Waves and energy

• Equilibrium


  Resting position of particles when not vibrating
• Frequency


  Number of waves passing a point per second
• Waves

  • All waves transfer energy but don't transfer matter
  • Waves are oscillating around a resting, equilibrium position
• Types of waves

  • Mechanical waves
  • Electromagnetic waves
• Mechanical waves


  Oscillations of particles in a solid, liquid or gas, require a medium to travel through
• Electromagnetic waves


  Cause oscillations in a magnetic and electrical field, can travel through a vacuum (no medium required), travel at the speed of light
• Wave period


  Time taken for one wave cycle
• Calculating wave speed


  Wave speed = Frequency x Wavelength
• Speed of sound: 330 m/s, Speed of light: 300,000,000 m/s
• Ripple tank practical

  1. Set up ripple tank with 5cm depth of water

  2. Adjust wooden rod to just touch surface of water

  3. Switch on lamp & motor, adjust until low freq waves seen

  4. Measure wavelength of a set number of waves

  5. Count number of waves passing a point in 10 seconds

  6. Calculate wave speed = Frequency x Wavelength
• Measuring waves in solids practical

  1. Attach cord to vibration generator

  2. Use 200g hanging mass and pulley to pull string

  3. Switch on vibration generator and adjust until stationary waves seen

  4. Measure set amount of waves and divide by how many measured

  5. Frequency = Frequency of power supply

  6. Calculate wave speed = Frequency x Wavelength
• Longitudinal waves


  Oscillations are parallel to the direction of the wave, examples include sound waves, ultrasound waves, seismic P-waves
• Longitudinal waves


  • Show areas of compression and rarefaction
  • Particles oscillate back and forth but do not move position
• Transverse waves


  Oscillations are perpendicular to the direction of travel, examples include electromagnetic waves and light, Mexican waves, seismic S-waves
• Electromagnetic waves


  • Oscillations are changes in electric and magnetic fields at right angles to the direction of wave travel
  • Transfer energy as radiation (from source to absorber)
  • Can travel through a vacuum
  • Travel at the speed of light (300,000,000 m/s)
• Electromagnetic spectrum


  • Radio waves
  • Microwaves
  • Infrared
  • Visible light
  • Ultraviolet
  • X-rays
  • Gamma rays
• Radio waves


  Produced by oscillations in electrical circuits, can be reflected to change direction, used for communication (TV, radio)
• Microwaves


  High frequency microwaves can be absorbed by molecules in food, causing heating
• Infrared


  Frequencies can be absorbed by chemical bonds, causing heating, all objects emit infrared
• Visible light


  Used in fibre optic communications, coded pulses of light travel through glass fibres
• Ultraviolet


  Has damaging effects on the body, causes skin to tan or burn, used in energy-efficient lamps
• Gamma rays and X-rays


  Produced by changes in atoms and nuclei, high energy waves that can pass through the body with little absorption, used for internal imaging
• Ionising radiation


  Gamma, X-ray and UV radiation that can remove electrons forming positive ions, has hazardous effects on the body
• Radiation dose

  Measure of the risk of harm caused by exposure to ionising radiation, measured in Sieverts (Sv) or millisieverts (mSv)
• Reflection


  Waves (including sound and light) are reflected, the angle of incidence equals the angle of reflection
• Specular reflection


  Reflection from a smooth, flat surface like a mirror, forms a virtual, upright image
• Diffuse reflection


  Reflection from a rough surface, light is scattered in all directions and no clear image is formed
• Refraction

  Change in direction of a wave at a boundary between two materials with different densities
• Wave speed slows down


  Wavelength decreases, frequency remains the same
• The Leslie cube practical investigates how the amount of infrared radiation absorbed by different surfaces affects the temperature rise
• What happens when light travels from air into glass at an angle

  1. Light speed and direction changes

  2. Light bends away from the normal

  3. Light wave slows down

  4. Wavelength decreases

  5. Light ray bends back to original direction as it leaves the glass
• As light goes from air into glass


  The angle of refraction is less than the angle of incidence
• Wave speed, frequency and wavelength change during refraction
• For a given frequency of light, the wavelength is proportional to wave speed
• If wave speed decreases, the wavelength will decrease
• Required practical: Investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface

  1. Place Leslie cube on heat resistant mat

  2. Fill cube with boiling water and place lid

  3. Leave for 1 min

  4. Use infrared detector to measure intensity of infrared radiation emitted from each surface
• No matter the temperature, all objects absorb and emit infrared radiation
• Good absorbers of infrared are also good emitters
• Infrared is absorbed by black, matte surfaces and reflected by white, shiny surfaces
• The temperature of a body is linked to the balance between the amount of radiation absorbed and emitted


  • If rate of absorption > rate of emission, temp increases
  • If rate of absorption = rate of emission, temp is constant
  • If rate of absorption < rate of emission, temp decreases