Physics P4

Physics P4

1. P4a

   • You charge an insulating material by rubbing it with a cloth
   • the two types of charge are positive and negative
   • the transfer of electrons cause electrostatic effects
   • some charged materials attract others such as small pieces of paper to a comb
   • like charges repel and unlike charges attract
   • you can get a shock from
     • synthetic clothing
     • earthing yourself after becoming charged
   • static electricity in terms of movement of electrons
     • negative = gain
     • positive = loss
   • the chance of shock can be reduced by
     • earthing
     • insulating mats
     • insulated footwear

2. P4b

   • defibrillator
     • A defibrillator is a machine that can be used by paramedics to stabilise an irregular heartbeat. They work by discharging electric charge.
     • Two paddles with insulated handles are charged from a high voltage supply. They are put in good electrical contact with the patient's chest. It is important that only the patient gets a shock:
     • The defibrillator passes charge through the patient to make the heart contract.
   • dust precipitators
     • Many power stations burn fossil fuels such as coal and oil. Smoke is produced when these fuels burn. Smoke comprises tiny solid particles, such as unreacted carbon, which can damage buildings and cause breathing difficulties. To avoid this, the smoke is removed from waste gases before they pass out of the chimneys. The electrostatic precipitator is the device used in chimneys for this job.
     • Smoke particles pick up a charge as they pass the charged metal grid. These smoke particles are attracted to oppositely charged collecting plates.
   • paint spraying
     • The paint spray goes past a high voltage positive needle
as it leaves the spray gun and the tiny droplets of paint
pick up a positive charge. They do this by losing negative
electrons. It is only the electrons that can move.
     • This improves efficiency in two ways.
       • 1. The paint droplets spread out more as they leave the gun. This happens because they all get the same positive
charge and so they all repel each other. This is better than
coming straight out of the gun as the paint will cover
a wider area more evenly as shown in the picture below.
The same thing happens with insecticide crop spraying.
       • 2. The paint droplets are attracted to the negative
metal car body, and so less paint will be wasted
by landing on the floor or the walls of the paint shop.

3. P4c

   • resistors / variable resistors can change the current in a circuit
   • equation Resistance = voltage / current
   • live, neutral and earth wires
     • live is brown
     • Earth is Yellow and green
     • Neutral is blue
   • double insulated appliances do not need earthing
     • plastic casing cannot become live
   • how a wire fuse protects an appliance by splitting the circuit if the current becomes too high
   • fuses and circuit breakers prevent house fires and other simillar risks from occurring
   • a complete circuit is needed for current to flow
   • an earthed conductor cannot become live