What Are Electric Current

electric current

For elementary physic we know, atom are the smallest matter consist of protons, electrons, and neutrons. So what is the relationship with the electric current?

The charge e on electron is negative and has magnitude of 1.602 x 10-19 and the proton carries the same magnitude as electron but positive charge.

Make sure you have read Basic Direct Current (DC) Circuit Theory first and continue to linear circuit analysis.

Electric Charge

Every electrical phenomenon is based on the basic concept of electric charge. Electric charge is also the most fundamental quantity of electric circuit.

The electric charge’s meaning is shown below :

Charge is an atomic particles in electrical system, measured in coulombs (C)

Some points we need to take note are shown below :

  1. Large unit of charges is Coulomb. For every 1 C of charge contains 1/(1.602 x 10-19) = 6.24 x 1018 electrons.
  2. From experiment observatory, the only charges that happen in nature are integral multiplies of electronic charge e = -1.602 x 10-19 C.
  3. From “Law of Conservation of Charge” states that charge can not be created or destroyed, it can only be transferred from a point to another where it can be converted to another energy form.

Let us see Figure.(1) again to analyze it with our theory about the electric charge.

When a battery (a source of electromotive force) is connected with conducting wire or electrical wire (consist several atoms), the charge is moving, the positive charge is moving in one direction and the negative charge is moving to the opposite direction.

This charge motion phenomenon creates electric current.

This circuit element is useful for calculating electric power.

For the conventional purpose, the current flow will be determined by positive charge flow direction even the electron itself (negative charge) moving in the opposite direction.

The current flow direction can be seen in Figure.(1).

electric current flow direction
Figure 1. Flow of electric current

Electric Current

Before moving to the next part, let us take a look a short explanation below :

Electric current is the change of charge in a period of time, measured in amperes (A)

In the mathematic equation, the relationship between current i, charge q, and time t is expressed as :

mathematic current equation

where :

i = current (A)


1 ampere = 1 coulomb/second

We can obtain the transferred charge between time to and t using integral for both sides. Equation.(2).

charge electric equation


Looking back to Equation.(1), the current do not need to be in constant-value function.

For further explanation, we will find there will be a variation changes of charge. There are two types of current.

After learning about electric current, you need to learn about electrical voltage.

If current has constant value over time or its value does not change with time, we call it direct current (dc) as we can see in Figure.(2).

a direct current (dc) is a current which has constant value with time changing.

The symbol of constant current is I.

direct current waveform

Figure 2. Direct current waveform

A changing current over time is represented by i symbol. This time-changing current is represented by alternating current (ac).

The alternating current has the sinusoidal waveform shown in Figure.(3).

An alternating current (ac) is a current that varies sinusoidally in a time period.

alternating current waveform

Figure 3. Alternating current waveform

The current types in Figures.(2) and (3) are the common types of current. Later discussion will reveal another type of current which are more complicated.

For a reminder, the current direction will refer to positive charge direction of flow.

Electric Current Example

For the starter of the concept, let us review these electric circuits examples below :

1. How many charges in 4.600 electrons?

Solution :

Every electron has -1.602 x 10-19 C. Thus 4.600 electrons will have -1.602 x 10-19 x 4.600 = -7.369 x 10-16 C.

2.Total charge entering one terminal is q = 5t sin4πt mC. Calculate the current at t = 0.5 s.

Solution :

total electric charge calculation

3. Calculate the total charge entering a terminal for a period time from t =1 s to t = 2 s if the current is i = (3t2 – t) A.

Solution :
electric total charge calculation