Electricity Flashcards
State what it meant by alternating current (a.c.)
Analternating currentis onewhich changes directionand instantaneous value with time i.e.the charges arecontinually changing direction.
State what is meant by the peak voltage of an ac supply
Peak voltage it the maximum voltage supplied by the alternating source
When performing calculations of power in ac circuits, do you use peak or rms values of current and voltage?
rms
Current is the same in ______
series
Voltage is the same in _______
Parallel
The total current entering a junction must _______ a __________
leave a junction
State what is meant by the EMF of a supply
EMF is a measure of the energy given to each coulomb of charge passing though the supply
State what is meant by the t.p.d. of a supply
T.p.d. is the voltage measured across the terminals of a battery i.e. it is the voltage dropped across the load connected to the circuit
State what is meant by internal resistance
Internal resistance is the resistance of the supply itself
State what is meant by an “ideal supply”
An ideal supply has zero internal resistance i.e. r=0
State what is meant by the term “open circuit voltage”
Open circuit voltage is the voltage of a supply when no current is drawn (due to there being infinite load resistance R). Open circuit voltage is equal to the EMF of the supply
How might you make an “open circuit”?
Connect nothing across the terminals of the battery or provide no path for current to flow between the terminals
The open circuit voltage of a supply is equal to…
its EMF
When a supply is short circuited it has a load resistance of…
zero i.e. a wire is connected directly from the positive terminal to the negative terminal.
short circuit current is…
the maximum current that can be provided by a supply.
How do you find internal resistance from a t.p.d vs current graph?
Internal resistance is the negative gradient of a t.p.d. against current graph.
Capacitance is…
Capacitance is the number of coulombs of charge stored per volt.
What does the gradient of the graph of charge vs potential difference tell you?
capacitance
What does the area under the graph of charge vs potential difference tell you?
energy stored on a capacitor (i.e. the work done in charging it)
What will the voltage across a capacitor be when it is fully charged?
Equal to the supply voltage
How much current will flow through a circuit with a fully charged capacitor that is connected in series with a battery and a resistor?
none
When a capacitor is fully discharged how much voltage is across it?
0 V
When does a resistor connected to a charging capacitor have the maximum flow of current through it?
As soon as it starts charging i.e. when it has no voltage across it and all the voltage is across the capacitor which it is connected to
The current starts/ends big for both charging and discharging a capacitor
The current starts big for both charging and discharging a capacitor
State two things which change the time taken for a capacitor to charge
- Bigger capacitance 2. Bigger load resistance
Increasing the load resistance connected to a capacitor increases/decreases the maximum current in the circuit
Increasing the load resistance connected to a capacitor decreases the maximum current in the circuit (I=V/R)
State two things which can affect the energy stored on a capacitor
- Capacitance 2. Supply voltage (i.e. E=0.5CV^2)
State what is meant by the term “band gap”
The gap between conduction and valance bands where no states exist
What is an energy band?
A region where it is possible for charge carriers to occupy
Electrons cannot move in the conduction/valance band
Electrons cannot move in the valance band
Electrons are free to move in the conduction/valance band
Electrons are free to move in the conduction band
Describe the energy bands in a conductor
There are free electrons in the conduction band of a conductor due to them overlapping or being partially filled
Describe the energy bands in an insulator
The valence band is full. The conduction band is empty. The band gap is too big for electrons to cross
Describe the energy bands in a semiconductor
A semiconductor is like an insulator with a much smaller band gap. The gap is small enough that room temperature is enough to give some electrons the energy to jump up from the valence to conduction band and increase the conductivity of the solid.
How does the temperature of a semiconductor affect its conductivity?
As temperature increases its conductivity increases
What is the name given to adding impurities to a semiconductor to increase its conductivity?
doping
What is an n-type semiconductor?
An n-type semiconductor has had an impurity with extra electrons added. As these are not used for valency (holding the solid together) they are added to the conduction band and so it conducts more.
What is a p-type semiconductor?
A p-type semiconductor has had an impurity with ‘missing’ electrons added. There are gaps in the valence band. This also increases conductivity.
What is a p-n junction?
when p-type and n-type semiconductor materials are side by side
What is a depletion layer?
A depletion layer forms which has virtually no free charge carriers.
What is forward bias?
p-type side of a p-n junction is connected to the positive terminal of a power supply and the n-type side is connected to the negative terminal
What is reverse bias?
means p-type to negative and n-type to positive
What is a solar cell and how does it work?
A solar cell is an unbiased p-n junction. Photons of light enter the depletion layer and give electrons enough energy to move from the valence band of the p-type into the conduction band of the p-type. The potential barrier of the depletion layer provides the electric field needed to move across to the conduction band of the n-type and a voltage is produced. This is the photovoltaic effect.
What is an LED?
An LED is a forward biased p-n junction. Electrons cross from the conduction band of the n-type to the conduction band of the p-type where they release energy as a photon of light as they ‘fall’ into the valence band of the p-type.
