electricity Flashcards
relationship between current through resistor and potential difference
it’s directly proportional, resistance does not increase if current increased.(ohmic conductor) but this is only true when temperature is constant
why are resistors used?
to control potential difference in circuits
relation between current through filament lamp and potential difference
it is not directly proportional,
- bcs filament gets hot which increases resistance
-bcs atoms in filament vibrate more, electrons in current now collide more with the atoms,
-more energy needed to push current thru fil
what is resistance?
anything in circuit opposing flow of current
what is the difference between direct and alternating current?
dc current only flows in one direction.
-the flow of ac current is constantly changing. the mains supply for ac is at 230V and 50 Hz per second.
why are step up transformers used?
-when elec is transmitted, energy lost thru heating in cables, higher for high current.
-trans elec at high voltage reduces current for the same amount of power,
-so step up transformers are used to increase voltage, decrease current and reduce energy loss
explain why electricity is transmitted at a high pd
-to trans high power, either high pd or high current is needed. high current is less efficient as it wastes more energy as thermal from wires heating up. trans at high pd means lower current can be used which reduces energy loss meaning trans more efficient
high power trans= +pd OR +current => +energy loss,,,,, +pd= -current= - energy loss= +efficient
why can a large charge on an isolated object result in a spark
-large charge causes large pd betw obj and earth, pd can get large enough for spark to jump betw obj and an earthed conductor
explain with ref to elec fields how car gives man electric shock when putting hand near it
-car has a large static charge, pd betw car and earth is large.
-this causes strong electric field, which ionises the air particles around, so air can conduct elec,
-man is an earthed conductor, so spark jumps across gap from car to man
what is charge
measure of total current that flowed within a certain period of time
In a series circuit, the current is:
In a series circuit, the potential difference of the battery is:
-same everywhere
-shared across components
in a parallel circuit, every component has the same …. and shares the ….
-p.d.
-current
in a parallel circuit, loops w/ greater resistance, take a ……. …. of current
lower share
more components in a parallel circuit…
less total resistance
step up transformers increase….. and decrease ….. to minimise …..
-voltage / p.d.
-current
-energy loss
the uk mains supply is
230 V and 50 Hz, alternating current
state the three wires colours and functions
-live, brown
provides the alternating p.d. (230V)
-blue, neutral
acts by completing the circuit and carries away current, has a p.d. of 0V
-green yellow, earth
stops appliance from becoming live in case live wire touches casing, by providing alternate pathway for current to flow
what causes an electric shock
a difference in voltage causes electricity to pass from one to another
humans have a p.d. of 0V so touching live wire will cause a large current to flow through
a sudden increase in current is called a
surge
could happen when you turn an appliance on or off
or because of a fault in the appliance
can damage appliance, cause fires, electric shocks
how do fuses/ circuit breakers promote safety
-breaks the circuit when current gets too high
-very thin piece of wire that’s connected to live wire
-if there’s surge, huge amount of current will flow through the live wire and fuse
-fuse heats up and melts, breaks circuit
benefit and disadvantage of fuses
simple and cheap
but permanently broken after a single surge, have to be replaced each time
circuit breakers vs fuse
-CB doesn’t get perm. damaged
-it just gets tripped and turns off circuit
-can be reset
-more expensive than fuses
what is double insulation
-appliance is covered in plastic coating
-no exposed metal parts
-plastic doesn’t conduct elec so no shock
-these appliances usually only have live & neutral wire
what is static electricity
build up of charge on insulating materials
what happens when two materials are rubbed against each other
-in terms of insulating and conducting materials
the electrons form one material rub off to the other material
-in conducting materials the electrons are able to move, so can just flow straight back again, thr4 no charge ever builds up (can but it is very uncommon)
-in insulating materials electrons not able to flow back so one is left with a positive static charge and other with negative static charge
in static charges, only the ……. are being transferred
electrons
what happens when an object keeps on gaining negative electrons
-size of the charge increases
-p.d build up betw obj and any earthed obj (any object with 0V, such as humans)
-if the p.d is big enough, electrons can jump betw the gap, causing a spark
the field lines in a positive and negative particle
-goes into the negative
-comes out of positive
what could happen when there is a strongly charged piece of metal surrounded by air
-air is an electrical insulator and doesn’t conduct electricity
-however, strong electrical field, like the one around metal can cause surrounding air particles to lose electrons and form positive ions, (ionisation)
-once air ionised, it can conduct electricity
this is how sparks are able to go betw objs, travelling through the air
IV Characteristics practical
-current, p.d vary resistance, temp, switch, graph
- measure the current in R using the ammeter
- measure the p.d. across R using voltmeter
- vary resistance of variable resistor
- record a range of values of current and p.d.
- ensure current low, avoid temp increase
- switch circuit off betw/ readings
- reverse connection of R to power supply
- repeat measurements of I and V in negative direction
- plot a graph of current against p.d.
what is the error regarding eye positioning called
Parallax Error
strong magnetic fields can ionise…
air around them, causing sparks
