Electricity Flashcards
Terminal Potential Difference
Potential difference that appears across the terminals of a source
It is this potential difference that appears acrossthe external resistance
Lost Volts
The difference between emf and the tpd
The potential difference used to drive a current through the internal difference of a source
Given by Ir
Electromotive Force
The energy given by the source to each one of the charges passing through the source
Is equal to the sum of all the potential differences across all resistors
Including Pd across internal resistance r
What is the DC equivalent of 2.0v rms?
2.0v
The TPD is always
Less than the emf of the source
Ideal supply
Has no internal resistance
Short circuit
Has zero or little resistance
Current flows through a short rather than the component, removing it from the circuit.
Open circuit
Has infinite or high resistance
Gap in the circuit
No current flows
Capacitance of a capacitor found by
The slope of the line of a charge against potential difference graph
Energy stored in a capacitor given by
Area under a charge against potential difference graph
Emf= (Due to conservation of energy)
Vlost=
Vtpd + vlost
Ir
Emf on a graph
Y-Intercept (Terminal Pd) axis
Internal Resistance on a graph
Slope of the line = -r
Factors determining time for capacitor to charge
Resistance of a circuit - higher limits current and takes longer for capacitor to charge
Capacitance - Higher means that it will take longer to charge as it can hold more coulombs of charge per volt - charge takes longer to fill capacitor
Ohm’s Law (V=IR)
The potential difference across a component is proportional to the current flowing through it
Why Terminal potential difference falls when source current increases
Number of lost volts also increases
As lost volts is equal to the current multiplied by (the constant) internal resistance
A larger current causes more lost volts
Lost volts are not available at the terminals and so terminal pd falls
Why car batteries have a very low internal internal resistance
Cells which deliver a high current have a higher number of lost volts which means lower tpd
These Cells must have a very low internal resistance in order to deliver enough current to start the starter motor
Short circuit current formula
Derived from E=I(R+r)
Ishort= E/r
Wheatstone bridge circuit
When two potential dividers are connected in parallel
Frequency of an AC supply
Using the period
Number of boxes across the way for a cycle X Time Base setting gives period - T
Use f=1/T
Peak voltage of an AC supply
Using peak to calculate rms voltage
Height of trace above middle X voltage gain
Use formula Vrms= VPeak /sqrt2
1F is equivalent to
1cv^-1
State an application of capacitors
Flashing indicators
Reducing the value of a capacitor/resistor (For an application of capacitors) will
Allow a capacitor to charge in less time
The lamp will flash more frequently
Time between each flash will be less
As capacitance is less the capacitor discharges in less time and so the light is lit for less time
AND the capacitor will store less energy so the flash will be less bright
Metals are
Good conductors
Highest occupied band is not completely full - Conduction Band
In an insulator
Highest occupied band is full
First unfilled band is conduction band
Large gap between valence and conduction band
Not enough energy for electrons to move from valence to conduction band
In a semiconductor
Gap between valence and conduction band is smaller
At room temp there may be enough energy to move some electrons from valence to conduction band
An increase in temp in a semi conductor
Increases the conductivity of the semi conductor
An electron is a (Pn junctions)
Negative charged carrier
A hole is a
Positive charged carrier
A position where an electron is missing
n-type semiconductor
Material which has an excess of free electrons
Is made by doping with atoms having 5 electrons in their outer shell
They have more electrons than a pure semiconductor of the same size
p-type semi-conductor
Material which has an excess of free holes
Majority charge carriers are positive holes
They have fewer electrons than a pure semi-conductor of the same size
All n-type and p-type conductors are
Electrically neutral
Hole movement really is
The movement of electrons filling holes and leaving new ones in the atom they came from
Doping has the effect of
Lowering the resistance of the semiconductor
Depletion Layer
The area surrounding the p-n junction of a diode
Electrons have combined with holes
No excess charges
As bias voltage is increased
Current through the diode will also increase
Current IS NOT directly proportional to the voltage
Diodes referred to as ‘non-ohmic’ conductors
When supply voltage is greater than depletion layer voltage - Forward Bias only
Free electrons in n-type conduction band can be pushed across the depletion layer
Up into conduction band of p-type semi-conductor
Leakage current
The tiny amount of current in a reverse-biased diode
Reverse Bias
Acts as a very high value resistor
When p type material is connected to the negative of the supply
Electrons at that side have more potential energy than under no bias
Raises bands on p-type side
Increases slope of depletion layer and is harder for electrons to cross that barrier
No current as no conduction
Forward Bias
Junction voltage opposes the supply battery voltage
As supply voltage increases majority charge carriers are able to flow
When p type material is connected to the positive of the supply
Electrons at that side have less potential energy than under no bias
Lowers bands on p-type side
Conduction can take place
Points about LED’s
Efficient light production as little heat produced
Low power devices - Work on small voltages
Protected by a resistor connected in series
Monochromatic Light has
One energy, one frequency, one wavelength, one colou
Why light produced is not completely monochromatic
Some electrons will fall from bands above the bottom of the conduction band to lower bands than the top of the valence band
These electrons undergo a greater energy transition
Resulting photon will be of greater frequency
Light produced nearer to blue end of spectrum
Difference between circuit symbols of LED’s and Photodiodes
LED’s have arrows outwards - Light out
Photodiodes have arrows inwards - Light shining onto
Voltage against light level graph (Solar Cell)
Output voltage quickly rises and remains constant
While voltage quickly reaches a maximum the same is not true of current delivered
When light level is increased the current delivered by the cell continues to rise
Differences in design of LED’s and Photodiodes
P-type section at the top of the photodiode is much thinner
N-type covered with a material which transmits light
Both of these maximise light reaching the junction region
Photovoltaic Mode
Photodiodes can be used to provide energy for solar powered equipment eg calculators
Photodiode is the power supply and so does not require a bias voltage
Amount of energy available depends upon area exposed to the light and on the intensity and frequency of the light source
What is the overall charge on an unbiased piece of semi conductor material?
Neutral
The number of valence shell electrons in a
N-type
Pure Semiconductor
P-Type
5 in n-type
4 in pure semiconductor
3 in p-type
Bias conditions required for an LED to emit light
Forward Bias
The farad is equivalent to the
Coulomb per volt
Changing the resistance in a circuit with a capacitor
Has no effect on the maximum energy stored as C and V are still the same
Connecting resistors to give max and min resistance
Max - Series
Min - Parallel
Explain in terms of charge carriers how an LED emits light
Electrons in n-type material combine with holes in p-type material, they lose energy as they cross the junction. This energy is emitted as quanta of visible radiation.
Capacitor Facts
Used to block DC signals
Can be used to store energy
Can store electric charge
