## Friday, January 4, 2013

### Potentiometer and its Principle of Working

It is consist of long uniform wire of manganin or constantan fixed parallel to one another connected in series by copper strips. A meter is fixed parallel to wires (each one meter) and two ends are fixed with binding screws. The potentiometer jockey can be touched to wire at any location. A battery E connected across A and B sends the current through the wire which is kept constant by using rheostat Rh.

Principle: The working of potentiometer is based upon the fact that fall of the potential across any portion of the wire is directly proportional to the length of the wire provided wire has uniform cross section area and constant current flowing through it.

Consider a length of wire l and current flowing through it is I. Cross section area is A and ρ is Resistivity and R is resistance. Then we can say potential across the wire length is

if I and A is constant then

V/l =K = constant, which also called potential gradient that is fall of potential per unit length of wire.

It is called potentiometer because it is used to measure potential difference.

1. Why Potentiometer is chosen over Voltmeter to measure the potential of a cell?

Ans : When Voltmeter is used, current flows through the circuit, and because of the internal resistance of the cell, always terminal potential will be less than the actual cell potential. But in a potentiometer circuit, when the potential difference is balanced, no current flows in the circuit, so terminal potential will be equal to actual cell potential. i.e, Voltmeter measures the terminal potential of a cell, but Potentiometer measures actual cell potential.

2. HOW DOES A FALL IN POTENTIAL OCCUR IN THE CIRCUIT?

1. Electric potential = work done per unit charge (C). The charge gains energy when it travels from negative to positive terminal of a battery, as it has to do some work against the electric field. So, when it is energized, it passes through any resistance (could be the negligible resistance of the wire or of, say a resistor etc) it loses energy. which brings us back to our 'defining equation' of voltage; Joules per charge (Coulomb). if energy is given out, then joules are less, then the numerator of the equation is less so voltage is less. now think that unit charge passes thorough more resistance (say another resistor), with already less energy, so eventually the voltage drops even more, until it literally has no energy (voltage at that point) just before it enters the cell again. you may think by comparing it with a water current, where there is motor pushing the current, and there are obstacles in a CLOSED route.

3. Why the current does not flow in the secondery circuit at balancing length

6. Is this the same potentiometer you are talking about ?:
https://en.wikipedia.org/wiki/Potentiometer

7. A potentiometer can refer to two types of devices: an instrument used to measure electromotive force, or an electronic component with an adjustable resistance. In the latter case, a potentiometer is generally a resistor with a sliding contact that is mounted on a rotating shaft. This device serves as a voltage divider that allows the user to alter the level of resistance offered to the current flowing through it, which means that the output voltage can be modified to anywhere between the maximum level of incoming current and zero volts.

In a typical potentiometer used for low-power applications, a shaft-mounted wiper contact slides across a resistive fixture. The placement of the wiper contact on the resistive element determines the level of output voltage, which is sent through a fixed contact. Another fixed contact provides the input voltage. The resistive part of a potentiometer can be made from graphite or a similar semi-conductor material. Regular applications for these devices are in audio devices, where they control volume levels, or in other low power electronic devices that require level adjustments. Certain potentiometers may also include an “off” position, and therefore act as an on/off switch as well as means to adjust volume or voltage output levels.

As measurement devices, potentiometers are designed in four categories: constant resistance, constant current, thermocouple and microvolt potentiometers. These instruments are used to measure voltage levels in low power circuits. Essentially, a potentiometer uses a known resistance and a sliding contact to determine the voltage of the circuit. A metering device is attached to one end of the circuit, which registers when the sliding contact has reached a level that does not allow current to pass through it (at zero volts). The user then determines the position of the contact, and uses it to calculate the voltage of the circuit.

8. why does back flow of current will not take place from secondary circuit?

9. why does back flow of current will not take place from secondary circuit?

10. potentiometer work which effect?