Electro Motive Force – EMF
Each cell has to do some work to send electricity through the conducting path. This work is done by the electromotive force emf of the cell. If E is the electricity, volt (V) is the emf of a cell, the work it does to send one coulomb ( C ) of charge round the external and internal circuits is E joule (J); and to send Q coulomb, the work W done in joules is
W= E x Q.
For instance a galvanic or voltaic cell has emf 1.08 V, a Daniel cell 1.08 V and a dry or Leclanche cell 1.5 V and the rechargeable lead-sulphuric acid cell has an emf of 2.0 V.
What is a Battery?
A group of cells is called battery. A single cell, however big has an emf characteristic of the cell; the bigger the cell the greater the current it can supply without getting damaged. Series grouping of cells gives additive emf whereas parallel grouping gives only the emf of a single cell.
Ohm’s law of Resistance
This is a fundamental law in electricity which gives a simple relationship between the emf applied to a conductor under steady condition and the electric current passing through the conductor.
Ohm’s law Statement: Temperature remaining constant the electric current passing through a given conductor under steady conditions is directly proportional to the potential difference or emf applied between the ends of the conductor.
One volt per ampere was named one ohm in honour of George Simon Ohm, the German physicist who discovered the Ohm’s Law in 1826.
What is Resistance?
The ratio of potential difference applied to the current conducted by the conductor or resistor is called the resistance which is expressed in ohm when emf and current are expressed in volt and ampere respectively.
On account of the resistance offered by a conductor, it is also called resistor. In house wiring or street lamp wiring circuits each electric lamp is connected across the circuit. So each lamp gets the full circuit emf or voltage; such a connection is called parallel connection. Each lamp is independent of the others.
What is Voltmeter?
Instrument called voltmeter is used to measure voltage at the terminals of a source of emf, or in some portion of an electrical circuit. Voltmeter looks like ammeter and so to distinguish it from other meters (measuring instruments) it is usually marked with the letter V and one terminal is marked ‘+’. In circuit diagrams it is symbolized by a circle with the letter V inside.
Basically voltmeter is also a galvanometer (or microammeter or milliammeter ) with a resistance of high resistance in series connected so as to increase the overall resistance of the meter.
Internal Resistance, Terminal Electro Motive Force (EMF)
An electric circuit consists of two parts, (i) the external circuit and (ii) the internal circuit. The external circuit comprises the components like the conducting wires and other devices connected externally between the positive and negative poles of the sources of emf, like an electric bell. The internal circuit comprises the electrodes or poles and the internal parts like solutions in a chemical (or galvanic) cell.
The resistance offered by the components in the external circuit from the positive pole to the negative pole is called the external resistance ‘R’. Similarly, the resistance offered by the contents of the cell (or source of emf) between the poles is called the internal resistance ‘r’. The total resistance to the electric current is the sum of external and internal resistances (R+r).