Electrostatic and Mains Electricity Flashcards
when we rub a balloon against our sweater then put it to the wall, why does it stick?
because the balloon gets a negative charge, and the negatively charged particles repel in the balloon and wall, so overall they get attracted to each other as the part of the wall closer to the balloon is more positively charged
what kind of material can be charged through friction?
insulating material
rules of charges
- every object has positive and negative charges
- like charges repel, unlike charges attract
- only negative charges move from one object to another
why doesn’t a metal rod hold onto its charge?
because it isn’t insulating and conducts electricity
what is a fuse
thin piece of metal with a low melting point usually inside a cartridge, current rating has to be just above the normal working current of the appliance
what does a fuse do?
it’s a safety device, it melts if the current becomes too high and so breaks the circuit, preventing an electrical fire
what is a plug composed of?
earth wire, live wire, neutral wire, fuse, outer insulation, cable grip
what is double insulation?
some appliances (e.g. vacuum cleaners and electric drills) do not have an earth wire; because they have plastic casings, or they have been designed so that the live wire cannot touch the casing (but still has a fuse)
what does double insulation do?
the casing cannot give an electric shock, even if the wires inside become loose
what is static electricity?
means that electricity stays on the material and doesn’t move, builds up when electrons are rubbed off one material onto another
live wire
brown
netural wire
blue
earth wire
green and yellow stripes, a safety wire to stop the appliance becoming live
what type of appliances must be earthed and how?
all electrical appliances with outer metal cases, by connecting it to the earth pin in the plug through the earth wire
what is current?
the rate of flow of charged particles
equation of charge, current, time
charge = current x time
Q = I x t
unit of charge
coulomb
unit of current
ampere (amp)
good electrical conductors
metals: gold, silver, copper, aluminum, steel
poor electrical conductors/ insulators
rubber, plastic, glass, wood, marble
potential dangers of electrostatic charges (high voltage cables)
massive charge on cables compared to an earthed object nearby, the charge can “jump” across the gap in suitable conditions, contact is usually fatal with severe burns
potential dangers of electrostatic charges (electric shock)
heart and muscles are affected, causing possible muscle “spasm” or death
potential dangers of electrostatic charges (filling aircraft fuel tanks)
during refueling the fuel gains electrons from the fuel pipe, making the pipe positively charged and the fuel negatively charged, the resulting voltage between the two can cause a spark
prevention for the dangerous filling of aircraft fuel tanks
earth wire/ tanker and plane can be connected by conductor
when does discharge occur?
if the voltage (potential difference) between an isolated object and a nearby earthed conductor become high enough, the air molecules could ionise and result in a discharge spark
how do photocopiers work?
the image of the document is projected to a positively charged copying plate -> where the light falls onto the plate, the electricity charge leaks away -> negatively charged black toner particles are attracted to remaining positive areas -> paper is placed over copying plate, toner is transferred to the paper and heated to make it stick
maximum current before the wire melts (affected by the thickness of the wire)
the thicker the wire, the higher the maximum current before the wire melts, but the rate gradually gets less
how do circuit breakers work?
an electromagnet separates a pair of contacts as the soft iron cylinder is not attracted to it and its position keeps the contacts in position. when the current becomes high enough, the soft iron cylinder gets attracted to the electromagnet, and the contacts spring up so the circuit is incomplete
benefits of circuit breakers
work more quickly than a fuse, are easily reset by pressing a button
direct current
flows in one direction from the positive terminal of a battery to the negative terminal
alternating current
changes direction; the mains electricity flows one way and back again (e.g. 50 times a sec would be 50 hz)
cell symbol
—| I —
battery symbol
—|I|I —
variable power supply symbol
—|I —|I —
what machine is current measured with?
ammeter
conventional current
flows from positive to negative
what is ac supplied by?
mains electricity (the electrical supply coming from a power station to the household you live in)
what is dc supplied by
cell or battery
