PHYSICS: WAVES
Overview- A wave is a disturbance that travels across space and time
- Propagation of waves usually involves transference of energy without transferring mass. This is achieved by oscillations or vibrations around fixed locations
- Mechanical waves require a medium for transmission (e.g. sound)
- Electromagnetic waves do not require a medium and can travel in vacuum (e.g. light)
- Longitudinal waves are those with vibrations parallel to the direction of wave propagation. E.g. sound waves
- Transverse waves are those with vibrations perpendicular to the direction of travel. E.g. electromagnetic waves including light
- Waves on a string are an example of transverse waves
Properties of waves
- Reflection: It is the change in direction of a wave at the interface between two media. Examples include reflection of light, sound etc
- Refraction: It is the change in direction of a wave due to a change in its speed. Examples: refraction of light when it passes through a lens
- Diffraction: Bending of waves as they interact with obstacles in their path. Example: rainbow pattern when light falls on a CD or DVD
- Interference: Superposition of two waves that come into contact
- Dispersion: the splitting up of waves by frequency
- Polarization: the oscillation of a wave in only one direction. Exhibited only by transverse waves (like light), not exhibited by longitudinal waves (like sound)
Wave properties in everyday life
- The floor of a lake or the ocean appears closer than it actually is. This is because of refraction of light
- The red ring around the Sun is due to diffraction of light
- We can hear but not see across corners, this is because of diffraction of sound (e.g. we can hear but not see a person in the next room)
- The rainbow and the blue colour of sky are both due to dispersion of light
- Sunglasses use polarization filters to block glare
SOUND WAVES
About Sound- Sound is a mechanical wave that is transmitted as longitudinal waves through gases, plasma and liquids. However, in solids it can travel as both longitudinal and transverse waves
- Sound cannot travel in vacuum, it needs a medium for propagation
- The speed of sound in air is 330 m/s
Perception of sound
- The frequency range 20 Hz to 20 MHz is known as the audible range, where human beings can detect sound waves
- The upper frequency limit decreases with age i.e. as we get older, our ability to detect higher pitches (shrills) decreases
- Other species uses different ranges for hearing. E.g. dogs can perceive frequencies higher than 20 KHz
- Increased levels of sound intensity can cause hearing damage. Hearing can be damaged by sustained exposure to 85 dB or by short term exposure to 120 dB sound. A rocket launch usually involves about 165 dB
Sonar systems
- Sound Navigation and Ranging is a technology that uses sound propagation for navigation and communication
- Primarily used under water because light attenuates very quickly in water whereas sound travels farther
- First developed by R.W. Boyle and A.B. Wood in 1917 in Britain
- Applications include military, fisheries, wave measurement, ocean-floor mapping etc
- Sonar is used by marine mammals (like dolphins and whales) for communication as well
- Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range)
ELECTROMAGNETIC WAVES
Electromagnetic Spectrum
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Electromagnetic radiation and applications
| Radiation | Applications |
| Radio waves | RADAR, TV, cell phones, microwaves |
| Microwaves | Wi-Fi |
| Infrared (IR) | Night vision, thermography, imaging |
| Visible light | Sight |
| Ultraviolet (UV) | Sun burn, water disinfection |
| X-rays | Astronomy, medicine |
| Gamma rays | PET scans, cancer therapy, astronomy, food sterilization |
Radar systems
- Radio Detection and Ranging is a technology that uses radio waves to identify moving and fixed objects
- Developed by Robert Watson-Watt in 1935 in Britain
- Radar works by measuring the waves that are reflected back from an object. Radar can detect objects at ranges where sound or visible light would be too weak
- Applications include aircraft detection, air traffic control, highway speed detection, weather detection etc
More about electromagnetic waves
- Radio waves are reflected by the ionosphere and hence can be received anywhere on the earth.
- TV transmission penetrates the ionosphere and hence is not received like radio waves. Thus TV transmission is limited to line-of-sight
- At night, the radio reception improves because the ionosphere is not exposed to sunlight and hence is more settled
- Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range). Other animals like dolphins and whales use SONAR as well
PHYSICS: HEAT
Overview
- Heat is the process of energy transfer from one system to another
- Units of heat: Joules (J), Calories, British Thermal Unit (BTU)
- Temperature is a measure of internal energy (enthalpy)
- Heat transfer can happen spontaneously only from a warmer to a colder body. Reverse heat transfer can only happen with the aid of an external source such as a heat pump.
- Conduction is the most significant heat transfer mechanism in solids. It occurs as hot high energy molecules interact with neighbouring and transfer heat to them. Eg: heat transfer from one end of a metal rod to another
- Convection is most significant in liquids and gases. This happens when hot molecules move and transfer energy to other molecules. Eg: boiling of water. When water is heated on a stove, hot water from the bottom rises and displaces colder liquid which falls.
- Radiation is the only form of heat transfer possible in the absence of a medium. Heat is transferred in the form of electromagnetic radiation. Eg: heat from the sun reaching the earth.
- Copper is used in construction of boilers and cooking utensils because it is a good conductor of heat
- Air is a poor conductor
- Wool and cotton are good insulators i.e. poor conductors. Their insulation arises mainly due to air spaces between molecules
- Double-walled glass doors with air between them are better insulators than windows with a single thick glass layers
- Eskimos live in snow huts because snow is a poor conductor of heat, and hence protects them from the extreme cold outside.
- Land and sea breeze, ocean currents are arise due to convection
- The boiling point of water at sea level and atmospheric pressure is 100C. When extra heat is added, it changes the phase of water from liquid to gas (water vapour).
- Thermometers can be divided into two groups:
- Primary thermometers: measure temperature directly based on the property of matter. They are relatively complex and not used commonly. Eg: thermometers based on velocity of sound in gas, thermal noise of an electrical resistor etc.
- Secondary thermometers: measure temperature relative to a pre-calibrated quantity. They are easy to use and used commonly. Alcohol thermometer, mercury thermometer, medical thermometer are all secondary thermometers
- Primary thermometers: measure temperature directly based on the property of matter. They are relatively complex and not used commonly. Eg: thermometers based on velocity of sound in gas, thermal noise of an electrical resistor etc.
- In cold winter places, alcohol thermometers are used instead of mercury thermometers because the freezing point of alcohol is lower
- For extra-low temperature measurements (-200 C), Pentane is used
- Water is not suitable for use in thermometers because it freezes at 0 C and has irregular expansion
- Mercury is used for common medical thermometers because
- It does not cling to glass and hence reading is easy
- It is opaque and easily seen
- Its expansion is uniform and hence calibration is easier
- It is a better conductor of heat than alcohol and hence responds more rapidly to changes of temperature
- It has low specific heat capacity and hence is more sensitive
- It does not cling to glass and hence reading is easy
- Solar cooker: is a box made of insulating material such as wood, cardboard etc. The box has a glass cover to retain heat inside by greenhouse effect. The inside of the box is painted black to increase heat absorption.
- Pressure cooker: Pressure cooker increases the boiling point of water by increasing pressure. When the boiling point of water increases, food cooks faster. Pressure cookers are especially essential in hill stations because at higher altitudes the boiling point of water decreases due to lower atmospheric pressure
- Refrigerator and Air-conditioner: are heat pumps that transfer heat from inside to the external environment. They use a refrigerant which is a compound that undergoes reversible phase change from gas to liquid. Common refrigerants include ammonia, sulphur dioxide, carbon dioxide and methane. The use of chlorofluorocarbons has been phased out due to concerns regarding depletion of the ozone layer.
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