P3 ELECTRICITY Flashcards
what’s the formula for charge?
charge = current x time (q=it)
what is a property of all matter?
charge
why do most bodies have 0 net charge?
there are equal numbers of positive and negative charges
what can form when 2 surfaces are rubbed together?
static electricity, sparking
what is build-up of static charge caused by?
transferring electrons
where can static electricity build up on?
insulators
why can static electricity only build up on insulators?
in conductors, electrons flow back into/out of them, staying neutral but in insulators electrons can’t flow so a positive static charge is left on the object that gained electrons
where do electrons move for a polythene rod vs acetate rod?
polythene: electrons move from duster to rod
acetate: electrons move from rod to duster
what are sparks/shocks caused by?
when enough charge builds up, it can suddenly move which causes sparks/shocks
why does static electricity build up?
friction between 2 materials can cause the transfer of electrons, which causes charge to build up
why does static electricity only build up on insulators?
they don’t conduct, so charge can’t flow through them
what do electrically charge objects do to each other, and what happens when they are further apart?
exert a force on each other, forces get weaker when further apart
what is the force between 2 charged objects called?
electrostatic attraction/repulsion
which types of objects do electrically charged objects attract?
small neutral objects placed near them
how can you test if a rod is charged?
hold it near a stream of water from a tap- rod induces charge in water so stream is attracted to rod and bends towards it
OR use a gold leaf electroscope: if - charged insulator touches zinc plate, some charge is transferred to electroscope and conducted down to metal stem+ gold leaf, which repel each other and make the gold leaf rise. BUT if you touch the plate with a positively charged insulator, electrons flow into it from the plate, stem+ leaf. stem and leaf has same charge and leaf rises.
how can we prove that charged objects exert forces of attraction/repulsion on one another when not in contact?
ie why do small scraps of paper ‘jump’ towards a charged rod?
if you held a charged rod near small scraps of paper, the papers will ‘jump’ towards it. TIB the charged rod induces a charge in the paper: if the rod is positively charged, it attracts the electrons in the paper towards it and if it is negatively charged, it repels the electrons. this causes the surface of the paper to have an opposite charge to the rod, causing attraction.
what’s an electric field?
a region that surrounds charged particles and exerts forces on other charged particles in that field.
which direction do electric field lines go in?
positive to negative
what angle are electric field lines from the surface of the object at the point where they touch the surface
right angle
what do closer field lines mean?
stronger field, and stronger the force a charged object in the field experience
when you get further from a charged sphere what happens?
field lines get further apart, so force felt by the charged object due to an electric field decreases with distance
what does isolated mean
not interacting w anything
how do electric fields explain static electricity?
when the fields interact, they produce a force, which causes movement and allows the electrons to move
what happens if field lines between charged objects point in the same direction?
field lines join up and the objects are attracted to each other (happens when 2 charges are opposite: if charges are free to move, field lines straighten and shorten as charges move together)
what happens if field lines between charged objects point in opposite directions?
field lines ‘push against’ each other and objects repel each other (happens when 2 charges are same type)
what is current?
rate of flow of charge (electrons)
what is necessary for current to flow through an electrical component?
pd across component and if circuit is complete (closed)
what does a higher pd across a component vs higher resistance across a component mean?
higher current vs smaller current
what is the formula for charge?
q=it (charge=current x time)
what does potential difference do?
pushes current through resistance (driving force that pushes the current round)
what is pd?
energy transferred per coulomb of charge that passes between 2 points in an electrical circuit
what’s the circuit symbol for a cell or battery?
what’s the circuit symbol for an open vs closed switch?
what’s circuit symbol for filament lamp?
what’s circuit symbol for LED?
what’s circuit symbol for power supply terminals?
what’s circuit symbol for resistor?
what’s circuit symbol for variable resistor?
what’s circuit symbol for ammeter and voltmeter?
what’s circuit symbol for diode?
what’s circuit symbol for LDR?
what’s circuit symbol for thermistor?
which circuit can be used to investigate components?
series circuit with variable resistor, ammeter and resistor in series (with voltmeter in parallel to resistor). you replace the resistor with the component you’re investigating
what does ammeter measure and how should it be placed in the circuit?
current, in series
what does voltmeter measure and how should it be placed (and why)?
voltmeter measures pd across a component. must be placed in parallel with the component being tested, NOT the variable resistor/battery (so it can compare energy the charge has before and after passing through the component)
how should you change the circuit’s components to form an IV graph?
vary the resistance of the variable resistor - this alters current flowing through the circuit and pd across the component. take readings from the ammeter and voltmeter to see how pd across the component varies as current changes.
what’s the formula for VOLTAGE?
V=IR
what does the resistors and wires IV graph look like?
why does the IV graph for resistors and wires look like this?
current is directly proportional to pd (if temp stays same). resistor/wire has constant resistance.dif resistors have dif resistances, so IV graphs have dif slopes
what does a steeper IV graph mean for a resistor/wire? and if the line goes through the origin what does resistance of a component equal?
lower resistance. resistance of component = inverse of gradient or 1/gradient
what does the filament lamp’s IV graph look like?
why does the IV graph for filament lamp look like this?
increasing current increases temperature of the filament, making resistance increase, so IV graph is curved
what does the IV graph for a diode look like?
why does the IV graph for a diode look like this?
current only flows through a diode in 1 direction. diode has very high resistance in the opposite direction
what is affected as resistance increases?
temp
what are the conditions needed for charge to flow?
source of potential difference and a closed circuit
why does resistance increase with temp?
when electron flows through a resistor, some energy is transferred to the thermal energy store of the resistor, heating it up. but the thermal energy store of the substance is just the kinetic energy store of particles so as resistor heats up particles start to vibrate more. as the particles are moving more it’s harder for the charge-carrying electrons to get through the resistor: so current can’t flow as easily and resistance increases.
why does the graph for a filament lamp level off at high currents?
there’s a limit to current that can flow: as current increases, temp increases which causes resistance to increase, meaning the current decreases again
what is the current like in a single closed loop?
same value at any point
what’s the formula for pd?
pd=current x resistance (v=ir)
what’s the formulae for energy transferred?
energy = charge x pd (e=qv)
energy=pt
what are the equations for power?
p=iv
p=i²r
why should voltmeters be connected in parallel?
objects in parallel experience the same pd
how should ammeters be connected?
in series
which circuit consumes more power?
parallel
what’s the difference in current in series vs parallel circuits?
in series circuit, the same amount of current flows through all components so current is the same everywhere. in parallel circuits, current is split between the components (total current is sum of individual branches)
what’s the difference in pd in series vs parallel circuits?
in a series circuit, voltage of a battery is the sum of all the voltages of the components. in a parallel circuit, the voltage of every component is the same
what does current depend on?
resistance and pd
what is potential difference?
the difference in the amount of energy that charge carriers have between 2 points in a circuit
is the value of resistance constant in all resistors?
for some resistors, the value of r remains constant but in others it can change as the current changes
how do filament lamps vary resistance?
resistance increases as current through the filament increases and hence temp increases
how do diodes vary resistance?
has v high resistance in one direction, so current only flows in one direction.
what is a diode made from?
semiconductor material eg silicon
what 2 things can diodes be used in?
radio receivers, get direct current from an alternating supply
what does a thermistor depend on?
temperature
how do thermistors vary resistance?
in hot conditions, resistance drops and in cold conditions resistance increases.
what’s the IV and resistance vs temp graph of a thermistor?
opposite curve to filament lamp
what can thermistors be used for?
temp detectors eg thermostats, irons and car engines
what happens when the current of a thermistor increases?
current increases, thermistor warms up and resistance decreases
what does an LDR depend on?
intensity of light
how do LDRs vary resistance?
in darkness, resistance is highest and when light levels increase, resistance falls.
what 3 things can LDRs be used for?
automatic night lights, outdoor lighting and burglar detectors
which circuit elements are linear?
resistors and wires
the graphs of which circuit components are both downward curves?
thermistors and ldrs
does a thinner or thicker wire have more resistance? why?
thinner, bc fewer electrons can flow through and carry a current
why does net resistance increase when 2 resistors are in series but decrease when 2 resistors are in parallel?
in a parallel circuit, net resistance decreases because there are more paths for the current to pass through but in a series circuit there’s only one path
how is power transfer related to pd + current and energy changes?
a higher pd /current –> a higher power transfer+energy transfer
what are sensing circuits used for?
turning on/increasing power to components depending on the conditions they’re in
in a sensing circuit containing a thermistor and fixed resistor with a fan in parallel with the fixed resistor, what’s the difference in pd of the fan and fixed resistor and why?
same pd because they’re connected in parallel
in a sensing circuit containing a thermistor and fixed resistor with a fan in parallel with the fixed resistor, how is the pd given to the components?
pd of power supply is shared between thermistor and loop made of the fixed resistor and fan according to their resistances: the bigger a component’s resistance, the more p.d is takes up
in a sensing circuit containing a thermistor and fixed resistor with a fan in parallel with the fixed resistor, what happens as the room gets hotter/
the thermistor’s resistance decreases, meaning it takes a smaller share of pd from the power supply. this means pd across the fixed resistor and fan rises, making the fan go faster
in a sensing circuit containing a fixed resistor and LDR with a bulb connected in parallel to the LDR, what will happen when it is dark?
LDR has greater resistance when it’s dark, meaning it takes up more pd so the pd across the LDR and bulb will be high. the greater the pd across a component, the more energy it gets. so a bulb connected across an LDR would get brighter as the room got darker.
why would removing a component in a series circuit affect the other components?
current has to flow through all components to get round the circuit, so if you remove one of them it can affect the others
why would removing a component in a parallel circuit hardly have an effect on other components?
each component is connected separately to the +ve and -ve terminals of the supply
why are parallel circuits used in cars/household electrics?
you can switch everything on and off separately
if you want to add more cells to any circuit how should you do so and why?
connect in series. connecting several cells in series gives a bigger total p.d, because each charge in the circuit passes through each cell and gets a ‘push’ from each one. so two 1.5V cells in series supplies 3V total
what are the rules for pd, current and resistance in a series circuit?
PD: total pd of supply is shared between components. p.ds add up to equal pd across power supply. V=V1+V2
CURRENT: same everywhere I1=I2=I3
RESISTANCE: total resistance is sum of individual resistances R=R1+R2+R3
why is sum of pd around a series circuit equal to pd supplied by the power supply?
total energy transferred to charges in the circuit by the power supply equals total energy transferred from the charges to the components
why is the resistance of 2 resistors in series bigger than one of the resistance of one of the resistors on its own?
battery has to push each charge through all of them
what does bigger resistance of a component have a bigger share of and why?
total pd because more energy is transferred from the charge when moving through a large resistance than a small one
in a series circuit when resistance of 1 component changes (if variable resistor/LDR/thermistor), what happens to other components?
pd of all other components changes
what are the rules for pd, current and resistance in a parallel circuit?
PD: same across all branches V1=V2=V3
CURRENT: shared between branches i=i1+i2
RESISTANCE: total resistance is lower than branch with smallest resistance
which circuit will have a higher current: circuit w 2 resistors in parallel vs circuit with 1 resistor
circuit with 2 resistors in parallel will have lower resistance and higher current (bc charge has more than 1 branch to take - only some charge will flow along each branch)
how can you investigate series and parallel circuits?
set up a circuit with a power supply, bulb, voltmeter across bulb and ammeter in series. add 2nd bulb in series, measure current + pd and bulbs should look dimmer. add 3rd bulb in series, all 3 will look dimmer and measure pd +current.
repeat experiment but add bulbs in parallel on a new branch. notice that bulbs don’t get dimmer as you add more.
why do bulbs get dimmer when added in series?
pd across each bulb falls (bc pds across bulbs add up to source pd). current also falls each time you add a bulb bc ur increasing resistance of circuit (less current and pd decreases the power of each bulb)
why do bulbs NOT get dimmer when added in parallel (on new branches)?
pd across each bulb is equal to source pd. current on each branch is the same and doesn’t change when you add more bulbs because resistance of each branch remains the same
how can electrical circuits be thought about in terms of energy transfer?
charges travel around the circuit and when they go through an electrical component, energy is transferred to make the component work
how do electric circuits work?
energy is supplied to the charge at the power source to ‘raise’ it through a potential. the charge gives up this energy when it ‘falls’ through a potential drop in any components elsewhere in the circuit
why does a battery with a higher p.d. supply more energy to the circuit for every coulomb of charge tat flows round it?
charge is raised up ‘higher’ at the start, and more energy will be transferred in the circuit too
why is there are greater drop in p.d the greater the resistance of a component?
the more energy the charge has to transfer to it to pass through
what is power?
rate of energy transfer
