Published by :
Renji Elsa Jacob, M-Tech in Electrical & Electronics Engineering
A Circuit can be a closed circuit when it is complete and forms a loop that allows the current to flow through it. Electric current is the continuous flow of electrons in an electric circuit.
Electric Potential
The capability of the charged body to do work is called electric potential. When a body is charged, either electric electrons are provided to it or they are removed from it. The work done is stored in the body in the form of electric potential. By moving the charges either by attraction or repulsion, the charged body can do work. The ability of the charged body to do work is called electric potential.
The capacity of a charged body to do work is called its electric potential.
The electric potential is determined by the capacity of the charged body to do work. The greater the capacity of a charged body to do work, the greater is its electric potential. The measure of the electrical potential is the work done to charge a body to one coulomb so:
Units: Herework is measured in joules and charge is measured in coulombs. So the unit of electric potential is joules/coulomb or volts.
If W= 1 joules and Q = 1 coulomb then
Hence we can say that a body is said to have an electric potential of 1 volt if one joule of work is done to charge the body to one coulomb.
For example, if a body has an electric potential of 10 volts which means that 10 joules of work have been done to charge the body to 1 coulomb.
Potential Difference or Voltage
The electric potential difference is defined as the amount of work done to carrying a unit charge from one point to another in an electric field. Some work or energy transfer is required for the electron in a conductor to move in a particular direction. This work is performed by an external electromotive force (emf).
Potential difference is defined as the difference in the electric potential of the two charged bodies.
If two bodies have different electric potentials, a potential difference exists between the bodies. For example, consider two bodies X and Y have potentials of 7 volts and 5 volts respectively.
When the two bodies are joined through a conductor, then the electrons will start flowing from body Y to body X, while the conventional current will flow in the opposite direction (X to Y).
The flow of current stops when the two bodies attain the same potential. So we say that the current flows in a circuit only if a potential difference occurs. No potential difference, no current flow.
Unit: As the unit of electric potential is volt, the unit of potential difference is also volt.
EMF vs Voltage
| Electromotive Force | Voltage | |
| 1. | EMF is the amount of energy supplied by the source to the charge | Voltage is the energy requires to move the unit charge from one point to another. |
| 2. | Symbol ɛ | Symbol V |
| 3. | Formula E= I(R+r) Where R – External resistance of the circuit r- internal resistance of the circuit I – current | Formula V = I/R V- Voltage I – Current R - Resistance |
| 4. | EMF is measured between the endpoint of the source when no current flows through it | Voltage is measured between any two points of the closed circuit. |
| 5. | EMF is generated by electrochemical cells, solar cells, batteries, etc. | Voltage is caused by the electric and magnetic fields. |
| 6. | SI Unit - Volt | SI Unit - Volt |
Potential Rise and Potential Drop
The figure consist of a circuit with a cell and a resistor. The cell is an energy source that provides a potential difference of 1.5 V. So there will be a rise in potential related to a cell. So this cell’s potential difference denotes the emf (E). The resistor is also linked with a potential difference. As it is a consumer of the energy, there is a drop in the potential across the resistor. We can relate potential rise or drop with the term “voltage” then we can say that potential difference across the cell as a voltage rise and the potential difference across the resistor as a voltage drop.
Resistance
Resistance is the opposition to the current. The schematic symbol for resistance is shown below:
Current is the flow of free electrons while resistance is the opposition offered by the substance to the flow of free electrons. This opposition happens because atoms and molecules of the substance obstruct the flow of these electrons. Some substances (metals like silver, copper, etc.) offer very little opposition to the flow of electric current and are called conductors while some substances offer high opposition to the flow of electric current are called insulators. Eg: glass, rubber, mica, etc.
The free electrons start flowing when the voltage is applied across the resistor. These moving electrons collide with each other and it opposes the flow of electrons. The opposition of electrons is known as the resistance. The heat is generated when the atom or molecules are colliding with each other.
Units: The unit of resistance is ohm and is represented by the symbol Ω.
Factors upon which resistance depends
The resistance R of a conductor
R α l is directly proportional to its length
is inversely proportional to its area of X- section
R depends upon temperature
depends upon the nature of the material,
Where ρ (rho) is a constant and is called resistivity or specific resistance of the material
Specific resistance or resistivity
Resistivity can be defined as the resistance between the opposite faces of a metre cube of the material or in other words, we can say that resistivity is the ability of the conductor or the material to oppose the electric current.
Unit of resistivity:
The unit of resistivity will depend upon the units of are of cross-section (a) and length (l).
- If the length is measured in cm and the area of the cross-section in cm2 then the unit of resistivity will be ohm-cm (Ω cm).
- If the length is measured in metres and the area of the cross-section in m2 then the unit of resistivity will be ohm-m (Ω m).
Conductance
The reciprocal of resistance of a conductor is called its conductance (G). If resistance is R, then its conductance G is given by,
Resistance is the opposition to the current flow while the conductance of a conductor is the inducement to current flow.
SI unit of conductance is mho (ie, ohm spelled backward). But now we use siemen as the unit of conductance which denoted by the symbol S.
Conductivity
The reciprocal of resistivity of a conductor is called conductivity. It is denoted by symbol σ.
So Conductivity = 1/ Resistivity
SI unit of conductivity: Siemen metre-1
(Sm-1)
No Comments