Instruction Manual ISC Class XII Physics PHYSICS LABORATORY
Science Department
Good Earth School
January 2016
ISC PHYSICS PRACTICAL INSTRUCTIONS
The most important instruction comes first. So please remember that a lot of money, time, thought and energy have been spent in procuring and maintaining all of the instruments that you are using in the lab. It may be easy to misuse one of those instruments and ‘give money for the damage’. But getting the same quality instrument that too in India is very difficult. So please take care of them like your own, with utmost care and hence save it for the generations of students to come.
Always… FOLLOW INSTRUCTIONS – For ISC, you don’t always need to know ‘why’ something is done; you only need to learn ‘how’ it is done. So, remember that when your teacher tells you to do certain things it is always for a very clear reason that may not be explained to you always. But there is always a reason. Mostly the reasons will be scientific in nature and beyond the scope of the syllabus and your understanding for now. You are free to ask why something is the way it is, but remember that if you don't want to, that's just fine you still have to follow the instructions.
You are only tested in your ability to do the experiment as accurately and precisely as possible, as instructed in the question paper and on your ability to present the observations, calculations and results of your experimentation.
Read and prepare the experiment that you have to do before coming for the lab session. This means drawing the tabular columns, writing the Aim Apparatus etc and coming ready to do the experiment in the observation book. Don’t waste time in the lab doing all this as it could be spent more productively on just doing the experiment with a lot of care accuracy and precision which will improve not only your marks but also your scientific and experimental temperament and skills. You may want to think of the Theory behind each of the experiments that you do, that is a direct application of all that you learn in your theory classes.
You have two books in which all the written work will be done. One is called the OBSERVATION BOOK, which is just a simple graph note book. Bring this to every lab session. Without this book, coming to the lab is meaningless. Record all your observations, comments, readings etc in this book. After you have finished an experiment and got the signature of your teacher, you are free to write the experiment into your fair RECORD NOTE BOOK.
The Record Notebook should be written neatly in blue or black ink and all diagrams drawn in pencil. It should contain the following entries in the order given below… The pages on the RIGHT hand side should contain…
1. 2. 3. 4. 5. 6. 7. 8. 9.
The Name of the Experiment (Title) The Experiment Number (in the margin) Date on which the Experiment was conducted (in the margin) Aim of the experiment Apparatus used in the experiment An account of the Theory behind the experiment (if given and required) Formulae used for calculations, explaining every term with units Detailed account of Procedure and description of apparatus Result
The pages on the LEFT hand side must contain…
1. Ray diagrams or Circuit diagrams. 2. Tabular columns containing all the observations with appropriate units. 3. All calculations done, clearly and elaborately, in order to reach the final result. 4. Graphs drawn with proper axis marked an d scales mentioned.
You are free to write the procedure in your own words if you please. However, the procedure already given, will give you a clear idea as to how to go about doing the experiment.
Your Record Book carries 3 Marks for the final exam and the experiment that you do carries 20 Marks. Your Project carries 7 marks. This total of 3 0 Marks is very easy to get if you are systematic and follow instr uctions.
Most importantly, as you must with everything you do in your life, do enjoy doing your p racticals!!!
EXPERIMENT 1: CONVEX LENSES I DISTANT OBJECT METHOD & U-V METHOD Aim: To determine the focal length of a given convex lens by distant object method and u – v method and graphical method Apparatus: Given a Convex Lens, a 100 cm optic bench, two pins, a lens stand and a white screen. Least count of the optic bench is _______ = _______ . Formulae:
1 f
Diagram:
=
1 v
+
1 u
⟹
1 No
[]
[]
(× 10−2 ) [cm-1]
f=
uv u+v
cm
1 (× 10−2 ) [cm-1]
=
+ []
=
Procedure: 1. Distant Object Method: Fix the lens on the lens stand and make sure that the lens is perfectly vertical to the optic bench. Use plasticine if necessary to keep the lens from moving. Arrange the screen and the given convex lens on the optic bench such that the surface of the screen is coinciding with the zero of the optic bench. Now move the lens to and fro from the screen till you get a sharp image of any distant object on the screen. At this position, the distance between the lens and the screen will equal the focal length ( 1 ) of the lens. Take care to ensure that the distant object being focused is as far away as possible and in line with the principal axis. 2. U – V method: Arrange the Screen, the Object pin (O), the lens (L) and the image pin (I) on the optic bench in that order. Keep the object pin such that it coincides with the zero mark of the optic bench. Keep the Lens such that the object distance (u) is about 20 to 30 cm. Keeping this fixed, move the image pin to and from the lens till there is no parallax between the image of the object and the image pin. At this position, note the values of object distance ( u) and image distance (v). Record the values in a table form and also calculate the value of focal length using the formula… [] = +
Now, repeat the experiment about 5 times by increasing the value of object position (u) by 3 to 5 cm and for each value, adjusting the value of image distance (v) such that the parallax is removed. For each set of u’s and v’s, find the value of fo cal length ( f ). Find the mean value of focal length. Graphs: Plot the following graphs… a) v/s graph: The graph will be a curve in the first quadrant. Make sure that the scales are same on both the graphs and there is no kink used. Draw a straight line 45 ° to the positive x axis such that it touches the curve. Take the corresponding values of v and u were it touches the curve and use that to find the value of F using the equation given above. b) 1/ v/s 1/ graph: The graph will be a straight line with negative slope. Make sure that the scales are same on both the graphs and there is no kink used. Find the line of best fit and find 0 the intercept and 0 the y intercept. The reciprocal of 0 and 0 gives you the value of focal length in cm. c) vs v graph: The graph will be a straight line with positive slope. Draw the line of best fit and find the slope of the graph. The reciprocal of the slope will give you the value of focal length in cm. Results: •
The approximate focal length of the convex lens using distant object method is 1 = ____ cm.
•
The mean value of focal length from the v and u data i s found to be = _____ cm
•
The focal length of the given convex lens from v/s graph is found to be _______ cm.
•
The focal length of the given convex lens from 1/ v/s 1/ graph is found to be ______ cm.
•
The focal length of the given convex lens from vs v graph is found to be _______ cm.
EXPERIMENT 2: CONVEX LENSES II: DISPLACEMENT METHOD Aim: To determine the focal length of a given convex lens u sing displacement method. Apparutus: Given an optic bench, a convex lens, 2 Pins, a Lens holder and an optic bench. Least count of the optic bench is _______ = _______ .
Diagram:
[cm]
No
Position L 1 [cm]
Position L 2 [cm]
= 1 ~2 [cm]
=
2 − 2
100
2
[ ]
2 =
2 − 2
4
[]
Procedure: First, determine the approximate focal length ( 1 ) of the given convex lens using distant object method. Now arrange the object pin (O) and the image pin (I) on the optic bench such that the distance between them is about 10cm greater than 4 . Let this distance OI be ≥ 4 1 . The distance between I and O ( ) should not be changed during the following setting. Now position the lens in between I and O and move the lens towards the object pin O till you find another no parallax position in between the two pins where the image of I is real, inverted and magnified. Let this position be L 1 . Note this position down in the tabular column. Now without changing the the dista nce IO (= ), move it towards the image pin I till you find a no parallax position. The image of the pin here will be real inverted and diminished. Let this position be L 2 . Note this position in the tabular column. Let = 1 ~2 . Note this also in the table. Repeat the experiment to get 4 more sets of values of L 1 , L 2 and d increasing the value of each time by about 4 to 6 cm. For each setting, calculate the value of (up to 3 significant figures) and 2 using the formula
=
2 − 2
100
&
2 =
2 − 2
4
Draw a graph of versus plotting the five sets of values and draw a line of best fit. Find the slope of this line (S). Hence find the focal length of the lens using the formula = 25 × Result: •
The approximate focal length of the given convex lens using distant object method is 1 = ________ cm.
•
The mean focal length of the given convex lens using theoretical method is 2 = ________ cm.
•
The value of focal length of the given convex lens using gra phical method is found to be = ______ cm.
EXPERIMENT 3: COAXIAL COMBINATION OF CONVEX LENS 1 Aim: To determine the focal length of convex lens by co mbining it co-axially with another convex lens. Apparatus: Given two convex lenses, two lens stands, two pins, optic bench and a screen. Least count of the optic bench is _______ = _______ .
Diagram:
Formula: p=
100
; =
; =
100
v 100 √ = Object distance; = Image distance; = slope of the graph; = Focal length
Observations: Approximate focal length of Lens A (f 1 ) = ___________ cm Position of Lens A [cm]
No
Position of Lens B [cm]
Object Pin Position O [cm]
Image Pin Position I [cm]
=
=
[cm]
[cm]
=
100
=
100
Procedure: 1. 2. 3.
Find the approximate focal length of Lens A by distant object method (f 1 ). Set up the apparatus as shown in the figure. Place Lens A at the 50cm mark and place the lens B at a distance f 1 from A. Move the object pin away from lens A till a clear inverted image of the object is seen through the combination of the lenses. Now remove parallax between O and I by adjusting the image pin. At no parallax position, measure and record the values u = A O, v = BI, p =
100 u & q = . v 100
4.
Repeat the experiment with 4 more sets of u and v, increasing u about 5.0 cm in each set. Tabulate all 5 sets of u, v, p and q. Take care to ensure that the distance AB between the two lenses are maintained constant throughout the experiment. 5. Plot a graph o f p v/s q, draw the line of best fit and find its slope S. The focal length (F) of Lens B can be found by the equation 100 F= √ S Result: •
The approximate Focal Length of Lens A is found to be ________ cm
•
The Focal Length of Lens B is found to be ________ cm
EXPERIMENT 4: COAXIAL COMBINATION OF CONVEX LENS 2 Aim: To determine the focal length of convex lens by co mbining it co-axially with another convex lens. Apparatus: Given two convex lenses of different focal lengths (difference, higher the better), two lens stands, two pins, optic bench and a screen. Least count of the optic bench is _______ cm = _______ mm.
Diagram:
Formula:
x=
1
;
1
=
;
=
[cm]
[cm]
1
0 r 2 r = Distance between lens B and Image pin at no parallax position; = distance between the two lenses; 0 = The x intercept of the graph; F = Focal length of Lens A Observations: Approximate focal length of Lens A (F A ) = ___________ cm
No
u [cm]
v [cm]
FA =
uv
u+v [cm]
No
=
1
[cm ]
=
-1
1
2 [cm-1]
Procedure: 1.
4.
Find the approximate focal length of lens A using distant object method and then confirm the value by u-v method. Arrange the two lenses and the pins as shown in the diagram. Make sure that the distance between the object pin and Lens A is twice the focal length of A. Take care to ensure that this distance is kept constant throughout the experiment. Fix a distance ‘’ between the two lenses and find the no parallax position. Let the distance between Lens B and the Image pin at no parallax position be ‘r’. Note do wn ‘’ and ‘’ in the tabular column. Repeat the experiment for 4 more values of ‘ ’ and get corresponding ‘’ values.
5.
Calculate = and =
6.
The Calculate focal length of =
2.
3.
1
1
2 −
. Plot a graph of y v/s x and find the intercept of the graph 0 . 1
.
Result: •
The approximate Focal Length of Lens A is found to be ________ cm
•
The Focal Length of Lens B is found to be ________ cm
EXPERIMENT 5: FOCAL LENGTH OF CONCAVE LENSES Aim: To determine the focal length of a given concave lens. Apparatus: A concave lens, a convex lens, two lens stands, two pins, optic bench and a screen. Least count of the optic bench is _______ = _______ . Formula:
F=
uv u−v
Diagram:
No
Position of Convex Lens [L (cm)]
st
1 Position of image without concave lens (A) [cm]
Position of concave lens (C) [cm]
n
2 Position of image pin with concave lens (B) [cm]
u = A−C [cm]
v =B−C [cm]
F=
uv u−v
[cm]
Procedure: Keep the object pin at the zero mark and make sure it remains there throughout the experiment. All readings of position should be taken in centimeters. Make sure that the tip of both the object and image pins and the center of both the lenses coincide. Place the convex lens at a convenient position (L) on the optic bench till you can see an inverted image of the pin on the other side. Now place the image pin on the bench and adjust its position till there is no parallax between the image of the object pin and the image pin. Note down the position of the convex lens (L) and the image pin (A). Now place the concave lens at a convenient position (C) in between the convex lens and the image pin. You will now see that the position of the inverted image has shifted so adjust the position of the image pin till there is again no parallax between the image of the object pin as seen through the combination of both the lenses and the image pin. Note down the position of the concave lens (C) and the new position of the image pin (B). Calculate the value of virtual object distance u = A − C and image distance v = B − C and hence find the focal length of the given concave lens using the formula given above Repeat the experiment from the start by either changing the value of position either of the lenses L or C by a few centimeters. Make sure you have an inverted image in each case. Note d own the readings of L, A, C and B in each case and calculate the corresponding value of F. Take the average value of focal length of all the readings. Result: The mean value of focal length of the given
concave lens
was found to be = _ _______ cm
NOTE: That the values of focal length for each reading may be drastically different for the other as this experiment has proven time and again to be extremely difficult to do. However, you may go ahead and take the average value.
EXPERIMENT 6: CONCAVE MIRRORS: U-V METHOD Aim: To determine the focal length of a given concave mirror by distant object method and u – v method and graphical method Apparatus: Given a concave mirror, a 100 cm optic bench, two pins, a mirror stand and a white screen. Least count of the optic bench is _______ = _______ . Formula:
1 Diagram:
f
=
1 v
+
1 u
⟹
f=
uv u+v
No
cm
[]
[]
1
1
[cm-1]
[cm-1]
=
+ []
Procedure: U – V method: Arrange the Mirror, the Object pin (O), and t he image pin (I) on the optic bench as shown in the figure. Keep the mirror such that it coincides with the zero mark of the optic bench. Keep the object pin (O) such that the object distance (u) is about 30 cm. Keeping this fixed, move the image pin to and from the mirror till there is no parallax between the image of the object (I’) and the image pin (I). At this position, note the values of object distance (u) and image distance (v). Record the values in a table form and also calculate the value of focal length using the formula… = [] + Now, repeat the experiment 4 times by increasing the value of object position (u) by 4 to 6 cm and for each value, adjusting the value of image distance (v) such that the parallax is removed. For each set of u’s and v’s, find the value of focal length ( f ). Find the mean value of focal length. Take care to ensure that he principle axis of the mirror lies along the optic bench. It will be almost impossible to get values otherwise.
Graphs: Plot the following graphs… a) v/s graph: Take same scale on both the axis and do not use kink. Mark and name the 5 data points of v and u on the x and y axes as v 1 , v 2 ,etc and u 1 , u 2 etc. Draw straight lines joining the corresponding data points from axis to axis. (v 1 to u 1, v 2 to u 2 etc up to v 5 to u 5 ). All these points will pass through a single point on the graph. A straight line drawn from the origin to this point will be at45° to the x and y axes. b) / v/s 1/ graph: The graph will be a straight line with negative slope. Find the line of best fit and find 0 the intercept and 0 the y intercept. The reciprocal of 0 and 0 gives you the value of focal length in cm. Results: • • •
The mean value of focal length from the v and u data is = ______ cm The value of focal length of the given concave mirror from v/s graph is _______ cm. The value of focal length of the given concave mirror from 1/ v/s 1/ graph is ______ cm.
EXPERIMENT 7: BOY’S METHOD Aim: To find the refractive index of a liquid using a lens and plain mirror (Boy’s Method) Apparatus: Given, a convex lens, a plain mirror, transparent liquid, a pin and a stand to hold the pin, a meter scale, a vernier calipers and spherometer. Least count of the optic bench is _______ . Least count of the = _______ spherometer is _______ . = _______
Theory: A convex lens kept on top of a liquid makes the liquid surface into a concave lens of same radius of curvature. If the object is kept at the focus, then the light coming out the lens will be parallel to the principle axis and a mirror kept on its way will reflect it right back to the object position. Formulae: For combination of two lenses 1 1 1 1 1 1 = + = f f F f f F For focal length of a lens, 1 1 R f = ( 1) + = 1+ R R f [ R =R&R = ] For a spherometer, l h R= + 6h 2 = focal length of the liquid lens; = combined focal length of convex lens and liquid lens; = focal length of convex lens; = refractive index of the transparent liquid; = Radius of curvature of the convex lens (equal to the radius of curvature of the liquid lens); = average distance between the legs of the spherometer; = spherometer reading when placed on the lens.
⇒
c L L
∵
1
µ � 1
2
2
�C 2 ⇒ µ � L L
∞
ℎ
Procedure: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Use a vernier calipers to find the thickness ( ) of the convex lens. Use a spherometer, find and and hence find the radius of curvature ( ) of the convex lens. Keep the mirror flat on the table and the lens above it. Arrange the object pin on a stand and keep it vertically above the lens in such a manner that the tip of the pin is on the principle axis of the lens. The reflection from the mirror will be seen. Move the pin up and down carefully till you get no parallax between the object pin and the image of it reflected from the lens and mirror combined. At this position, using the meter scale, measure the distance from the top surface of the lens to the pin ( ). Include the transparent liquid between the mirror and the lens. Repeat steps 4 and 5. At this position, using the meter scale, measure the distance from the top surface of the lens to the pin ( ). Use the formula to find the focal length of the liquid lens. Use the formula to find the refractive index of the liquid.
ℎ
1 2
Observations: (a) The thickness of the given convex lens is
= __________ cm
6 2 2 1
(b) The radius of curvature of the given convex lens is
=
+
= __________ cm
(c) Distance between surface of lens and pin at no parallax when convex lens alone is used (d) The focal length of the give convex lens
=
+
= __________ cm
1
= _________ cm
2 = ______ cm The focal length of the combination of liquid lens and convex lens is = + 2 = _________ cm 2 The focal length of the liquid lens is = − = __________ cm The refractive index of the liquid is = 1 + = __________ cm
(e) Distance between surface of lens and pin at no parallax when convex lens and liquid le ns is used (f) (g) (h)
×
Result: The refractive index of the given transparent liquid is found to be _________
EXPERIMENT 8: OHM'S LAW AND POTENTIAL DROP Aim: To verify ohm's law for the given resistance wire of unknown resistance and hence to find its resistance using the concept of potential drop across an ohmic resistance. Apparatus: Given a battery eliminator, plug key, voltmeter, ammeter, rheostat, an unknown resistance wire and a few connecting wires. Least count of (i) Voltmeter = _______ V; (ii) Ammeter = _______ A
No
[Ampere]
[Volts]
=
[Ω]
Procedure: 1. Set up the circuit as shown in the circuit diagram. 2. Take care to ensure that all the connections are tight and the terminals of ammeter and voltmeter are correct (positive of battery goes to positive of both ammeter and voltmeter). 3. Keep the rheostat at maximum value. 4. Include about 2V in the battery eliminator and insert the plug key. 5. Adjust the rheostat from maximum to minimum value to ensure that both the ammeter and voltmeter readings remain within range. If the range of ammeter or voltmeter is small, increase the voltage supply to 4V. 6. Adjust the rheostat till you get a small value of current in the ammeter. The corresponding value of voltage across the unknown resistance must be within range. 7. Note the voltmeter (V) and ammeter (i) readings. 8. Now adjust the rheostat till you get a higher value of current 9. Repeat steps 7 & 8 four more times till you reach the maximum range of either of t he two instruments. 10. In the table, calculate resistance for each case using the formula =
Ω.
Find the mean value of all the
calculated resistances. 11. Plot a graph of versus and draw a straight line of best fit. Find the slope of this line, this would give you the resistance of the wire. Result: Potential drop was studied and Ohm's law was verified for the given Resistance Wire. Its resistance was calculated as follows... •
Mean Resistance from Observation
= __________ Ω
•
Resistance calculated from the Graph
= __________ Ω
EXPERIMENT 9: THE WHEATSTONE’S BRIDGE – FINDING UNKNOWN R ESISTANCE
Aim: To find the value of unknown resistance using the principle of Wheatstone’s bridge and hence find resistivity of the given resistance wire. Apparatus: Battery, key, rheostat, resistance box, ammeter, Metre Bridge, jockey, an unknown resistance wire of 50.0cm length and a few connecting wires. Least count of (i) Voltmeter = _____ V; (ii) Ammeter = _____ A (iii) Metre Bridge = _____ cm.
Theory: For a given set of fours resistances (P, Q, R and S) that form a Wheatstone’s bridge, the condition for balance of bridge is / = / . P is replaced with an unknown resistance wire (X) of 50cm length and Q with a Resistance box (R). Replace R and S is by a single long resistance wire that provide a constant resistance gradient and hence any two desired values of resistance R 1 and R 2 . The Resistance of a wire is given by the equation R =
1 1
where is the resistivity, l the length and A the area of cross section of the wire. Since a meter
2 2 − ⇒ 12 12 12 ⇒ � − 2 � − � ⇒
bridge is being used, the resistances will have values (100 ) R = = & R = = The bridge balance condition can now be replaced as…
=
=
=
=
No.
Ω
[ ]
[cm]
− �Ω −
100 [cm]
=
100
[ ]
=
100
Formulae:
=
100
=
=
4 0.5
Procedure: 1. Set up the circuit as shown in the circuit diagram. 2. Take care to ensure that all the connections are tight and the terminals of ammeter and voltmeter are correct (positive of battery goes to positive of both ammeter and voltmeter). 3. Since this experiment is independent of the current that flows in the circuit and only depend on potential drop across the resistances keep the rheostat at maximum value. 4. Include about 2V in the battery eliminator and insert the plug key. 5. Include a random value of Resistance (R) in the Resistance box and then tap the Jockey at both the ends of the meter bridge to ensure opposite deflection. 6. Now, include 1 in the Resistance Box and tap the jockey at different points sequentially to find the balancing length l for which the galvanometer shows no deflection. In the table, note the value of R and l . 7. Repeat step 6 for all values of R from 1 to 10 and for each, note the value of l in the table. 8. Find the value of unknown resistance X using the formula for given above for every reading and find the average value of X and note it down. 9. Find the diameter (d) of the sample wire given using a screw gauge < you have to include the appropriate tabular column for that also> and hence find the resistivity of the given unknown resistance wire using the equation for resistivity given above.
Ω
Ω Ω
Result: •
The resistance of the given wire is found to be ______
•
The Specific Resistance or Resistivity of the given wire is found to be _______
EXPERIMENT 10: POTENTIOMETER I: DETERMINING UNKNOWN EMF Aim: To study the use of Potentiometer and hence use it to determine the value of unknown emf using a known standard emf source. Apparatus: Given a Potentiometer, battery eliminator, Electronic substitute for Daniel cell (1.08V) and Leclanché cell (1.5V), ammeter, rheostat, key, galvanometer, unknown emf source (Dry Cell), j ockey and some connecting wires. Least count of (i) Voltmeter = _____ V; (ii) Ammeter = _____ A (iii) Metre Bridge = _____ cm. Theory & Formula: The potential drop V across a certain length L of a wire of cross sectional area A and specific resistance wire is given by the formula
= where i is the current
and R �= is the resistance. For
every different value of V , the balancing length L differs, while A and are constants for the given wire. So if a fixed current is passed through the wire, then for every potential drop, V is proportional to L. Therefore we have the following relations… V L L = = = = V = V1 1 1 & V1 L1 L1
� � ⇒ ∝ ⇒ ∝
∝ ⇒
⇒ ∴
Table: Balancing length for different current values Cell Used
i = 0.400A 1 [cm]
i = 0.450A 2 [cm]
i = 0.500A 3 [cm]
Ex for = 0.400 [V]
Ex f or = 0.450 [V]
Ex for = 0.500 [V]
) [1.08V] Leclanché Cell ( ) [1.50V] Dry Cell ( ) (Unknown emf) Daniel Cell (
Calculations:
Daniel Cell Leclanché Cell
× 1.08 × 1.50 Vx =
Vx =
Procedure: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Set up the circuit as shown in the circuit diagram. Take care to ensure that all the connections are tight and the terminals of ammeter and voltmeter are correct (positive of battery goes to positive of both a mmeter and voltmeter). Keep the rheostat at maximum value. Include about 2V in the battery eliminator and insert the plug key. Move the rheostat to both ends to make sure that you can get current readings in the ammeter ranging from 0.000 to 0.500 A. Adjust the rheostat to include a current of 0.400 A in the primary circuit. Include the Daniel cell in the secondary circuit. Find the balancing length for the bridge for which the galvanometer shows no deflection. Note this value in the table Repeat step 7 & 8 for = 0.45A and = 0.500A Remove the Daniel Cell and include the Leclanché cell in the secondary circuit. Repeat steps 8 & 9. Remove the Leclanché cell and include the Dry Cell in the secondary circuit. Repeat steps 8 & 9. Calculate the values for Unknown emf for each of the 3 cases (3 different current readings) using each of the readings for both Daniel Cell and Leclanché Cell. Find the average of all the 6 readings.
Result: The average value of EMF of the given Dry cell is = ________ V
EXPERIMENT 11: POTENTIOMETER II: DETERMINATION OF INTERNAL R ESISTANCE Aim: To determine the internal resistance of a given dry cell. Apparatus: Given a potentiometer, battery eliminator, ammeter, rheostat, key, galvanometer, resistance box, dry cell, jockey and some connecting wires. Least count of (i) Voltmeter = _____ V; (ii) Ammeter = ____ A (iii) Metre Bridge = _____ cm. Formula: Using the theory of potentiometer and potential drop, the internal resistance of a given dry cell can be found using the formula…
= � − 1
= �12 − 1
⇒
‘r’ is the internal resistance of the given dry cell, ‘R’ is the resistance included in the Resistance Box, ‘ 1 ’ and ‘ 2 ’ are the balancing lengths that correspond to emf and voltage of the drycell.
No
Ammeter Reading () [A]
Resistance Box Reading ( ) [ ]
Ω
1 2 3 4 5 6
Length ( 1 ) with K 1 open [cm]
Length ( 2 ) with K 1 closed [cm]
= �12 − 1 [Ω]
Procedure: 1. Set up the circuit as shown in the circuit diagram. 2. Take care to ensure that all the connections are tight and the terminals of ammeter and voltmeter are correct (positive of battery goes to positive of both ammeter and voltmeter). 3. Keep the rheostat at maximum value. 4. Include about 2V in the battery eliminator and insert the plug key K. 5. Adjust the voltage on the battery eliminator and/or rheostat till you get opposite deflection. 6. Now adjust the rheostat to keep the current at a convenient value such that you get opposite deflection on both ends of the potentiometer. Note this current value down in the table. 7. Keeping the key K 1 open, touch the jockey on the potentiometer wire to find the point of null deflection. Note this down in the table as 1 . 8. Keeping the key K 1 closed, touch the jockey on the potentiometer wire to find the point of null deflection. Note this down in the table as 2 . 9. Repeat steps 7 & 8 two more times using different values of R each time. 10. Repeat steps 6 to 9 once more using a different value of . 11. For each case, find the value of internal resistance ‘r’ using the given formula and calculate the average.
Result: The value of internal resistance of the given dry cell is = ________
Ω
EXPERIMENT 12: POTENTIAL GRADIENT AND SPECIFIC R ESISTANCE Aim: To determine the Specific Resistance of the given wire by usin g the idea of Potential Gradient. Apparatus: Given a Potentiometer, sample wire used in the Meter Bridge, DC power supply, rheostat, key, ammeter, voltmeter, jockey, and connecting wires Least count of (i) Voltmeter = _____ V; (ii) Ammeter = _____ A (iii) Metre Bridge = _____ cm.
Theory: For any given resistance (R), the potential drop across it is given by ohm’s law, = . If we are talking about a Resistance Wire of uniform circular area of cross section (A) and of a certain length, the formula can be amended as follows…
� �2 22 �2 2 �2 ⟹ �2 ⇒ Ω =
=
=
=
=
4
Introducing Potential Gradient ( ) which is the Potential drop per unit length of the wire the above equation can be we re-written as… 4 4 = = = = [ ] 4 V i R l
Potential Drop across the resistance Current through the Resistance Resistance offered by the length of the wire Length of the wire resistance wire
[V] [A] [ ] [m]
Resistivity / Specific Resistance Radius of the given wire Diameter of the given wire Potential Gradient
Ω
[ m] [m] [m] [V/m]
The experiment is set up to find the potential gradient of the given wire for a fixed value of current. The Specific resistance of the wire can be found from the equation for .
No
Ammeter Reading (i) [A]
Voltmeter Reading (V) [V]
Length of the wire (l ) [m]
Potential Gradient ( ) [V/m]
Procedure: 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11.
Set up the circuit as shown in the circuit diagram. Take care to ensure that all the connections are tight and the terminals of ammeter and voltmeter are correct (positive of battery goes to positive of both ammeter and voltmeter). Keep the rheostat at maximum value. Include about 2V in the battery eliminator and insert the plug key. Touch the jockey at end B of the Potentiometer AB and adjust the rheostat and/or voltage supply to maintain the ammeter and voltmeter reading within range. For a more accurate result, it is better to get an almost full scale deflection in the Voltmeter at this setting. So if required, increase the supply to 4V. Once the rheostat is fixed and a constant current flowing through the circuit, make sure that this value of current is maintained constant throughout the experiment. Tap the jockey at the 10.0cm position (L) and note the Volt meter reading (V). Record both in the tabular column. Repeat step 6 nine times more, each time increasing the L value by 10.0 cm For each reading calculate the value of potential gradient ( = / ). Find the mean value of all the readings. Plot a graph of V versus l and draw a straight line of best fit and find its slope. The slope will again give you the potential gradient ( ). Using a screw gauge, find the diameter of the wire ( d ) using the sample wire given by taking diameter readings at various positions. Use the value of current ( i), potential gradient (slope of graph ) and diameter of the wire ‘d’ obtained in the experiments to find the value of specific resistance usi ng the above formula.
Results: •
Value of potential gradient from table
= _______ V/m
•
Value of potential gradient from Slope of graph = _______ V/m
•
Mean value of diameter of the wire
= _______ m
•
Calculated value of Specific Resistance
= _________________
Ω
m
EXPERIMENT 13: POST OFFICE BOX: FINDING UNKNOWN R ESISTANCE
Aim: To find the value of unknown resistance using the principle of Wheatstone’s Bridge using a Post Office Box Apparatus: Given a post office box, an unknown resistance, a galvanometer, a battery eliminator and a few connecting wires. Least count of Post Office Box is ______ Ω
Theory & Formula: A Post Office Box is an instrument used to find resistance to high accuracy values using the principle of Wheatstone’s bridge.
=
⇒
=
×
where P,Q,R and X are the 4 resistance arms of a Wheatstone’s Bridge. The Post Office Box apparatus itself consists of terminals for direct connection of battery eliminator (B) galvanometer (G), unknown resistance (X), Press key for B and G and the three Resistances P,Q and R. While the value of R can be chosen to be any whole number between 1Ω and 11,110 Ω, P and Q are have only 3 values each – 10 Ω ,100 Ω & 1000 Ω. Together, P and Q are called the Ratio Arms in which P is the multiplier and Q is the divider, the names derived directly from the equation for calculating X. Calculations
No
Multiplier Resistance (P) [Ω]
Divider Resistance (Q) [Ω]
1
10
10
2
10
100
3
10
1000
Galvanometer Deflection to the Left [Ω]
Right [Ω]
Mean R [Ω]
=
×
[Ω]
Procedure: 1. 2. 3. 4. 5. 6. 7. 8.
Set up the circuit as shown in the diagram. The circuit diagram may be used for comparison. Keep the P knob at 10 Ω. Keep the Q knob at 10 Ω. Find that value of resistance in R for which the galvanometer just deflects to the left. Note this in the table. Find that value of resistance in R for which the galvanometer just deflects to the right. Note this in the table. Find the mean of the left and right values and note it in the table. Repeat steps 3 to 6 by changing the value of Q each time. Calculate the X value using the formula for all the 3 sets of values to the correct number of significant figures. 9. The last reading when P = 10 Ω and Q = 1000Ω is the final value. Result: The resistance of the given wire is found to be ______ Ω
