Section 16
Basic electricity
Every charge is an integer multiple of the elementary charge.
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The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance.
For a point charge
The electric field is the force per unit charge; that of a point charge is k|Q|/r².
The force on a charge in an electric field is the product of the charge and the field.
For a point charge
The electric potential is the potential energy per unit charge.
For two point charges
The electric potential energy of a charge is the product of the charge and the potential.
In a uniform field parallel to the displacement
The potential difference is minus the integral of E; in a uniform field, Ed.
Instantaneous
Current is the charge crossing a section per unit time.
The potential difference across an ohmic conductor is the product of the current and the resistance.
- ohmic element in the considered regime.
The resistance of a uniform conductor is proportional to the length and inversely proportional to the area.
Resistance varies linearly with temperature relative to a reference value.
- linear approximation in the temperature range considered.
Using Ohm's law
Electric power is the product of the potential difference and the current.
For constant power
Electric energy is the product of power and time.
The equivalent series resistance is the sum of the resistances.
For two resistors
The reciprocal of the equivalent parallel resistance is the sum of the reciprocals.
Equivalently
At a node, the sum of currents entering equals the sum of currents leaving.
Around a closed loop, the sum of the potential differences is zero.
included as a General Physics concept; the detailed analysis of capacitive circuits can be moved to Electronic Physics.
With dielectric
The capacitance of a parallel-plate capacitor is proportional to the area and inversely proportional to the separation.
The energy stored in a capacitor is (1/2)CV².