potentiometer experiment precautions

Write the two precautions for using potentiometer.

Precautions with potentiometer: 1. emf of the cell connecting in primary circuit must be more than or equal to the emf of the cell of secondary circuit otherwise zero deflection can not be obtained. 2. all the high potential points or positive terminals should be connected at $$a$$ 3. balancing length should be calculated from $$a$$ 4. area of cross-section of the wire should be uniform otherwise potential gradient will not be constant. 5. current should not be passing through potentiometer wire for a long time otherwise this will heat up the wire and will changes its resistance and hence potential gradient will also changed..

Write two precautions to be observed while using a beam balance.

Potentiometer

• Post author By Hemant More
• Post date January 2, 2020
• 2 Comments on Potentiometer

Science > Physics > Current Electricity > Potentiometer

In this article, we shall study the principle, construction, and working of a potentiometer and its uses.

Principle of Potentiometer:

When a steady current flows through a wire of uniform cross-section the potential difference per unit length of the wire is constant throughout the length of the wire (or p.d. across any two points of the wire is directly proportional to the length of the wire. It can be explained as below.

Let us consider a uniform wire AB of length l AB  and uniform cross-sectional area A. Let R AB be its resistance. Let ‘I’ be the steady current flowing through the wire. Let V AB be the p.d. across the ends of the wire. Let ‘ρ be the specific resistance of the material of the wire. Let there be a uniform potential drop across the length of wire.

Let us consider point P on the wire and the length of wire between A and P be ‘ l AP ’. Thus, the resistance of a wire of length ‘R AP ’ is given by

When a constant current flows through a wire, then the potential difference between any two points of the wire is directly proportional to the length of wire between these two points. In such a case, the p.d. per unit length of the wire is constant and called the potential gradient of the wire or voltage drop across the wire.

Precautions to be Taken While Using a Potentiometer:

The e.m.f. of the cell connected across the potentiometer wire should b greater than the e.m.f. to be compared.

The positive terminal of the cells whose e.m.f. is to be compared must be connected to that end of potentiometer wire where positive terminal of the battery (driving cell) is connected.

The potentiometer wire must be uniform.

The resistance of potentiometer wire should be high.

Advantages of a Potentiometer Over a Voltmeter:

• A potentiometer can be used to measure the internal resistance of cell which cannot be measured by the voltmeter.
• A Potentiometer can be to measure e.m.f of a cell which cannot be measured by a voltmeter. When a voltmeter is connected in a circuit it draws current through the circuit and thus can measure the potential difference across the cell terminals. When the potentiometer is connected in a circuit it draws no current when the null point is obtained. Thus it measures the e.m.f. of the cell.
• A potentiometer can be used to measure extremely small p.d. accurately which cannot be measured by a voltmeter. It can be done by using very long wire and adjusting a very small potential gradient.
• Potentiometer is more sensitive compared to voltmeter.
• The accuracy of the potentiometer can be increased by increasing the length of the wire. The accuracy of the voltmeter cannot be increased beyond the limit.

Disadvantages of a Potentiometer:

• A voltmeter is a direct reading instrument while potentiometer is not so. We have to perform calculations to find the result.
• A voltmeter is portable while potentiometer is non-portable

Construction of Potentiometer:

A potentiometer consists of a uniform wire AB several meters long.  It is stretched between two points A and B on the wooden board. A battery having a sufficiently large e.m.f. E is connected between A and B of the wire.  On closing, the key current will flow through the wire. The current in the wire can be adjusted by adjusting rheostat connected in series with the battery.  The battery maintains a uniform potential gradient along the length of wire.

Uses of Potentiometer:

To Measure e.m.f. of a Cell or to Compare e.m.f.s of Two Cells by Individual Method

Let E 1 and E 2 be the e.m.f.’s of the two cells to be compared by using the potentiometer. The positive terminal of the cell of e.m.f. E 1 is connected to end A and a negative terminal is connected to jockey through the galvanometer. By closing the key the jockey is moved along wire AB and null point P is determined such that galvanometer shows no deflection. The length of wire AP = l 1 is measured.  The p.d. across this length balances e.m.f. E 1

e.m.f. of the cell =  potential difference across AP

E 1 = K l 1 ………..  (1)

where K is the Potential gradient of the wire

Then cell of e.m.f. E 1 is disconnected and cell of e.m.f. E 2 is connected in circuit and procedure is repeated

E 2 = K l 2 ………..   (2)

Dividing equation  (1) by (2), we get,

Thus knowing the values of l 1 and l 2  we can compare e.m.f.s of two cells.

To Measure e.m.f. of a Cell or to Compare e.m.f.s of Two Cells by Sum and Difference Method:

Let E 1  and E 2 be the e.m.f.’s of the two cells to be compared by using the potentiometer. In this method both the cells whose e.m.f.s are to be compared are connected together.

When the two cells are connected in series such that the negative terminal of one cell is connected to positive terminal of the other, then the two cells are said to assist each other and their resultant e.m.f. is given by the sum of the e.m.f.s of the two cells. (E 1 + E 2 )

When the two cells are connected in series such that the negative terminal of one cell is connected to the negative terminal of the other, then the two cells are said to oppose each other and their resultant e.m.f. is given by the difference of the e.m.f.s of the two cells. ( E 1 – E 2 )

In the first step, the cells are connected to assist each other. The positive terminal of the combination of cells is connected to end A and another terminal is connected to jockey through the galvanometer. By closing the key the jockey is moved along wire AB and null point P is determined such that galvanometer shows no deflection.  The length of wire AP = l 1 is measured.  The p.d. across this length balances e.m.f. (E 1 + E 2 )

∴  e.m.f. of the cell  = potential difference across AP.

E 1 + E 2   = K l 1 . . . (1)

In the second step, the cells are connected to oppose each other and the procedure is repeated.

E 1 – E 2 = K l 2   . . . (2)

Thus knowing the values of l 1 and l 2 we can compare e.m.f.s of two cells.

To Find Internal Resistance of a Cell:

A battery B having an e.m.f. greater than the e.m.f. (E) of the cell whose internal resistance (r) is to be measured, is connected in series with the potentiometer wire AB, a key K 1 , and a rheostat. The positive terminal of the cell of e.m.f. E is connected to the end A of the potentiometer wire. The negative terminal of E is connected to a jockey through the galvanometer G. A resistance box and a keyK 2 are connected across the cell E.

Initially, the key K 2 is kept open.  By closing the key K 1 current is passed through the potentiometer wire so that uniform potential gradient is produced along the wire. By sliding the Jockey along the wire, a point of contact P 1 for which the galvanometer shows zero deflection is found.  The length of the wire AP 1 = l is measured. As the cell is in an open circuit, e.m.f. of the cell is equal to the p.d. across the length l, of the potentiometer wire.

E = K l          . . . (1)

Where K is the potential gradient of the wire.

Now a suitable resistance (R) is connected from the resistance box and the key K 2  is closed and once again null point P 2 is found on the potentiometer wire.  The length AP 2  = l 1   is measured.  Let V be the terminal p.d. of the cell

Thus knowing R, l and l 1 we can calculate the value of r i.e. the internal resistance of the cell using this formula.

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• Tags Advantages of potentiometer , Comarision of emf of cell , Construction of potentiometer , Current Electricity , Individual method , Internal resistance of cell , Principle of potentiometer , Sum and Difference method , Uses of potentiometer , Voltmeter , Working of potentiometer

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• To Compare the EMF of Two Given Primary Cells Using Potentiometer Experiment

EMF of Two Primary Cells Using a Potentiometer

A popular Physics experiment taught in schools is the experiment to compare the EMF of two given primary cells using a potentiometer.

In this article, we will learn how to compare the EMF of two given primary cells using a potentiometer. You will also learn about the potentiometer and its uses. The experiment will then be explained using the theory behind it. Each material required for the experiment is also listed clearly. The procedure is also explained in a simple and easy to understand manner. Refer to the official website of Vedantu or download the app for an elaborate and easy explanation.

The materials which we need for the experiments are a potentiometer along with a Leclanche cell, a Daniel cell,  ammeter,  voltmeter ,  galvanometer,  battery or battery eliminator,  rheostat of low resistance,  resistance box, one-way key, two-way key, jockey,  set square, then connecting wires and a piece of sandpaper.

What is a Potentiometer?

A potentiometer is a very useful instrument that is used to measure electric potential. It consists of three terminals and a slidable or rotating contact. This acts as a voltage divider in the potentiometer.

They are mostly used in audio equipment to control the volume. They can be also used to control inputs for other electric circuits.

Theory 

E1/E2=l1/l2

Where, the letter  E1 and E2 are the e.m.f. of two cells which are given and l1 and l2  are the corresponding balancing lengths on the wire of the potentiometer.

The Procedure

We will follow the following steps to do the experiments: 

First of all, arrange the apparatus as shown in the circuit diagram figure.

Then we need to remove the insulation from the ends of the connecting copper wires with sandpaper.

Next is to measure the e.m.f. That is denoted by E of the battery and the e.m.fs. That is E1 and E2  of the cells. See that E > E1  and also we will notice that E > E2.

Then we need to connect the positive pole of the battery or a battery of constant e.m.f. to the zero end that is denoted as P of the potentiometer and the negative pole through a one-way key. An ammeter and a low resistance rheostat to the other end which is denoted by Q of the potentiometer.

Next, we will connect the positive poles of the cells E1 and E2  to the terminal at the zero end denoted by P and the negative poles to the terminals a and b of the two way key.

Now connect the common terminal that is c of the two-way key through a galvanometer denoted by G and a resistance box denoted by letters R.B. to the jockey J.

We now need to take maximum current from the battery making rheostat resistance zero.

Then we will insert the plug in the one-way key denoted by letter K in the circuit and also in between the terminals a and c of the two-way key.

Now carefully take out a 2,000 ohms plug from the resistance box that is R.B.

 Now we will press the jockey at the zero end and then please note the direction of deflection in the galvanometer.

 Then press the jockey at the other end of the potentiometer. If the direction of deflection is showing opposite to that in the first case then we can say that the connections are correct. If the deflection is in the same direction itself then either connections are wrong or we can say that the e.m.f. of the auxiliary battery is less.

 Then we will slide the jockey gently over the potentiometer wires till we obtain a point where the galvanometer shows no deflection.

 Now we need to put the 2000 ohms plug back in the resistance box and obtain the null point position accurately by using a set square.

 We need to note that the length l1 of the wire for the cell E1 Also note that the current is as indicated by the ammeter.

 Now disconnect the cell which is E1  by removing the plug from gap ac of two-way key and then connect the cell E2  by inserting plug into gap be of two-way key.

 Now we need to take out a 2000 ohms plug from resistance box R.B. and slide the jockey along potentiometer wire to obtain no deflection position.

 Now put the 2000 ohms plug back in the resistance box that is R.B and obtain an accurate position of the null point for the second cell E2.

 We need to note the length l2  of wire in this position for the cell E2. However, we need to make sure that ammeter reading is the same as in step 14.

 Now repeat the observations alternately for each cell again for the same value of current which we want.

 Then increase the current by adjusting the rheostat and obtain at least three sets of observations in a similar way as mentioned above.

 Then record our observations.

Observation

For each observation, we will find the mean l1 and mean l2  and record in a column by naming them 3c and 4c.

Then we will find E1/E2 for each set by dividing mean l1  (column 3c) by mean l2  (column 4c).

Now find the mean of E1/E2.

Precautions of the Experiment 

The connections which we are making should be neat, clean and tight.

The plugs should be introduced in the keys only when we are doing the experiment and taking the observations.

The poles which are the positive ones of the battery that are E and cells E1 and E2  should all be connected to the terminal at the zero of the wires.

The key of the jockey should not be rubbed along the wire. We need to take care that it touches the wire gently.

The reading of the ammeter should remain constant for a particular set of observations. If necessary we can adjust the rheostat for this purpose as well.

The e.m.f. That is, the battery should be greater than the e.m.f. 's which is either of the two cells.

There are some high resistance plugs also which should always be taken out from the resistance box before the jockey is moved along the wire.

The Source of Error

 This could be one reason that the auxiliary battery may not be fully charged.

The potentiometer wire that we are using may not be of uniform cross-section and material density throughout its length.

Then the end resistances may not be zero.

FAQs on To Compare the EMF of Two Given Primary Cells Using Potentiometer Experiment

1. Explain how you can compare the EMF of two cells using a potentiometer?

While using a potentiometer we can determine that which is the emf of a cell by obtaining the balancing length that is denoted by l. Here we can say that the fall of potential along the length l of the potentiometer wire is equal to the emf of the cell. As there is no current which is being drawn from the cell.

2. Explain what is the primary cell in a potentiometer?

The two cells which are the primary cells whose EMFs are to be compared are so connected in the circuit that their terminals which are positive terminals are joined together to the end A of the potentiometer wire AB and their negative terminals are also said to be joined to a galvanometer through a two-way key a, b, c.

3. Explain why the EMF of a cell measured by a potentiometer is accurate?

A voltmeter draws generally current from a cell therefore voltmeter is said to measure terminal potential difference rather than emf. While we can say that a potentiometer at balance does not draw any current from the cell so the cell remains in an open circuit. Hence the potentiometer is said to read the actual value of emf.

4. Explain how we use a potentiometer experiment?

We introduce a sufficiently high resistance on the resistance box which is denoted by R.B. Then we place the jockey at the two endpoints of the wire. Then we press the jockey at both ends of the potentiometer wire and we need to note which is the deflection in the galvanometer. If the galvanometer shows opposite deflection then we can say that the connections are correct.

5. What principle of Physics must be understood well to comprehend the experiment of the potentiometer?

The potentiometer works on the simple principle that the drop of potential across a wire segment of the uniform cross-section is directly proportional to the length of the wire. This is an important principle that must be thoroughly understood to be able to successfully understand the above-mentioned experiment. The potentiometer workings can be understood better using the Vedantu website. Our experts have explained the workings of the potentiometer, its characteristics and applications in a simple manner along with the diagram of the device.

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Calibration of Ammeter, Voltmeter, and Wattmeter using Potentiometer

We know that voltage, current, and power are measured in volts, amps and, watts and voltmeter, ammeter, and wattmeter are used to measure these parameters. Though these measuring instruments are manufactured with the care they may still give error readings at the customer end. So these instruments are calibrated to minimize the error. Here in this article, we will explain how to calibrate Voltmeter, Ammeter, and Wattmeter using a potentiometer .

Before going into detail, let us first discuss the important concept used in this article.

If we have two voltage sources of the same value connected in parallel as shown below, then there will be no current flow between them. This is because the potential values of both sources are the same and neither of the sources can push charge on to the other. So in the circuit, the galvanometer does not show any deflection.

We will use the same phenomenon of balancing two voltage sources in the calibration process.

Calibration of Potentiometer

The above figure shows the circuit diagram for potentiometer calibration.

In the figure, a standard cell with voltage 1.50V is used which doesn’t produce voltage fluctuations even in millivolts upon loading. This kind of stable source is necessary for calibrating potentiometer without any error.

The conductive scale is scaled accurately to avoid miss reading during measurements. The conductive scale also has a smooth surface with clean-cut dimensions for equal resistance distribution along all its length.

The rheostat is present for adjusting the flow of current in the circuit loop and thereby we can adjust the voltage drop per unit length along the conductive scale. A galvanometer is also connected here for visualizing the defection that happens in case of current flow between the standard cell loop and conductive scale loop. The unknown EMF here is connected to the galvanometer for measurement after the calibration of the potentiometer.

First, turn ON the power and adjust the rheostat to allow a current of a few hundred milliamperes to flow in the main circuit loop. Because the conductive scale is also in the main loop the same current flows through it producing a voltage drop. Although the voltage drop appears across the metal scale will be distributed all along its body evenly.

After the appearance of the voltage drop along the conductive scale, if we take the sliding contact and move along the metal scale from zero, then current flows from secondary circuit to primary circuit because of circuit imbalance. And as the sliding contact moves further away from zero, the magnitude of this current flow decreases. This is because, as the contact area increases, the voltage drop across the scaled area will get close to the voltage of the standard cell. So at a certain point, the voltage drop across the scaled area will be equal to the voltage of the standard cell and at that point, there will no current flow between two circuits.

Now that a galvanometer is connected in the secondary circuit, it will show a deviation on its display because of current flow and higher the current more will be the deviation. Based on this, the galvanometer will show no deviation only when both the circuits are balanced and this is the state we will be trying to achieve for calibrating the potentiometer.

For better understanding, let’s see the circuit shown below which shows the state of balance.

If we assume the resistance of metal contact from length 0 to 100 cm as ‘R’, then the voltage drop across the entire 100cm length metal contact is V=IR. Since we assumed a balanced circuit, this voltage drop ‘V’ must be equal to the voltage of the standard cell and there will be zero deviation in galvanometer reading.

Now by measuring this exact length at which galvanometer shows zero, we can calibrate the potentiometer scale based on the standard cell voltage value.

After knowing the voltage drop per centimeter in the potentiometer scale, connect the unknown voltage to the secondary circuit and slide the contact to measure the length at which we will have zero deviation. After knowing this length of scale at which balance takes place, we can measure the value of unknown EMF as,

Applications of Potentiometers

In addition to the measurement of unknown voltage, the potentiometer can also be used to measure the current and power, it just needs a couple of extra components for measuring them.

Other than measuring voltage, current, and power, the potentiometers are mainly used for calibration of voltmeters, ammeters, and wattmeter . Also, since the potentiometer is a DC device, the instruments to be calibrated must be DC moving iron or electrodynamometer types.

Calibration of Voltmeter using Potentiometer

In the circuit, the most important component for the calibration process is a suitable stable DC voltage supply. This is because any fluctuations in supply voltage will cause an error in the voltmeter calibration thereby leading to an entire failure of the experiment. So standard voltage cell with stable terminal value is taken as a source and connected in parallel with voltmeter which needs to be calibrated. The two trim pots ‘RV1’ and ’RV2’are used for adjusting the voltage that is to appear across the voltmeter as shown in the figure.

A voltage ratio box is also connected in parallel with the voltmeter to divide the voltage across the voltmeter and get appropriate value suitable for connecting the potentiometer.

With the entire setup in place, we are ready for testing the accuracy of the voltmeter . So to start, just provide the power to the circuit to get a reading on the voltmeter and an unknown voltage at the voltage ratio box output. Now we will use a calibrated potentiometer to measure this unknown voltage.

After getting the potentiometer reading, check whether the potentiometer reading matches the voltmeter reading. Since potentiometer measures the true value of voltage, if the potentiometer reading does not match with the voltmeter reading, then a negative or positive error is indicated. And for correction, a calibration curve can be drawn with the help of the readings of voltmeter and potentiometer.

Also, for accuracy of measurements, it is necessary to measure voltages near the maximum range of the potentiometer as far as possible.

Calibration of Ammeter using Potentiometer

As mentioned above, we will use a suitable stable DC supply voltage to avoid the errors in calibration which do not produce voltage fluctuations during the entire experiment. A rheostat is used for adjusting the magnitude of the current flowing through the entire circuit. Also, a standard resistance ‘R’ of suitable value with sufficient current-carrying capacity is placed in series with the ammeter (which is under calibration) for getting a voltage parameter which relates to the current flowing in the circuit.

Now after the power is turned ON, a current ‘I’ flows through the entire circuit and with this current flow reading will be generated by the ammeter present in the loop. Also, a voltage drop will take place across the standard resistance ‘R’ because of this current flow.

Now we will use a potentiometer to measure the voltage across the standard resistor and then use ohms law to calculate the current through the standard resistance.

Since we are using the standard resistor, the resistance will be accurately known and the voltage across the standard resistor is measured by the potentiometer. The calculated value will be the accurate value of the current flowing through the loop. Then compare this calculated value with ammeter reading to check the accuracy of the ammeter. If there are any errors, we can make necessary adjustments for the ammeter to rectify the errors.

Calibration of Wattmeter using Potentiometer

As mentioned above for an accurate calibration process, we will use two suitable stable DC voltage power supplies as sources. Usually, low voltage supply is connected in series with the current coil of a wattmeter and a moderate voltage supply is connected to the potential coil of the wattmeter. A rheostat in the top circuit is used for adjusting the magnitude of the current flowing through current coil and trim pot in the bottom circuit is used for adjusting the voltage across the potential coil.

Do remember that a trim pot is preferred for adjusting the voltage and rheostat is preferred for adjusting the current in a circuit.

Also, a standard resistance ‘R’ of suitable value and sufficient current-carrying capacity is placed in series with the current coil of the wattmeter. And this standard resistance will generate a voltage drop across it when current flows in the current coil circuit.

After the power is turned ON we will get two unknown voltage readings, one is at the voltage divider output and the other is across the standard resistance ‘R’. Now if a potentiometer is used to measure the voltage across the standard resistor then we can use ohms law to calculate the current through the standard resistance. Since the current coil is in series with the standard resistance the calculated value also represents the current going through the current coil. In a similar way, use the potentiometer second time to measure the voltage across the potential coil of the wattmeter.

Now that we have measured the current through current coil and voltage across the potential coil using a potentiometer, we can calculate the power as

After calculating we can compare this calculated value with wattmeter reading to check for errors. Once the errors are found, make necessary adjustments to the wattmeter to adjust the errors.

This is how a potentiometer can be used to calibrate Voltmeter, Ammeter, and wattmeter to get accurate readings.

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• Physics Practicals
• Physics Viva Questions With Answers
• physics class 12 viva questions with answers
• class 12 to determine the internal resistance of a given primary cell using a potentiometer viva questions

Physics Practical Class 12 - To Determine the Internal Resistance of a Given Primary Cell Using a Potentiometer. Viva Questions with Answers

1) What is Electromotive Force?

The electric potential generated by either changing the magnetic field or electrochemical cell is known as electromotive force. It is commonly known as EMF.

2) Define potential difference.

The energy that degenerates as the unit charge passes through the components is known as the potential difference. During the measurement, the potential difference depends on the resistance between the two points.

3) What is the difference between EMF and terminal voltage?

Following are the main difference between EMF and terminal voltage:

• When the circuit is on, the potential difference across the terminals of a load is known as terminal voltage. On the other hand, the maximum potential difference that is delivered by the battery when there is no flow of current is known as EMF.
• A potentiometer is used for measuring the EMF, whereas a voltmeter is used for measuring the terminal voltage.

4) What is a Potentiometer?

A potentiometer is basically a resistor with three terminals that forms a variable voltage separator by having either descending or revolving contact. A potentiometer should have only two terminals with one end and the wiper in order to use it as a rheostat or variable resistor.

5) Write two applications of a potentiometer.

• Audio control: For changing the loudness and other audio related signals, both rotary and linear potentiometers are used to control audio equipment.
• Television: To control the picture brightness, contrast and colour response, potentiometers are used.

6) What is electrical resistance?

Electrical resistance is the hindrance provided by a material in the flow of current; it is the physical property of a substance which allows it to oppose the flow of electrons, that is, current. Resistance is inversely proportional to the cross-sectional area of the material and directly proportional to the temperature and length of the material.

7) What is direct current?

Direct current is the current whose direction remains the same. From a region of high electron density to a region of low electron density, the flow of electrons in a direct current is constant. Direct current is widely used in applications that involve a battery and many household appliances.

8) Why is Ohm’s law not applicable to semiconductors?

The semiconductors are nonlinear devices, and this is the reason why Ohm’s law is not applicable to semiconductors. This means that for variations in voltage, the ratio of voltage to current doesn’t remain constant.

9) When does Ohm’s law fail?

When semiconductors and unilateral devices such as diodes come into play, Ohm’s law fails to give the desired result because, in these materials, the physical conditions, such as temperature or pressure, do not remain constant.

10) What is a Wheatstone bridge?

A Wheatstone bridge is a particular type of electrical circuit that is used in measuring the unknown electrical resistance of the circuit by balancing the two legs of the bridge circuit, where the unknown component includes one of its legs.

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• Measurement of Electromotive Force and Potential Difference
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