Electricity Flashcards
electric current definition
flow of charge
rate of movement of charge
will only flow through a component with potential difference across it
cell definition
single electrical energy source
supplies circuit with power, provides potential difference
how cell applies potential difference to circuit
transfers energy to charges passing through cell
voltage definition
potential difference
electrical pressure that causes current to flow in a circuit
potential difference formula
potential difference (volts) = work done (J) / charge (C) V = W/Q
current formula
current (A)= charge (C) / time(s)
I = C/t
resistance definition
how difficult for charge to flow through it
resistance formula
resistance (ohms, omega)= potential difference (V) / current (A)
R = V/I
how to measure resistance
connect ammeter before component
voltmeter across both ends of it
Ohm’s law
ratio of voltage to current is constant
R = V/I
ohmic conductor definition
conductor that obeys Ohm’s law
current-voltage graph of ohmic conductor
straight line as resistance is fixed
resistance is equal to inverse of gradient of line
steeper graph, lower resistance
filament lamp current-voltage graph
more current = more heat = more resistance as resistance increases with current, gradient decreases to reflect this curved graph (looks like S, sigmoid)
power definition
rate of energy transfer
power formula
P (W) = E (J) / t (s)
power = work done / time taken
power (electricity) formula
power (W) = potential difference (V) x current (A)
P = I x V
static electricity
insulating materials rubbed together
electrons rubbed off one material and deposited to other material
charges build up
spark/shock happens when enough charges build up and move
material that gains electrons become negative and vice versa
direction of electron transfer in static electricity
depends on material
examples:
cloth duster -> polythene
acetate -> cloth duster
static electricity in medicine
defibrillators
produced electrical shocks, makes heart contract when placed on patient’s chest
static electricity in dust precipitates
dust precipitators remove dust from chimneys
cleans emissions from factories
negatively charged grid in chimneys makes dust particles negative
-ve particles attracted to +ve metal plates, form large particles
static electricity in paint sprayers
paint charged so droplets repel each other
gives fine spray
surface being painted given opposite charge to attract paint droplets
gives even coat + less waste
switch function
opens/closes circuit
allows current to flow or not flow through it
cell/battery function
battery = series of cells
supplies circuit with power
provides potential difference needed to move charge around circuit
lamp function
thin wire in inert gas
wire gets hot and flows when current flows through
fuse function
stop current from flowing through circuit when current gets very high (melts and opens circuit)
diode function
only allows current to flow in one direction
restricts current in parts of circuit
light-emitting diode function (LED)
only allows current to flow in one direction
glows when current flows through it
fixed/variable resistor function
oppose flow of current
helps set current in circuit to wanted value
thermistor function
temperature-dependent resistor
changed resistance according to temperature
voltmeter function
measure potential difference in volts across given component
always in parallel with components
ammeter function
measure current in amps in component
placed in series with components
light deadens resistor function (LDR)
resistor changing resistance depending on amount of light shining on it
current across a series circuit
same at any point in the circuit
potential difference across series circuit
potential difference shared between components
energy transferred from battery in circuit
total energy transferred from battery to charge = total energy transferred from charge to components
resistance across series circuit
resistance of circuit = sum of resistance of each component
current across parallel circuit
current shared between components of circuit
potential difference across parallel circuit
potential difference is same at any point of circuit
resistance in parallel circuits
more components = in parallel = lower total resistance p
each individual circuit gives electrons more than one way to get around circuit
also provided higher current
efficient versions of filament light bulbs
compact fluorescent lamps (CFLs)
light-emitting diodes (LEDs)
how to save money with electricity
some electricity is converted to waste energy
using more efficient appliances keeps this to a minimum, decreases energy bill in long term
magnets definition
objects that produce magnetic field
have opposite north and south poles
hard magnets definition
permanent magnets
generate own magnetic field
able to maintain it constantly
examples of hard magnets
pure iron
nickel
cobalt
steel
soft magnets definition
induced magnets
objects that generate magnetic field only if within magnetic field
can only be attracted by other magnets, cannot be repelled
permanent magnets induces only opposite pole on soft magnets
soft magnet examples
some alloys of iron and other magnetic metals
iron fillings
electromagnetic effect definition
circular magnetic field generated around wire when wire carries current
direction of magnetic field in straight wires
anti-clockwise and perpendicular to direction of current
direction of magnetic field in solenoids
south pole at positive end
north pole at negative end
like a bar magnet
how to increase strength of magnetic field around wire
increase current (directly proportional) increase number of coils in solenoid (each coil has own magnetic field, adding coils increases amount of individual magnetic field)
permanent magnets vs electromagnets
permanent magnet retains magnetism once magnetised
electromagnet only has magnetism when current passes through it
what is produced when wire’s magnetic field interacts with permanent magnetic field
force
Fleming’s left hand rule
finger = direction of thumb = force index = magnetic field middle = current
used for motors
magnitude of force for straight wire in uniform field when all quantities are perpendicular) equation
force (N) = magnetic field stretch (T) x current (A) x length of wire (m)
F = BIL
how DC motor works
DC current flows through armature on commutator between north and south pole permanent magnets
overall turning motion as 2 opposite forces on two sides of coil
how to increase force produced in DC motor
more turns of coil
stronger magnetic field
stronger current
longer length of wire
electromagnets in speakers
variation of input current in soil moved core at specific frequency via motor effect
cone attached vibrates at same frequency
generates pressure waves in air (produces sound)
electromagnets in microphones
same as speaker except in reverse
generator effect in place of motor effect
electromagnets in doors
entering correct passcode / using keycard disables electromagnet holding door close
electromagnets in toasters
magnet holds bread down
roast pops out when time is up and power to electromagnet is cut off
electromagnetic induction definition
product of a potential difference across an electrical conductor in a changing magnetic field
how potential difference can be induced in an electrical conductor
magnet moved into coil of wire
electrical conductor moved in a magnetic field not parallel to field lines
how direction of induced potential difference/current can be changed
changing direction of movement of conductor relative to magnetic field
changing direction of magnetic field (polarity)
how AC current is induced
magnet moved backwards and forwards in the coil
how voltage is increased in induced AC currents
moving magnet faster
stronger magnet
more turns of coil
higher area of coil
how AC generators work
induces current by spinning coil of wire inside magnetic field of magnet or vice versa
current changes every half turn as direction of motion changes due to alternating current
how current and voltage increases in AC generator
strength of magnetic field
number of turns of coil
rate at which coil/magnet spins (also affects frequency)
Fleming’s right hand rule
finger = direction of thumb = force index = magnetic field middle = current
used in generators
output voltage of AC generators
generator effect only exists when wire is cutting field lines
maximum when 90° or 270° as coil is most perpendicular to magnetic field lines
0 when coil is not cutting magnetic field lines (0°, 180°, 360°)
transformer definition
device that changes voltage of AC supply
step-up transformer
changes low voltage supply to high-voltage supply
increases voltage as more turns on secondary coil than first
step-down transformer function
changes high voltage supply to low voltage supply
increases voltage as less turns on secondary coil than first
Structure of transformer
primary and secondary coils wound around soft iron core (coils not connected to each other)
primary coil supplies AC to iron core, induces magnetic field which is transferred to secondary coil where current is induced
relationship of voltages of primary and secondary coils
inversely proportional to each other
ratio of voltage across primary and secondary coils
depends on number of turns on primary and secondary coil
V1/V2 = N1/N2
power in coils formula
transformers nearly 100% efficient
power in primary coil = power in secondary coil
power = potential difference x current so
V1 x I1 = V2 x I2
transformers in the national grid
high power supply = very high voltage or high current
step-up transformers used to produce very high voltage and low current for transmission of electricity (minimises energy loss through heat)
step-down transformers used to reduce voltage to safe value before arriving in homes
power loss due to resistance in cables formula
power loss (V) = current^2 (A) x resistance (ohms)
why high voltage is used
high power is made by high voltage or high current
increasing current instead of voltage for high power = more heat loss as power loss is dependent on current
switch more transformer
reduced voltage of main supply converts AC to DC operates at high frequency lighter and smaller used very little power when no device connected to output terminals
isolating transformer
same number of turns in primary and secondary coils
same voltage
used to isolate connections from mains
