Electricity and Current

Electric Potential and Potential Difference

Electric potential is the work done per unit charge in bringing the charge from infinity to that point against electrostatic force.

In a conductor, electrons flow only when there is a difference in electric pressure at its ends. This is also called potential difference. In a circuit, this potential difference is created by a battery.

• A battery or a cell has a potential difference due to reaction of chemicals inside it even when it is not connected to any circuit. Once connected, the potential difference makes the electrons flow and the current is generated in the circuit/conductor. A battery generates this potential difference till the chemicals inside it are not completely spent.

• Potential difference is also defined as work done to move a unit charge from one point to another.

V = W/Q   Or Potential difference =Work done/Charge

Electric current is rate of flow of electric charge or the amount of charge flowing through a particular area in unit time.

• Until the discovery of electrons, the electric current was considered to be a flow of positive charge and the direction was considered from positive terminal of the battery to the negative terminal.

• In reality, electric current is flow of negatively charges particles called electrons. These flow from negative terminal of the battery to the positive terminal.

• As a convention, the direction of current is taken opposite to the direction of flow of electrons.

• Measurement of Electric Current

  Ammeter is an instrument used to measure electric current. It is always connected in series in a circuit.

  • SI Unit of electric charge is Coulomb (C).Charge is usually denoted by letter Q.

  • SI Unit of electric current is Ampere (A).Current is usually denoted by letter I.

  I = Q/t, which means current is net charge flowing through a cross-section in time t.

  Hence, 1 A = 1 C/ 1s.

  • 1 Coulomb is equivalent to 6 X 1018 So an electron possess negative charge of 1.6 X 10-19 C.

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship

I = V /R ,

where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

Ohm's law is an empirical relation which accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However some materials do not obey Ohm's law, these are called non-ohmic.

• Resistance is a property of conductor to resist flow of charge through it. Its SI unit is Ohm (Ω).

R = V/I, 1 Ohm = 1 Volt/ 1 Ampere

• I = V/R denotes that Current is inversely proportional to Resistance. Hence, greater the resistance lesser will be the flow of current and vice-versa.

• A rheostat or a variable resistor is a device or a component which allows changing of resistance in a circuit keeping the voltage same.

• In a conductor, electrons are attracted by the atoms. This is a resistive force which is lower for a good conductor but very high for an insulator. A conductor having some resistance is called a Resistor.

Factors on which Resistance depends

Resistance of a conductor depends:

1. On its length

2. On its area of cross-section

3. On the nature of its material

• Resistance of uniform metallic conductor is directly proportional to its length and inversely proportional to area of cross-section.

R∝l

R∝ 1/A

Therefore, R = ρ(l / A),ρ(rho) is constant of proportionality.

• ρ is called electrical resistivity of the material of the conductor. SI unit is Ωm.

• Resistivity of conductors is very low whereas the insulators have a very high resistivity.

• Resistivity varies with Temperature.

• Alloys having higher resistivity than metals are used in electrical heating devices, like iron and toasters, tungsten is used in filament of electric bulbs and copper and aluminium are used for electrical transmission lines.

Resistivity of Conductors < Resistivity of Alloys < Resistivity of Insulators

Resistors in series

When two or more resistors are connected in series:

• The current through the circuit remains the same.

• The potential difference becomes sum of the individual potential difference across each resistor.

• Equivalent resistance of the circuit is the sum of individual resistances.

Disadvantages of series arrangement of resistors

• Two different electrical appliances, having different current requirements, cannot be connected in series as the current is constant in a series circuit.

• If one of the components fails in a series circuit, the circuit gets broken and none of the other components get the current.

Resistors in parallel

When two or more resistors are connected in parallel:

• The current through the circuit is the sum of currents through each branch of the circuit.

• The potential difference across the two points of the circuit remains the same.

• The reciprocal of equivalent resistance of the circuit is the sum of reciprocal of the individual resistances.

Advantages of parallel arrangement of resistors

• Parallel arrangement divides current in different branches and hence each component receives the required amount of current.

• If one of the components fails in the parallel circuit, the rest work as usual.

eating effect of Current

When a voltage source like a battery generates current for a circuit, most of the energy is used up in maintaining the current inside the circuit. However, a portion of the energy is dissipated in the form of heat. In a circuit containing only battery and resistance, the entire energy is dissipated in the form of heat. This is heating effect of current.

Power = Work Done / Time Taken  orP = W / t

Since, Potential Difference = Work Done / Charge  or V = W / Q

P = VQ / tor P = VI, where I (current) = Charge / Time Taken

Energy supplied by power source to a circuit in time t is, H = P x t = VIt

Or from Ohm’s law, H = I2Rt

This is known as Joule’s law of heating. It states that heat produces in a circuit is directly proportional to the square of current flowing, resistance for current and time for which current flows.

Disadvantages of heating effect

• Loss of energy in the unwanted heat.

• Wear and tear of components.