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Electromotive force – (e.m.f.)
Circuit below shows set up of an electric source of e.m.f. = E volts, driving electric charges round the closed the circuit. The e.m.f. drives the electric charges through the internal resistance of source as well as the external resistance of the circuit.
Hence if the e.m.f. of the cell is 2 V, the total energy dissipated (given out) is 2 joules when one coulomb of charge flows through the closed circuit. Since energy = charge (Q) X potential difference (V). The energy given out will be in the form of heat and light at the bulb with some heat generated within the cell, and along the connecting wires. If Ohm’s law is applied for the whole circuit, we have
E = I (r + R) i.e. current X total resistance in the whole circuit
Where E is the e.m.f. of the source in volts
I the current flowing through the closed circuit in the lamp
r the internal resistance of the cell in ohm
R the total external resistance in ohm
Potential difference
It is the amount of energy that one coulomb of charge transfers when it moves from one point to the other point. It is measured in volts and it is often referred as voltage.
One Volt
The potential difference is said to be 1 Volt, if one coulomb of charge transfers 1 J amount of energy when its moves from one point to the other point.
Explanation of potential difference
The electrons moving round a circuit have some potential energy. As electrons move around a circuit, they transfer energy to the various components in the circuit. Total amount of energy transferred to the different components of the circuit is always equal to energy provided by the power source (battery). For example, when electrons move through a circuit having one lamp as shown below.
Measurement of potential difference
Potential difference can be measured by connecting a voltmeter parallel to those points, across which we have to determine the potential difference as shown in the diagram below.
A voltmeter can be used to show how the potential difference varies in the different parts of the circuit. In a series circuit you find different values of the voltage depending on where you attach the voltmeter. It can be assumed that the energy is only transferred, when electric current passes through electrical components such as lamps and motors. The energy transfers as the current flows through copper connecting wires is very small. It is only possible therefore to measure a potential difference or voltage across a component.
Voltmeter
Voltmeter is an instrument used to measure potential diference across a resistor or between any two points in a in a circuit.
A voltmeter is always connected in parallel to a resistor or device across which potential difference is to be measured because potential difference across all components (resistors voltmeter etc.) connected between same two points would be same.
Voltmeter is basically a galvanometer which is modified to measure potential difference. A galvanometer is converted into voltmeter by connecting a high resistance in series with it. Then the total resistance of voltmeter will be very high According to Ohm’s law
V=IR or I = v / R
Since potential difference ‘ V ‘ across voltemeter and resistors across which it is connected will be same, current drawn by it will be small as it has high resistance. This current will produce sufficient defection in voltmeter (or the galvanometer used in it).
This deflection will be proportional to the current passing through it and hence to potential differenc across it. By connecting high resistances of differenet siutable values in series with galvanometer, it can be converted into voltmeters of different ranges.