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Potentiometer is a measuring instrument used to measure electric potential, basically a three-terminal with a sliding contact that acts as a voltage divider.
- They are mainly used in audio control equipment.
- The working principle of a potentiometer is that the potential difference across any part of the uniform wire is directly proportional to the flow of constant current through it.
- It is based on the null method or zero deflection method.
Ques. Define a cell.
Ans. A cell is a single unit within a battery that provides the necessary potential difference to an electric circuit to maintain a continuous flow of electric current in it.
Ques. Define the internal resistance of a cell.
Ans. The internal resistance of a cell is defined as the opposition offered by the electrolytes and electrodes of a cell to the flow of current through it.
Ques. On what factors does the internal resistance of a cell depend?
Ans. The internal resistance of a cell depends on the nature of the electrolytes and electrodes of a cell.
Ques. Define Potentiometer.
Ans. A potentiometer is an electrical device used to measure the emf of a source (say a cell) without drawing any current from the source.
Ques. On which principle does the potentiometer work?
Ans. A potentiometer works on the principle that potential difference across any part of the uniform wire is directly proportional to the length of that portion when a constant current flows through the wire.
Ques. Which material is used for the potentiometer wire?
Ans. The alloys of constantan and manganin are used to make potentiometer wires because they have high resistivity and low-temperature coefficient of resistance.
Ques. Why do we prefer a potentiometer with a longer bridge Wire?
Ans. Since the potential gradient decreases with the increase in the length of the wire, therefore a potentiometer with a longer bridge wire is preferred because they have a small potential gradient that makes it more sensitive.
Ques. Why copper is not used as a potentiometer wire?
Ans. Copper has a high-temperature coefficient of resistance and low resistivity. As a result, a very small change in temperature results in a large change in resistance which affects the circuit.
Ques. What is the balance point in the potentiometer?
Ans. The balancing point on the slide wire of a potentiometer in the point where the galvanometer attached to the potentiometer shows zero deflection.
Ques. How can be a potentiometer made more sensitive?
Ans. A potentiometer can be made more sensitive by decreasing its potential gradient which can be obtained by
- By decreasing the potentiometer current with the help of rheostat.
- By increasing the length of the wire of the potentiometer.
Ques. What is electromotive force (emf)?
Ans. The electromotive force of a cell, commonly pronounced as emf of a cell may be defined as the potential difference between the terminals of a cell when no current is drawn from it. The name electromotive force is misleading because it is not a force but the work done per unit charge.
Ques. Define potential difference.
Ans. Potential difference is the difference in the electric potential energy per unit charge between the two points in an electrical circuit. It is also known as Voltage.
Ques. What is the terminal potential difference of a cell?
Ans. The terminal potential difference of a cell is defined as the potential difference between its terminals in a closed circuit i.e. when current is drawn from it.
Ques. Why is Ohm’s law not applicable to semiconductors?
Ans. The conductivity of semiconductors lies between a conductor and an insulator. The voltage-current characteristics of semiconductors are nonlinear which means the variation of voltage applied to a semiconductor device is not directly proportional to the current flowing through it. Hence Ohm’s law is not applicable to semiconductors.
Ques. What is the difference between a potentiometer and an ordinary voltmeter?
Ans. Following are the difference between a potentiometer and an ordinary voltmeter
| Potentiometer | Ordinary Voltmeter |
|---|---|
| It acts like an ideal voltmeter of infinite resistance. | It is an ordinary instrument of finite resistance. |
| It worked on the null method. | It worked on the deflection method. |
| No current is drawn by it from the cell to be measured. | Some current is dawn by it from the cell to be measured. |
| Sensitivity is high | Sensitivity is low |
| It is a very accurate device. | It is an approximate device. |
Ques. What are the applications of a potentiometer?
Ans. A potentiometer is used to
- Compare the emf of two cells
- Determine the internal resistance of a cell
- Measure unknown resistance
Ques. What is direct current?
Ans. An electric current is said to be a direct current if its magnitude and polarity remain constant over time. The direction of flow of electrons in a direct current circuit is in one direction only i.e. from high potential to low potential.
Ques. What is a Wheatstone bridge?
Ans. Wheatstone bridge is an arrangement of four resistors in the form of a bridge used for measuring the resistance of one unknown resistor in terms of the known resistances of the other three resistors.
Ques. What is the difference between emf and terminal voltage?
Ans. Following are the differences between emf and terminal voltage
| EMF | Terminal Voltage |
|---|---|
| It is defined as the difference in potential between two terminals of a cell when no current is drawn from it. | It is defined as the difference in potential between two terminals of a cell when current is drawn from it. |
| Its value is independent of the resistance external resistors. | It is directly proportional to the value of the external resistance. |
| The emf of a cell is greater than the terminal voltage. | Its value is less than the emf. |
Ques. What is electrical resistance?
Ans. Electrical resistance is the opposition offered by a material or conductor to the flow of electric current through the circuit. The resistance offered by a material is directly proportional to its length and temperature and inversely proportional to the area of the cross-section.
Ques. Define the temperature coefficient of resistance of a material.
Ans. It is defined as the change in resistance per unit of original resistance per degree rise in temperature of a material. It is denoted by alpha (????). The SI unit of the temperature coefficient of resistance is per kelvin (K-1). Its value is positive for metallic conductors and negative for insulators and semiconductors.
Ques. When does Ohm’s law fail?
Ans. Ohm’s law fails for the devices which do not follow a linear relationship between voltage and current. In a semiconductor, the relationship between voltage and current is nonlinear because the physical condition like temperature and pressure changes with the applied voltage. Hence Ohm’s law fails in these materials.
Ques. How does the temperature affect the internal resistance of a battery?
Ans. The internal resistance of a battery is the opposition to the flow of ions within a battery. As the temperature increases, the ions move faster which produces more current and results in a decrease in the internal resistance of the battery.
Hence, with the increase in temperature, the internal resistance of a battery decreases.
Ques. Define electric energy.
Ans. The work done by a source to maintain a current in an electric circuit is known as electric energy. The formula of electrical energy is given by
- E = VIt
- E = I2Rt
- E = (V2/R)t
Where
- E = electrical energy
- I = electric current
- R = Resistance
- V = Potential difference
- t = time
Ques. Define electric power.
Ans. Electric power can be defined as the heat energy produced per unit of time in an electric device of resistance R, when current I is passed through it. The formula of electrical power is given by
- P = E/t
- P = I2R
- P = V2/R
- P = VI
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