What is Electric Current?
When there is a potential difference between two points in a conductive medium, electric charge starts flowing from the higher potential point to the lower potential point to balance the charge distribution between the points. The rate of flow of charge in respect of time is known as electric current.
If q Coulomb electric charge gets transferred between these two points in time t sec, then the current can be calculated as
In differential form, the current can be represented as
Unit of Current
As the current is the ratio of transferred charge to the time taken for this charge transferring we can say one unit current is such a rate of charge transferring in which one Coulomb charge is transferred from one point to another in one second. Hence, unit of current is coulomb / second and it is well known as ampere after the great physicist Andrew Marie Ampere. This is SI unit of electric current.
Theory of Electricity
Current in Metallic Conductor
The main cause of current through a metallic substance is the flow of electrons that is the directional drift of free electrons. In metal, even at room temperature, there are plenty of free electrons exist inside the metallic crystal structure. When electric potential between two points in the metal differs the free electrons which were randomly moving at equilibrium potential condition and also the free electrons supplied by the source (if a source is connected between these two points) now get drifted towards higher potential point due to electrostatic attraction. As each of the electrons has a negative charge of – 1.602 × 10 – 19 coulomb, it can be said that the negative charge gets shifted towards higher potential point. The rate of flow of this negative charge in respect of time is the current in the metal.
Conventional Direction of Current
Although the flow of electrons or negative charge is from lower potential point to higher potential point but the conventional direction of current is considered from higher potential to lower potential. Although current is mainly caused by the flow of electrons that is the flow of negative charge but previously it was thought that the electrical current is due to the flow of positive change.
But now it is proved that the current in a metallic conductor is due to flow of electrons or negative charge but the direction of current is still considered as it was accepted previously that is opposite of flow of electrons. The direction of the current which is considered from a higher potential point to a lower potential point is known as the conventional direction of
Types of Current
When current flows in one direction either in constant or fluctuating manner the current is called direct current.
When current flows in either direction alternatingly in a frequency is called alternating current. The average value of an alternating current is zero. The alternating current is measured in RMS value. One main parameter of alternating current is frequency.
Magnetic Effect of Current
When current flows through a conductor there will be a magnetic field surrounding the conductor. The direction of the lines of force of the magnetic field can be determined by right-hand grip rule. If we imagine that we have held the current carrying conductor with our right hand with extended thumb along the direction of the current then four fingers of our right hand indicate the direction of lines of force of the magnetic field.
When we make a coil with a conductor and current flows through the coil then due to magnetic effect of each conductor of the coil there will be an overall magnetic field surrounding the coil. Here we can also determine the direction of the field by right-hand grip rule. If we hold the current carrying coil with our four fingers along the direction of current in the turns of the coil then the extended thumb indicates the direction of the magnetic field.
Current in Magnetic Field
When we place a current carrying conductor or a current carrying coil in a magnetic field, a mechanical force acts on the current carrying conductor or coil. This mechanical force depends on the current through the conductor or coil.
Measurement of Current
Depending on the principle of interaction between current and magnetic field one can measure the current. One of the basic instruments to measure the current is pmmc instrument or permanent magnet moving coil instrument. The pmmc instrument is only able to measure direct current. The alternating current can be measured by moving iron instrument where magnetic field created by current through the instrument coil causes movement of a soft iron piece either by attraction or repulsion force. This instrument can also measure direct current. Rectifier type instruments are also used to measure alternating current. Here bridge rectifier is used to rectify alternating current then it is measured with pmmc instrument. Wherever may be the types of current measuring instrument in one word all current measuring instruments are called ammeter. An ammeter is always connected in series with the path of which the current to be measured. When very high current is to be measured we use current transformers to step down current for measuring purpose.
Heating Effect of Current
When current flows through a conductor there is a heating effect in the conductor. The loss of power in the conductor is i2R watts. The loss of energy is i2Rt joules. This loss of energy is converted to heat. Hence,
This is known as Joule’s Law of Heating.