Notes on the whole of electricity and circuits. Includes notes on Kirchoff's laws, capacitors, current, voltage, emf and solving circuit questions. Made using lecture slides, textbook and online resources and is in accordance with the study guide. Made for course physics (bio) 154 at Stellenbosch U...
Electric charge flowing through a conductive medium or the rate at which
electric charge flows through a conductor. Current flows from points of
high voltage to low voltage (from + terminal to - terminal) and is measure
in Amps. A large current means that a large charge is moved in a short
amount of time whereas a small current means that a small charge is
moved over a long period of time.
💡 1 Amp = 1 C/s
What is a conductor?
A material that allows electric current through flow through it
What causes current to flow?
When a voltage source is connected to a conductor, it exerts a potential
difference. This potential difference creates a field around the conductor
and this field exerts a force onto the charges and causes current to flow.
What does potential difference refer to?
Difference in potential energy of a charge between 2 points in a circuit
divided by the charge. Measured in Volts.
💡 V = delta PE/q
What does resistance refer to?
Electricity 1
, Opposition to flow of electric charge. A very high resistance allows a small
amount of current to flow. A very low resistance allows a large amount of
current to flow. Measured in Ohms
💡 RV/I
Resistors in series: Total resistance of resistors in series = sum of all
resistors
Resistors in parallel: 1/Total resistance of resistors in parallel = sum of
reciprocal of all resistors
Use these equations only when the circuit obeys Ohm's law
An objects resistance depends on the material it is composed of as well as
its size
💡 An objects resistance is directly proportional to its length and
inversely proportional to the segment's area
From https://courses.lumenlearning.com/boundless-physics/chapter/resistance-and-
resistors/ accessed on 30/07/2020
p=resistivity. Resistivity is independent of size and shape but is true for
anything made of a specific material. An object's resistance is directly
Electricity 2
, proportional to its resistivity. Resistivity of all materials is dependent on
temperature.
Resistance and resistivity are directly proportional to temperature. A
linear pattern is followed for small temperature changes while a non
linear pattern is observed for large temperature changes.
💡 Over temperature changes of 100ºC or less, resistivity (ρ) varies
with temperature change ΔT as: p=p01αΔT
where ρ0 is the original resistivity and α is the temperature coefficient of
resistivity. Resistance will follow an similar patter since it is directly
proportional to resistivity.
Combination resistors:
Two parallel resistors in series with one resistor:
💡 1/R11/R2 or R1 x R2/R1 + R2
R3 is connected in series to both R1 and R2, so the resistance would be
calculated as:
💡 RR1 x R2/R1 + R2R3
Ohm's law
Defines relationship between power, voltage, resistance and current
States: "The direct current across a conductor is directly proportional
to the potential difference between its ends at a constant temperature"
💡 VI x R
Electricity 3
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