electricity Flashcards
direct current
flow of charge in one direction only; negative to positive
alternating current
the terminals alternate their roles continuously, meaning the electrons effectively oscillate in place, back and forth
timebase and voltage gain
timebase is x-axis
voltage gain is y-axis
peak and r.m.s equations
Vrms = Vpeak/root 2
Irms = Ipeak/root 2
potential difference definition
the energy given to each unit of charge by a power supply
current definition
rate of flow of charge
power definition
(electrical) the rate at which electrical energy is converted from one form into another
resistance definition
the opposition of flow of charges.
Ohm’s Law
V = IR
(voltage = current x resistance)
internal resistance
resistance exhibited by power supplies
EMF
- stands for electromotive force
- energy supplied to each coulomb of charge passing through the supply, before the effects of internal resistance are taken into account
lost volts
the voltage ‘lost’ due to internal resistance and therefore not available to the rest of the circuit, generally converted into heat energy
t.p.d
- terminal potential difference
- the voltage actually available to the circuit
= EMF - lost volts
what would an ideal supply have?
no internal resistance
short circuit
occurs when a power supply is directly connected to itself, there is no internal resistance and current becomes very high.
open circuit
occurs when there’s no connection between the terminals of the power supply (switch opened).
- no current
- no lost volts
- t.p.d and EMF equal
what is a capacitor?
component that can be used to store energy, consists of two metal plates facing each other, separated by an insulator.
how does a capacitor work?
a cell or battery does work to move charges away from one plate of the capacitor to the other, meaning one plate is positively charged and the other negatively. if it is connected to a different component, the capacitor will discharge and the charges will redistribute until both plates are neutral again.
what is capacitance?
the measure of how much charge can be stored in a capacitor per volt connected across it.
measured in Farads
capacitance equation
C = Q/V
( capacitance = charge/voltage)
effect of varying capacitance
increasing capacitance increases the time taken to charge, initial current is not affected
effect of varying resistance
increasing resistance increases the time taken to charge, initial current is lower
band theory
energy bands exist in a very similar way to energy levels in individual atoms, when a material has very many atoms, it makes less sense to think about each possible energy level and more sense to treat the group as a band
conduction band
outermost band, has the highest energy.
when it’s partially filled, the material can conduct
valence band
band below conduction band
properties of insulators
no electrons in conduction band, band gap too big for electrons to be promoted from valence band.
no conduction takes place
properties of semiconductors
normally no electrons in conduction band but band gap is small enough that electrons could be promoted from valence band.
conduction will take place under correct circumstances
properties of conductors
no gap between bands, they overlap so electrons are already in conduction band, this allows movement of electrons. conduction will take place
production of n-type semiconductors
- group 5 element is introduced, this will result in a “free” electron, allowing a flow of charge within the material
- not electrically negative as each electron is still accompanied by a proton
production of p-type semiconductors
- group 3 element introduced, which leaves a “hole” that can be occupied by an electron. provides a stepping stone-like path for electrons, allowing a flow of charge within the material
- not electrically positive as each electron is accompanied by a proton in the nucleus
formation of p-n junctions
when a substrate is doped with both a group 3 and 5 element, an interface called a p-n junction forms
- the electrons from n-type will migrate to fill holes in the p-type
- this results in the n-type becoming slightly positive and the p-type slightly negative, the region between them is called a depletion layer
forward bias
- diode is connected to d.c power supply in order to allow the flow of charge (n-type to negative terminal, p-type to positive terminal)
- increasing voltage causes depletion layer to shrink and when it’s overcome, the material will conduct
reverse bias
- diode is connected to d.c power supply the opposite way round (n-type to positive terminal and p-type to negative terminal)
- power supply effectively increases depletion layer, so increasing voltage does not overcome depletion layer
how do LEDs work?
- electrons in n-type move across to p-type
- this transition results in a decrease in electron energy which, in turn, causes a photon of the same energy to be emitted
how do solar cells work?
- light incident on the junction within the solar cell causes an electron to be promoted to conduction band
- electrons migrate from p-type to n-type, creating a voltage across the solar cell
- this production of electricity is known as a photovoltaic effect
band theory of electron energy
