Studied by 1 person
charge
a fundamental property (like mass)
symbol Q
measured in Coulombs, C (1 coulomb = 1.8 × 1019 electrons)
current (symbol, def, unit and eqn)
the rate of flow of charge (how many coulombs pass per second)
symbol I
measured in amps, A
current = charge/time (amps = coulombs/second)
conventional current flows in the ___ direction to electrons
opposite, from positive to negative.
electrons are negatively charged, so flow negative to positive
how do metals conduct electricity?
metals have delocalised e- that can move freely
normally, there are the same amount of e- moving forwards and backwards
when a metal is connected to a power supply the negatively charged e- move towards the positive terminal of the supply
This movement of electrons causes a current
How is potential difference set up
the terminals of a cell make one end of a circuit positive and the other negative
potential difference (voltage)
energy transferred per unit of charge passing through terminals or components
measured in volts (V)
1 volt = 1 joule per Coulomb
potential difference formulae (give 2)
V = E/Q
potential difference = energy transferred / charge
V = IR
potential difference = current x resistance
a current will flow in a circuit if:
the circuit includes a source of potential difference
the circuit is closed
in a closed-loop circuit, current is ___ at ___ point. why?
current is the same at any point
same number of electrons per second at any point
what happens to current at a junction?
split between each branch (because each unit of charge can only go one way or another).
if branches are identical, the current divides equally
if the branches are different, the current divides unequally
It is the same before and after the junction, and the sum of the current between branches is the same as the main circuit. CONSERVED
what happens to potential difference in a series circuit
split- shared between components (because each unit of charge has to bring voltage to every component)
what happens to potential difference in parallel circuits
it remains the same for each branch of circuit (because each unit of charge carries the same voltage)
resistance
the ability of a material to slow or stop the flow of electric current
the larger the resistance, the ___ the current
smaller
what happens to the resistance of a variable/fixed resistor if the current changes. What about the resistance of other components?
nothing- the resistance of a resistor can only be changed by physically adding more wire/more resistant components
however, the resistance of other components can change with the current (lamps, LEDs, LDRs)
resistance/current/pd equation
V = IR
voltage = current x resistance
R = V/I
resistance = voltage / current
when multiple resistors are in series, their resistances….
combine to form the total resistance
when multiple resistors are in parallel, the total resistance…
is lower (the more loops we add, the more options there are for current to travel, so overall more current flows)
disadvantages of series circuits
if one component breaks, the whole circuit stops working
components can’t be individually switched on and off
IV graph for a filament lamp (how does current vary with pd in a filament lamp)
current and pd are not directly proportional
as current increases, the temp increases
higher temp causes atoms to vibrate more
increases resistance as e- can’t pass through
IV graph for a fixed resistor
current and pd directly proportional (as one increases, the other increases at a constant rate)
resistance constant
IV graph for a diode (how does current vary with pd)
diode only allows flow in one direction (forward bias)
high resistance in backwards direction, so no current flows
low resistance in forwards direction (sharp increase in current on RHS)
threshold voltage is the pd that has to be reached for current to flow (a bit after the origin)
how is resistance affected in an LDR
as light intensity increases, resistance decreases
how is resistance affected in a thermistor
as the temp increases, the resistance decreases
how to investigate the variation of resistance with current in a filament lamp or diode
place in a series circuit with a cell, variable resistor, ammeter and voltmeter(in parallel)
vary the current with a variable resistor (the current is the independent variable)
record the voltage for each chosen value
calculate resistance for each: R = V/I
investigate variation in resistance in LDR/thermistor
series circuit with cell, LDR/thermistor, ammeter, and voltmeter (in parallel) Dimmable lamp/heater near the LDR
start in dark room (LDR only), lamp/heater turned off, record voltage and current
slowly increase light intensity/temp, wait a few seconds and record V and I
calculate resistance for each value
why do components (e.g. resistors) heat up when a current passes through them?
electrical → heat energy transfer heats the component
flowing e- collide with ions in metal lattice, resulting in heating
when an electrical current does work against electrical resistance, electrical energy is ___ in the surroundings as ___
dissipated, thermal energy
advantages and disadvantages of the heating effect of electrical current
advantages:
generating heat for electrical appliances e.g. ovens/kettles
disadvantages:
makes devices less efficient
can be dangerous
factors affecting resistance
temperature
higher temp = higher resistance
length of wire
longer wire = more resistance (more time for collisions)
thickness of wire
thinner wire = more resistance (less x-sectional area means more collisions)
energy transferred (ALSO WORK DONE) equation
E = VIt
energy transferred (J) = pd (V) x current (A) x time (s)
E= VQ
energy transferred (J) = pd (V) x charge (C)
E = Pt
energy transferred (J) = power (W) x time (s)
power meaning
energy transferred per second
measured in Watts, W
Watts are the same as Joules per second
the THREE power equations
P = E/t
Power = energy transferred over time
P = IV
Power = current x pd
P = I2R
Power = current2 x resistance
2 ways household appliances get energy and 2 ways it is used
energy comes from either batteries or a.c. mains electricity
energy is used in motors or heating devices
direct current + what is it produced by
a current that is steady and constantly flowing in one direction only (positive to negative)
has fixed negative and positive terminals
produced by cells and batteries
alternating current
a current that continuously changes direction, going back and forth around a circuit
has 2 identical terminals that switch between positive and negative
therefore current can be positive OR negative (depending on direction at that time)
frequency = no. of times direction changes per second
UK mains electricity
a.c. supply, frequency 50Hz (pd around 230V)
live wire (mains input wire)
carries alternating potential difference from supply to appliance circuit
most dangerous
neutral wire (mains input wire)
forms the opposite end of the circuit to the live wire, to complete the circuit
less dangerous
earth wire (function)
Acts as a safety wire to stop the appliance itself from becoming live:
provides a low-resistance path to the earth
causes a surge of current in the Earth wire + also live wire
high current causes fuse to melt and break
cuts off supply of electricity to appliance, so it’s safe
fuse
safety device that cuts off flow of electricity if current becomes too large
contains a thin wire that heats and melts if current too strong
circuit breaker
safety device that cuts off flow of electricity if current too large
automatic electromagnet switch, breaks if current exceeds a certain value
don’t melt + break, so can be reset and used again
faster than a fuse
why should switches and fuses be connected in the live wire?
it will cut off any electricity supply to the circuit, immediately cutting off the appliance
the live wire is also the most dangerous with a voltage of 230V, so it is important to cut it off
potential difference of the 3 wires
live - 230V
neutral - close to 0V
earth (not in use) - 0V
what happens if the live and earth wires come into contact?
the potential difference is very large between them, around 230V
this will cause a very large current if they are connected
if they are connected through a person, it will cause a very large, possibly lethal shock
what does the power rating of an appliance mean?
how quickly it transfers energy
so the higher the power rating (in watts), the more joules per second it will transfer from one store to another
Note 
Flashcard (67)