Charge
The most basic quantity of electric circuit is charge.
It is an electrical property of atomic particles of the matter and its unit is coulombs (C).
Charge on an electron = - 1.6 × 10–19 C.
Coulomb is a large unit of charge.
In 1 C charge, there are:
Current
The flow of electrons or the time rate of charge of charge is called as current. Its unit is ampere (A)
The direction of current flow is conventionally taken as the direction opposite to the flow of electrons (e–).
Types of current:
A current that is constant in time is termed as direct current (DC).
Figure 1: Direct current (DC)
A current that changes its direction with respect to time is alternating current (AC).
Figure 2: Alternating current (AC)
Voltage
An external force is required to move an electron (e–) from one point to another in a direction and in an electrical circuit this force is provided by an electromotive force (emf) and is given by:
Note: Voltage does not exist at any point by itself, it is always determined with respect to some other point.
This is the reason that voltage is also called potential difference.
Power
It is the time rate of change of energy (expending or absorbing) and its unit is watt (W).
Current direction and voltage polarity are important in determining sign of power.
Energy
It is the ability or the capacity to do work.
Resistor
Resistance (R) can be described as that property of circuit element which offers opposition to the flow of current. The energy is dissipated in it in the form of heat energy.
voltage across it is linearly proportional to the current through it.
Its unit is ohm (Ω)
Ohm’s Law
It states that at constant temperature, the current density is directly proportional to the electric field intensity.
Figure 3: Ohm's Law
Inductor
Inductor is a circuit element which opposes the change in current.
The voltage across inductor is proportional to the derivative of the current with respect to time.
→ It stores the energy in the form of magnetic field.
Faraday Law of electromagnetic induction
- If a flux linking a coil varies as a function of time then an emf is induced between its terminals (statically induced emf)
or
when a conductor cuts the magnetic line of force, an emf is induced in the conductor (dynamically induced emf)
- Value of induced emf is directly proportional to the time rate of charge of flux linkage (Ψ)
Capacitor
If two conducting plates are separated by an insulator (an dielectric), they will constitute a capacitor which stores the energy in the form of electric field.
The amount of charge stored in a capacitor is directly proportional to the voltage difference between two plates.
Figure 4: Capacitor
q ∝V
q = CV
Where, C is capacitance, q (charge) can be written as:
Hence, the terminal voltage across capacitor is proportional to the integral of current with respect to time through it.
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