5.1 Basic concepts and principles of electricity


This page reviews only a few basic concepts. It’s meant to add to your already existing knowledge. If you know nothing about electricity I suggest you learn the basic concepts from here: http://www.s-cool.co.uk/a-level/physics. At the bottom of the page I added resources from where you can learn more about electricity. Furthermore, I have also compiled a list of the most important formulae – yes, it is not complete.


In terms of electric properties, substances can be classified into three groups:

  • Conductors – substances containing free electric charges, such as a metallic lattice.
  • Non-conductors/Insulators – substances which do not contain any free electric charges.
  • Semiconductors – substances in which the number of free charges depends on the physical conditions they are in.

 

Charges.

  1. Electric charges are always associated with elementary particles, such as electrons (e¯ ) or protons.
  2. Electric charges will exert a force on one another.

Electric charge(Q) is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges: positive and negative. Positively charged substances are repelled from other positively charged substances, but attracted to negatively charged substances; negatively charged substances are repelled from negative and attracted to positive. An object is negatively charged if it has an excess of electrons, and is otherwise positively charged or uncharged. The SI derived unit of electric charge is the coulomb (C).

– wikipedia.com

If the charge is static, it will exert an electrostatic field. However, if the charge is in motion then it will exert an electrodynamic field (or magnetic field).

Principles of Electric Charges:

  1. The conservation of charge principle states that the total electric charge of an isolated system remains constant.
  2. The superposition principle states that if more than one charge is present, the total electric field at a point is equal to the vector sum of the respective electric fields.
  3. Charge quantisation principle states that all charges are integer multiples of an indivisible elementary charge. e¯=1.602×10^(-19)
  4. Coulomb’s Law states that

    The magnitude of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distance between them.[12]

    The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive.

    -wikipedia.com


 

Capacitance and Capacitors:

An important property of a conductor is its capacitance (C). This is the amount of charge stored per volt.

Capacitance

where V is voltage (also U). The unit of Capacitance is Coulombs per Volt or the Farad [F]

Charge is stored in a capacitor as work is done by electric charge – as current flows through an electric circuit containing the capacitor. This work/energy can be expressed as:

Work done by capacitor

where εr is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates (for a vacuum, εr = 1);ε0 is the electric constant (ε0 ≈ 8.854×10−12 F⋅m−1)


 

Work and Electric Power of Constant Current:

If a constant current flows through a conductor with resistance R, then it does work.

Work done by charge

where I is current, V is voltage, and t is time. The unit of work is the Joule [J].

Power is the quantity the expresses the amount of work done per unit time.

Electric Power

The unit of Power is the Watt [W].


 

Magnetic Fields:

Magnetic fields form either around permanent magnets, or around conductors containing moving changes within.

A magnetic field can be described using magnetic force lines. The total number of force lines passing through an area A is defined as the magnetic flux.

The magnetic flux density is the magnetic flux in a unit area placed rectangular to the direction of force lines.

Magnetic Flux Density

Magnetic Flux where B is the magnetic flux density and S is the surface area. The unit of the magnetic flux density is Tesla [T].

Magnetic fields can be classified according to two principal criteria – time and space.

According to time changes, magnetic fields can be:

  • static – the magnetic field intensity value remains unchanged
  • alternating – the intensity value regularly changes from zero to a positive maximum, back to zero, to a negative minimum and again back to zero. This type of field is specific to conductors carrying AC.
  • impulse – arises around conductors carrying electric impulses

According to the distribution of magnetic field lines in space we distinguish:

  • homogeneous magnetic fields – exerts a force of the same magnitude and direction over an area
  • non-homogeneous magnetic fields – the magnitude and direction of the field varies

Electromagnetic induction is the production of an electromotive force across a conductor exposed to time varying magnetic fields. Electromagnetic induction results in electric voltages, and as a consequence, in electric currents. These arise by either Lorentz’s forces or Faraday’s Currents.

The electromagnetic induction is directly proportional to the magnetic field strength, H [Amps per metre]. The proportionality factor is μ, the magnetic permeability.

Based on the relative magnetic permeability, materials can be divided into three groups:

  1. Ferromagnetic materials – μ > 1
  2. Diamagnetic materials – μ < 1
  3. Paramagnetic materials – μ ≈ 1

List of necessary formulae:

Electric Current

Resistance

Resistivity

Electric Power

Work done by charge

Capacitance

Work done by capacitor

Coulomb

Electric field Strength

Magnetic Flux

Take a break 😉

Recommended reading to help you understand this chapter:

Coulomb’s Law:

http://www.physicsclassroom.com/class/estatics/Lesson-3/Coulomb-s-Law

Electric Fields:

https://burningscience.wordpress.com/asa2-cie-tips/physics/electric-fields/
http://www.s-cool.co.uk/a-level/physics/electric-fields-and-forces/revise-it/electric-field-strength-e

Capacity and Capacitors:

http://www.electronics-tutorials.ws/capacitor/cap_4.html

Electric Current:

https://en.wikibooks.org/wiki/A-level_Physics/Electrons,_Waves_and_Photons/Electric_current

http://www.physicsclassroom.com/class/circuits/Lesson-2/Electric-Current

Ohm’s Law: http://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law
Kirchhoff’s laws: http://www.electronics-tutorials.ws/dccircuits/dcp_4.html