction of travel. The wave has crests and troughs.

Examples include water waves, waves on strings or rope, light and other electromagnetic waves.

longitudinal wave has vibrations parallel to the direction of wave travel. It has compressions and between these are stretched parts called rarefactions.

There are two types of seismic waves called P-waves and S-waves:

  • P-waves are longitudinal waves that travel through solid or liquid rock. They travel faster than S-Waves.
  • S-waves are transverse waves that can only travel through solid materials in the Earth.
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Sound Waves

For sound and ultrasound waves:

  • The pitch of the sound is the frequency of the vibrations.
  • The loudness of the sound depends on the amplitude of the vibrations

Sound travels much faster in solids than in liquids and faster in liquids than in gases.

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Electromagnetic Spectrum

The Electromagnetic spectrum consists of 7 types of waves which are:

  • Gamma radiation
  • X-ray radiation
  • Ultraviolet radiation
  • Visible radiation
  • Infrared radiation
  • Microwave radiation
  • Radio waves
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Radio waves and Microwaves

Radio Waves - are produced when an alternating current flows in an aerial. They spread out and travel through the atmosphere.

Another aerial is used as a detector. The radio waves produce an alternating current in it, with a frequency that matches that of the waves.

Microwaves - transmitted through glass and plastics. They are absorbed by water, though how well depends on the frequency (energy) of the microwaves.

Microwave ovens use a microwave frequency which is strongly absorbed by water molecules, causing them to vibrate, increasing their kinetic energy. This heats materials containing water, for example, food. The microwaves penetrate about 1 cm into the food. Conduction and convection processes spread the heat through the food.

Microwave oven radiation will heat up our body cells and is very dangerous at high intensity because it will burn body tissue. The radiation is kept inside the oven by the reflecting metal case and metal grid in the door.

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Use of Electromagnetic Waves

  • Wavelength of the ElectroMagnetic spectrum continually changes
  • high frequency = short wavelength
  • high frequency = high energy
  • high energy = more dangerous

Radio Waves (communications)

  • TV and FM radio (short wavelength)
  • Direct line of sight with transmitter (do not diffract)
  • Medium wavelength – travel further because they reflect from layers in the atmosphere

Satellite signals (Microwaves)

  • Frequency of microwaves pass easily through atmosphere and clouds

Cooking (Microwaves)

  • Microwaves are absorbed by water molecules.
  • These water molecules become heated > heat food
  • Dangers: microwaves are absorbed by living tissue Internal heating will damage or kill cells
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Uses of Electromagnetic Waves

Ultraviolet

  • Dangers:
    • Over-exposure to UVA and B damages surface cells and eyes and can cause cancer.
      • There is a problem with current sunscreens which protect against skin burning from high UVB but give inadequate protection against free radical damage caused by UVA.
      • Dark skins are not necessarily safer from harm.
      • Sun exposure for the skin is best restricted to before 11 am and after 3 pm in the UK in summer months.
  • Benefits:
    • Sanitary and therapeutic properties have a marked effect on architecture, engineering, and public health and have done so throughout history.
    • UVC is germicidal, destroying bacteria, viruses, and moulds in the air, in water, and on surfaces.
    • UV synthesises vitamin D in skin, controls the endocrine system and is a painkiller.
    • Used in state of the art air-handling units, personal air purifiers and swimming pool technology.
    • Used to detect forged bank notes: they fluoresce in UV light; real bank notes don’t. Used to identify items outside visible spectrum areas, known as 'black lighting'.
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Uses of Electromagnetic Waves

Infrared Radiation (remote controls, toasters)

  • Any object that radiates heat radiates Infrared Radiation
  • Infrared Radiation is absorbed by all materials and causes heating
  • It is used for night vision and security cameras as Infrared Radiation is visible in daytime or night-time 
  • Police use it to catch criminals, the army use it to detect enemy
  • Dangers: damage to cells (burns)

X-rays

  • X-rays detect bone breaks
  • X-rays pass through flesh but not dense material like bones
  • Dangers: X-rays damage cells and cause cancers. Radiographer precautions include wearing lead aprons and standing behind a lead screen to minimise exposure

Gamma Rays

  • Gamma Rays cause and treat cancers
  • In high doses, gamma can kill normal cells and cause cancers. Also used to kill mutated cells 
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Reflection

All waves can be reflected, but this does not prove they are waves because particles also show these effects. According to the law of reflection, the angle of incidence equals the angle of reflection. This can be shown on a wave diagram or a ray diagram. Echoes are reflections of sound waves from hard surfaces.

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Refraction

When waves enter a different medium they change speed. The wavelength changes, but the frequency stays the same. They change direction unless they are travelling along the normal to the boundary. This is called refraction.

As light waves enter a denser medium they slow down and bend towards the normal. As light waves enter a less dense medium they speed up and bend away from the normal. Water waves slow down as they go from deep water to shallow water.

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Ultrasound

  • Ultrasound is sound with very high frequency 
  • Frequency above 20kHz - inaudible
  • High frequency = short wavelength

Uses of Ultrasound

1. Industrial Cleaning

  • Ultrasound can be used to clean delicate equipment.
  • Short ? = narrow, focused beam.
  • Vibrations remove dirt effectively by “shaking” dirt loose.
  • No need to dismantle the equipment.

2. Industrial Quality Control

  • Ultrasound waves can detect cracks inside metals.
  • When a sound wave travels from one substance to another, some waves are reflected back as echoes.
  • The reflected waves (echoes) are detected by a computer.
  • An echo = a flaw in the metal > a crack has formed.
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Uses of Ultrasound

3. Pre-Natal Scanning

  • X-rays can be used to see inside the body - (unsafe for a baby)
  • Ultrasound can create images and is safer.
  • Passes through new substance (skin, muscle, bone) > waves are reflected as echoes.
  • The reflected waves (echoes) are detected by a computer.
  • These build up a picture from each echo.

4. SONAR

  • Bats produce ultrasound squeaks > pick up the reflections using their big ears.
  • Brain processes the reflected signals into a mental picture of the surroundings.
  • Ships use SONAR to detect items on the seabed.
  • The pattern of the reflections indicates the depth and shapes.
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Seismic Waves

seismic waves

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Seismic Waves

P waves (primary)

  • They are longitudinal so they cause the ground to move up and down
  • They pass through solids and liquids
  • They go faster through more dense material

S waves (secondary)

  • They are transverse so they cause the ground to move sideways
  • They ONLY pass through solids (hint: s wave = solids)
  • They go faster through more dense material
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Visible Light

When electromagnetic radiation from the Sun arrives at the top of the atmosphere some of it is reflected, some is transmitted to the Earth’s surface and some is absorbed by the atmosphere.

The electromagnetic radiation that arrives at the Earth’s surface is:

  • High energy infrared.
  • Visible light.
  • Low energy ultraviolet.

Some of the ways visible light is used include:

  • By our eyes when images are formed on the retina at the back of the eye.
  • Photography when images are formed on the light-sensitive film in a camera.
  • Digital photography when images are formed on the light sensitive screen and stored electronically.
  • Plants use light as an energy source for photosynthesis.
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Visible Light

The human eye is similar to a camera. In both, light rays pass through a small hole and produce a small upside down image. A lens can be used to gather and focus more light.  The image is formed on a light-sensitive surface:

  • On photographic film, a chemical reaction occurs which changes the colour of the film.
  • On the cells of the retina, a chemical reaction causes electric signals to travel along the optic nerve to the brain.
  • On the screen inside a digital camera, electronic signals are sent to the memory. The amount of information to store the picture is measured in bytes.
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