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501303
Unit 1 - Electricity
Description
Mindmap of AS Physics Unit 1, Section 3 - Electricity (AQA), from the AQA exam board. If you need any help or have any suggestions, contact me directly on ExamTime or email me on: cmcclintock977@gmail.com
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a-level
physics
physics
a-level
Mind Map by
Callum McClintock
, updated more than 1 year ago
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Callum McClintock
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Resource summary
Unit 1 - Electricity
Circuit Diagrams
Series & Parallel
Symbols
Current & Potential Difference
Current is the rate of flow of charge
I = Q / t
Pd is the work done in moving a unit charge between two points
V = W / Q
Resistance
The hindrance to the flow of charge
R = V / I
Ohmic Conductors
Obey Ohm's law
I is directly proportional to V
If temperature is constant
I-V Characteristics
I-V Graphs
The lower the gradient, the higher the resistance
Metallic Conductors
Filament Lamps
Semiconductors
Quite good conductors
But not as good as metals due to fewer charge carriers
However, if energy is supplied to a semiconductor (e.g. a rise in temperature), more charge carriers can be released and the resistance decreaes
Components
Thermistors
Diodes
LDRs
Resistivity & Superconductors
Resistivity is the resistance of a 1m length with a 1m^2 cross-sectional area
ρ = RA / L
You can lower the resistivity of some metals by cooling them down
Reach a transition temperature and resistivity disappears entirely - superconductors
Without any resistance, no electrical energy is wasted as heat, so none is lost
This transition temperature is below 10 kelvin (-263°C) for most 'normal' conductors
It is tricky and very expensive to reach such temperatures
Superconductors can be used for cables with no loss of power, really strong electromagnets, and high-speed circuits
Power & Electrical Energy
Power Equations
P = E / t
P = VI
P = V^2 / R
P = I^2R
Energy Equations
E = VIt
E = (V^2 / R)t
E = I^2Rt
E.m.f & Internal Resistance
E.m.f is the amount of electrical energy a battery produces and transfers to each coulomb of charge
You can use a V-I graph to calculate ℰ and r
The y-intercept (c) is ℰ and the gradient (m) is -r
Equations
ℰ = E / Q
ℰ = I(R + r)
ℰ = V + v
V = ℰ - v
V = ℰ - Ir
Conservation of Energy & Charge
Kirchhoff's Laws
First Law
The total current entering a junction = the total current leaving it
Second Law
The total e.m.f. around a series circuit = the sum of the p.d.s across each component
ℰ = ΣIR
In circuits
Parallel
1 / R total = (1 / R1) + (1 / R2) + (1 / R3)...
Series
R total = R1 + R2 + R3...
ℰ total = ℰ1 + ℰ2 + ℰ3...
The Potential Divider
At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
You can use potential dividers to supply a p.d., V out, between zero and the p.d. across the power supply.
V out = (R2 / R1 + R2)Vs
Uses
Light sensors
Temperature sensors
Potentiometers
Alternating Current
An AC or AV is one that changes with time
Oscilloscopes
An AC source gives a regular repeating waveform
A DC source is always at the same voltage, so you get a horizontal line. You'd get a dot on the voltage axis if the time base was turned off
Equations
f = 1 / T
V rms = V0 / sqrt2
P = V rms * I rms
R = V rms / I rms
R = V0 / I0
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