electricity Flashcards
condition for electrical charge to flow
for electrical charge to flow through a closed circuit, the circuit must include a source of potential difference
define the size of the electric current
the rate of flow of electrical charge
does current have different values at different points in a single closed loop?
no, a current has the same value at any point in a single closed loop
what does the current through a component depend on and how
the current through a component depends on both the resistance of the component and the potential difference across the component. the greater the resistance of the component, the smaller the current for a given potential difference across the component
function of a thermistor
a component whose resistance is related to temperature
function of an LDR
a component whose resistance is related to to the amount of light falling in it
function of a diode
they only allow current to flow through them in one direction
function of an LED
LEDs emit light when they conduct electricity; current can only flow through them in one direction.
function of an on/off switch
an on-off switch allows current to flow when closed
function of a cell
a cell supplies electrical energy
function of a battery
batteries supply electrical energy and are made up of multiple cells
function of a resistor
it restricts the flow of current in a circuit
function of a variable resistor
a component used to vary and control the current in a circuit
function of a lamp
a component that transforms electrical energy into light
function of a fuse
a safety device that will break the circuit and stop current flowing
function of a voltmeter
used to measure potential difference between two points in a circuit. connected in parallel
function of an ammeter
used to measure current in a circuit. connected in series
relationship between resistance and ohmic conductors; what would this look like on a graph?
the current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. this means that the resistance remains constant as the current changes
on a graph plot potential difference against current; the resistance is a straight line through the origin (extends on both negative and positive values)
relationship between resistance and components; what would this look like on a graph?
the resistance of components, such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. the resistance of a filament lamp increases as the temperature of the filament increases
on a graph plot potential difference against current; the resistance is a curved line from the negative values that goes through the origin and curves in the positive values
relationship between resistance and diodes; what would this look like on a graph?
the current through a dioxide flows in one direction only; the diode has a very high resistance in the reverse direction
how does resistance change in
a) a thermistor
b) an LDR
a) the resistance of a thermistor decreases as the temperature increases
b) the resistance of an LDR decreases as light intensity increases
rules for components connected in series
- there is the same current through each component
- the total p.d. of the power supply is shared between the components
- the total resistance of two components is the sum of the resistance of each component e.g. Rₜₒₜₐₗ = R₁ + R₂
rules for components connected in parallel
- the p.d. across each component is the same
- the total current through the whole circuit is the sum of the currents through the separate components
- the total resistance of two resistors is less than the resistance of the smallest individual resistor
- total resistance is the reciprocal of all of the resistances added together
why does adding resistors in parallel decrease total resistance
in a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through
why does adding resistors in series increase total resistance
in a series circuit, the net resistance increases as more components are added; the current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow
why are parallel circuits more advantageous than series circuits
if a wire were to be faulty in the series circuit, the whole circuit would be broken, whereas if a wire were to be faulty in a parallel circuit, only the junction(s) affected would be broken, and the other lamps would still function
what is the mains electricity
an ac supply, which in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230V
difference between direct and alternating current
direct current is the movement of charge in only one direction, whereas alternating current is the backward and forward movement of charge
describe alternating current
the p.d. constantly changes, reaching a peak of 230V in one direction, reverting to 0, then reaching a peak of 230V in the opposite direction and repeating. there are 50 of these cycles per second, so the frequency is 50Hz
function and colour of the live wire
brown; the live wire carries the alternating potential difference from the supply. this wire has a p.d. of 230V
function and colour of the neutral wire
blue; the neutral wire completes the circuit. this wire has a p.d. of 0V
function and colour of the earth wire
green and yellow stripes; the earth wire is a safety wire to stop the appliance becoming live. this wire would be at 0V
why is the insulation covering each wire colour coded
for easy identification
how does a fuse work
fuses have different capacities for how much current can pass through; if it surpasses the amount, then the fuse overheats and melts which breaks the circuit, to ensure that other components which could be dangerous with excessive amounts of current do not receive it
what happens when charge flows
energy is transferred as it does work against the resistance of the circuit (work is done)
what do all domestic appliances do
transfer electrical energy into other forms of energy, typically kinetic and thermal energy; hence we can divide appliances into two main categories: those that transfer electrical energy to the kinetic energy of motors, or those that transfer electrical energy to the thermal energy of heating devices
energy transfer to the mains in a blender
they are powered by electrical energy, which is then transferred to the kinetic energy of electric motors to allow for movement (some energy will be dissipated as thermal energy due to friction)
energy transfer to the mains in a kettle
they are powered by electrical energy, which is transferred to thermal energy in the heating element
examples of appliances that fit under transferring electrical energy into both kinetic and thermal energy categories
hair dryers, washing machines
what are everyday appliances designed to do
bring about energy transfers
define power rating
the maximum power that appliances can safely operate at / the maximum rate of energy transfer
describe the difference in power rating between appliances designed to transfer electrical energy to thermal energy vs kinetic energy
usually, appliances designed to transfer electrical energy to thermal energy have a much higher power rating than those designed to transfer to kinetic energy, as it takes much more energy per second to heat than to move
misconception about power
a higher power does not necessarily mean that an appliance transfers more energy usefully i.e. appliance 1 might have a very high power but a very low efficiency
define the national grid
a system of cables and transformers linking power stations to consumers
what is transferred using the national grid
electrical power
what is the issue with getting electricity to homes? how can this issue be reduced; what is the problem with this resolution?
energy is lost in power cables due to the resistance of the wires; we can reduce energy loss by building power stations near to homes, but this isn’t feasible, as people don’t want to live near power stations
how are unwanted energy transfers reduced in the national grid
by using step-up and step-down transformers; step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use
why is electrical power transferred at a very high p.d. across step-up transformers
in our P=IV equation, if we increase the potential difference keep the power (rate of energy transfer) the same, then we have a lower current. a lower current results in less energy being lost
define insulators; give examples
insulators are materials that do not conduct electricity, as electrons cannot move through them e.g. cloths and plastic rods
why are metals good conductors of electricity
electrons can easily flow through metals
what happens when you rub certain insulating materials together
when they are rubbed against each other (e.g. a cloth and a plastic rod), they become electrically charged. negatively charged electrons are rubbed off one material and on to the other. the material that gains electrons becomes negatively charged and the material that loses electrons becomes positively charged. these positive and negative charges are equal and opposite
why is static electricity called that
because the two insulators now become electrostatically charged
what happens when two electrically charged objects are brought close together
they exert a force on eachother; two objects that carry the same type of charge repel, two objects that carry different types of charge attract. attraction and repulsion between two charged objects are examples of non-contact force
how can static electricity cause a spark under certain conditions
when we rub two insulators and electrons are transferred from insulator A to insulator B, insulator A becomes positively charged and B becomes negatively charged. the electrons from insulator B are therefore strongly attracted to insulator A’s positive charge, so sometimes, these electrons can jump across the gap when they are brought close to each other, causing a spark
how do you prevent sparks leading to fires e.g. on aircraft fuelling
the refuelling truck and the aeroplane are earthed to prevent any charged building up and leading to a spark
how do sparks commonly take place
between an object with a static charge and an earthed object
describe how sparks take place between static charged objects and earthed objects - what assumptions do you make
as electric charge builds up on an object, the p.d. between the charged object and the earthed object increases. with a high enough p.d., the electrons can jump across the gap and be transferred from the charged object to the earthed object. you must assume that the uncharged object is earthed, not positive
when is an electric field created
a charged object creates an electric field around itself
determine the strength of an electric field
the electric field is strongest close to the charged object; the further away from the charged object, the weaker the field
what happens when another charged object is placed in the electric field
it experiences a non-contact force; the force gets stronger as the distance between the objects decreases
how can we draw electric field lines
draw lines perpendicular to the surface with arrows that point in the direction of a positive particle that is placed within the field i.e. field lines for a positive object will go away from the object (if you place a positive object in the field it will be repelled) and field lines for a negative object go towards the object (if you place a negative object in the field it will be attracted)
