Notes to a video lecture on http://www.unizor.com
For a short introduction to the International System of units (SI) see the previous lecture "SI Intro & Time" within this topic.
Electricity Units
All SI units used in the science of Electricity are derived from base units introduced in the previous chapter Base SI Units of this part Units in Physics of the course.
Electric Charge
As we know, electric charge is excess (negative charge) or deficit (positive charge) of electrons relatively to the number of protons in the atoms of an object.
Electric charge (that is, excess or deficit of electrons) can travel along a conductor forming an electric current.
There is a base SI unit of electric current - ampere (A).
An amount of electrons transferred by a current of 1 ampere during 1 second is a unit of measuring the electric charge, it's called coulomb (C)
1 C = 1 A·s
When electrons are going away from an object by a current of 1A during the time of 1s, this object is positively charged with 1C of electric charge.
When electrons are going into an object by a current of 1A during the time of 1s, this object is negatively charged with 1C of electric charge.
Voltage
Consider any electrostatic field. For example, an electrostatic field around some negatively charged point-object.
Every positively charged point-object that carries a charge of +1 coulomb (test charge) will experience certain force in this field. In our example of a field produced by a negatively charged point-object the force will be an attraction towards the center of a field.
Therefore, to move our test charge from point A to point B of the electrostatic field might require certain energy to spend against the force of a field. In our example of a field produced by a negatively charged point-object, to move a test charge away from a center of the field requires to do some work against the force of attraction.
Obviously, since the force of attraction or repelling between charges is proportional to the charges themselves (the Coulomb's Law), the amount of work to move any charge from point A to point B will be proportional to this charge.
If our test charge of +1 coulomb requires to perform 1 joule of work to move it from point A to point B in the electrostatic field, we say that there is a 1 volt (V) of difference in electric potential between these points A and B.
This difference in electric potential is called voltage.
So, the unit of voltage is 1 volt, which can be expressed as
1 V = 1 J/C
Resistance
Measurement of electric resistance is based on the Ohm's Law. Stronger electric field produced by a greater difference in electric potential (voltage) between the ends of a conductor should cause proportionally more intense movement of electrons, greater number of electrons going from one end of a conductor and reaching the other end per unit of time (amperage)
I = U/R
where
I is electric current,
U is voltage,
R is electric resistance.
If the voltage between two opposite points of a conductor is constant 1 volt (V) and the electric current running through this conductor is 1 ampere (A), we say that a conductor's resistance equals to 1 ohm (Ω).
So, the unit of resistance is 1 ohm, which can be expressed as
1 Ω = 1 V/A
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