Electric Current Definition
Electric current meaning is the movement of free electrons from one point to another. An electron in an atom has an electric charge but it can’t contribute in the electric current until it leaves the atom (Read My Lecture on Electronics Materials to clear this concept). In this lecture I will try to cover electric current basics.
How current flows in a conductor?
With the help of the following figure-1a and 1b you can understand very easily how electric current flows in a material.
Fig-1a: Free electrons are moving randomly
In the above diagram blue circles represent atoms of an electronic conductor. Small red circles show free electrons. You can observe that electrons are moving between the spaces of the atoms. Every conductor has free electrons. These electrons move randomly between the spaces of atoms with approximate speed of 10^5 m/s. During their motion they collide with each other and with the atoms of the material again and again. After each collision they change their direction of motion. You can compare this situation with the molecules of the gas. The molecules of a gas move randomly without any specific direction. Similarly the direction of free electrons is also not specific in fact average of their direction is zero.
But when we apply potential difference across the two ends of the conducting wire (See Fig-1b). It creates an electric field. This electric field accelerates the electrons and electrons start moving towards the positive terminal. During this process electrons again and again collide with the atoms so these collisions do not let their velocity to become very high. This velocity is larger than the velocity of the their random motion. The average speed or velocity gained by the electron due to the electric field is known as drift velocity of electron. Which is approximately equal to 10^-5 m/s.
Fig-1b: Electric Current Diagram
Electric Current Direction
One important thing to be noted. The direction of electrons (or current) inside the conductor is from the negative terminal of the battery to the positive terminal. But conventional current flows from high potential (+ve) terminal to lower potential (-ve) terminal. Before the discovery of electric current it was assumed that current flows from the positive terminal to the negative terminal.
Conventional Current Definition
If the direction of current is assumed from positive terminal to the negative terminal in the circuit this is called conventional current.
The assumption that current flows from positive to negative terminal is still in use. In fact we solve electric circuit problems using conventional current theory.
Electric Current Formula
As the electrons carry charges and move from one point to another in some time. So electric current equation can be written as
I (current) = Q/t (1)
Where “I” is electric current symbol and “Q” is the charge on electrons. Magnitude of charge on one electron is 1.6 x 10^-19 Coulomb (C).
Electric Current Unit
Electric current SI unit is Ampere (A). So according to equation (1) if one coulomb charge passes through a point in one second is equal to one ampere current. Similarly if 2 coulomb charge passes in one second then current is 2 amperes.
How to calculate Current?
Using the equation (1) you can find the electric current in the circuit. Following examples will guide you how to find electric current in the circuit.
How much current is flowing through the circuit if 25 coulomb charge passing in 2 millisecond.
If 1.33 x 10^12 electrons are passing through a point in 50 ms. Calculate current
Number of electrons = n = 1.33 x 10^12
Charge on one electron = e = 1.6 x 10^-19
Q = ne = 2.128 x 10^ -7 C
Using electric current formula given in equation (1)
I = 0.425 mA
Electric current types
Generally electric current is divided into two types. But mainly you can divide it into three categories
1. Steady State Current:
Steady current means a current which doesn’t change its magnitude with respect to time. Following graph shows a relation between time and current. You can observe the current remains constant throughout the time. Another name of steady current is Direct Current. The current provided by a battery is a steady state current.
Graph-1: Steady Current
2. Variable Current:
If the magnitude of current changes with time then its a variable current. In the following graph you can observe the currents’s amplitude is varying with respect to time.
Graph-2: Varying Electric Current
3. Alternating current
This is another form of variable but in this form current changes its magnitude and direction periodically. In other words in one complete time period (T), half of the time current flows in one direction and half time current flows in opposite direction. Following graph will explain alternating current
Graph-3: Alternating Current
Commercial companies produce alternating current to supply electric energy to the homes or factories. One reason is that its economical and can be distributed to longer distances as compared to direct current. The current produced by the generator is the alternating current.
At home we can convert alternating current to direct current using rectifier circuits which are usually built in power supplies of electronic devices work on direct current.
Some Common Questions and Their Answers
Current is scalar or vector?
Sometimes students ask me a question either current is a scalar or vector quantity. Refer to the equation (1) above. The charge and the time are scalar quantities so the current is a scalar quantity. The arrows used in figure-1 only indicate the direction of flow of charges. These arrows are not vectors. They don’t follow the rules of vector addition or subtraction.
How to determine direction of current in a circuit?
You can observe in the figure-1b the electrons in the material are moving from negative to positive terminal. Outside the material I mean in the circuit electrons are going from positive to negative terminal. This is conventional current as we discussed before.
Can electric current flow through air?
As discussed above, electric energy needs free electrons or charged particles to move from one point to another. The atoms of the material stop charge particles from moving to the other point. In other words atoms show some resistance to the flow of electrons. If resistance is very high i mean the spaces between atoms are very small. Very few number of electrons will pass through. It means small amount of electric current has passed. Another case is the resistance is low so more electrons will pass and hence the current.
Similar to the above case the resistance of air is very high. Air consists of many gases. We need to create free and charged particles (ions) to move the current through the air otherwise we can’t. The ions can be created using very high voltage. Sometimes you see high tension wires sparking in the air. When the wire breaks the high voltage in the wire causes ionization in the air and produce ions of the air gases. These ions let the current move in the air to some distance.
Can electric current flow in an open circuit?
An open circuit is just like a break in the wire. And open circuit resistance is considered infinite or in other words resistance is very high. So the current cannot pass through the open circuit. That in case of very high voltage (let say 30kV) as we discussed in the above question current can flow through some distance.
Can current be produced by magnetic field?
As we know that by providing external energy the electrons can be freed from the valence shell of the atom. These free electrons contribute in electricity. Similarly electric current can be generated by magnetic force. The magnetic lines of force can extract electrons from the valence band to the conduction band by providing sufficient energy to them.
You can see the following video to understand this answer
Who invented electric current?
Some people give credit to Benjamin Franklin for electricity invention. But electricity is a form of energy which was already present in nature. I mean electrical current is only the movement of free electrons in the particular direction. So many scientists studied electric current. Its been discovered not invented. You can read the history at Wikipedia.
Voltage vs Current
It is evident from the above discussion that there is a clear difference between voltage and current. Current is the flow of electric charges from one point to another. And voltage or potential difference is the cause that builds pressure on the electric charges to move.