Physics Revision

Notes on Electricity and Mechanics for Edexcel AS Physics

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ELECTRICITY
Kirchhoff's First Law
The sum of the currents entering a point is equal to the sum of
the currents leaving the point.
Charge is conserved ­ regarded as a law of conservation of charge.
No charge is lost in a circuit or at any junction in a circuit.
Resistors
Resistors oppose the flow of current. Reduce the current through
every component that they are in series with.
Large series resistance = current everywhere is small
Small series resistance = current everywhere is large
Extra resistance by:
Making part of the wiring thinner
Include a much longer wire
Use a piece of material which electrons find hard to move
through/very few charge carriers that can move
Light-dependent resistor (LDR) ­ changes resistance with
illumination level
Thermistor ­ Changes resistance with temperature
Voltage
If you connect cells in series to a lamp a voltmeter will show a
larger voltage the more cells that add.
This is because the lamp glows brighter which shows a greater
current. If the current is increased then the charge is flowing
faster. If the charge is flowing faster (resistance is constant), it is
being pushed harder, and the higher voltage shows this.
Flow ­ Drift Speed
With a steady current, average drift speed of the charge carriers is
a constant. Therefore, average resultant force on the carriers is 0.

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Potential Difference (Voltage)
A voltmeter is connected across a component in order to measure
the voltage difference.
Voltages are the same at each end = no voltage across
component
Therefore no current will flow e.g. connecting both ends of a
component to the same terminal of a battery.
Sum of all voltages across components pushing
=
Sum of the voltages across components resisting
Electrical Power
Electromotive forces are energy givers because they push charge
carriers in the direction that they are moving.
e.g.…read more

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Power
Voltage = Work/Charge
Therefore, Work = Voltage x Charge
Power = Work done per second
So, Power = Work/Time
Therefore, Power = Voltage x (Charge/Time)
Power = Voltage x Current (P = VI)
For components in series:
Total voltage across all the components
=
Sum of the voltages across each individual component
For components in parallel, voltage across each component is the
same.…read more

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High resistance ­ Need a large voltage for a given current
Low resistance ­ Need a small voltage for a given current
The resistance indicates the voltage you need for each amp of
current:
Resistance = Voltage/Current (R = V/I)
Unit = Volt per Amp = Ohm
In series:
Total resistance for a number of components in series
=
Sum of the individual resistances
In parallel:
For each extra resistor in parallel, an additional path is provided
for current to go through.…read more

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Resistance of an LDR decreases as the light intensity falling on it
increases.
Calculating the speed of charge
Charge carrier density is the number of electrons per cubic metre.
It has the symbol n.
A wire has a charge carrier density (n), a charge (q) moving at a
speed (v ­ the drift speed), a cross sectional area (A), with a
current I.
Volume of charge carriers passing a point in 1s = Av.…read more

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This is the dominant effect in pure metals and means that
resistance increases with temperature.
Potential Dividers
Experiments with identical resistors in series show that the
voltage across each resistor is the same and adds up to the
voltage across the whole chain. For a series of resistors you can
divide a voltage into any fraction.
If two resistors in series are different, they have different
voltages across them.
e.g.…read more

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When it is dark, the resistance is high and voltage is large.
When it is light, resistance is low and voltage is small.
Thermistor to control voltage
When it is cold, the resistance is high and voltage is large.
When it is warm, the resistance is low and voltage is small.
Ohm's Law
All components resist the flow of current (in some way)
Resistance is given by R = V/I
For some components, R is constant.…read more

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Scalar quantities have size only and no direction.
All base quantities are scalar.
Vector quantities have size and direction.
Speed
Speed = Distance/Time
Average Speed = Total distance travelled/Total time taken
Both speed and average speed are scalar quantities ­ no
direction.
Velocity
Displacement is the vector that corresponds to distance.
Velocity is the vector that corresponds to speed.…read more

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Average Velocity
Gradient at a point = Instantaneous Velocity at that point
Velocity-Time Graphs
Gradient = Velocity/Time = Acceleration
Area under graph = Displacement
Newton's First Law
If you want a freely moving body to accelerate you need a
push/pull
When you apply a push/pull you are applying a force.
Forces are measured in newtons (N).
If you apply a pair of equal and opposite forces then the object
will not accelerate because the forces cancel out and are
balanced.…read more

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