Electricity Flashcards
Charge, current, time equation
Q=It
Current
Rate of change of charge
Measured in Amperes (A)
What is one coulomb
The amount of charge that passes in 1 second through a current of 1 Ampere
Electron flow vs conventional current
E: what actually happens, electrons flowing from negative terminal of cell to positive terminal
C: what is shown in diagrams, current going positive to negative
Which part of a battery symbol is positive and which is negative
Positive is the long terminal
Negative the short terminal
What causes electrons to flow
A potential difference
Flow from a higher potential to a lower potential
What happens to current if a kink is made in a wire
Causes resistance (difficulty in the flow of electrons) so decreases the current
What is Ohm’s law
The current flowing through a metallic conductor is directly proportional to the potential difference applied across it
At a constant temperature
Conditions for Ohm’s law
Metallic conductor
Constant temperature
Equation for Ohm’s law
V=IR
V∝I
Charge of one electron
-1.6x10^-19C
How many electrons needed to produce a charge of 1C
6.25x10^18
1/(1.6x10^-19)
Potential difference
The potential difference between two points is the amount of energy transformed when one coulomb of charge is moved between the points
What is resistance
A ratio of voltage to current
Measured in Ohm’s
1 Volt =
1 Joule per coulomb
Equation for work done, charge and voltage
W=QV
W=E
E=QV
Power
The rate at which work is done
The rate at which energy is transferred from one type to another
Measured in joules per second or watts
Equation for power
P=W/t or P=E/t
P=VI
P=I^2R
P=V^2/R
How is P=VI obtained
Q=It (1)
W=QV (2)
W=Pt (3)
2 and 3;
QV=Pt
P=QV/t
1;
Q/t=I
1 and 2 and 3;
P=IV
How is P=I^2R obtained
P=IV
V=IR
P=IIR
P=I^”R
How is P=V^2R obtained
P=IV
V=IR
I=V/R
P=(V/R)V
P=V^2/R
Energy transformed by a component
E=VIt
Since E=Pt and P=VI
Current voltage graph for a diode
X=V
Y=I
Threshold voltage of around 0.6V
Very little current can flow in reverse
For very large voltages, current will be forced through diode in wrong way and will eventually break it
Gradient of a current voltage graph
NOT RESISTANCE
Ratio of current to voltage
How do you get negative results
Flip battery
Current voltage graph for a fixed resistor
X=V
Y=I
Straight line through origin
Current voltage graph for filament bulb
X=V
Y=I
Initially a straight line through 0 where it acts as an ohmic conductor with constant resistance
Then at high positive and negative voltages it starts to bend towards X axis
What is a short circuit
Zero/very small resistance
Infinite/excessive current flows
May damage the component and break the circuit
What is an ammeter
Piece of equipment that measures the current in a circuit
Its resistance should be zero in order for it not to contribute to the resistance of the circuit or remove any voltage/no potential difference, lower the current and hence give an inaccurate reading of current
What is a voltmeter
Piece of equipment that measures the potential difference between two points in a circuit/across a component
It acts as an observer so connected in parallel
Resistance is infinite so current is zero
What is a rheostat
A piece of equipment that can be used as a variable resistor (using 2 terminals) or a potential divider (using all 3 terminals)
Pros vs Cons for a variable resistor circuit
Simple to set up
Used in sensor circuits
Series circuit so current can be higher than in a potential divider circuit
Can’t ever get the voltage to be zero
Pros vs Cons for potential divider circuit
Allows full range of supply voltage to be used
Current/p.d can be reduced to zero
Used for volume control and dimmer switches
More complicated to set up than a variable resistor
Current has to be split due to the parallel branches so maximum current lower
Ohmic component
Follows Ohm’s law
Where the current flowing through a metallic conductor is directly proportional to the potential difference applied across it at a constant temperature
So the resistance is constant
Non-Ohmic component
Does not follow Ohm’s law
So current and potential difference are not directly proportional
And there is varying resistance
What is a series circuit
Only one path current can take that connects the positive terminal to the negative terminal
No junctions or branches
What is the current like in a series circuit
Constant at every point
What is the potential difference like in a series circuit
May vary through different components if the resistance is different
Bu the total voltage adds up to the supply voltage
What is a parallel circuit
Multiple paths that current can take from the positive terminal to negative terminal
Contains junctions and branches
Current in a parallel circuit
Current in each branch may be different but the total current is the same as the supply current
Potential difference in a parallel circuit
p.d across each branch is the same and equals the supply voltage
Kirchoff’s first law
At any junction in a circuit the sum of current flowing into the junction is equal to the sum of current flowing away from it
Kirchoff’s second law
In any complete loop of a circuit the sum of potential difference is equal to the source potential difference
How do you work out the total resistance in a parallel circuit
Rt=(1/R1 + 1/R2 + 1/R3 + …)^-1
What determines brightness
Power
Not current or voltage
Effect of connecting two cells/power supplies in series
Potential difference is equal to the sum of the individual potential differences
Effect of connecting two cells/power supplies in parallel
Potential difference will be the same as the potential difference of each source
But the current flowing through each will be half so they will last twice as long
NTC
Negative temperature coefficient thermistor
As temperature increases the resistance of the thermistor decreases
Assume all NTC unless told otherwise
PTC
Positive temperature coefficient thermistor
As temperature increases resistance of the thermistor increases
LDR
Light dependent resistor
As light intensity increases the resistance of an LDR decreases
Uses of potential dividers
Sensor circuits, often with NTCs and LDRS
If the temperature increases explain how the voltage changes in an NTC in series
As temperature increases the thermistor resistance decreases
So now receives a smaller proportion of the total resistance
Hence receives a smaller proportion of the total voltage
Since V=IxR, and I is constant
If the temperature increases explain how the voltage changes in another resistor in series with an NTC
As temperature increases the thermistors resistance decreases
So now receives a smaller proportion of the total resistance
So the other resistor receives a larger proportion of the total resistance
Hence receives a larger proportion of the total voltage
So voltage increases
Since V=IxR, and I is constant
If the temperature decreases, explain how the voltage changes in an NTC in series
As temperature decreases the thermistor resistance increases
So receives a larger proportion of the total resistance
Hence receives a larger proportion of the total voltage
Since V=IxR, and I is constant
If the temperature decreases explain how the voltage changes in another resistor in series with an NTC
As temperature decreases the thermistors resistance increases
So now receives a larger proportion of the total resistance
So the other resistor receives a smaller proportion of the total resistance
Hence receives a smaller proportion of the total voltage
So voltage decreases
Since V=IxR, and I is constant
If the temperature increases explain how the current changes in another resistor in series with an NTC
As temperature increases the thermistor resistance decreases
So total resistance decreases
From Ohm’s law, if the total resistance decreases the total current increases
Since I=V/R and V is constant
So the current increases
If the temperature decreases explain how the current changes in another resistor in series with an NTC
As temperature decreases the thermistor resistance increases
So total resistance increases
From Ohm’s law, if the total resistance increases the total current decreases
Since I=V/R and V is constant
So the current decreases
If the temperature increases explain how the current changes in an NTC in series
As temperature increases the thermistor resistance decreases
So total resistance decreases
From Ohm’s law, if the total resistance decreases the total current increases
Since I=V/R and V is constant
So the current increases
If the temperature decreases explain how the current changes in an NTC in series
As temperature decreases the thermistor resistance increases
So total resistance increases
From Ohm’s law, if the total resistance increases the total current decreases
Since I=V/R and V is constant
So the current decreases
If the light intensity increases explain how the voltage changes in an LDR in series
As intensity increases the resistance of the LDR decreases
So receives a smaller proportion of the total resistance
Hence a smaller proportion of the total voltage
So voltage decreases
If the light intensity increases explain how the voltage changes in another resistor in series with an LDR
As intensity increases the resistance of the LDR decreases
So receives a smaller proportion of the total resistance
Hence a smaller proportion of the total voltage
So the other resistor receives a larger proportion of the total resistance and hence total voltage
So voltage increases
If the light intensity decreases, explain how the voltage changes in an LDR in series
As intensity decreases the resistance of the LDR increases
So receives a larger proportion of the total resistance
Hence a larger proportion of the total voltage
So voltage increases
If the light intensity decreases explain how the voltage changes in another resistor in series with an LDR
As intensity decreases the resistance of the LDR increases
So receives a larger proportion of the total resistance
Hence a larger proportion of the total voltage
So the other resistor receives a smaller proportion of the total resistance and hence total voltage
So voltage decreases
If the light intensity increases explain how the current changes in another resistor in series with an LDR
As the light intensity increases the LDR’s resistance decreases
So the total resistance decreases
From Ohm’s law if the total resistance decreases then the total current increases
Since I=V/R and V is constant
So the current increases
If the light intensity decreases explain how the current changes in another resistor in series with an LDR
As the light intensity decreases the LDR’s resistance increases
So the total resistance increases
From Ohm’s law if the total resistance increases then the total current decreases
Since I=V/R and V is constant
So the current decreases
If the light intensity increases explain how the current changes in an LDR in series
As the light intensity increases the LDR’s resistance decreases
So the total resistance decreases
From Ohm’s law if the total resistance decreases then the total current increases
Since I=V/R and V is constant
So the current increases
If the light intensity decreases explain how the current changes in an LDR in series
As the light intensity decreases the LDR’s resistance increases
So the total resistance increases
From Ohm’s law if the total resistance increases then the total current decreases
Since I=V/R and V is constant
So the current decreases
What is 1 Volt
The energy transformed per coulomb of charge between 2 points
Explain why the resistance of an NTC thermistor decreases when its temperature increases
As temperature increases the weakly bound electrons going enough energy to become free
They can carry a current
Lattice ion vibrations still increase but the freeing of electrons is more significant
What is resistivity
A measure of how much a particular material resists current flow
Measured in Ohm metres
And is a property of the material
The resistance of a 1m length wire with a cross sectional area of 1m^2
Explain the resistivity for a material that conducts electricity
Low resistivity
How does thickness effect resistance
Larger cross sectional area
Bigger lattice ion structure
Increases number of paths for electrons to travel in
Resistance decreases
How does thickness effect resistivity
It doesn’t
Resistivity is constant and is a property of the material
It will instead affect resistance
How can you stop a circuit overheating
Use a switch and turn off the circuit when not in use/between readings
What is a superconductor
A material that conducts electricity with zero resistance below a critical temperature
Hence can carry a current without losing energy
What is a superconductor
A material that conducts electricity with zero resistance below a critical temperature
Hence can carry a current without dissipating energy as heat
Uses of superconductors
Maglev trains
Speed up connections between computer chips
Superconducting coils make possible the very powerful electromagnets in MRI machines
Uses of superconductors
Power cables that transmit electricity without any power losses
Really strong electromagnets like those in maglev trains or MRI
Electronic circuits that work very fast with minimal energy losses since there is no resistance to slow the current down
EMF
Electromotive force
The amount of energy per unit charge produced inside a cell
Or
The voltage across terminals of a cell when no current is flowing through it
TPD
Terminal Potential Difference
The voltage measured across terminals of a cell when current is flowing through it
What is internal resistance
The resistance of the cell itself
So it cannot be separated from it
Why is TPD less than EMF
Some voltage from the cell is used up by the cells own internal resistance
This voltage is know as the lost volts
TPD, EMF and lost volts equation
V=E-U
TPD = EMF - lost volts
EMF, current and resistance equation
E=I(Rxr)
R=Resistance of circuit
r=Internal resistance
EMF, current and resistance equation
E=I(R+r)
R=Resistance of circuit
r=Internal resistance
Equation for the energy wasted due to total internal resistance
EMFI=I^2R+I^2r
EMFI=Energy per second supplied by the source
I^2R=Energy per second supplied to external components
I^2r=Energy per second used up by internal resistance/Dissipated as heat in the battery
EMFI=I^2R+I^2r derivation
P=IV and EMF=IR+Ir
So to get power, you multiply everything by I
Explain the ideal internal resistance for a car battery
Cars need to supply a large current to have a large power
But internal resistance limits current
So having a low internal resistance means it can provide a higher current
Also means less energy wasted on internal resistance
So batteries last longer between charges
A higher TPD would be possible since less p.d lost across internal resistance
Explain the graph of a diode
Below the threshold voltage of 0.6V the resistance is infinite so current is zero
As the voltage increases beyond the threshold value, the weakly bound electrons in the semiconductor gain energy
So become free and able to conduct electricity
Lattice ion vibrations will still increase (working to increase resistance) but this has a lesser effect than the freeing of electrons
Explain the graph of a filament bulb
Initially acts as an ohmic conductor
But as the current increases so does the temperature of the filament
Causing lattice ion vibrations to increase
Increasing the number of electron collisions per second with the lattice ion structure per second
The current will continue to increase but at a lesser rate
Explain the graph of an ohmic conductor
As voltage increases by fixed amounts so does current
Resistance at each point calculated by dividing the voltage by the current
This value of resistance stays constant for the graph
What is a semiconductor
A group of materials that aren’t as good as conducting electricity as metals due to having less electrons
If energy is supplied to them (e.g. increasing the temperature) more charge carries/electrons can be released from their weakly bound electron structure
And the resistivity of the material decreases
Hence make good sensors for detecting changes in their environment
Such ass thermistors, LDR’s and diodes
Energy, emf and charge equation
emf=E/Q
Since emf is just a voltage
4 different emf equations using the same variables
E=I(R+r)
E=V+U
V=E-u
V=E-Ir
What does resistivity depend on
The structure of the material
Environmental factors such as temperature and light intensity
What is a potential divider
A circuit with a voltage source and more than one resistor in series
Useful equation for potential divider circuits
I=V/Rtotal
I=Vout/Rout
V/Rtotal = Vout/Rout
Define the volt
1 joule per coulomb
Advantage of using a potential divider to control current vs advantage for a variable resistor
Potential divider can provide sensitive control from zero to max
Variable resistor can provide a larger current but can’t get near 0
Explain internal resistance
Comes from electrons colliding with atoms and losing their energy
In a battery chemical energy is used to make electrons move
As they move they collide with atoms in the battery
Meaning it must have resistance
