electricity

Created by

Katie F

Cards (35)

charge is a fundamental property of some types of particles, such as protons and electrons
charge is measured in coulombs
current is the rate of flow of charge
$I =$ $ΔQ/ Δt$
one coulomb is defined as the amount of charge that passes a point in 1 second when the current is 1 ampere
potential difference is defined as the work done per unit charge $V=$ $W/Q$
power is the rate of transfer of energy
$P=$ $W/t$
power is measured in watts (W), where 1 watt is equivalent to 1 joule per second
a component has a resistance of 1Ω if a pd of 1V makes a current of 1A flow through it
resistance increases with length and decreases with diameter or cross-sectional area
$R=$ $V/I$
Ohm's law states that, provided the physical conditions, such as temperature, remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
$I∝V$
the resistivity of a material is defined as the resistance of a 1m length with 1m^2 cross-sectional area
$ρ=$ $RA/L$
resistance is a property of an object and it depends on the material and dimensions of the object. resistivity is a property of a material, not a specific sample
negative temperature coefficient (ntc) thermistors are those whose resistance decreases as the temperature increases
superconductivity is a property of certain materials which have zero resistivity at and below a critical temperature. such critical temperature depends on the material
uses of superconductors include power cables that transmit electricity without any power loss and really strong electromagnets, such as those used in medicine and Maglev trains
voltmeters are assumed to have infinite resistance so that no current flows through them
ammeters are assumed to have no resistance so that there is no pd across them
semiconductors are a group of materials that do not have as many free charge carriers as regular conductors
e.g thermistors, diodes and LDRs
the direction for which the current is allowed to flow is known as 'forward bias"
diodes are made from semiconductors, and are designed to let current flow in one direction only
in reverse bias the resistance of the diode is very high such that the current flowing is incredibly small
the electrical energy produced in a power source per unit charge is known as the electromotive force (emf,ε). it is measured in volts 
$ε=$ $E/Q$
for cells in series, you can calculate the total emf of their combination by adding their individual emfs
$εtotal=$ $ε1+$ $ε2+$ $ε3...$
for identical cells in parallel, the total emf of the combination of cells is the same size as the emf of each of the individual cells
Kirchhoff's first law states that the total current entering a junction is equal to the total current leaving the junction
Kirchhoff's second law states that the sum of the emfs round a loop in a circuit is equal to the sum of the potential drops round the loop
For components in series:
$Rtotal=$ $R1+$ $R2+$ $...$
for components in parallel:
you have to take the reciprocal in order to find Rtotal
$1/Rtotal =$ $1/R1+$ $1/R2+$ $...$
the potential divider used to supply constant or variable potential difference from a power supply
for an unloaded potential divider, the current through each resistor is the same so the potential difference is proportional to the resistance
$V1/V2=$ $R1/R2$
$Vout=$ $R2/(R1+R2)*$ $Vin$ in this case R2 is the resistor across which you are measuring the pd
potential difference is where charges do work in the component, but emf does work to the charge
$E=$ $IVt$
not on data sheet
current flows through all parts of a circuit, including the cells. since the cells themselves are made of a material with resistance there will be some heating in the cell, this is known as internal resistance
the energy wasted per coulomb overcoming internal resistance is called lost volts (v)
the p.d drop across the internal resistor
the potential difference across the terminals of the real cell, accounting for the lost volts, is known as the terminal potential difference