Galvanometer 35c20

Physics Project 1

Physics Project

Anushrav Rai,

XII-

H

LAB CERTIFICATE This is to certify that this project is completed by Anushrav Rai of class XII-H, titled as physics project, for the year 2010-2011 for submission during the practical examination of physics CBSE Examination 2010-2011

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Physics Project 3

Teacher’s Signature

ACKNOWLEDGEMENT

My strength is all due to S.S Mishra Sir, P.G.T Physics, H.A.L School.

Physics Project He was the person who helped me with the strenuous and 4 straining task of preparing the physics project. It was he who guided me and inspired me to work relentlessly on improvement no matter what. He is a living example of what a humble person you can be, no matter how great you become. I would also like to pay gratitude to my parents for their help and cooperation. I would also like to thank our physics lab assistant, to perform the experiments required to attain the proficiency. I would also like to thank Mrs S. Jaiswal, Principal, H.A.L School, for her never ending help which she pours to every one of her student. Suggestions for improvement for this project are always welcome.

Anushrav Rai

Physics Project

Galvanometer

D'Arsonval/Weston galvanometer movement. A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.

Operation

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Physics Project D'Arsonval/Weston galvanometer movement. Part of the magnet's left pole piece is broken out to show the coil The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval/Weston form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position. When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer settles quickly to its position without oscillation. The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop of, say, 50 millivolts at full current). Such meters are often calibrated to read some other quantity that can be converted to a current of that magnitude. The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the voltage required to generate a full scale current. A meter can be configured to read other voltages by putting it in a voltage divider circuit. This is generally done by placing a resistor in series with the meter coil. A meter can be used to readresistance by placing it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor adjusted to produce full scale deflection. When an unknown

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Physics Project resistor is placed in series in the circuit the current will be less than full scale and an appropriately calibrated scale can display the value of the previously-unknown resistor. Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading from a mirrored scale is improved by positioning one's head while reading the scale so that the pointer and the reflection of the pointer are aligned; at this point, the operator's eye must be directly above the pointer and any parallax error has been minimized. Tangent galvanometer

Tangent galvanometer made by J.H.Bunnell Co. around 1890. A tangent galvanometer is an early measuring instrument used for the measurement of electric current. It works by using a com needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a com needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Pouillet in 1837.

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Physics Project A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis ing through its centre. A com box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the com needle. In operation, the instrument is first rotated until the magnetic field of the Earth, indicated by the com needle, is parallel with the plane of the coil. Then the unknown current is applied to the coil. This creates a second magnetic field on the axis of the coil, perpendicular to the Earth's magnetic field. The com needle responds to the vector sum of the two fields, and deflects to an angle equal to the tangent of the ratio of the two fields. From the angle read from the com's scale, the current could be found from a table.[2] The current supply wires have to be wound in a small helix, like a pig's tail, otherwise the field due to the wire will affect the com needle and an incorrect reading will be obtained. [edit]Theory The galvanometer is oriented so that the plane of the coil is parallel to the local magnetic meridian, that is the horizontal component BH of the Earth's magnetic field. When a current es through the galvanometer coil, a second magnetic field B perpendicular to the coil is created, of strength:

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Physics Project where I is the current in amperes, n is the number of turns of the coil and r is the radius of the coil. These two perpendicular magnetic fields add vectorially, and the com needle points along the direction of their resultant, at an angle of:

From tangent law,

, i.e.

or

or

, where K is called the Reduction Factor of

the tangent galvanometer. One problem with the tangent galvanometer is that its resolution degrades at both high currents and low currents. The maximum resolution is obtained when the value of θ is 45°. When the value of θ is close to 0° or 90°, a large percentage change in the current will only move the needle a few degrees.

From galvanometer to ammeter Class Experiment Adding a shunt in parallel with a galvanometer converts it to an ammeter with a higher range. This is a trial and error method, not one involving calculations. Apparatus and materials For each student group:

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Physics Project •

Galvanometer

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Ammeter, 0 to 1 A, DC

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Cell, 1.5 V in holder

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Power supply, low voltage, DC

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Crocodile clips, 2

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Lamp in holder, 12 V, 36 W or 24 W

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Eureka wire, 28 SWG or thicker

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Leads, 4 mm, 6

Procedure Making an ammeter

a Your galvanometer is designed to measure small currents of a few milliamps. When the pointer is at the end of the scale, the current through the little coil which moves with the pointer must be, say, 0.01 A (or whatever your galvanometer is built to measure there). Suppose you wish to use it to measure much larger currents, say 1 A, at the end of its scale. The rest of that large current (1 A minus 0.01 A, for example) must travel by an alternative route a loop line in parallel.

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Physics Project b For that loop line or shunt, connect a short piece of alloy wire across the terminals of your galvanometer, as in the diagram. Take care! If, when adjusting the shunt, you let the whole big current go through the galvanometer, even momentarily, you might damage the galvanometer badly.

c Start with a very short shunt, straight across from terminal to terminal. Make a very rough test of that by connecting in series a lamp, your shunted galvanometer, a commercial ammeter (for comparison) and one 1.5-volt cell just for a safe first trial.

d Switch on the current just for a moment, to see whether the pointer moves too far or too little. e Adjust the length of shunt by trial and error. Shorten or lengthen the shunt until your home-made ammeter seems to read roughly what you want it to read. f Disconnect the battery from your test circuit and replace it with the power

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Physics Project supply, set to give 12 V. Adjust the shunt more carefully till you have a good ammeter. g A commercial ammeter is constructed like this. It is a milliammeter with a shunt. Sometimes the basic instrument has several removable shunts to make it an ammeter with a choice of several ranges-as in the case of multimeters where you can select a range by turning a dial. Teaching notes 1 Those who are keen to use their knowledge of resistance can have a go at converting a milliammeter to an ammeter. This is the job which a meter shunt does. The correct resistance has to be connected in parallel with the milliammeter in order to allow it to amps. It does this by sending most of the circuit’s current through the shunt and tapping a small fraction of it to send through the meter. 2 The commercial ammeter enables students to adjust their home-made one to read as they wish, by a trial and error method rather than one in which the resistance of the shunt is calculated.

Focal length

The focal length of an optical system is a measure of how strongly the system converges (focuses) or diverges (defocuses) light. For an optical system in air, it is the distance over which initially collimatedrays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance. In telescopy and most photography, longer focal length or lower optical power is associated with largermagnification of distant objects, and a narrower angle of view. Conversely, shorter focal length or higher optical power is associated with a wider angle of view. In microscopy, on the other hand, a short objective lens focal length leads to higher magnification.

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For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses and/or mirrors (e.g., a photographic lens or a telescope), the focal length is often called theeffective focal length (EFL), to distinguish it from other commonly-used parameters:

Physics Project 

Front focal length (FFL) or Front focal distance (FFD) is the distance from the front focal point of the system to the vertex of the first optical surface.[1][2]

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Back focal length (BFL) or Back focal distance (BFD) is the distance from the vertex of the last optical surface of the system to the rear focal point.[1][2]

For an optical system in air, the effective focal length gives the distance from the front and rear principal planes to the corresponding focal points. If the surrounding medium is not air, then the distance is multiplied by the refractive index of the medium. Some authors call this distance the front (rear) focal length, distinguishing it from the front (rear) focal distance, defined above.[1] In general, the focal length or EFL is the value that describes the ability of the optical system to focus light, and is the value used to calculate the magnification of the system. The other parameters are used in determining where an image will be formed for a given object position. For the case of a lens of thickness d in air, and surfaces with radii of curvature R1 and R2, the effective focal length f is given by:

where n is the refractive index of the lens medium. The quantity 1/f is also known as the optical power of the lens. The corresponding front focal distance is:

and the back focal distance:

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Physics Project 15 In the sign convention used here, the value of R1 will be positive if the first lens surface is convex, and negative if it is concave. The value ofR2 is positive if the second surface is concave, and negative if convex. Note that sign conventions vary between different authors, which results in different forms of these equations depending on the convention used. For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. The focal length is positive for a concave mirror, and negative for a convex mirror. In the sign convention used in optical design, a concave mirror has negative radius of curvature, so

, where R is the radius of curvature of the mirror's surface. Measurement of Focal Length of Lenses

Objective : -

To study the image formed by a convex lens

- To determine the focal length of a convex lens by graphic method Apparatus : Ray box

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Convex lens with holder

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Concave lens with holder

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Metre rule

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Object slide (A piece of transparent ruler) 1

Physics Project White screen

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12 V d.c. power supply

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Theory : Lenses are made of glass or of similar transparent material, which refracts light at the surfaces. They are shaped so that they diverge or converge a bundle of parallel rays. When a bundle of parallel light rays enters a converging lens, the rays are focused at a point in space a distance f, the focal length, from the lens. f

A converging lens can be used to form an image on a screen of an object. The lens formula relates the focal length f of a lens, the object distance u and the image distance v.

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1 1 1 = + f u v The formula can be used for both converging and diverging lens; the only difference is that the focal length f is positive for converging lenses, negative for diverging lenses. Procedure : 1. lens by projecting the image of a distant

Determine the focal length of the

object (building)onto a white screen. (See figure below.) White screen

Metre rule

Using a far away building as a distant

f

The focal length of the lens is about ____________ cm. 2. Place the object slide on the front of the ray box. 3. Place the ray box at a distance about 1 m from the screen. 4. Place the convex lens on a lens holder between the screen and the ray box as shown below.

Physics Project Screen Ray v u box conve Metre xrule lens

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5. Adjust the distance of the lens from the screen until a sharp image of the object is formed on the screen. 6. Record the object distance u, the image distance v from the lens, the size of the object ho and the size of the image hi. 7. Keep the screen and the ray box in the same position and move the lens back

Physics Project and forth between the object and the screen to find another position resulting in a focused image. 8. Record the object distance, image distance and size of the image for each position. 9. Move the screen about 10 cm closer to the object and again adjust the position of the lens to find two position of focused image. Record the object distance, image distance and size of the image for each position. 10. Continue moving the screen closer and adjusting the lens until you determine the lens position such that the image distance is equal to the object distance. Tabulate the results.

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