**Circuit symbols and diagrams**

Electric components and their circuit symbols

Electric Current

It is the rate of flow of electric charge past a point.

current=charge/time

I=ΔQ/Δt

So, we can get equation for charge as: ΔQ = IΔt

Charged Particles

Current is the flow of charge particle called charge carriers. Electrons carry negative charge which is approximately equal to -1.6 × 10^-19C.

The magnitude of charge is elementary charge:

Elementary charge e = 1.6 × 10^-19C

Equation for current

Number density n:

It is the number of free electrons, per unit volume in a material.

A current *I *in a wire of cross sectional area *A*. The charge carriers are mobile conduction electrons with the drift velocity *v*.

Number of electrons = number density × volume of wire

= n × A × l

Charge of electrons = number × electron charge

= n × A × l × e

We can find current I as it is the charge that flows in time t, and current = charge/time

I=n×A ×l ×e⁄t

Substituting v for l/t

I=nAve

I=nAvq

Where v is drift velocity and q is the charge of each particles.

From these equations we can know that:

If the current increases, drift velocity must increase i.e. v α I

If the wire is thinner, the electrons move more quicker for given current v αI/A

**Potential Difference**

Potential difference V is defined as the energy transferred per unit charge. The potential difference between two points, A and B, is the energy per unit charge as charge moves from point A to point B.

Electromotive force e.m.f. is defined as the total work done per unit charge when charge flows round a circuit.

**Electrical Resistance**

It is defined as the ratio of potential difference to the current.

resistance=(potential difference)/current

R=V/I**Ohm**

It is the unit of resistance. The ohm is equivalent to “1 volt per meter” that is:

1Ω = 1V A^-1 **Electric Power**

It is the rate at which energy is transferred. Power P is measured is Watt (W).

Power= (energy transfered)/(time taken)

P= W/(∆t )= V∆Q/∆t=V(∆Q/∆t)

The ratio of charge to time, ∆Q/∆t, is the current I. so,

P = VI

Power = Potential difference × current

P = I^2R

P = V^2/ R