Electricity notes

Notes on Unit 1 of AS AQA Physcis A

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The flow of electrons around a complete circuit
Conventional current
As electricity is the flow of electrons, which are negatively charged, the flow of the current
is from negative to positive
However before this was discovered the opposite was thought. Therefore conventional
current is used despite it being incorrect.
Current is the rate of the flow of charge around a circuit. It is measured in amperes.
The equation: Q=I*t can be used to connect these
Charge (c)= Current (A) * time (s)
To find the Number of electrons flowing past a point, use the following:
Charge (C)/charge of an electron. This is expressed as Q/e.
For example:
1.65 c / 1.6 x 10-19
This equals 1.03 x 1019
Series and parallel circuits
In series circuits, the current is the same throughout
In parallel circuits, the current is shared between branches in proportion to the
resistance of each branch. The path of least resistance will have the greatest
current flowing through it.
Potential difference
Opposite charges attract. If the positive charge is fixed but the negative is able to move, the
potential energy which the negative charge has will decrease as it travels towards the positive.
This is the potential difference-from starting at the negative side to leaving at the positive side.
In a circuit, the cell supplies the energy required to move the electrons. Without this source of
energy, no work is done to move the electrons. How much charge goes into the component and
how much goes out is the potential difference.

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This is the energy transferred to the current per unit of charge. This is how much energy is
transferred to the electrons. This can take into account resistance, such as that from the battery
which will decrease the charge in the circuit.
Rules in a circuit
As charge flows through a component it loses energy. In a circuit with one component, all energy
is lost in the component, so the voltage in the cell is equal to the voltage across the component.…read more

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In an exam, if there is a diode in the circuit and a current is flowing, the potential difference across
the diode is always 0.6 V
In thermistors, as the temperature increases, the
resistance of the component decreases. These
components are made of intrinsic semiconductors
(not metal) and have a negative temperature
coefficient (they do the opposite of what you
would expect). Most other components have a
positive temperature coefficient and their
resistance increases as temperature increases.
Resistance (ohms) = P.…read more

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This is true until the temperature exceeds critical, which
results in the conductor gaining resistivity rapidly.
Uses of these super conductors include:
High power electromagnets
Power cables- as they allow energy to be transferred efficiently as no energy will be lost through
Resistance in a circuit
In series
R = R1 + R2 ...…read more

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Terminal Potential difference
"The electrical energy per unit of charge delivered to the circuit"
This is given by:
T erminal pd = I R
The lost voltage due to the internal resistance of a battery is given by:
Lost pd = I r
Therefore, EMF can be written as:
= IR + Ir
Measuring internal resistance
When the graph intercepts
the Y axis, this is the EMF of
the cell.
The gradient of the line is
the internal resistance.…read more

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This is worked out through P = I 2R which when subbed into P = VR can be written as the
P = (R+r)2
gives the current.
This works as I = R+r
Maximum power is delivered to the load when the load resistance is equal to the internal
resistance of the source.…read more

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Supplying a variable pd
Length of wired with a sliding contact is used. This can be used for audio volume controls and to
vary the brightness of light bulbs.
Sensor circuits
A sensor circuit produces an output pd which varies when a physical variable changes
Temperature sensor
This consists of a thermistor and a variable resistor. This means the variable resistor can be set to
have a certain pd across the thermistor at a certain temperature.…read more

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Heating effect
Power changes as current changes. Maximum power occurs where the peak current is supplied.
Root mean square
This is the value of direct current that would produce the same heating effect as the alternating
current at the same resistance.…read more


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