Electricity and magnetism

Electric fields

Electric Charge

We now use electricity to power many devices but the idea of charge was originally developed to explain static electricity.


Electric force

The electric force is the force that occurs due to charges interacting. For example, the attraction between a proton and an electron. The formula for this force is almost identical to the formula for gravity. This attraction or repulsion between two charges is equal to


q1 and q2 are the magnitudes of the charges. k is a constant, and r is the distance between both charges. 

Electric field constant (k)

K is easily calculatable, but it depends on the medium. Precisely, it depends on the permittivity of the medium through which the charges are interacting. It is calculated by 


Permittivity of free space

Epsilon represents the permittivity of space in the medium. So the formula for electric force becomes


Permittivity is the amount of charge needed to create 1 single unit of electric flux in a given medium. A charge will yield more electric flux in a medium with high permittivity.  Electric flux is the rate of flow of electric field a given area. It is calculated as the integral of the dot product of the perpendicular electric field vectors and the perpendicular vector of an infinitely small area dA. 

Charge & Power

Charge: a fundamental property of all particles.  A particle can have a positive charge, negative, or no charge.  Like charges will repel while opposite charges will attract.


Coulomb's law



Definition: The time rate (△t) at which the charge (△q) moves past a particular point in a circuit.

Conversion from electrons per second to amps

Inorder to convert from electrons per second to amps, multiply by the charge of an electron in coulombs (1.6×10-19C/1e1.6 \times 10^{-19} C/1e)

I=ΔqΔt(1.6×10-19)I=\frac{\Delta q}{\Delta t}(1.6\times10^{-19})

Direct Current vs Alternating current

Direct current: A circuit where the current flows in one direction. Used for short distance circuits because it is safer.

Alternating current:  A circuit where the current flows alternate directions over time. Used for long distance circuits because of its higher voltage capacity.

Potential difference

Potential difference is the difference in electric potential between two parts of a circuit. It is this difference which causes current to move through the circuit.

Units: Voltage (V)


Heating effect of electric currents

Circuit diagrams

Some important circuit diagrams to remember are:

NameWhat it doesDiagram
CellA component which pushes the electric current (Voltage) from positive to negative.

BatteryA component which consists of several cells.

ResistorA component which reduces the current flow.(Almost every component is technically a resistor)

Variable resistorA components which controls the current flow.

Light bulbA component which emits light, by heating up filament inside the bulb.

Kirchhoff’s circuit laws


Ohms law


Resistance and resistivity



Factors that affect resistance

There are five things which can affect resistance, also as current is inversely proportional to resistance these five things will also change current but inversely.




Length of wire

The longer, the wire, the more times the electrons hit the sides slowing down.



The lower the conductivity of the material(σ) the higher the resistance.


Wire thickness

The smaller the diameter(⌀) of, the wire, the more times the electrons bump into each other



Heating a wire makes the atoms in the wire vibrate increasing the number of collisions.


Resistance formula: R=ρLA (resistance = resistivity * (length/current)

Series vs parallel circuits overview



Voltmeters and Ammeters

Type of meter

What it measures

How it is connected

Ideal resistance



In parallel

Very high resistance, so electricity does not flow through it.



In series

Very low resistance, so electricity flows easily through it.

Potential Dividers

A  potential divider is a circuit made of two or more resistors that allow us to tap off any voltage we want that is less than the battery voltage. You can imagine as using resistors to create a ratio of the voltages.

Formula: \(V_{out}  = \frac{V_{in} R_1}{R_1+R_2} \)

Power and power dissipation

Power: The rate of energy required to drive electrical current through a circuit.  It proportional to the potential difference and to the electrical current that flows through the circuit.

Power dissipation

Electric cells


Internal resistance

Secondary cells

Terminal potential difference

Electromotive force (EMF)

Magnetic effects of electric currents

Magnetic fields

Magnetic force


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