electric currents Flashcards
insulator
all electrons firmly bound to nuclei, does not cause flow of electrons elsewhere
conductor
de localized or free electrons that are not bound to any atom of he metal (redistributes charges)
electric current
the force per unit length between parallel current carrying conductors (amps)
speed of electric field in a current
around the speed of light
proportionality in a wire
the current is proportional to the drift velocity and number of electrons per mm of wire
what effect does current always have
heating (because of acceleration and collisions of electrons and electric potential energy is transferred to heat energy)
what happens to electric potential energy when charges separated
it increases (hence electrons gain electric potential energy through a cell)
electromotive force (ε)
the energy per unit charge converted into other form from a battery or power supply (V(=j/coulomb))
Electric potential difference
the change of electric potential energy when 1 C (coulomb) moves between two points
Kirchoff’s second rule
emf=sum of p.d.s
electron-volt
the energy gained or lost by an electron when it moves through a voltage of 1 volt
eV=0.5mv^2 1eV=1.6*10^-19j
voltage
energy available per electron, therefore if more available it will speed up
The power provided by a cell=
the total power emitted by the circuit components
power
the power (energy/second) supplied = the amount of electrical energy each coulomb of charge acquires (emf) and the number of coulombs passing through per second (current) P=emf*current (Amps) the power dissipated (such as from resistor) = the amount of electrical energy each coulomb transfers (p.d) and the current P=p.d*current P=VI
work-done on a charge when moving through a voltage
W=qV
resistance
the opposition to the flow of current (V/coulomb)
ratio of loss of electric Ep/per unit charge to the quantity of mobile charge per mm by the drift velocity
drift velocity
average velocity gained from an electric field
3 elements of resistance
internal structure of the metal, resistivity
resistance proportional to length
inversely proportional to the cross-sectional area of the wire (ie thinner wire is faster with more cars)
resistance=resistivity*length/cross-sectional area
=pl/A
resistivity
constant in a material, but increases with temperature
ohm’s law
the resistance of a conductor constant if temp constant or current proportional to pd if temp constant
ohmic components
components that obey Ohm’s law
thermister
temperature variable resistor
non-ohmic components
do not obey ohm’s law because of heating effect (eg filament lamps)
I-V curve
straight if obeys ohm’s law
curves to point towards v axis as temperature increases with resistance
specifications of lamps
operating voltage, the current at the voltage and power (input/output) required. under these the lamp is at normal brightness, lower will be dimmer and higher brighter but less long lasting
Ammeters
connected in series, zero resistance
measures current
Voltmeter
connected in parallel, infinite resistance to measure the change in voltage from one side of a component to another
internal resistance in a cell
chemical reactions still provide same energy, but electrons collide in the cell and the chemical energy is transferred to thermal energy as well as electric potential energy
ε=I(R+r) where R is the total resistance of external circuit and r internal resistance of the cell. ε is the emf
effects of current (terminal voltage and collisions)
larger current increases collisions in cells
larger effect on terminal voltage at lower current
optimum resistance
when power delivered to the external resistance will be a maximum and this happens to be when the external resistance= internal resistance
series
in a line, one after the other
Rt=R1+R2+R3…
parallel
division of current to parallel components
1/Rt=1/R1+1/R2+1/R3
potential divider circuit
potential difference is divided between the various resistances in the circuit
Light Dependant resistors (LDR)
low resistance in well lit conditions
high resistance in dark
Negative temperature coefficient thermistors
low resistance in warm conditions
high resistance in cold conditions
Strain gauges
as the gauge is stretched, then the resistance increase with smaller diameter wire.
