# physics- Module 4

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## Electrons

-An atom consists of a small positively charged nucleus, surrounded by an equal number of negatively charged electrons.

-In a stable, neutral atom there are the same amounts of positive and negative charges.

-All electrostatic effects are due to the movement of electrons.

-Like charges repel, unlike charges attract.

-When a polythene rod is rubbed with a duster, electrons are transferred from the duster to the polythene rod, making the rod negatively charged. When an acetate rod is rubbed with a duster, electrons are transferred to the duster, leaving the acetate rod positively charged.

-In general an object has: a negative charge due to an excess of electrons. OR. A positive charge due to the lack of electrons. atoms or molecules that have become charged are called ions.

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## Electrostatic Shocks

-When inflammable gases or vapours are present, or there is high concerntration of oxygen, a spark from static electricity could ignite and cause an explosion.

-If someone touches something at high voltage, large amounts of electric charge may flow through their body to Earth. even small amounts of charge flowing through the body can be fatal.

-Static electricity can be a nuicanse but not dangerous. Dust and dirt are attracted to insulators such as T.V screens and clothing made from synthetic materials, these often cling to skin or other materials.

-Electric shocks can be avoided : if an object is likely to become charged, only build up of charge would be sent down the Earth wire. In a factory, where machinery is at risk of becoming charged, the operator stands on a rubber mat. Shoes with insulating souls are also given to workers.

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## Dust Precipitators

-Removes harmful partical from chimneys of factories and power stations. A metal grid or wires are placed in and given a large charge from a high voltage supply.

-Plates inside a chimney are oppositely charged to the grid...Like charges repel... dust particles repel away from the wires because they are attracted to the oppositely charged plates. At intervals, the plates are vibrated and dust falls down into a collector.

-The dust particles gain or lose electrons to become charged. The charge on the dust particles induces a charge on the earthed, metal plate... Opposite charges attract, so the dust goes to the plate.

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## Paint Spraying

-Static electricity is used in paint spraying. The spray gun is charged, paint particles become charged with the same charge, like charges repel, giving out a fine spray.

-The object being painted is given an opposite charge/ The object then recieves an even coat with limited paint wastage.

-If the object to be painted is not charged, the paint moves onto it, but the object becomes charged from the paint. Further paint drops repel away.

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## Difibrillators

-Difibrillation is a procedure to restore regular heart rythm by delivering a shock through the chest wall.

-Great care is taken to ensure the operator does not get a shock.

-If a difibrilator is switched on for 5 milli-seconds, the power can be calculated by: Power=energy/time= 400/0.005= 80,000 W

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## Resistance

-A variable resistor or rheostat, changes the resistance.

-Longer lengths of wire have more resistance

-Volatge is measured in Volts

Resistance = Voltage/ current

this formula for restsiance can be rearranged to find out:

Voltage(V)= I x R

Current (I) = V / R

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## Electrical Power

-The rate at which energy transfers is

Power = voltage x current

-Formula for electrical power: Power = v x c       and       Current = P / V

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## Live, Neutral and Earth wires.

-Live wire carries a high voltage around houses.

-Neutral wire completes the circuit, providing a return path for the current.

-Earth wire is connected to the case of the appliance to prevent it becoming live.

-A fuse contains a wire which melts, breaking the circuit if the current becomes too large. No current can flow, preventing overheating and further damage to the appliance.

-Earth wires and fuses stop a person recieving an electrical shock. As soon as the case becomes live, a large current flows into the earth and live wires and the fuse 'blows'.

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## Uses of Ultrasound

-When ultrasound is used to break down kidney stones, a high powered beam is aimed at the stones, the ultrasound energy then breaks down the stones onto tiny peices of crystal which is then released from the body naturally.

-When ultrasound is used in body scans, a pulse of ultrasound is sent into the body at each boundary at different tissues. Some ultrasound is refelected and the rest is transmitted. The  returning echoes are recorded and used to build an image of an internal structure.

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## Ultrasound body scans

-Ultrasound can be used for body scans becasue when ultrasound is reflected from different interfaces of the body, the depth of each structure is calcualated by: distance= speed x time. Knowing the speed of the ultrasound for different tissue types and the time for the echoe to return.

-The proportion of the ultrasound reflected at each interface depends on the densities of each of the ajoining tissues and the speed of sound in the tissue.

-If the tissues are very different (E.g. Blood/Bone) most of the ultrasound is refelected, leaving very little to penetrate further into the body. The imformation gained is used to produce an image of the part of the body.

-Ultrasound is preferred to X-rays because it's able to produce an image of soft tissue and it doesn't damage living cells.

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## Longitudinal Waves.

-Ultrasound is a wave above 20,000Hz. Which is a high frequency which humans cannot hear.

-It travells as a pressure wave containing compressions and rarefactions.

-Compressions are regions of higher pressure and  rarefactions are regions of low pressure.

-Longitudinal waves cannot travel through a vaccuum. The higher the frequency, the smaller the wave. the louder the sound, the more powerful ultrasound.

-In a longitudinal wave, the vibrations of the particles are parallel to the  wave.In a transverse wave the vibrations of the particles are at right angles to the direction of the wave.

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