Electricity

current

• the rate of the flow of charge

conventional current vs actual current

conventional current: positive to negative

actual current: negative to positive

resistance and temperature

• temperature increases - amplitude of vibrations increases - more collision of electrons with metal ions - resistance increases

• current increases - more electrons collide with the positive ions of the metal - transfer of the electron kinetic energy to the positive ions - component / wire heats up to to amplitude of vibrations of positive metal ions

• the resistance of a filament lamp will be lowest when it is first switched on, hence the initial current will be at its largest value, causing a sudden rapid change in temperature - this is why it is more likely to fail when it is first switched on.

potential difference

the work done per unit charge between two points when charges when charge moves between them

insulator, conductor and semiconductor

• insulator - electrons are not free to move - don’t flow when a potential difference is applied

• metals - conductor - some delocalised electrons - will flow when a potential difference is applied

• semiconductor - number of charge carriers increases with temperature or light intensity as electrons are liberated - reducing resistance

number of electrons

(current x time) / e = number of electrons

potential difference in series vs in parallel

series: Vs = V1 + V2 + …

parallel: Vs = V1 = V2 =…

electromotive force: emf

• the amount of electrical energy per unit charge produced inside a source of electrical energy

• similar to voltage but takes into account internal resistance of the power supply, making it greater than the voltage

power

work done per unit time

power = current x potential difference

resistance and ohms law

• a measure of how difficult it is for current to flow through a component in a circuit

• R = V/I

• the pd across a metallic conductor is proportional to the current it, provided the physical physical conditions do not change

measuring resistance

• set up a series circuit

• ammeter placed in series used to measure current flowing through it

• ammeter should have zero resistance to avoid altering the reading due to internal resistance

• voltmeter placed in parallel and used to measure pd across it

• voltmeter should have infinite resistance to stop current flowing though it

• each time variable resistor is adjusted, a reading is taken from the ammeter and the voltmeter

• voltage current graph plotted - gradient is the resistance of the resistor

factors affecting resistance

• resistance is directly proportional to length

• inversely proportional to cross sectional area

• the constant of proportionality is the resistivity

• current takes the path of least resistance

resistivity

the resistance per unit length x the cross sectional area of a material

superconductors and the critical temperature

• in some metals and alloys, when the material is cooled down to to a certain temperature, the resistance of the material falls to zero - superconducting

• the critical temperature is the temperature at which the electrical resistivity drops to zero

applications of superconductors

• superconducting power cables - high temperature superconductors - lossless transmission of electrical power

• superconducting electromagnet - uses coils of superconducting wire - cooled to low temperatures during operation. can produce stronger magnetic fields than ordinary iron core electromagnets and can be cheaper to operate

oh mic and non ohmic components

• ohmic components have a constant value of resistance as the current through it varies

• non ohmic component have resistance varying with current

positive and negative temperature coefficient

ptc - resistance increases with increasing temperature

ntc - resistance decreases with increasing temperature