### Charge and current

- The law of conservation of energy states that the charge can neither created nor destroyed, it can only transferable from one place to another place.
- Therefore the algebraic sum of all charge in an electrical circuit is equal to zero.
- The electrical charge is mobile therefore it can be transferred to one place to another.
- The positive charge moves in one direction whereas the negative charge moves in the other direction.
- The motion of electrical charge creates electrical current in a closed circuit.
- The Benjamin Franklin invented that the conventional direction of electrical current is opposite to negative charge or electron. There is a following relation between charge and current.

i = dq / dt

q = ∫ i dt

### Direct current ( DC ) or direct voltage

- If the magnitude of current or voltage does not change with time, it is called as direct current.
- The magnitude of direct voltage and current remains constant with respect to time.

### Alternating current ( AC ) or alternating voltage

- If the magnitude of current or voltage changes sinusoidal with time, it is called as alternating current.
- The alternating supply may be single phase or three phase.

### Voltage or Potential

- It is defined as the work done in bringing a positive charge of one coulomb from infinity to that point against magnetic field.
- The potential is equal to work done per unit charge.
- The earth is taken as reference potential zero because the potential of earth remains constant in spite of it keeps and losing charge.

One voltage =
Work done / Charge

= Joule / Coulomb

### EMF ( Electro – motive Force )

- It is force that requires moving electrons to produce an electrical current.
- It is better known as EMF or battery source. It is measured in voltage.
- The word force is used for electrical potential not for mechanical force.

OR

- It is source of energy that causes electrical current to flow in the given closed circuit.

### Potential difference

- It is defined as potential difference of one voltage between two points if one joule of work is done for shifting a point charge from one point to other point.
- The potential difference between two points is considered by considering one point as reference potential.
- Let us consider that the potential of point is + 10 V as compared to potential of point B. Similarly the potential of point P is – 10 V as compared to potential of point Q.

### Power

- It is defined as rate doing work. It is a scalar quantity. The unit of power is watt or joule per second in the honour of James watt.

P = dW / dt

dW = Rate of
work

dt = Rate of change of time

Therefore

P = dW / dt

P = dW / dt

= ( dW / dq ) (
dq / dt )

= VI

= I

^{2}R ( V = IR )
= V

^{2}/ R ( I = V / R )- If the voltage and current are time varying quantity, the power is called as instantaneous power.
- Therefore the power is defined as the product of voltage across element and current passes through element.
- The power has positive sign if it is delivered to load it means that voltage and current both have positive sign.
- The power has negative sign if it is delivered from load it means that either voltage or current has negative sign not both.

Therefore

- Power absorbed = – Power supplied

### Law of conservation of power

- The algebraic sum of power in a circuit is equal to zero. Therefore

∑ P = 0

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You may also like :

Law of Conservation of Energy

Armature Reaction in the DC Generator

Characteristic of DC Generator

Principle of DC Motor