# Current Electricity

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Conventional current flows around a circuit from positive to negative
However the electrons around a circuit flow in the opposite direction
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Charge
(Q) measured in coulombs (C)
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Charge of an electron
1.6x10^-19C
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Current
(I) measured in A - current is the rate of flow of charge (1A = 1JC^-1)
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Potential Difference
(V) and is measured in V - P.D. is the work done per unit of charge
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V=W/Q
V=p.d. in volts... W= work done/energy transferred in joules(J)... Q= charge in coulombs, C
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Resistance
The ratio of p.d.across a component to the current flowing through it and is measured in ohms
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A resistor at a constant temperature is an
Ohmic conductor
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What is the effect of doubling the p.d. on the current? Why?
Current is directly proportional so it doubles the current too
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What does an IV graph look like of a resistor at a constant temperature?
It gives a straight line passing through the origin
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Ohm's Law
"The electrical current in a conductor is proportional to the p.d. applied to it provided the temperature remains the same"
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Ammeters measure
Current
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Voltmeters measure
P.d.
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IV graph of a filament lamp?
The graph curves (like an s) because as the filament heats up, the resistance goes up too (meaning the resistance of the filament is chanigng)
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IV graph of a diode?
Because the diode only allows current to flow in one direction through it, therefore the graph shows the current going up only when the p.d. is 0.7V
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What happens if the diode is reversed biased and the p.d. is continually increased?
The diode will eventually begin to conduct in the reverse direction (this is known as the break down voltage- about 50V)
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Graph of a thermistor?
The resistance decreases as its temperature increases which can be seen on a graph where resistance is plotted against temperature
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Thermistors can have...
A positive or a negative coefficient.
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A negative coefficient of a thermistor means...
Resistance decreases with an increase in temperature, this is caused by the release of extra charge carriers in the thermistor
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LDR
Light Dependent Resistor - graph of resistance plotted against light intensity looks similar to a thermistors (negative curve which levels off at a high light intensity) LDRs are used in circuits which automatically switch on lights as it gets dark
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What does the resistance of a piece of wire depend on?
The length of wire (the longer the wire, the greater the resistance) and the cross-sectional area of the wire (the greater the cross sectional area, the smaller the resistance)
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Resistivity
(p) a property of materials which takes into account their resistance (R) Length (L) and their cross-sectional area (A)
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Unit of resistivity
Ohm meters
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Resistivity and temperature
In metals, increases in temperature make the atoms in the structure of the metal vibrate more and this makes it more difficult for the electrons to move through the material, so the resistance of the material goes up. e.g. a filament bulb
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In an IV graph the line curves because as the filament heats up
Its resistance also goes up
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In semi conducting materials, the increase in temperature...
Releases more charge carriers so the resistance of the material goes down e.g. a thermistor
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Why is a thermistor a negative coefficient thermistor?
The resistance decreases as the temperature increases
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Uses of a thermistor?
Thermostats, in fridges and fridges (to monitor and control the temp in the room they are in
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Superconductivity
This is when metals or alloys are at or below a critical temperature which reduces their resistance to zero
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What is the critical temperature?
Varies depending on the material but the sorts of temperatures are such like -196 degrees celcius
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When are superconductors used?
When very strong electromagnets are required, in MRI scanners or to reduce losses in power cables
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Cells in series...
When cells are in series in the same direction, the total p.d. of the circuit = the p.d. of the cells added together
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Identical cells in parallel...
When identical cells are in parallel to one another the total p.d. supplied to the circuit is the same as the p.d. of just one cell. e.g. if three 2V cells are in parallel the p.d. supplied to the circuit is 2V
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Resistors in series...
When resistors are in series with each other, the total resistance is just their individual resistance added together
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Resistors in parallel...
When they are in parallel, the total resistance is found using: 1/Rtotal=1/R1+1/R2=1/R3
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Conservation of charge
"The total charge flowing into a junction of wires must equal the total charge flowing out of the junction
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Kirchoff's first law
"The sum of the currents flowing into a junction of wires must equal the sum of the currents flowing away from the junction of wires
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Current in series circuits
The current is the same wherever the ammeter is put
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Current in parallel circuits
The current divides between each branch proportionally to how much resistance it has
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Kirchhoff's second law
"The sum of the EMF's in any closed loop in a circuit must be equal to the sum of the potential differences in the closed end loop circuit"
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P.D. in a series circuit
The total p.d. is shared out between components - if the components all have the same resistance they will have an equal p.d. across them
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P.d. in parallel
The p.d. supplied by the cell is the same p.d. as that across each component in parallel
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Energy
(E) Measured in joules (J)
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Power
(P) is measured in watts (W). Power is the rate at which energy is transferred.
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Potential divider equation
V=R/Rtotal x Vtotal
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What is a potential divider?
A simple circuit which uses resistors (or thermistors/LDRs) to supply a variable potential difference
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What are potential dividers used for?
Volume controls, temperature controls or monitor changes of light in a room
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What does it do?
Divides up the pd supplied to them from a cell. The proportion of the available p.d.that the two resistors get depends on their resistance values
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Equation to work the V(out)
V(out)=(V(out)xR1)/(R1+R2)
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What is EMF
Electromotive force which is the energy provided by the cell or battery per coulomb of charge passing through it (measured in Volts). It is equal to the pd across the terminals of the cell when no current is passing through
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Internal resistance
Something that all batteries and cells have, when electricity flows around a circuit the internal resistance of the cell itself resists the flow of current and so thermal energy is wasted in the cell itself
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How can the EMF equation be rearranged?
E=V+Ir
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DC current
Cells and batteries provide an electrical current which always flows around the circuit in the same direction
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AC current
Alternating current is what the UK mains electricity supplies at about 230V, meaning that the current flows one way and then the other around the circuit. It constantly changes direction. In the UK the frequency of mains electricity is around 50Hz
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What represents AC signals?
Oscilloscopes
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What is an oscilloscope?
The oscilloscope trace can be used as a voltmeter
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Why is an RMS value necessary?
The pd and current are varying continuously in an AC signal so we need to represent an average value and the RMS represents the effective value
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What does it mean if we see a spot in the middle of the screen in an oscilloscope?
Input connected to ground and timebase switched off
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... The spot is 3 squares up?
3 Volts from a DC power source and timebase off
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...There is a horizontal line in the middle of the screen?
0 volts, DC source and timebase on
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...There is a vertical line in the middle of the screen 6 squares long?
3V, AC source and timebase off
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## Other cards in this set

### Card 2

Charge

#### Back

(Q) measured in coulombs (C)

### Card 3

#### Front

Charge of an electron

Current

### Card 5

#### Front

Potential Difference