What is Electric Current and Theory of Electricity

What is Electric Current and Theory of Electricity

What is Electric Current?
The electric current is the rate of flow of electric charge through a conducting medium with respect to time.
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.

Current Formula

If q Coulomb electric charge gets transferred between these two points in time t sec, then the current can be calculated as
current formula
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.
Conventional Direction of Current
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 current.

Types of Current

Direct Current

When current flows in one direction either in constant or fluctuating manner the current is called direct current.

Alternating 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.
right hand rule
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.

Comments

Post a Comment

Popular posts from this blog

SAFETY VALVE FLOATING

SAFETY VALVE FLOATING PROCEDURE FOR SAFETY VALVE FLOATING 1.0 OBJECTIVE The objective of this procedure is to outline the general procedures involved in setting the safety valves to the designed set pressure before allowing the boiler to go for commercial operation. Follow the vendor’s manual for adjustment of set pressure. 2.0 PRE-REQUISITES Ensure proper mounting of the safety valve exhaust pipe, such that no load is coming to the safety valves. Check the drip pan assembly and its drains are as per the requirements. Follow the instructions of the vendor for safety valve exhaust piping and its support. 3.0 PROCEDURE 3.1 Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valve is gagged. Start the boiler as per cold start-up the procedure by modulating the firing. When the drum pressure reaches about 75% of operating pressure gently tighten gage on other safety valves. Raise pressure slowly by throttling the startup ...
Read more

Steam Condenser of Turbine

Steam Condenser of Turbine Steam condenser is a device in which the exhaust steam from steam turbine is condensed by means of cooling water. The main purpose of a steam condenser in turbine is to maintain a low back pressure on the exhaust side of the steam turbine . After releasing from nozzles, the steam has to expand to a great extent for converting available energy into it to usable mechanical work. So, if the steam after doing its, work, does not get condensed, it will not give required space to other steam behind it, to expand to its required volume. Condensation of steam in a closed system, creates an empty place by reduction of volume of the low pressure steam. It is found that, 1 Kg of dry steam at 1.033 kg/cm 2 absolute pressure has a volume of 1.673 m 3 when it is condensed into water at 100 o C in a steam condenser, its volume becomes 0.001044 m 3 . The volume of steam would be thus 1/1644 parts of the space inside the vessel, and the pressure would fall to 0.2...
Read more

Rankine Cycle

Image
Rankine Cycle Ideal Rankine Cycle Principles of thermodynamics are useful for power cycle for of electrical power generation (i.e net power output) and to study refrigeration & heat pump which requires input of net power. Classification of thermodynamics power cycles can be done into two types: Vapor cycle working fluid exists in liquid phase during one part of the cycle (i.e from condenser outlet to Boiler) and mixed phase wit in the steam boiler and in vapor phase at the Boiler outlet. Gas cycle working fluid during the cycle remains in gas phase. Steam power generation units run on vapor power cycle using water as the working fluid. Under this section attempt is made to familiarize the readers with the concepts of ideal vapor cycle called Rankine cycle . Typical Power Plant Cycle Electrical power is generated by using vapor cycle power plants by using Coal, Lignite, Diesel, Heavy furnace oil as fuel depending upon the availability and cost. The flow s...
Read more