To understand electrical currents, we must first understand what an electric current consists of and what makes it 'go'.
An electric current is the movement of charges. Charges move when they experience an electric force. While charge is the more axiomatic quantity, current is the fundamental quantity with an SI base unit.
Key Concepts
Charge is a conserved quantity, like mass. It is measured in Coulombs. The smallest amount of charge that exists independently is 1.6 x 10 -19 C; this is called the fundamental unit of charge and is the charge associated with the components of the atom. All matter contains charge and all things that are charged have mass; they are inseperable.
However, unlike mass (which is always attractive across the universe), charges fall into one of two categories: positive and negative.
An atom is uncharged as it contains equal numbers of positive charges and negative charges:
- The nucleus is charged by positive protons, with a charge of +1.6 x 10-19 C.
- The outer shells are charged by negative electrons, with a charge of -1.6 x 10-19 C.
- The number of protons = number of electrons.
- Neutrons, also contained in the nucleus, are uncharged.
- A particle that has electrons added or removed is called an ion, a charged particle.
An electric field is a region of space where a charge experiences a force. Electric fields cause a non-contact force between the charges:
- Objects with the same type of overall charge (e.g. positive) repel one another
- Objects with unlike charges (i.e. positive and negative) attract
As with graviatational forces (see Gravity), electric forces are vector quantities. Just like with gravitational fields, electric fields can be radial or uniform. Here we will consider just radial fields (HL students encounter uniform fields later).The direction of the electric force depends on the electric field.
Field lines are drawn to show the direction and strength of the field:
- Arrows show the direction of the force experienced by a small positive charge placed in the field
- The concentration of field lines (i.e. closer together next to the charge) represents the strength of a field
How could you use field lines to show the repulsion of two charges?
The electron volt is a convenient unit of energy. 1 eV is the amount of energy gained by an electron accelerated through a potential difference of 1 V.
Coulombs law tells us which quantities affect the size of the electric force between two point charges (or between charges that can be modelled as having all of the charge at their centres).
The force experienced by two point charges is directly proportional to the product of their charges and inversely proportional to their separation squared:
\(F_E \propto {{q_1 q_2}\over r^2}\)
The proportional sign can be replaced with an equals sign by multiplying by a constant:
\(F_E=k{{q_1 q_2}\over r^2}\)
k is known as Coulomb's constant or the electric force constant. Its value is 8.99×109 N m2 C−2.
NB: This is similar to Newton's law of gravitation.
Electric field strength (E) is defined as the the electric force per unit charge experienced by a small positive test charge placed at the point.
\(E={F_E\over q}\)
In this case, let us assume that the charges in the equation are Q (the charge creating the original field) and q (the charge placed in the existing field), in order to distinguish more easily between them:
\(E = {k{Qq \over r^2} \over q}\)
\(E = {k{Q \over r^2}}\)
Electric field strength has the units N C-1 (since it is defined as force per unit charge).
The significance of the test charge is that the electric field strength is calculated as though the placed charge were not there (and indeed, the size of this charge cancels out!). However, if a charge were actually placed there, it would change the shape of the field in that location.
How much of Electric fields have you understood?
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