Science - Physics - Electricity - P2 Flashcards
what is the circuit symbol for a cell ?
+ I
-I l-
what is the circuit symbol for a battery ?
+I l l
-I l ….l l-
what is the circuit symbol for an open switch ?
-./ ._
what is the circuit symbol for a closed switch ?
-.–.-
what is the circuit symbol for a filament lamp or bulb ?
-x-
what is the circuit symbol for an ammeter ?
-A-
what is the circuit symbol for a voltmeter ?
-V-
what is the circuit symbol for a resistor ?
rectangle with lines either side
what is the circuit symbol for a variable resistor ?
same symbol as a normal resistor, but with an arrow through it
what is the circuit symbol for a fuse ?
rectangle with a line going through it
what is the circuit symbol for a thermistor ?
same as a normal resistor but with a line going halfway across under it, then diagonal up
what is the circuit symbol for a diode ?
circle with a triangle and a line inside of it, with a line going through the middle
what is the circuit symbol for a LED (light emitting diode) ?
same symbol as diode, but with 2 arrows diagonally above it pointing upwards
what is the circuit symbol for an LDR ( light dependant resistor) ?
a circle with the resistor symbol in it, and 2 diagonal arrows above it pointing down
what does LED stand for ?
light emitting diode
what does LDR stand for ?
light dependant resistor
what does a voltmeter measure ?
potential difference
what does an ammeter measure ?
current
how is a voltmeter connected in a circuit ?
across a component - in parallel
how is an ammeter connected in a circuit ?
in line with a component - in series
what is potential difference ?
the driving force that pushes electric charge around a circuit
what is an electric current ?
a flow of electric charge
does current flows from positive to negative or negative to positive around a circuit ?
positive to negative
which terminal line in cells is positive and which is negative ?
longer terminal line - positive
shorter terminal line - negative
what affects the rate of flow of charge ?
the size of the current
what is the rate of flow of charge measured in ?
amperes
how do you calculate rate of charge flow ?
Q = I X T
Q- charge flow
I - current
T - time
does more charge pass around a circuit when a bigger or smaller current flows ?
a bigger current
a battery charger passes a current of 2.5 through a cell over 4 hours, how much charge does the charger transfer to a cell altogether ?
I = 2.5 T=4 x 60 x 60s = 14 400 s
Q = IT = 2.5 x 14 400 = 36 000 C
what is needed for a current to flow in a circuit ?
potential difference
how is a circuit complete ?
if you can follow a wire from one end of the battery, through any components to the other end of the battery
what is resistance ?
anything in the circuit which reduces the flow of current
what is resistance measured in ?
ohms
what does the current flowing through a component depend on ?
resistance of the component, and the potential difference across it
the greater resistance, the ……. the current that flows ?
smaller
how do you calculate resistance ?
V= IR
V- potential difference
I- current
R- resistance
a voltmeter reads 6.0 v and a resistor has a resistance of 4.0, what is the current ?
rearrange the formula to I = V/R
I = V/R = 6.0/4.0 = 1.5 A
does the resistance of an ohmic conductor change with the current ?
no
the resistance of an ohmic conductor is directly proportional to what ?
the potential difference across it
what is an ohmic conductor ?
a conductor with resistance that is constant at a constant temperature
name 2 factors that affect resistors
- whether the components are in series or parallel
- the length of the wire in a circuit
as the length of the wire increases, what happens to the resistance ?
it increases
what do I - V characteristics show ?
how the current varies as potential difference changes across the component
what happens to the resistance of non-linear components ?
the resistance changes depending on the current
how do you find a components I-V characteristic?
vary the resistance of the variable resistor
this alters current flowing through the circuit and the potential difference across the component
explain the resistance of a filament lamp
when electrical charge flows through a filament lamp it transfers some energy to thermal so the lamp heats up and glows, the higher the temperature, the higher the resistance so as the lamp heats up more the resistance increases so less current can flow
explain the resistance in a diode
a diode is a component that only lets current flow through it in 1 direction, so the resistance depends on the direction the current is flowing, the diode will happily let the current flow one way, but will have a very high resistance if the current is reversed
what are series circuits ?
different components connected in a line, end to end, between the positive and negative ends of the power supply, except for voltmeters which are always connected across a component
if you remove or disconnect 1 component in a series circuit, what happens ?
the circuit is broken, and they all stop
what are series circuits used for ?
- to measure quantities
- test components
how do you calculate potential difference in a series circuit ?
the sum of the potential differences of all the batteries in the circuit
the potential difference around a series circuit is always what ?
equal
in a series circuit, the current is what ?
the same in all parts of the circuit
what is the size of the current determined by in a series circuit ?
the total potential difference of the cells and the total resistance of the circuit
in a series circuit, how do you calculate the total resistance ?
add up all the resistances
in a series circuit, the bigger the resistance of a component, the bigger the what ?
its share of the total potential difference
which circuits are generally more useful, series or parallel ?
parallel
what is a parallel circuit ?
components each have their own branch in a circuit connected to the positive and negative terminals, except ammeters which are always in series.
if you disconnect one branch of components in a parallel circuit, what happens to the other branches ?
it won’t affect them as current can still flow in a complete loop from one end of the power supply to the other branches that are still connected
where are parallel circuits used ?
in household electrics
what is the potential difference like in parallel circuits ?
it is the same across all components connected in parallel
why is parallel connection useful in cars ?
- everything can be turned on and off separately
- everything as the full potential difference of the battery across it so you can have music on full blast without it having much effect on the brightness of your lights
what is current like in parallel circuits ?
the total current flowing around the circuit is equal to the total of all the currents going through the seprate branches
can the current be different in the mini branches in parallel circuits ?
yes
is the current the same thought each mini branch in a parallel circuit ?
yes
how do you calculate the total current in the parallel circuit ?
add up the total currents from each branch
adding a resistance in parallel does what to the total resistance ?
decreases it
why does the resistance decrease the more resistors you have in a parallel circuit ?
-both resistors have the same potential difference, so the pushing force making the current flow is shared between the resistors
-current has more than 1 direction to go in, increasing the total current that can flow around the circuit but decreases the total resistance
in a series circuit, when you add resistors what happens to the overall resistance of the circuit ?
it increases
in a parallel circuit, when you add resistors, what happens to the overall resistance of the circuit ?
it decreases
what is an LDR ?
a resistor that is dependant on intensity of light
what happens to the resistance of LDRs in bright light ?
it decreases
what happens to the resistance of LDRs in the dark ?
it increases
when are LDRs useful ?
where a function depends on light levels
give an example on how LDRs are used
street lights use LDRs so when the light level falls, the resistance of the LDR increases to a level that triggers the light to turn on
what is a thermistor ?
a type of resistor, which depends on temperature, a thermistors resistance decreases as the temperature increases
in hot conditions, what happens to the thermistors resistance ?
it decreases
in cold conditions, what happens to the thermistors resistance ?
it increases
when are thermistors useful ?
in temperature detectors called thermostats
when are thermostats used ?
in car engine temperature sensors to make sure the engine isn’t overheating
how do thermostats work ?
thermistors are connected in a circuit where their resistance is measured
as resistance varies with temperature, knowing the resistance means you can detect the temperature of the thermistor
if the resistance of a LDR or thermistor increases, what happens to the resistance of the output resistor ?
it decreases
if the resistance of the thermistor or LDR decreases, eg the thermistor gets hotter or more light shines on the LDR what happens to the resistance of the output pd ?
it increases
what are the 2 types of electricity supplies ?
alternating current (ac)
direct current (dc)
what is the current like in ac supplies ?
the current is constantly changing direction
how are alternating currents created ?
by alternating voltages in which positive and negative ends keep alternating
what is the uk domestic mains supply ?
the electricity in your home
what is the uk domestic mains supply in volts ?
230 v
what is the frequency of the ac mains supply (how often the current changes direction) ?
50 cycles per seconds or 50hz
what current do cells and batteries supply ?
direct current
what is direct current ?
a current which is always flowing in the same direction
how is direct current produced ?
a direct voltage - where the positive and negative ends of the source are fixed
how are electrical appliances connected to the mains supply ?
by the 3 core cables
what does the colour of the insulation of each cable show ?
its purpose
which 2 wires does the mains supply normally flow through ?
the brown live wire
the blue neutral wire
how many volts is the live wire at ?
230 v
how man volts is the neutral wire ?
0 v
what is the green and yellow wire called ?
the earth wire
how many volts is the earth wire usually at ?
0 v
what is the earth wire used for ?
protecting the wiring and safety - it stops the appliance casing from becoming live - it only carries a current when there’s a fault
what happens if you touch the live wire ?
your body is at 0 v
if you touch the live wire a large potential difference is produced across your body and a current flows through you
this causes an electric shock which could injure or kill you
why is there still danger in a turned off plug socket or light switch ?
there is still a pd in the live wire and if you make contact with it your body would provide a link between the supply and the earth so a current could flow through you
if the link between the live and earth creates a low resistance path to the earth what could happen ?
a huge current would flow which could result in a fire
how do kettles transfer energy ?
from the mains ac apply to the thermal energy store of the heating element inside the kettle
what is energy efficiency ?
how much energy is transferred usefully
why is energy transferred when a charge moves ?
the charge works against the resistance of the circuit
what is classed as unuseful energy ?
thermal energy
what does the total energy transferred depend on ?
how long the appliance is on for
the power at which it is operating
what is the power of an appliance ?
the energy that it transfers per second
the more energy transferred in a given time, the greater the what ?
power
how do you calculate energy transferred ?
power x time
what is the unit of power ?
watts
what is the maximum operating power ?
the maximum safe power an appliance can operate at
what does a power rating tell you ?
the maximum amount of energy transferred between stores per second when the appliance is in use
how do power ratings help customers choose which appliance to buy ?
lower the power rating, less electricity an appliance uses in a given time = cheaper
why may a higher power not necessarily mean more energy transfers usefully ?
its efficiency may be low
when an electrical charge goes through a change in pd what happens ?
energy is transferred
what is the pd between 2 points ?
the energy transferred per unit charge passing between 2 points
what does a bigger change in pd mean ?
more energy is transferred for a given charge passing through the circuit
what is the national grid ?
a network of cables and transformers that covers the UK and connects power stations to consumers
where does the national grid transfer electrical power ?
from power stations anywhere on the grid to anywhere else on the grid where its needed e.g homes
how do you transmit lots of power ?
you need a high pd or a high current
what is the problem with a high current ?
lots of energy is lost as the wires heat up and energy is transferrd to the thermal stores
what is the better way to transmit lots of power through the national grid ?
boost up the pd really high (400,000) and keep the current really low
- inceasing the pd,
- decreases the current
- decreases energy lost by surroundings
what is a transformer ?
a device used in the national grid to change the pd of an electrical supply
what does getting the pd to 400,000 v for transmition require ?
transformers and pylons and huge insulators
why is getting the pd to change using transformers better ?
its cheaper than transmitting it through smaller wires at a high current
how do transformers work ?
- transformers step up the pd at one end for efficient transmission
- pd is brought down to safe usable levels at the other end
how is pd increased ?
using a step up tranformer
how is pd reduced ?
using a step down transformer
when is demand for power from power stations high ?
- when people get up in the morning
come home from school
in winter when heating, lights are used
why are power stations often used below their maximum power output ?
so there’s space capacity to cope with a high demand even if there’s an unexpected shut down from another station
what is a transformer made up of ?
primary coil
secondary coil
wrapped around an iron core
how are transformers made ?
when a current flows the the primary coil, it causes a current to flow in the secondary coil
what are transformers efficiency levels ?
nearly 100%
what is power output = to
power input
what is an electrical conductor ?
a material which an electrical charge can move easily through
give an example of a good electrical conductor ?
metal
what is an electrical insulator ?
a material which an electrical charge cannot move easily through
give an example of an electrical insulator ?
rubbery or plastics
what is a static charge ?
an electrical charge which cannot move
where are static charges often found ?
on electrical insulators where charge cannot flow easily
what can a static charge build up on ?
a conductor if its isolated - there’s no where for the charge to flow to
what can happen when certain insulating materials are rubbed together ?
friction causes negatively charged electrons to be scraped off an dumped of one another
- this leaves a positive static charge on the one which loses electrons
-and a negative static charge on the one that gains electrons
what does the way electrons are transferred depend on when 2 insulating materials are rubbed together ?
the materials involved
what are produced by movement of electrons ?
positive and negative electrostatic charges
what are positive static charges caused by ?
electrons moving away elsewhere
the material that loses the electron loses negative charge and is left with an equal positive charge
what builds up as electric charge builds up in an object ?
the pd between the object and the earth increases
how is a spark caused ?
when electrons jump across the gap between the charged object and the earth
2 things with opposite electric charges are ….
attracted to each other
2 things with the same electrics charge will ….
repel each other
what is electrostatic attraction/repulsion ?
when electrically charged objects exert a force on each other which causes the objects to move
what type of force is electrostatic ?
non - contact
when do electrostatic forces get weaker ?
when the objects move further apart
what is an electric field ?
created around any electrically charged object
the closer to the object you get - the stronger or weaker the field gets ?
stronger
how are electric fields shown ?
by field lines
how do electric field lines point ?
away from positive charge
towards negative charge
they are at right angles to an objects charged surface
when an object is placed in the electric field of another charged object what happens ?
it feels a force which causes the attraction or repulsion, it is caused by electric fields of each charged object interacting with each other
what does the size of force between the objects depend on ?
the strength of the electric field at that point
if you move further from a charge what happens to the field ?
it gets weaker
how are sparks formed ?
when there’s a high enough pd between a charged objcet and the earth
what will a high pd to do an electric field ?
it will cause a strong electric field between the charged object and the earthed object
what does the strong field do to the electrons when sparks are caused ?
it causes the electrons in the air particles to be removed
what happens to air when it is ionised ?
it is more conductive so charge can flow through it causing a spark
