Topic 2 – Electricity Fundamentals Flashcards
in a circuit, the electric current flows from _____ to the____
negative end of the cell to the positive end
in a lamp, electrical energy is transferred to what?
to light energy and thermal energy
when electrons return to the positive end of the cell, they’re carrying ___ energy than when they left the negative end
less
whats the conventional current?
the oppositely drawn flow of charge (silly stupid thing)
what is current
the flow of electric charge around a circuit. the unit is aperes
in a series circuit, current is ______ all way round
the same
in a parallel circuit, current _____
in each branch ADD UP to the total current leaving the cell
the cell has a tranfer of_____ energy which is transfered to ______ energy
chemical
electrical
a potential difference of 1 volt tells us that:
1 joule of energy is transferred for each coulomb of charge that is moving through the circuit
placing 2 lamps in a series circuit would make them____
dimmer
how does voltage work in a series circuit
the total potential difference across both lamps is the SAME a the p.d across the cell. (NOT ALWAYS EQUALLY SHARED but split)
how does voltage work in a parallel circuit?
for component connected in parallel, the potential difference across each component is the same.
(2 lamps in 1 branch counts as series)
whats the voltage of 2 cells facing the right way and the wrong way?
the voltage cancels out, making 0
whats electrical charge measured in?
coulombs
a current of 1 ampere means what in coulombs
1 coulomb of charge flowing per second
the size of the elctric current is the rate of______
flow of an electrical charge
whats the equations for charge?
charge (Q) = current (I) x (t) time
whats the equation for energy transferred?
E = Q x V
what does resistance tell you
the potential difference required to drive a current through a component
whats the equation for resistance ?
R = V/I
current through a resistor is______ to potential difference
directly proportional
resistance is _____ . It _______ if we change the current
constant
does not change
resistance will only stay constant if:
temperature is constant
what do we call resistors which have a constant resistance?
ohmic conductors
the current through a. filament lamp is _____ to potential difference
not directly proportional
Why is current through a filament lamp not directly proportional to the potential difference
because the filament gets hot which causes resistance to increase.
at high temps, the atoms in the filament vibrate more so electrons in the current now collide more with atoms so more energy is needed to push the current through filament
as potential difference increases, the current______
the current does NOT increase as much
the current through a diode___
flows in one direction only
why does current through a diode flow i only 1 direction?
it has a v high resistance in the reverse direction
what are diodes useful for?
controlling the flow of current
what does an LED do when current flows through it?
gives off light
why are LEDs useful?
theyre extremely energy efficient sources of light
resistors in series___
add together because current must pass through each resistor and cant bypass any resistor
whats it called when we replace two resistors which add to 150 ohms with a 15oo ohm resisitor
equivalent resistance
the total resistance in parallel is less than:
the resistance of the smallest individual resistor
why is the total resistance in parallel less than the resistance of the smallest individual resistor?
because theres two pathways for the current to take so more total current will flow through the circuit
in parallel, if the current has increased but the potential difference hasn’t changed, what does that mean for the resistance?
total resistance must have decreased
whats the resistance like in an LDR in dark conditions?
high resistance
whats the resistance like in an LDR in light conditions?
very low
whats an example of an LDR
a mobile phone turning the screen off during a conversation
in the light, the resistance of the LDR is very low, meaning what?
What does that mean for the potential difference?
it takes v little energy for the current to pass through the LDR
its low
(LDR) because potential difference is shared between components in series, the potential difference across the lamp is_
large so lamp now lights up screen
explain how a phone screen lights up using an LDR
if held to a persons ear, LDRs in darkness, the resistance of the LDR rises sharply now it requires a great deal of energy for current to pass through ldr so potential difference across the LDR is v high meaning less electrical energy is available for the lamp. potential difference across lamp is v low meaning lamp becomes v dim
in a thermistor, the resistance of a thermistor decreases, if the temp___
increases
under cool conditions whats resistance like in a thermistor
The resistance of the thermistor is high meaning it takes lots of energy for the current to pass through , this causes high potential difference
how is a thermistor related to a fan in low temps
in low temps, The resistance of the thermistor is high meaning it takes lots of energy for the current to pass through, this causes a high potential difference,, bc the potential difference is shared, potential difference across fan is small meaning the fan operates at v low speed
how is a thermistor related to a fan in high temps
if the computer gets hot, the resistance of the thermistor falls now it takes much less energy for the current to pass through the thermistor meaning potential difference across the thermistor is now low so more electrical energy is available for fan making p.d across fan v high so fan powers to high speeds, cooling computer high
how does an incubator for premature babies work
if the temperature falls, this is sensed by the thermistor, this causes a lamp to turn on and an alarm to sound
what’s the method of the resistance required practical
attach the wire to a metre ruler using tape, and connect wire to rest of circuit using two crocodile clips (only wire between two clips is part of the circuit) move clips further apart and continue calculating resistance
in resistance required prac, the resistance of the wire is_____ to the length
directly proportional
what is a zero error
when a reading on a measuring instrument should be zero, this is a systematic error which means we cant reduce it by carrying out repeats
in the case of resistance, how do you bypass the zero error?
subtract the zero error from all readings
how does the zero error occur in the resistance required practical
its v difficult to get crocodile clip exactly at zero
there is some resistance caused by the contact between the crocodile clip and the wire
how do heating effects change the results of resistance practical
if the temperature of the wire increases, the temperature also increases
how to reduce heating effects in resistance required practical
using a low potential difference - this will keep the current low reducing any heating in the wire
only turn on the current when taking readings, turn off between readings
what does a variable resistor contain
a long piece of wire and a coil, using a slider, we can change the length of the wire that the coil runs through
why do we use a variable resistor
to control potential differences across the lamp
by increasing resistance we can make lamp dimmerl
whats the method for the current required practical
connect the battery to the resistor using wires, a resistor in series with an ammeter and a variable resistor and voltmeter in parallel across the resistor.
use vm to read p.d across the resistor.
Then use ammeter to read current through the resistor, record in table.
Adjust variable resistor and record new readings on vm and am
do this several times to get range of readings.
Switch direction of battery meaning p.d is reversed, vm and am have negative values, take several readings and plot graph ( I against p.d)
current through resistor is ______ to potential difference
this is only seen if ______ is kept _______
directly proportional
(resistor is ohmic conductor)
temperature, constant
how does required practical for current change with a diode
an extra resistor because diodes are veery easily damaged by a high current, extra resistor keeps current relatively low to protect diode
Because of this we need a sensitive ammeter (milliammeter) (then adjust the variable resistor and record p.d and current do in forward and reverse direction
in case of diode, we only get current wen the potential difference is around _______ volts
as p.d rises above this, the current ____
0.6-0.7
rises sharply
in a diode , we get _______ if the p.d is ________
why?
no current, reversed
in reverse direction, the diode has a high resistance
in things like blenders and fans, wheres the energy transfer?
electrical energy –> kinetic energy of motors
in things like irons or kettles, wheres the energy transfer?
electrical energy –> thermal energy
in things like hair dryers and washing machines, where is the energy transfer?
electrical energy —> kinetic energy of motors & thermal energy
what is power
the rate at which energy is transferred
appliances designed to produce thermal energy tend to have ____ power rating than appliances designed to produce kinetic energy
a much higher
whats the equation for energy
energy = power x time
whats equation for power
power = voltage x current
whats a direct current
when current from the cell only moves in 1 direction
whats alternating current?
when the current is constantly changing direction
whats the main benefit of AC
its v easy to use a transformer to increase/decrease potential difference
whats the frequency of uks alternating current
50 Hz
whats the UK’s potential difference
230V
what does the height of the peak tell us in an oscillascope
the maximum potential difference
in an oscilloscope with a DIRECT current the potential difference ______
does not change
how to find the frequency from a voltage-time graph (oscillascope)
1/time between peaks
whats the brown wire in a plug and what does it do
the live wire
carries alternating potential difference from the supply
whats the live wire connected to in the plug?
the fuse
whats the blue wire and what does it do
the neutral wire
completes the circuit
p.d of neutral is 0V compared to live
whats the green/ yellow striped wire and what does it do
the earth wire
this is a safety wire to stop the appliance from becoming live
why can appliances with a metal case be dangerous?
if the live wire comes loose and touches the metal case, the case can become live
(could give fatal shock)
why does can touching the live wire we fatal
current would flow through the person into the earth and theyd be electrocuted
how does an earth wire work
earth wires attached to the metal case of the appliance and is connected into the ground with a metal rod so if case becomes live, a huge current flows to earth, the fuse melts and shuts off the circuit (preventing shock)
whats the national grid
a system of transformers and high voltage cables
whats the issue with electricity in homes
energy is always lost in the power cables due to the resistance of the wires
how can we reduce energy lost from power stations to homes
building power stations near to homes
what does a step up transformer do
increases the potential difference to several hundered so that less energy is lost in power cable
what happens in a step down transformer
reduces voltage to 230V to be safer
why are metals good conductors
electrons can easily pass through them
what happens in a vanderwhatever generator
as belt moves, electrons pass from top plastic roller onto the belt
as electrons moves to bottom roller, electrons pass off the belt and are transferred to earth
so top region of generator becomes positively charged
positive charge builds up on metal dome
why does hair go static when touching generator
man holding onto dome has overall positive charge, hairs on head are + so repel each other and move apart
whats an example of static electricity being dangerous
electrostatic charge can build up as fuel flows through a refuelling pipe, this could cause a spark and trigger an explosion
how to prevent aeroplane and refueling trucks from exploding?
They’re both earthed. this prevents any charges from building up and leading to spark
when two insulators are rubbed together electrons can___
pass from one to the other
what type of force is the force of attraction?
non contact as two spheres arent touching
how do scientist show an electric field
what angle must they be touching the centre
field lines
90’
if you decrease the distance between objects (field lines) what happens
the forces get stronger
