unit two: electricity Flashcards
current unit
amperes (A)
charge unit
coulomb (C)
resistance unit
ohm
voltage unit
volts (V)
power unit
watts (W)
purpose of the earth wire
to protect you if an appliance develops a fault by. providing a path fo current to escape through the user
voltage supplied to homes in the UK
230 V
what is a fuse
a cylinder or cartridge whcih obtains a thin piece of wire made from a metal with a low melting point
what is the fuse there for
if there is too large a current then the fuse wire becomes very hot and melts. the circuit is now incomplete and reduces the chance of getting a shock or electrical fire
size of fuses
3A
5A
13A
why do many appliances have a metal casing
if the live wire becomes damaged or breaks and comes into contact with the casing the earth wire provides a low resistance path for the current, which would probably be big enough to blow the fuse and turn the circuit off.
where should a switch always be placed and why
in the live wire so when the switch is o pen no energy in the form of electricity can reach an appliance. if the switch is placed in the neutral wire, energy can still enter a faulty appliance and possibly cause an electric shock
if we want wires to become warm (heating elements) what do they have
high resistance
if we dont want wires to become warm what do they have
low resistance
why will a 70J bulb be brighter than a 50@ oe
it transfers 70J of electrical energy every second as opposed to 50J
what is power measured in
joules per second or watts
1kW is
1000W
power (W) =
P =
current (A) x voltage (V)
I x V
energy (J) =
E =
power (W) x time (s)
P x t
direct current
current/voltage that isi always in the same direction and have the same value. on a graph it would be a straight line
alternating current
an electric current that reverses its direction many times a second at regular intervals, typically used in power supplies.
what is an electric current
a flow of charge
electrons flow through all metals , so metals are
conductors of electricity
voltage (V) =
V =
current (A) x resistance (ohms)
I x R
what usually happens in a conductor
the electrons are free to move between the atoms. the number of electrons flowing in any one direction is equal to the number of electrons flowing in any other direction.
what happens in a conductor when a cell or battery is connected across a conductor
the electrons start flowing away from the negative terminal and to the positive terminal. now there is a net flow of charges or electric current.
how do you measure current
using an ammeter
the size of an electric current indicates…
the rate at which the charge flows
if there is a current of 1A then it means
1C of charge is passing along the wire each second
charge (C) =
Q =
current (A) x time (s)
I x t
what is a volt
a joule per coulomb
energy (j) =
charge (c) x voltage (v)
Q x V
how do you measure volts
using a voltmeter
series circuit
only one path the current can take
parallel circuit
multiple paths and junctions that the current can take
if one bulb in a series breaks,
it causes a gap int he circuit and all of the other bulbs will stop working
the energy in a series circuit is shared between all the bulbs, so the more bulbs you add,
the dimmer they all become
if one bulb in parallel breaks,
only bulbs in the same branch of the circuit break so not all of the bulbs will break
in a series circuit how is the current distributed
the current is the same in all parts and not used up
in a parallel circuit how is the current distributed
the current will be different in different parts of the circuit,
voltage (V) =
V =
current (A) x resistance (ohms)
I x R
practical: how to investigate how current varies with voltage with different components
have a circuit with a switch, variable resistor , voltmeter, ammeter and piece of wire. turn up the variable resistor to maximum and close the switch to take readings from the. ammeter and voltmeter. alter value of variable resistor or and take a new reading. repeat at least six times
on a current vs voltage graph what does a flat slope indicate
a high resistance
what does a fixed resistor do
to control the sizes of currents and voltages. without it the voltage across the bulb may cause too large a current and the bulb will break
how is a variable resistor different to a fixed resistor
you can vary the resistor, allowing the bulb to shine more brightly or less brightly.
what is a thermistor
a resistor whose resistance changes quite.a lot even with small changes in temperature
what is a light dependent resistor
it has a resistance that changes when light is shone on it. in the dark its resistance is high
where are thermistors used
temp sensitive circuits like fire alarms
where are LDRs used
automatic lighting controls and burglar alarms
what are diodes
special resistors that allow charges to follow through them in one direction.
when diodes are used in rectifier circuits what do they do
they convert alternating current into direct current
PRACTICAL: investigate how insulating materials can be charged by friction
- Before beginning the experiment, ensure the plate of the electroscope is uncharged (you
can do this by touching it with your finger). The leaf should hang straight down next to the
stem. - Charge up either a perspex or polythene rod by rubbing it with a cloth.
○ The polythene rod should become negatively charged as the cloth deposits
electrons onto its surface.
○ The perspex rod should become positively charged as the cloth removes electrons
from its surface. - Hold the rod near the plate of the electroscope and the leaf should be repelled from the
stem, showing that the rod is charged.
conclusion of the practical: investigate how insulating materials can be charged by friction
The charge from the rod is transferred to the metal plate and travels down the stem and leaf of the
electroscope. The stem and leaf therefore carry the same charge and repel each other.
PRACTICAL: sticking a balloon to a wall
- Take the inflated balloon and hold it against the wall for a few seconds and then let go of it.
If it is uncharged it should fall to the ground. - Using the same balloon, rub it vigorously with a cloth or against your hair
- Once again hold it against the wall and release it. If it has been successfully charged, it will
stick to the wall. - Electrons have rubbed on or off the balloon, giving it an overall charge. This overall charge
repels the same charges on the wall but attracts the opposite charges. It is this attraction of
opposite charges that causes the balloon to stick.
PRACTICAL: deflecting a stream of water
- Rub the polythene rod with the cloth so that it becomes charged.
- Run the tap so that there is a strong stream of water and then hold the charged rod
alongside the stream. - The water should deflect. This is because the charged molecules in water are attracted or
repelled according to the charge of the rod.
is current the same in all parts of a series circuit
yes there is only one route for the electrons to flow so the current must be the same in all parts of a series circuit.
is voltage the same in all parts of a series circuit
no, the energy supplied by cell is ‘shared’ between the components so the voltage across each component will vary. However, the voltages across each component must sum to the voltage of the cell
is current the same in all parts of a parallel circuit
no, there is several routes for the electrons to flow so the current will vary in the different branches, although the currents in the branches must sum to the current in the main circuit.
is voltage the same in all parts of a parallel circuit
yes each component has its own connections to the cell so does not have to ‘share’ the energy. The voltage across each branch in the circuit will be the same.
when does the resistance of a thermistor increase
when temperature increases
what would happen if the current flowing through a diode in a circuit was reversed
the current would not flow anymore
Explain the meaning of insulated wire
covered with a non conductor
Explain the meaning of a 5A fuse
a thin wire that melts when theres a current in excess of 5A to break the circuit
Explain the meaning of earthed
outside case made of metal. Direct connection to ground
Explain the meaning of double insulated
outside case made of an insulator so cannot get a shock from it
why type of energy does coal use to generate electricity
thermal energy
electrons are __________ charged
negatively
protons are ___________ charged
positively
if an atom is neutral/no overall charge what must it have
equal numbers of protons and electrons
what are electrical insulators
substances that dont allow electricity to flow through them
what are electrical conductors
materials that allow electricity to flow through them
e.g. of electrical insulators
plastic, rubber, glass and wood
e.g. of electrical conductors
copper, graphite, tin and gold
if two charged objects have similar charges
the objects repel eachother
if two charged objects are oppositely charged
they attract eachother
explain the force between charged and uncharged objects (like a balloon)
after the balloon has been charged with static electricity, (before being close to the wall), the charge is evenly distributed. the wall is uncharged the balloon is negative. this means that some of the balloons electrons will be repelled from the wall’s surface. this gives the surface of the wall a slight positive charge that attracts the negatively charged baloon.
how does static electricity help electrostatic paint spraying
as the drops of paint emerge from the spray gun, they are charged. as the drops all carry the same charge they repel and spread out as a thin spray. the metal object has a wire attached to it with an electrical supply that attracts the paint drops
how does static electricity help inkjet printers
each spot of ink is given a charge. when it falls between a pair of deflecting plates, electrostatic forces direct it to the right position. the charges on the plates change hundreds of times a second, so that each drop falls in a different position, forming pictures and words on the paper.
how does static electricity help photocopiers
positive charges are sprayed onto a turning drum whose surface is covered with selenium (metal). a bright light is shone on to the sheet of paper to be copied. the white parts of the paper reflect the light, causing the selenium to lose its charge but dark parts do not. a negatively charged carbon powder called toner is blown across the drum, and sticks to just those parts of the drum that are charged. a sheet of paper is now pressed against the drym and picks up the pattern of the carbon powder. the powder is then fixed in place by a heater.
how does static electricity help electrostatic precipitators
mant heavy industrial plants produce a lot of smoke. electrostatic preciptators can reduce the risks of these to humans and the environmenta and to buildings. as the smoke intially rises, it passes through highly charged wires. (-50,000 V). as they pass through, the ash and dust particles become negatively charged. higher up the chimmney, these charged particles are attracted by and stick to, large metal earthed plates. the cleaner smoke si then released. when the earthed plates are completely covered in dust and ash they are tapped hard, and all the dust and ash falls into a collection box to be emptied.
how many tonnes of dust and ash can be removed from the smoke each hour in large coal fired power stations
50-60 tonnes
problems with static electricity
friction causes aircrafts to become charged with static electricity. after landing, there is the possibility of charges escaping to earth as a spark of flash of electricity. if this takes place during refuelling it could cause an explosion.
solution to the gain of static electricity by aircraft/fuel tankers
earth the plane with a conductor as soon as it lands, and before refuelling, allowing the charge that has built up to flow to earth.
