Topic 5 - Electricity Flashcards
what is a voltmeter?
measures p.d and connected in parallel to the component
what is a ammeter?
measures current and connected in series to the component
what is current?
the rate of flow of electric charge ( A )
which flows from positive to negative terminal
charge equation
Q = I t
charge = current x time
potential difference
a source of potential difference allows current to flow through a circuit ( driving force ). Meausred in V
How does changing the resistance affect the size of the current?
the greater the resistance of the component the smaller the current ( for a given p.d )
equation linking current, p.d , resistance
V = IR
p.d = current x resistance
I-V characteristic of ohmic conductor
the current through ohmic conductor is directly proportional to the p.d across it.
- it has a constant resistance ( for given temp )
I-V characteristic of filament lamp
- as the current increases the resistance also increases
- less current can flow per unit of p.d and so the graph gets shallower
why does the resistance of a filament lamp increase with current
- when an electrical charge flows through it transfers energy to thermal energy store of filament
- this heats up and glows
- resistance increases with temperature
- as more current flows the lamp heats up more, increasing resistance
I-V characteristic of diode
- only lets current pass through one direction
- in the opposite direction R initially high but drops drastically as p.d increases letting current flow
safety precaution with diode
- use a protective resistor to stop the current becoming too high and damaging the diode
p.d in series circuits
total V = V₁ +V₂ + …..
current in series circuits
total I = I₁ = I₂ = …etc
resistance in series circuits
total R = R₁ +R₂ + …..
p.d in parallel circuits
total V = V₁ = V₂ = …etc
current in parallel circuits
total I = I₁ +I₂ + …..
resistance in parallel circuits
adding more resistors in parallel decreases the overall resistance of the circuit
total resistance less than smallest individual resistance
resistance of LDRs
- in bright light the resistance falls
- in darkness the resistance is highest
resistance of thermistors
- in hot conditions the resistance falls
- in cold conditions resistance is highest
uses of LDRs
automatic night lights, burglar detectors
uses of thermistors
thermostats
sensor circuits
- if it gets hot, the resistance of the thermistor decreases so takes a lesser share of the p.d
- this means the p.d across the output ( e.g : fan ). will increase, meaning it will get switched on ( faster )
what is a static charge?
- an electric charge that can’t move
- found on electrical insulators where charge can’t flow freely or isolated conductors
what happens when two insulators are rubbed together?
- creates friction which gives the electrons energy
- negatively charged electrons transferred from one materila to the other
- gain electron = N , lose electron = P
how do sparks form?
- when charge builds up on an object the p.d between it and the earth ( 0V ) increases.
- if the p.d is large enough, electrons can jump across the gap between charged object and earth/ earthed conductor
what is an electric field ?
- a region around an electrically charged object in which forces can act on an object, causing it to attract or repel
what happens when two electrically charged object are brought close?
they exert a force on each other called electrostatic attraction / repulsion
electric field strength
- further away the weaker the force of attraction/repulsion
- field lines further apart
how do sparks work in terms of electric fields
- the object has a large static charge and so there is a large p.d between it and earth
- causes a strong electric field
- electric field ionises air particles so the air can conduct electricity
- spark jumps from object to earthed conductor
alternating current ( A.C )
current is constantly changing direction as the positive and negative terminals alternate
direct current ( D.C )
current always flows in the same direction ( uni - directional )
why is A.C. used for mains electricity
- easier to generate
- easy to increase/decrease p.d of transformer
voltage of mains electricIty in UK
230V
frequency of mains electricIty in UK
50 Hz
why is each wire in a three core cable colour coded?
so that it is easy to tell different wires apart
live wire colour
brown
neutral wire colour
blue
earth wire colour
green and yellow
purpose of live wire
carries the A.C from supply to appliance at 230V
purpose of neutral wire
carries the A.C away from appliance and completes the circuit
purpose of earth wire
for protection and safety
- provides an alternate low-resistance path for the high current to flow in case of a fault
- this prevents the current from going to the person
purpose of fuse
fuse connected to the live wire
if a fault occurs high current will cause fuse to heat up and melt
- this will break the circuit and stop dangerously current from flowing
power equation
P = VI
energy equation
E = Pt or VIt
step up transformer
- increases p.d and
- decrease current
step down transformer
- decreases p.d to safe domestic levels
- and increases current
why is the p.d increased first
- for a given power, if you increase p.d you decrease current
- less current means wires heat up less and less thermal energy is dissipated
- this makes National grid more efficient
