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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.


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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

Therefore no current will flow e.g. connecting both ends of a
component to the same terminal of a battery.

Sum of…

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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…

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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

Resistance = Voltage/Current (R = V/I)

Unit = Volt per Amp = Ohm

In series:


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Resistance of an LDR decreases as the light intensity falling on it

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…

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of the electrons through the material is limited which tends to
reduce the drift speed.

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…

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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…

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Internal Resistance is part of the total resistance of the circuit ­
behaves like any other resistance in the circuit.

However, you cannot measure it directly. As current flows through
the cell, there is a voltage drop across the internal resistance.

This voltage drop is the lost volts when you…

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Scalar quantities have size only and no direction.

All base quantities are scalar.

Vector quantities have size and direction.


Speed = Distance/Time

Average Speed = Total distance travelled/Total time taken

Both speed and average speed are scalar quantities ­ no


Displacement is the vector that corresponds to…

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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

When you apply a push/pull you are applying a…


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