physics

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  • Created by: Sophiw
  • Created on: 11-03-13 18:07

P4-1: ELECTROSTATICS

  • when some materials are rubbed together they become charged with static electricity and attract other materials
  • when insulating materials are rubbed together they become charged and is known as an electrostatic charge. Insulating matierials can be charged positively or negatively. The charge is caused by negative electrons being transferred from either material
  • if a polythene rod is rubbed with a woollen cloth then the electrons move from the cloth to the rod and the rod becomes negatively charged as electrons have a negative charge 
  • if a acetate rod is rubbed with a woollen cloth then electrons move from the rod to the cloth, and the rod becomes positively charged as it has lost electrons which are negatively charged
  • a negative charge means there are extra electrons, a positive charge means there is a lack of electrons
  • charged atoms/molecules are called ions
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P4-2: DANGERS OF STATIC ELECTRICITY

  • conductors like metals allow charge to move freely but insulators like polythene don't allow charged electrons to move freely so a charge builds up
  • you can become charged by wearing clothing made from synthetic material or walking over a floor covered in synthetic carpet. If you then touch a conductor that is connected to the earth such as a metal door handle the built-up charge is conducted away very quickly and you then get an electrostatic shock. You can also get a shock if you touch something that is charged as the charge is conducted to earth through you
  • static electricity can be dangerous is that is a charge is transferred between two objects a spark can jump across the gap between them. This spark can have a high temperature as could cause an explosions where glammables gases or vapours are being used 
  • static electricity also makes lightning, it builds up on clouds and a bolt occurs when a large amount of the charge flows quickly to the earth
  • you can reduce the chance of electrostatic discharge by connecting to the earth through a conductor so the charge does not build up in the first place
  • vehicles that contain flammable gases, liquids of powders have to be connected before loading / unloading
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P4-3: USING STATIC ELECTRICITY

  • electrostatic dust precipitators are used to remove smoke particles from waste gases coming out of chimneys. Inside the chimney there is a charged wire grid and when the dust passes through the grids the dust becomes charged.to make sure that lots of dust is collected the grids have a high voltage. Further up the chimney are metal plates on each side which are oppositely charged to the grids or are earthed. The charged dust particles are attracted to the plates and stick to them. Several times during the year the plates are struck by metal hammers and the dust falls.
  • a paint spray gun has a charge and the paint particles that pass through it when sprayed also get that charge. The object being painted has an opposite charge and the particles stick to it. Paint particles all have the same charge so the repel eachother making it a finer spray 
  • defibrillators are used to restart a person's heart if they have had a heart attack or an electric shock. Two paddles are electrostaically charged using a high-voltage supply and are places on the patient's chest and discharged. This charge passes through the patient's chest to make the heart contract and start beating again. The paddles have insulated handles so the operator does not get an electrostatic shock
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P4-4: CURRENT AND RESISTANCE

  • when an electric current flows around a circuit the current is a movement of charge. This charge is carried by electrons, the higher the current the higher the number of electrons moving past a point each second
  • resistors can be used to change the current in a circuit, variable resistors can change the resistance and hence the current in the circuit. Increasing the resistance decreases the current and decreasing the resistance increases the current. 
  • resistance can be changed by different wires; long and thin wires have a high resistance whereas short thick wires have a low resistance
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P4-5: MAINS ELECTRICITY IN THE HOME

  • every plug has a fuse with a rating which is connected to the live wire and if the current passing through exceeds the rating then the wire in the fuse heats up, melts and breaks the cicuit
  • the earth wire is connected to the metal case of the appliance so if the live wire came loose and touched the case it wouldn't become live as the current passes down the earth wire and surges the current to melt the fuse
  • circuit breakers can also be used. They are attached to the live wire but rather than heating up and melting the fuse they just break the circuit. They switch it off much faster and can easily be reset and used again
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P4-6: LONGITUDINAL WAVES AND ULTRASOUND

  • sound waves are series of oscillations or vibrations which travel through air in the same direction as the speaker movement. Sound waves are examples of longitudinal waves. Within longitudinal waves the oscillations are parallel to the wave movement
  • when a speaker cone moves out it creates a compression because the air is bunched up and there is high pressure, but when it moves back in it causes a rarefaction, where the air is more spread out and there is low pressure
  • like all waves longitudinal waves have wavelengths and frequency. The wavelength is the distance between each compression. The frequency is the number of compressions which pass a given point each second
  • particles in a longitudinal waves vibrate from side to side. If the frequency increases then the air particles vibrate more rapidly.
  • soundwaves above the frequency of a human ear [20,000 Hz] are called ultrasounds
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