electricity Flashcards
what does an ammeter read?
current measured in A
equation for charge, currrent, and time
Q= It
what is voltage?
energy given to each coulomb of charge passing through a power supply
what is voltage also known as?
potential difference
equation for energy given to each coulomb of charge (or work done to fully charge a capacitor)
W= QV
ohms law formula
V= IR
equation for power from current flowing and potential difference
P= IV
potential dividers formula
V= (R1/ R1+R2) x Vs
equation for power, energy, and time
P= E/t
DC signals current direction
one direction only
AC signals current direction
changes direction periodically
what does the y-gain of an oscilloscope tell you?
voltage value per box
what does the time base (x) of an oscilloscope tell you?
how long wave takes to cover one box in x-direction
peak voltage equation
pv= no divs x y-gain
period of wave equation
T= no divs x timebase
frequency equation
f= 1/ T(period)
notation for time base
10^-3
v peak formula
vpeak= √2 x Vrms
I peak formula
ipeak= √2 x Ims
when can you use V= IR?
using both peak voltage in current
or
using both rms voltage and current
what is EMF?
the electromotive force of a source is;
energy supplied to each coulomb of charge passing through the source
what happens when the switch in a circuit is open?
no current flows and voltmeter reads EMF
what are lost volts?
energy wasted inside a source due to its internal resistance
lost volts formula
V= Ir
what is terminal potential difference?
voltage that remains after lost volts are subtracted from the EMF
EMF formula
E= V= Ir
what happens in a short circuit?
external resistance effectively zero
what happens due to a short circuit in E= Ir?
current is incredibly high
why is the EMF and terminal potential difference different?
current flowing with switch closed means r must be considered and there are lost volts
parallel resistors formula
1/Rt= 1/R1 + 1/R2
what happens to the voltmeter reading if all resistors are connected in parallel?
total external resistance decreases
voltmeter reading decreases
current flowing increases
lost volts increases
what 2 things does a graph of 1/i against R give?
gradient gives EMF
y-intercept gives -r
what 2 things does a graph of V/i give?
gradient gives -r
y-intercept gives EMF
how do you find the short circuit current?
where line cuts x axis
what is a capacitor?
a device designed to store charge
what is capacitance measured in?
farads
micro-farad notation (μF)
1x10-⁶
nano-farad notation (nF)
1x10-⁹
what happens when capacitor is fully charged?
potential difference has reached the same point as supply voltage
what happens in the directly proportional relationship between a capacitor and voltage?
the more charged a capacitor is, the higher the potential difference across the capacitor
equation for capacitance
C= Q/V
how is the energy stored in a capacitor found?
area under a graph
energy stored in a capacitor formula
E= ½QV
when is the only time E= ½QV can be used?
energy in a capacitor
why is the work done to fully charge a capacitor and the energy stored in a capacitor different?
only half work is used to charge capacitor
other half given off as heat in resistor R
what is a smoothing capacitor?
smooth or even out fluctuations in a signal
electron availability in conductors, insulators, and semi- conductors
conductor- many free electrons
insulator- very few free electrons
semiconductor- very few free electrons when pure, but conduct when impurities present
2 examples of semi-conductors
silicon
germanium
2 highest energy bands in order
conduction band
valence band
when will a solid conduct?
when there are electrons in the conduction band
band theory in an insulator
valence band completely full- electrons can’t move
energy/ band gap is large so electrons can’t gain enough energy to move up to conduction band
band theory in a semi- conductor
valence band completely full
energy/ band gap is small so electrons can move up to conduction band if they gain enough energy, and then be free
band theory in a conductor
valence and conduction band usually overlap, meaning electrons free to move
what are intrinsic semiconductors?
have four outer electrons
all outer electrons are bonded so there are very few free electrons- making for a large resistance
2 places electrons come from in intrinsic semiconductors
imperfections in the lattice
thermal ionisation due to heating
what happens when temperature increases in intrinsic semiconductors?
produces more free electrons
2 changes due to increased temperature in intrinsic semiconductors
conductivity increases
resistance decreases
what is a positive hole?
when an electron leaves its space in the valence band and leaves a space that is positively charged
what may happen to this hole?
may be filled by electron from neighbouring atom
this would then leave its own hole
what happens if an impurity atom with 5 outer electrons is present in the lattice?
an extra free electron is introduced
2 effects of the 5 outer electron impurity atom
conductivity increases
resistance decreases
what is this process of adding an impurity atom called?
doping
why is it called an n-type semiconductor?
majority charge carriers are negatively charged electrons
where does this extra electron from the n- type semiconductor lie?
within the band gap- but a small amount of energy away from jumping to the conduction band
example of a 5 outer electron impurity atom
arsenic
example of a 3 outer electron impurity atom
indium
what happens when a 3 outer electron impurity atom is introduced to the lattice?
hole is introduced where an electron is missing
how can conduction take place in a 3 outer electron impurity atom?
electrons from adjoining atoms through the movement of positive holes created
why is it called a p-type semiconductor?
majority of charge carriers are positively charged holes
what does the small potential difference across the junction do?
opposes any further movement of charge
what occurs in the region around the junction?
there is no charge and is an insulator
called the depletion layer
what does “biasing” a semiconductor device mean?
apply voltage to it
2 ways of semiconductor bias
forward biased
reverse biased
what happens in a forward bias?
electrons attracted to positive terminal of battery
holes will be attracted to negative terminal of battery
current flows
what happens in a reverse bias?
electrons move toward positive terminal
holes move toward negative terminal
depletion layer grows
no current flows
what does the result of a reverse bias mean for current and what is this known as?
it can only flow one way
diode
what is an LED?
consist of p-n junction diode connected to a positive and negative terminal
what is the difference between a p-type and an n-type semiconductor and why?
p-type semiconductor valence and conduction bands are higher up
due to electrons needing higher energies to jump to conduction band
3 movements describing how and LED works
electrons at junction move from n-type to p-type conduction band
holes move from p-type to n-type valence band
electrons drop down to valence band to fill holes
what happens when the electrons drop?
photon is emitted
what does the colour of light emitted from photon when electrons drop depend on?
band/energy gap
bigger the band/energy gap, the higher the frequency of light emitted
what is a photodiode?
a p-n junction with a transparent coating that reacts to light
what happens when light of a higher energy than the band gap falls on the junction and what is produced?
excites an electron from the valence band and into the conduction band
leaves behind a hole
electron-hole pair produced
what happens after electron-hole pair made and what is produced?
electron accelerated into n-type
hole accelerated into p-type
photocurrent produced
what is happening as this photocurrent is produced?
light energy being converted to electrical energy
what can this photodiode be used as and what is this the basis of?
as a power source
basis of a solar cell
full photovoltaic effect process
light of higher energy than band gap falls on junction
this excites electron from valence into conduction band, leaving behind hole, producing electron-hole pair
electron accelerated into n-type and hole to p-type
this causes photocurrent to be produced
light energy converted to electrical energy
photodiode used as power source- basis of solar cell
