# Relationship of galvanometer deflection and overlap length

### Physics Class XII Practical lines of attack and assumptions used by others, might reveal relations .. the mean magnetomotive force per unit length of the magnet will be. 4«nl. —. —. + H recording galvanometer deflections directly, the galvanometer is acces- sible and Eq. ously two separate spectral regions which may or may not overlap. The. Plot the average galvanometer deflection versus overlap length in Figure W1. is the expected relationship of the galvanometer deflection and overlap length?. ID: My name: Albina Age: 25, Eyes: Brown, Hair Color: Light Coloured, Height: cm, Residence: Saratov, Russian Federation As for my interests.

The optical bench is placed on a rigid table ,making it horizontal using a spirit level and leveling screws. The concave mirror is clamped on an upright and mounted it vertically near one end of the optical bench. An object pin P1 is moved on the optical bench back and forth so that its image is formed at the same height by making slight adjustments of the height of the pin or the mirror inclination.

This procedure ensures that the principal axis of the mirror is parallel to the optical bench. To determine index correction, a thin straight index needle is placed so that its one end A1 touches the tip of the pin and the other end B1 touches the pole P of the mirror. The positions of the uprights are readed on the scale. Their difference gives the observed distance between tip of the pin and the pole of the mirror. Length of the needle A1B1 is measured by placing it on the scale which is the actual distance between the points in question.

The difference between the two gives the correction to be applied to the observed distance. The index correction is found for both the pins P1 and P2 for all measurements.

The pin P1 is moved away from the mirror and is placed almost at 2F till an inverted image of same size as the pin should be visible. A piece of paper is placed on the tip of one pin, taking this as the object pin. The pin is placed with paper at a distance lying between F and 2F. The image of the pin is located using the other pin by removing parallax between the image and the pin. The values of u and v i. The experiment is repeated for at least five different positions of the object and the corresponding values of v is recorded in tabular form.

After tabulating them and the mean value of the focal length of the given concave mirror is found. The uprights supporting the optical elements should be rigid and mounted vertically. The object pin should be kept in between the centre of curvature and the focus of the mirror. The aperture of mirror should be small otherwise the image formed will not be distinct. Eye should be placed at a distance of distinct vision 25 cm from the image needle. The tip of the inverted image of the object pin must touch the tip of the image pin and must not overlap. It should be ensured while removing the parallax. The image and the object pins should not be interchanged during the course of the experiment. The corrected values of u and v must be put in the formula for calculating f and then a mean value off should betaken. Calculations for f must not be made using the mean values of u and v. A white screen or plane background may be used for seeing the clear image of the pin.

Image of the Sun should not be seen directly as it may hurt your eyes. An error may arise in the observations if the top of the optical bench is not horizontal and similarly if the tips of pins and pole of the mirror are not at the same horizontal level.

The concave mirror should be front-coated, otherwise multiple reflections will come from the reflecting surface of the mirror. THEORY Fig-1 Fig-2 For an object placed at a distance u from the optical centre of a thin convex lens of focal length fa real and inverted image is formed on the other side of the lens at a distance v from the optical centre. Obtain approximate value of the focal length of the thin convex lens by focusing the image of a distant object.

It can be found by obtaining a sharp image of the Sun or a distant tree on a screen, say a plane wall, or a sheet of paper placed on the other side of the lens and measuring the distance between the lens and the image with a scale. This distance is a rough estimate of the focal length, f of the convex lens. Do not look at the image of Sun directly as it may hurt your eyes.

The optical bench is placed on a rigid table or on a platform, and using the spirit level to make it horizontal with the help of leveling screws provided at the base of the bench. The convex lens is clamped on an upright and mounts it vertically almost near to the middle of the optical bench such that its principal axis is parallel to the optical bench. In this position, the lens would lie in a plane perpendicular to the optical bench. Index correction is found for both the pins.

As the value of u changes from 2f to f, v changes from 2f to infinity. Since the values of u and v are interchangeable, i. The upright position of the object pin, convex lens and image pin on the optical bench are recorded observation table.

Length of the index needle as measured by the metre scale. Taken u along x-axis and v along y-axis. Scales of x- and y-axis should be same. A hyperbola curve is drawn for various values of u and v Note that six sets of readings For u between f and 2f, give 12 points on the graph by interchanging values of u and v.

The lengths AZ and BZ are both equal to distance 2f. Thus by plotting the u — v graph, the focal length of the lens can be obtained. The aperture of the lens should be small otherwise the image formed will not be distinct. Eye should be placed at a distance more than 25 cm from the image needle. An error may arise in the observations if the top of the optical bench is not horizontal and similarly if the tips of pins and optical centre of the lens are not at the same horizontal level.

The image and object needles should not be interchanged during the performance of the experiment, as this may cause change in index corrections for object distance and image distance.

The tip of the inverted image of the object needle must touch the tip of the image needle and must not overlap. This should be ensured while removing the parallax. The general instructions to be followed in all optical bench experiments as given in the description of optical bench must be taken care of.

The corrected values of the distances u and v must be put in the formula for calculating f and then a mean of f should be taken. The uprights may not be vertical. Parallax removal may not be perfect. If the knitting needle or index rod for finding index correction is not sharp like a needle, its length may not be accurately found on scale. A highly diminished and point image is located at the focus behind the convex mirror Fig. A diminished virtual image is produced between the pole and focus behind the mirror Fig The image formed by a convex mirror iis s virtual and erect.

Therefore, its focal length cannot be determined directly. However, it can be determined by introducing a convex lens in between the object and the convex mirror Fig.

This is possible if the light rays starting from the tip of the object, after passing through the lens, fall normally on the reflecting surface of the convex mirror and retrace their path. Any normal ray perpendicular to a spherical surface has to be along the radius of that sphere so that point C must be the centre of curvature of the convex mirror.

Therefore, the distance P C is the radius of curvature R and half of it would be the th focal length of the convex mirror. In case, if the focal length of the given thin convex lens is not known then approximate value of its focal length should be estimated first.

The optical bench is place on a rigid table or on a platform. Using the spirit level, it is made Horizontal with the help of leveling screws provided at the base of the bench. The index correction is determine between upright holding of the convex mirror and image pin respectively, using an index needle.

This occurs when the rays starting from the tip of pin P1, after passing through the lens strike the mirror normally and are reflected back along their original paths.

The parallax between the image and object pins is removed. The convex mirror is removed from its upright and the image pin P2 is fixed on it. The height of pin is adjusted such that the tip of it also lies on the principal axis of the lens. That is, the tips of the pins P1 and P2 and the optical centre O of the convex lens, all lie on a straight horizontal line parallel to the length of the optical bench.

A small piece of paper may placed on image pin P2 to differentiate it from the object pin P1. The position of the image pin is noted.

In this manner, five sets of observations are taken. The uprights supporting the pins, lens and mirror must be rigid and mounted vertically. The apertures of the given convex lens and convex mirror should be small, otherwise the image formed will be distorted. Eye should be placed at a distance of about 25 cm or more from the image pin. Optical bench should be horizontal. The tips of pins, centre of convex lens and pole of the mirror should be at the same horizontal level. The tip of the inverted image of the object pin should just touch the tip of the image pin and must not overlap.

Personal eye defects may make removal of parallax tedious. The convex mirror should preferably be front-coated. Otherwise multiple reflections may take place. The focal length of the lens used in this experiment should neither be too small nor too large.

The line along which any two faces refracting surfaces of the prism meet is the refracting edge of the prism and the angle between them is the angle of the prism. For this experiment, it is convenient to place the prism with its rectangular surfaces vertical. The principal section ABC of the prism is obtained by a horizontal plane perpendicular to the refracting edge Fig. The dotted lines in the figure represent the normal to the surfaces. The advantage of putting the prism in minimum deviation position is that the image is brightest in this position.

A white sheet of paper is fixed on a drawing board with the help of cello tape or drawing pins. A straight line XY,is drawn using a sharp pencil nearly in the middle and parallel to the length of the paper. Points O1, O2, O3. The boundary of the prism is drawn with a sharp pencil. Two alpins Pl and Q1 are fixed with sharp tips vertically about 10 cm apart, on the incident ray line Pl Ql such that pin Q1 is close to point O1.

Closing one eye say left and looking through the prism, right eye is brought in line with the images of the pins Pl and Ql. Alpins Rl and Sl are fixed about 10 cm apart vertically on the white paper sheet with their tips in line with the tips of the images of pins Pl and Ql.

In this way pins R1 and S1 will become collinear, with the images of pins P1 and Q1. Removing the pins Rl and Sl and encircling their pin pricks on the white paper sheet with the help of a sharp pencil ,the pins P1 and Q1 are removed and their pin pricks encircled also. The points or pin pricks Rl and Sl is joined with the help of a sharp pencil and scale, to obtain the emergent ray Rl S l.

R1S1 is produced in backwards to meet the incident ray Pl Ql produced forward atT1. Arrowheads are drawn on Pl Ql and R1 S1 to show the direction of the rays. Observations are recorded in tabular form with proper units and significant figures. Angle of incidence, i degrees.

Alpins should be fixed vertically to the plane of paper. Distance PQ and RS should be about 10 cm in order to locate incident and emergent rays with greater accuracy. Same angle of prism should be used for all observations. Position of the prism should not be disturbed for a given set of observations. There may be an error in measuring the values of the angles. In such situations, more readings should be taken in the minimum deviation region to be able to obtain the value of angle of minimum deviation accurately.

### Galvanometer - MagLab

Taking more readings in this region will help in drawing a smooth curve. This will enable you to locate the position of the lowest point on the graph more accurately.

The graph does not show a sharp minimum. We have same deviation for a range of angle of incidence near minimum deviation. To determine refractive index of material of glass slab using a travelling microscope. The above steps 4 and 5 is repeated for other two glass slab.

The parallax should be properly removed.

## Physics laboratory experiment: Electromagnetic Induction

The microscope should be moved in upward direction only to avoid back lash error. Putting the value of f 2 and R, n can be calculated. The rough focal length of the convex lens is found and then it is placed a plane mirror which is already placed on the horizontal base of the iron stand. An optical needle is horizontally clamped with the stand with its tip equal to the rough focal length above the pole of the convex lens.

Adjusting the height of the needle the tip of the needle is coincide with its image by removing parallax.

By the help of plumb line and half metre scale the distance between the tip of needle and upper surface of lens is measured. Considering the refracting index of material of lens as 1. The liquid taken should be transparent 2. Only few drops of liquid should be taken for thin layer. The parallex should be removed tip to tip. Depletion region — The immobile space space-charge charge region on either side of the junction together is Known as depletion region.

Barrier potential-- The potential difference across the junction that tends to prevent the movement of electron from thee n region into the p region, called a barrier potential. The The direction of the applied voltage V is opposite to the built-in in potential V0. The depletion layer width decreases decreases. The barrier height is reduced to V V0 — V. When the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium value, and only a small number of carriers in the uppermost energy levels—will possess enough energy to cross the junction.

So the current first increases very slowly, almost negligibly, till the voltage across the diode crosses a certain value. This voltage is called the threshold voltage or cut-in voltage When an external voltage V is applied across the diode such that n-side is positive and p-side side is negative, it is said to be reverse biased.

The The direction of applied voltage is same as the direction of barrier potential. The depletion region widens 3 a 3 b 3 c Fig 3 a Diode under reverse bias, b Barrier potential under reverse bias. Even a slight increase in the bias voltage causes large change in the current.

The range, least count and zero error of both the voltmeters ,micro ammeter and millammeter are recorded. After identifying the P and N terminals of given diode, it is connected between the given 2. After Knobs for forward bias in the PP-N junction diode characteristics apparatus.

Supply is given to P-N N junction diode characteristics apparatus and the switch is set to on position. The potential difference across the diode is gradually increased and the voltmeter ,corresponding milliammeter readings are recorded after suitable interval up to the specified limit.

The diode is disconnected from forward bias knob and reconnected between the given Knobs for reverse bias. The potential difference across the diode is gradually increased and the voltmeter ,corresponding micro ammeter readings are recorded after suitable interval up to the specified limit.

V 4 Voltmeter R. The I-V characteristic curve of the given p-n junction in forward bias and reverse bias are shown in the attached graph paper. The reverse break down voltage is ………………….

Voltmeter and milliameter should have appropriate range and least count. The pointer of meters should either be adjusted to zero in the absence of current or zero error of the instrument should be taken in to count. The variation in V should be done in steps of 0. The terminals of diode should be cheeked and connected to appropriate knob for F. Never cross the limit s specified by the manufacturer ,other wise the diode get damaged.

Zener diode is a special purpose semiconductor diode designed to operate under reverse bias in the breakdown region. It is fabricated by heavily doping both p- and n- sides of the junction. Due to this, depletion region formed is very thin and the electric field of the junction is extremely high even for a small reverse bias voltage. When an external voltage V is applied across the Zener diode such that n-side is positive and p-side is negative, it is said to be reverse biased.

The angle of twist can be measured by attaching a pointer to the coil, or, even better, by mounting a mirror on the coil, and reflecting a light beam off the mirror. Sincethe device can easily be calibrated by running a known current through it. There is, of course, a practical limit to how large the angle of twist can become in a galvanometer. If the torsion wire is twisted through too great an angle then it will deform permanently, and will eventually snap.

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Thus, there is a maximum current which a galvanometer can measure. This is usually referred to as the full-scale-deflection current. The full-scale-deflection current in conventional galvanometers is usually pretty small: So, what do we do if we want to measure a large current? What we do is to connect a shunt resistor in parallel with the galvanometer, so that most of the current flows through the resistor, and only a small fraction of the current flows through the galvanometer itself.

This is illustrated in Fig. Let the resistance of the galvanometer beand the resistance of the shunt resistor be. Suppose that we want to be able to measure the total current flowing through the galvanometer and the shunt resistor up to a maximum value of. We can achieve this if the current flowing through the galvanometer equals the full-scale-deflection current when. In this case, the current flowing through the shunt resistor takes the value. The potential drop across the shunt resistor is therefore.

This potential drop must match the potential drop across the galvanometer, since the galvanometer is connected in parallel with the shunt resistor. It follows that which reduces to Using this formula, we can always choose an appropriate shunt resistor to allow a galvanometer to measure any current, no matter how large. For instance, if the full-scale-deflection current isthe maximum current we wish to measure isand the resistance of the galvanometer isthen the appropriate shunt resistance is Most galvanometers are equipped with a dial which allows us to choose between various alternative ranges of currents which the device can measure: All the dial does is to switch between different shunt resistors connected in parallel with the galvanometer itself.

Note, finally, that the equivalent resistance of the galvanometer and its shunt resistor is Clearly, if the full-scale-deflection current is much less than the maximum current which we wish to measure then the equivalent resistance is very small indeed. Thus, there is an advantage to making the full-scale-deflection current of a galvanometer small. A small full-scale-deflection current implies a small equivalent resistance of the galvanometer, which means that the galvanometer can be connected into a circuit without seriously disturbing the currents flowing around that circuit.

Circuit diagram for a galvanometer measuring current. A galvanometer can be used to measure potential difference as well as current although, in the former case, it is really measuring current. In order to measure the potential difference between two points and in some circuit, we connect a galvanometer, in series with a shunt resistor, across these two points--see Fig.

The galvanometer draws a current from the circuit. This current is, of course, proportional to the potential difference between andwhich enables us to relate the reading on the galvanometer to the voltage we are trying to measure.

Suppose that we wish to measure voltages in the range 0 to.