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Edexcel GCSE Physics Topic 3 + 8 - Conservation of Energy + Forces doing Work
Edexcel GCSE Physics Topic 4 + 5 - Waves + Light and the Electromagnetic spectrum
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Electricity and Circuits CGP Topic 5
Electric Current
An electric current is a stream of charged A delocalised electron in a metal is an
particles, such as electrons or ions, moving electron that is not attached to any one
through an electrical conductor or space. atom and is free to move between atoms.
Q = I ×t
Current is the flow of charge per second. charge (coulomb, C) = current (ampere, A) × time (second, s)
When a closed circuit includes a source of potential difference there will also be current. Two or more cells
are referred to as a battery
There are two types of current - direct and alternating:
In a direct current à the flow of electrons is consistently in one direction around the circuit
In an alternating current à the direction of electron flow continually reverses
Voltage and Resistance
Voltage is the pressure from an electrical circuit's power source that pushes charged electrons.
Potential difference is the difference of electrical potential between two points.
Resistance is a measure of the opposition to current flow in an electrical circuit and is caused by collisions
that occur between electrons and the metal lattice. It tells us the voltage required to create 1amp current;
the higher the resistance, the greater voltage is required and the lower the current.
A resistor has a fixed resistance which does not change. A variable resistor is a single unit in the circuit
where the resistance can be altered (LDR or thermistor – as light intensity/temperature increases,
resistance decreases).
E =Q×V
energy transferred (joule, J) = charge moved
(coulomb, C) × potential difference (volt, V)
V=I×R
potential difference = current × resistance
(volt, V) (ampere, A) (ohm, Ω)
In a 1 volt cell each coulomb of receives 1
joule of electrical potential energy
To measure the potential difference across a component, a voltmeter must be
placed in parallel with that component in order to measure the difference in
energy from one side of the component to the other.
Meanwhile ammeters measure the current and are connected in series.
, Red dots represent the
fixed metallic ions blue dot
shows the free electron
pin ball as it collides,
slowing down Low voltage
and high voltage shown by
steeper/shallow gradient.
Temperature
• The positive ions in a metal actually vibrate. When current flows through an electrical component
the free electrons collide more frequently with the fixed ions hence they vibrate more.
• Temperature is a measure of the average kinetic energy of the particles within an object. Hence if
the free electrons collide more frequently within the fixed positive ions, the ions vibrate more
rapidly and the temperature increases and also the resistance.
• Electrical energy is therefore transferred into thermal energy
When the length of an electrical component is increased,
there are more frequent collisions so a higher resistance.
If the cross sectional area of a wire is increased there would
be more space for electrons to pass between fixed positive
ions meaning less collisions and a lower resistance.
The more frequent collisions between the free electrons
and the fixed positive ions, the greater the resistance of a
given electrical component.
How do electrons really move in a metal?
1. In a metal free electrons move around randomly between the fixed positive
ions.
2. The electron movement is similar to that of multiple pinballs launched into a
pinball machine. The positive ions are equivalent to the fixed obstacles on the
pinball machine.
3. When a voltage is applied the electrons start to get attracted to the positive
terminal and repelled by the negative terminal.
4. The electrons do not move in a straight line towards the positive terminal but
start to gradually move towards the positive terminal as they continue to
bounce between the fixed positive ions.
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