Taking Measurements Alters The Circuit
When you use a voltmeter or ammeter, you are connecting another resistor to an existing circuit and, thus, altering the circuit. Ideally, voltmeters and ammeters do not appreciably affect the circuit, but it is instructive to examine the circumstances under which they do or do not interfere.
First, consider the voltmeter, which is always placed in parallel with the device being measured. Very little current flows through the voltmeter if its resistance is a few orders of magnitude greater than the device, and so the circuit is not appreciably affected. .) If, however, the voltmeters resistance is comparable to that of the device being measured, then the two in parallel have a smaller resistance, appreciably affecting the circuit. .) The voltage across the device is not the same as when the voltmeter is out of the circuit.
An ammeter is placed in series in the branch of the circuit being measured, so that its resistance adds to that branch. Normally, the ammeters resistance is very small compared with the resistances of the devices in the circuit, and so the extra resistance is negligible. .) However, if very small load resistances are involved, or if the ammeter is not as low in resistance as it should be, then the total series resistance is significantly greater, and the current in the branch being measured is reduced. .)
Working Of Moving Coil Galvanometer
Let a current I flow through the rectangular coil of n number of turns and a cross-sectional area A. When this coil is placed in a uniform radial magnetic field B, the coil experiences a torque .
Let us first consider a single turn ABCD of the rectangular coil having a length l and breadth b. This is suspended in a magnetic field of strength B such that the plane of the coil is parallel to the magnetic field. Since the sides AB and DC are parallel to the direction of the magnetic field, they do not experience any effective force due to the magnetic field. The sides AD and BC being perpendicular to the direction of field experience an effective force F given by F = BIl
Using Flemings left-hand rule we can determine that the forces on AD and BC are in opposite direction to each other. When equal and opposite forces F called couple acts on the coil, it produces a torque. This torque causes the coil to deflect.
We know that torque = force x perpendicular distance between the forces
= F × b
Substituting the value of F we already know,
Torque acting on single-loop ABCD of the coil = BIl × b
Where lx b is the area A of the coil,
Hence the torque acting on n turns of the coil is given by
The magnetic torque thus produced causes the coil to rotate, and the phosphor bronze strip twists. In turn, the spring S attached to the coil produces a counter torque or restoring torque k which results in a steady angular deflection.
Under equilibrium condition:
k = nIAB
How To Convert A Galvanometer Into An Ammeter
A galvanometer can detect only small currents.; Thus, to measure large currents it is converted into an ammeter.; It can be converted into an ammeter by connecting a low resistance called shunt resistance in parallel to the galvanometer.
Let G be the resistance of the galvanometer and Ig be the current for full scale deflection in the galvanometer, the value of the shunt resistance required to convert the galvanometer into an ammeter of 0 to I ampere is,
Ig is calculated using the equation, Ig = nk, where n is the number of divisions on the galvanometer and k is the figure of merit of galvanometer.
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Procedure For The Conversion Of A Galvanometer Into A Voltmeter:
Connect the resistance box in a series combination across the galvanometer and then collect the plugs of resistance R.
In the above diagram, A and B are the fixed ends/terminals and C is the variable terminal of the rheostat .
We can see that the galvanometer works as a voltmeter under the range of V Volts.
Now, take out the plugs of calculated resistance R from the resistance box.
Now, use the key to adjust the movable contact of the rheostat such that the deflection of the galvanometer reaches the maximum ranger.
Note the readings of both the galvanometer and voltmeter.
Convert the readings of the galvanometer into V or volts.
Check if there is a difference in the reading and this difference between voltmeter reading and galvanometer reading may show an error.
Now, by moving the variable contact of a rheostat, take six readings covering the range of voltmeters from 0-to-3 V.
Figure Of Merit Of A Galvanometer
;Figure of merit is in general is the numerical value representing the degree of effectiveness or efficiency of an instrument approximated by different estimation techniques.;
The figure of merit of a galvanometer is the current required to produce a deflection of one division in the galvanometer scale. It is represented by the letter k, and is given as,
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Conversion Of Galvanometer To Ammeter
A galvanometer is converted into an ammeter by connecting it in parallel with a low resistance called shunt resistance. Suitable shunt resistance is chosen depending on the range of the ammeter.
In the given circuit
RG Resistance of the galvanometer
G- Galvanometer coil
I Total current passing through the circuit
IG Total current passing through the galvanometer which corresponds to full-scale reading
Rs Value of shunt resistance
When current IG passes through the galvanometer, the current through the shunt resistance is given by IS = I IG. The voltages across the galvanometer and shunt resistance are equal due to the parallel nature of their connection.
Therefore RG .IG= .Rs
The value of S can be obtained using the above equation.
Factors Affecting Sensitivity Of A Galvanometer
a) Number of turns in the coil
b) Area of the coil
c) Magnetic field strength B
d) The magnitude of couple per unit twist k/nAB
Solved Question: Increase in current sensitivity results in an increase in voltage sensitivity of a moving coil galvanometer. Yes or no? Justify your answer.
Solution: No. An increase in current sensitivity of a moving coil galvanometer may not necessarily result in an increase in voltage sensitivity. As the number of turns is increased to increase the current sensitivity of the device, the resistance of the coil changes. This is because the resistance of the coil is dependent on factors like the length and area of the coil.
As we know that voltage sensitivity /V = ; the overall value of voltage sensitivity remains unchanged.
Conversion Of Galvanometer Into Voltmeter Practical Observations
Question 1: What is a Galvanometer? Why Does it Become Difficult for a Galvanometer to Function as an Ammeter?
Answer: A galvanometer is an electromechanical device that is used for detecting and indicating an electric current of a smaller magnitude. A galvanometer can function as an actuator by generating a rotary deflection of a pointer in the effect of an electric current flowing via a coil in an invariant magnetic field.
Working principle of a galvanometer: Galvanometers are developed from the observation that the needle of a magnetic compass deflects near a wire through which the current flows.
A galvanometer is a device with high resistance and low current capacity. So, when a large amount of current passes through it as required in an ammeter, the galvanometer may get damaged. So, this is the reason why galvanometer conversion into an ammeter becomes a little tough task.
Question 2: How Does a Galvanometer Work? How Sensitive are Galvanometers?
Answer: If we connect a galvanometer in a Wheatstones bridge circuit, the pointer in the galvanometer shows null deflection, which means that no current flows through the device. However, if the pointer deflects to the left or right that means the current starts flowing through it and the direction of deflection depends on the direction of the current.;
Galvanometer How Does It Work
The galvanometer works on the basis of electromechanical transduction, which responds to the current that it is subjected to on the basis of the strength of the current and its rate of flow. Other than these two factors, it also responds to any sort of stimuli, which strengthens the current, or blockage, which weakens the current. Among the various types of galvanometers, the two most commonly used ones are ..
Moving-coil galvanometer with the coil either mounted on pivots or suspended by thin metal strips. Moving-magnet galvanometer with the needle mounted on the pivot in itself being a permanent magnet surrounded by the coil.
When the instrument is subjected to a source of direct current, the current flows through the coil. This flow of charge produces a magnetic field in the surrounding, which acts against the permanent magnet and makes the coil twist and push against the spring. The fluctuation, in turn, moves the pointer, which indicates the scale showing the electric current. The direction of the current through the coil directly determines the direction of rotation, while the strength of the current determines the amount of rotation. In order to ensure that the magnetic field is uniform, the pole pieces need to be carefully designed. Failing to do so would result in disproportionate angular deflection of the pointer to the current.
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+ Top Mcqs On Moving Coil Galvanometer
Physics Multiple Choice Questions on Moving Coil Galvanometer.
1. What is moving coil galvanometer used for?a) Measurement of voltage onlyb) Measurement of resistancec) Measurement of small currentsd) Measurement of electric fieldAnswer: cClarification: Moving Coil Galvanometer is an instrument used for the detection and measurement of current. It is sensitive instrument and can measure current even if it is only a few microamperes. It was invented by Johann Schweigger in the 1800s.
2. Pick out the expression for galvanometer constant from the following?a) G = b) G = k × NABc) G = d) Answer: aClarification: In a moving coil galvanometer, the current passing through the galvanometer is directly proportional to its deflection , i.e.I = GWhere G = Galvanometer constantN = number of turns in the coil; A = area of coil; B = strength of the magnetic field; k = torsional constant of the spring that means restoring torque per unit twist.
4. Voltage sensitivity and current sensitivity are related.a) TrueClarification: Yes, voltage and current sensitivity are related to each other. Current sensitivityIS = ; Voltage sensitivity = Therefore, voltage sensitivity VS = ) x IS.
Clarification: Given: V = 10 V; Ig = 0.010A; R = 950 Required equation R = ) G G = ) RG = ) 950G = 1000 950G = 50 Therefore, the resistance of galvanometer is 50 .
To Convert The Given Galvanometer Of Known Resistance And Figure Of Merit Into A Voltmeter Of Desired Range And To Verify The Same Experiment
Lets suppose that the water has to be transferred from one tank to another. The energy/push given for the water supply is the potential difference and the measure of the same is done by a voltmeter.
If the waterfalls in minute drops , then this water flow is measured by a galvanometer.;
The conversion of galvanometer into voltmeter is done by adding a highly-resistive multiplier in series.;
How Does It Work
When the current is passed through the coil, a couple of two equal and opposite forces acting on two different points act on the coil and the coil tends to rotate. Such a couple is known as deflecting couple.If N is the number of turns of the coil and A is the area then the torque due to the deflecting couple is given by:
= NIBA cos
Since the pole pieces of the magnet are made concave to make the field radial, therefore the plane of the coil is always parallel to the field, so; =0º
= NIBA cos0º
= NIBA .
Restoring torqueAs the coil turns under the action of deflecting torque, the suspension wire is twisted which gives rise to the torsional couple which tends to untwist the suspension wire and restores the original position. This couple is known as a restoring couple.The torque due to restoring the couple is proportional to the angle of deflection as long as the suspension wire obeys Hooks law. Thus
Restoring torque = C
Where C is the constant, known;as twisting or torsional constant and it depends upon the nature of suspension wire.When the deflecting couple balances the restoring couple then the coil will come to rest. So, in equilibrium.Deflecting torque =Restoring torque
So the current passing through the coil is directly proportional to deflection;.
How Exactly Galvanometer Scale Is Calibrated To Function As An Ammeter
Today, I read in my textbook that in order to convert a Galvanometer into an Ammeter we need to attach a small Resistor with it in parallel for obvious reasons. I understood completely that why we need to do that. So there’s no problem with that.
The thing which I’m not getting is how the original scale of the Galvanometer is modified in order to use it as a current meter? Kindly explain this with an example.
I’m attaching some screenshots of my book in which this line is highlighted in green
PS: I’m just a high school student so pardon me if you find any mishaps in my description.
A galvanometer is an ammeter which is designed to detect and measure small currents.Like any other ammeter to extend the range of a galvanometer a shunt resistor is connected in parallel with the galvanometer.
Since the potential difference across the galvanometer and the shunt is the same and using the symbols defined in the text
$I_R_ = I_r_$
where $I_$ and $I_$ are the currents through the galvanometer and shunt.
The current $I$ that you wish to measure is the sum of the currents through the galvanometer and the shunt resistor.
$I = I_+I_$
Combining these two equations gives $I = I_ \left $.
If $R_$ and $r_$ are known and the current through the galvanometer $I_$ is read off its scale the current $I$ can be found.
Another way would be to recalibrate the galvanometer scale so that one could directly read the current $I$ off the scale.
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Moving Coil Galvanometer Construction And Diagram
The moving coil galvanometer is made up of a rectangular coil that has many turns and it is usually made of thinly insulated or fine copper wire that is wounded on a metallic frame. The coil is free to rotate about a fixed axis. A phosphor-bronze strip that is connected to a movable torsion head is used to suspend the coil in a uniform radial magnetic field.
Essential properties of the material used for suspension of the coil are conductivity and a low value of the torsional constant. A cylindrical soft iron core is symmetrically positioned inside the coil to improve the strength of the magnetic field and to make the field radial. The lower part of the coil is attached to a phosphor-bronze spring having a small number of turns. The other end of the spring is connected to binding screws.
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The spring is used to produce a counter torque which balances the magnetic torque and hence helps in producing a steady angular deflection. A plane mirror which is attached to the suspension wire, along with a lamp and scale arrangement, is used to measure the deflection of the coil. Zero-point of the scale is at the centre.
Connections: Limits To Knowledge
Making a measurement alters the system being measured in a manner that produces uncertainty in the measurement. For macroscopic systems, such as the circuits discussed in this module, the alteration can usually be made negligibly small, but it cannot be eliminated entirely. For submicroscopic systems, such as atoms, nuclei, and smaller particles, measurement alters the system in a manner that cannot be made arbitrarily small. This actually limits knowledge of the systemeven limiting what nature can know about itself. We shall see profound implications of this when the Heisenberg uncertainty principle is discussed in the modules on quantum mechanics.
There is another measurement technique based on drawing no current at all and, hence, not altering the circuit at all. These are called null measurements and are the topic of Chapter 21.5 Null Measurements. Digital meters that employ solid-state electronics and null measurements can attain accuracies of one part in \boldsymbol.
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Galvanometer To Voltmeter Formula
We know that for the conversion of a galvanometer into a voltmeter, a high resistance is required. So, if the resistance of the galvanometer is G and that of the high resistance is R, when they are connected in series, the total resistance of the arrangement becomes the following:
;;;;;RSeries;; = ; G + R;;;;
Now, the galvanometer behaves as a voltmeter. How does this happen? Now, we will look at the same thing in the form of the following experiment.;;;;;;;;
Construction Of The Galvanometer
The construction of the potentiometer is shown in the figure below.
The moving coil, suspension, and permanent magnet are the main parts of the galvanometer.
Moving Coil ;The moving coil is the current carrying part of the galvanometer. It is rectangular or circular and has the number of turns of fine copper wire. The coil is freely moved about its vertical axis of symmetry between the poles of a permanent magnet. The iron core provides the low reluctance flux path and hence provides the strong magnetic field for the coil to move in.
Suspension ;The coil is suspended by a flat ribbon which carries the current to the coil. The other current carrying coil is the lower suspension whose torque effect is negligible. The upper suspension coil is made up of gold or copper wire which is made in the form of a ribbon. The mechanical strength of the wire is not very strong, and hence the galvanometers handle carefully without any jerks.
Mirror ;The suspension carries a small mirror which casts the beam of light. The beam of light placed on the scale on which the deflection is measured.
Torsion Head ;The torsion head is used for controlling the position of the coil and for adjusting the zero setting.
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