Electricity Flashcards
in a circuit which way does conventional current flow
Positive to negative
in a circuit which way do electrons flow
negative to positive
what is current
the rate of flow of charged particles around a circuit or how many coulombs are flowing per second
what is 1 coulomb equivalent to
6x10^18 electrons
how does current work in series
because there’s no junctions all coulombs flow through all components , therefore current is the same throughout the circuit
what is voltage
amount of work done per coulomb by a component
how does voltage work in series
a voltmeter will measure how much energy a coulomb will have before and after a component which is why it needs to be connected in series
what is resistance
a component will have 1 ohm of resistance if 1V is required for 1A to pass through it
what is kirchhoffs second law
in a series circuit the sum of the voltages across each component must equal the voltage across the power supply
what is EMF also know as
electro motive force
the voltage across the power supply
what should resistance be in an ideal circuit
wires should have no resistance
ammeters in series and have no resistance to not impede electron flow
voltmeters in parallel and have infinite resistance to stop current flowing through
what makes a circuit parallel
it has multiple routes that a coulomb could go. these are called branches
how does voltage work in parallel
when a coulomb reaches a junction it can split it goes one way or another. so the voltage in each branch will be the same
how does current work in parallel
because voltage is the same in each branch the current will split in an even ratio between branches. however the greater ration will go down the path with least resistance
what is kirchhoffs 1st law
the current going into a junction must equal the current leaving the current AKA no current is lost in the circuit
what are tips for tricky circuit solving questions
write all value on the diagram
OHMS LAW
keep it simple
add series resistors first
what is an ohmic conductor and give an example
a component that follows ohms law
voltage is directly proportional to current (resistance is constant)
eg. fixed resistor
what are important features of an I against V graph
gradient is 1/R
steep line=low resistance
shallow line=high resistance
how do you check that a component is an ohmic conductor
you should reverse the direction of the current through it (switch the connection leads on power supply)
if the line remains straight (gradient) then its an ohmic conductor
what is a variable resistor
a long coil of wire where current flows through one end and out through a moveable slider. this is needed to check for an ohmic conductor.
a longer wire= greater resistance
what is a non-ohmic conductor and give examples
a component that does follow ohms law- there resistance isn’t constant.
eg. filament bulb, thermistor, diode, LDR
what is a filament bulb and draw an IV graph for it
a very long piece of wire is coiled inside a bulb. current flows through making it so hot it glows.
(check century for graph)
why does a filament bulb IV graph look like that
at the beginning the bulb is cold so there’s low resistance and electrons can flow through easily. when current is increased so is electron flow.
more electrons bump into the atoms in the wire passing them energy and vibrating harder heating up the wire. this means resistance is increasing
why might a filament bulb break
resistance starts off very low and heats up very quickly. the quick temperature change can cause the bulb to break
what is a semiconductor
Thermistors and LDRs change resistance depending on light and temp. they are made of silicon which is a semiconductor.
insulators dont carry current well at all and conductors carry current easily. a semiconductor is somewhere in between.
what are the graphs for thermistors and LDRs against resistance
check century
what is diode and what safety should there be when using one
its like an electrical valve, it only lets current flow in one direction. but only when the voltage is above the threshold voltage (0.6v-0.7v)
when using a diode you should always use a safety fixed resistor to prevent current getting too high
what 4 factors effect resistance in a wire
length
temperature
cross-sectional area
material
how does temperature effect resistance in a wire
in a wire there are lattice of atoms, when temp increases the lattice vibrates more, increasing the chance of electrons hitting it (resistance increases)
more current=more electrons= more chance of hitting lattice=more resistance
how does length effect resistance in a wire
in a longer wire there are more atoms. the more atoms vibrating so more chance of collision so higher resistance. (short wire=less resistance)
how does cross-sectional area effect resistance in a wire
if you make a wire wider, more change can flow through per second, so resistance is lower than a thin wire.
how does material effect resistance in a wire
resistivity is a property of a material that measures how easily can current flow through that material.
conductors have low resistivity (copper)
insulators have high resistivity (plastic)
therefore insulators have more resistance
how do you find resistivity from a graph
plot a resistance against length graph. then multiple the gradient by the cross-sectional area
what are superconductors
conductors that have no electrical resistance below there critical temperatures (mostly hundreds of degrees below 0 celsius)
what are some uses of superconductors
no resistance means no energy loss and allows larger currents
-power cables
-strong electromagnets
-electronic circuits
what is power
the rate of transfer of energy
what is EMF
the amount of energy in joules given to each coulomb that passes through the battery
what is internal resistance
as coulombs pass through a battery it will have some resistance due to it being made of atoms. this is internal resistance
what is load resistance
the resistance of all the components outside of the battery
how does internal resistance effect rechargeable batteries
it needs a very high current to start the energise which would cause loss of power dissipation to thermal energy. therefore they make internal resistance very low.
how do you find EMF in a circuit
you put a voltmeter in series with a battery. because voltmeters have infinite resistance no current will flow through, no energy is loss through internal resistance therefore measuring EMF
what are important features of a terminal voltage and current graph
gradient is -r (internal resistance)
y-intercept is EMF
what is terminal voltage
when a resistors is connected to the terminals of a battery. or when a voltmeter is connected parallel to the resistor would also read terminal voltage (voltage in the battery)
why does terminal voltage decrease when current increases
the EMF is shared between a resistor is series and the battery. when the resistor has a high resistance it will have more voltage and therefore a lower current. so the battery will have more current (and less voltage)
what is a potential divider
a simple circuit with a power source and 2/3 resistors, splitting the voltage between them
what terminology is used when talking about potential dividers
EMF=Vin
output voltage (trying to find)=Vout
what is the potential divider equation
Vout =R2
Vin R1 + R2
what are uses for potential dividers
only useful if a variable resistor is used as you would need to change this easily. eg, speaker volume
what is a potentiometer
a configuration of a variable resistor where it effectively splits it into two resistors making it easy to change to resistance ratios
what are the differences between a variable resistor and a potentiometer
variable resistor- not very sensitive and cant reach 0A/0V
-easier to connect up
potentiometer- can sensitively move current and voltage from 0A/0V all the way to their maximums
-tricky to connect up
