Section a Practicals

8/9/2019 Section a Practicals

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C.C.H.S. PHYSICS PRACTICALS CLASS-XII EXPERIMENT No. 1 Date-

Aim-To determine resistance per cm of a given wire by plotting a graph of potential difference versus current.

Apparatus-The given resistance wire, an ammeter, a voltmeter, a rheostat, a cell or battery, one-way key, meter scale,

connecting wires.

Circuit Diaram

!ormu"a #se$

According to Ohm's law, the potential difference across the ends of 'a conductor is directly proportional to the current

flowing through it provided the physical conditions of the conductor and its temperature remain the same. Let be the

current flowing through a conductor and ! be the potential difference across its ends, then

! or ! " # or # " !$ , where # is a constant of proportionality and is called resistance of the

conductor. t depends upon material of the wire and its dimensions %length and radius&.A graph between current along -ais and potential difference ! along y -ais comes out to be a straight line and the

slope % V/ I & of this line gives the value of the resistance of the given wire.

%&ser'atio(s

Step ".To find out range, least count and (ero error of instruments )

#ange of ammeter - .......... A, Least count of ammeter " .......... A

#ange of !oltmeter -.......... !, Least count of voltmeter - .......... !

*ero error of the ammeter, e, ".......... A

*ero error of the voltmeter, e+ -.......... !

Step-)

*ariatio( i( pote(tia"+'o"tae $i,,ere(ce * it curre(t I

l.o

.

Ampere reading %Amp& !oltmeter reading %in volt& !$ " # %in ohm&

Observed orrected Observed ! orrected !

+.

/.

0.

1.

2.

3.

4.

step /.Length of wire, l "..........cm.

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C.C.H.S. PHYSICS PRACTICALS CLASS-XII

Ca"cu"atio(s. 5y choosing suitable scales, plot a graph between current along -ais and potential difference !

along y-ais. The graph comes out to be a straight line as shown in 6ig. 7etermine the slope % V/ I & of thegraph which gives the value of resistance of the given wire. Thus, # " lope of !- graph " % V/ I & " 5$A "8. 9.

The resistance per unit length " #$ l " 8. 9$cm.

Resu"t- The resistance per cm of the wire by plotting graph between potential difference versus current is ........ 9$cm.

Precautio(s

l. The voltmeter and ammeter should be of proper ranges.

/. The ammeter should be connected in series and the voltmeter should be connected in parallel with their positive

terminals towards the positive pole of the battery and negative terminals towards the negative pole of the battery.

0. The length of the resistance wire should be measured from where it :ust comes out of the terminals of the voltmeter.

Sources o, Errors

l. The resistance wire may have kinks in it.

/. The pointers of the ammeter and voltmeter may lie in between the marking on the scales.

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EXPERIMENT No. ) Date-

Aim-

To find resistance of a given wire using meter bridge and hence determine the specific resistance of its material.

Appparatus-;eter bridge, resistance wire %length about + m& whose specific resistance is to be determined, resistance

one-way key, :ockey, galvanometer, battery or eliminator. screw gauge, thick connecting wires, sand paper, etc.

Circuit Diaram

!ormu"a #se$

%i& The unknown resistance


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C.C.H.S. PHYSICS PRACTICALS CLASS-XIItep 0 Least count of screw gauge " 88. mm

*ero error if any " 88.. mm %with sign&

tep 1-

l.o

.

;ain scale

reading %mm&

ircular scale reading %mm& Observed diameter %mm& ;ean diameter

%mm&

7ivision

coinciding

!alue "

division L..

Observed orrected

+.

/.

0.

Ca"cu"atio(s

The specific resistance of the material of the given wire is given by by > " ( d2

4 l)X " 88. 9@cm " 88.9@mtandard value %from tables& of specific resistance, >?" 88888 9@m

ercentage error "

(

0 )

+??" 88.B

Resu"t

+. The resistance of the given wire, " ... 8 9@m

Precautio(s

+. All the brass plugs of the resistance bo should be made tight by giving them screw motion.

/. The :ockey should be made to slide gently on the wire of the meter bridge. t should not be pressed hard on the wire

otherwise it may change uniformity of the wire.

0. The balance point should lie near the middle of the wire of the meter bridge. so as to minimi(e error due to end

resistances.1. The resistance wire %whose resistance is to be determined& should not make a loop or touch at certain points.

Causes o, Error

+. The wire of the meter bridge may not have uniform area of cross-section throughout its entire length.

/. The wire of meter bridge may get heated up due to passage of electric current, thus, changing resistance of the wire.

0. 5acklash error may be present in the screw gauge due to loose fitting of the screw.

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EXPERIMENT No. / Date-

Aim- To verify the laws of combination %series$parallel& of resistances using a meter bridge.

Apparatus-A meter bridge or slide wire bridge, two resistance coils, resistance bo, one-way key, :ockey, battery or

battery eliminator, connecting wires and sand paper.

Circuit Diaram

!ormu"a #se$

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%i& The resistance of a wire for resistance coil using meter bridge is given by r " ( 100ll )R , where l is lengthof the meter bridge wire from the (ero end upto null point and # is the resistance from resistance bo

%ii& Chen two resistances r+and r/are connected in series, their combined resistance is given by rs3 r14 r)

%iii& Chen two resistances r+and r/are connected in parallel, their combined resistance is given by rp3

r1r2

r1+r 2 %&ser'atio(s#esistance

coil

.o. #esistance

from

resistance bo

%#& %ohm&

5alancing

length

A7 " l %cm&

length

7 "

(100ll ) #%cm&

#esistance

r " ( 100ll ) # %ohm&

;ean resistance

%ohm&

r+ +. r+

/.

0.

r/ +. r//.

0.

r+and r/in

series

+. rs

/.

0.

r+and r/in

parallel

+. rp/.

0.

Ca"cu"atio(s a($ *eri,icatio( o, Las

%a& !erification of the law of resistances in series.;ean value of r+" 88..ohm, ;ean value of r/" 88..ohm

;ean value of %r+D r/& " 88..ohm,

;ean value of rs" 88..ohm

Cithin eperimental errors, rs " %r+D r/&. Therefore, the law of resistances in series is verified.

%b& !erification of the law of resistances in parallel )

;ean value of r+" 88..ohm, ;ean value of r/" 88..ohm

;ean value ofr1

r2

r1+r 2 " 88..ohm, ;ean value of rp" 88..ohm

Resu"t-%i& Cithin eperimental errors, rs " %r+D r/&. Therefore, the law of combination of resistances in series stands verified.

%ii& Cithin eperimental errors, rp3r1

r2

r1+r 2,. Therefore, the law of combination of resistances in parallel stands

verified.

Precautio(s

+. All the brass plugs of the resistance bo should be made tight by giving them screw motion.

/. The :ockey should be made to slide gently on the wire of the meter bridge. t should not be pressed hard on the wire

otherwise it may change uniformity of the wire.

0. The balance point should lie near the middle of the wire of the meter bridge. so as to minimi(e error due to end

resistances.1. The key E in the battery circuit should remain inserted only when observations are to be taken and then the key

should be taken out to avoid unnecessary heating of the resistance wire of the meter bridge.

2. The resistance wire %whose resistance is to be determined& should not make a loop or touch at certain points.

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C.C.H.S. PHYSICS PRACTICALS CLASS-XIICauses o, Error

+. The wire of the meter bridge may not have uniform area of cross-section throughout its entire length.

/. The wire of meter bridge may get heated up due to passage of electric current, thus, changing resistance of the wire.

0. 5acklash error may be present in the screw gauge due to loose fitting of the screw.

EXPERIMENT No. Date-

Aim-To compare the emf of two given primary cells %7aniell cell and Leclanche cell& using potentiometer.

Apparatus- otentiometer, 7aniell cell, Leclanche cell, ammeter, voltmeter, galvanometer, battery, or battery-

eliminator, rheostat, resistance bo, one-way key, two-way key, :ockey, connecting wires and a piece of sand paper.

Circuit Diaram

!ormu"a use$

Let l1 and l2 be the balancing lengths of the potentiometer wire using cells fo emfFs G +and G/respectively.

Then according to the principles of potentiometer, G+" E l1 and G/" E l2 where k is a constant of

proportionality. 7ividing eHuations, we getE

1

E2"

l1

l2

%&ser'atio(s

Step ".

''o determine potential differences G, G+, and G/ .

Least count of the ammeter " ......... A

Least count of the voltmeter " ......... !

otential difference across the battery, G " .........!otential difference across Leclanche cell, G+" ......... !

otential difference across 7aniell cell, G/" ......... !

Step ). 6I6a&"e ,or compariso( o, em,6s o, to ce""s.

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l.o

.

Ammeter

reading

%Amp&

5alancing length when Leclanche

cell G+is in the circuit l1 %cm&

5alancing length when 7aniel e

cell G/is in the circuit l2 %cm&E

1

E2"

l1

l2

+.

/.

0.

1.

2.

Mea( 'a"ue o, E18 E) 3 .

Resu"t- Theratio of the emfFs of the given two cells using potentiometer is E18 E) 3

Precautio(s

+.The current should be passed through the circuit only when observations are to be taken to avoid unnecessary

heating of the potentiometer wire.

/. The positive terminal of the auiliary battery G , G+, and G/should be connected at the (ero end of the

potentiometer wire.0. The emf of of the auiliary battery G should be greater than the emfFs of the either of the two cells.

1. The :ockey should be made to slide gently along the potentiometer wire.

2. The balance point should be at large distance from the end of the potentiometer wire preferably be on the last wire

of the potentiometer. This can be done by ad:usting the sliding contact of the rheostat.

3. ome high resistance plug %say,/??? 9&&should be taken out from the resistance bo before the :ockey is moved

along the wire for the safety of the galvanometer. This plug should be inserted back which noting the accurate position

of the null point.

4. The ammeter reading should remain constant for a particular set of observation.

Sources o, Error

l. The potentiometer wire may not have uniform area of cross-section along its whole length.

/. The emf of the auiliary battery may not remain constant.0. Ieating of the potentiometer wire may cause some error.

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EXPERIMENT No. 2 Date-

Aim-To determine the internal resistance of given primary cell using potentiometer.

Apparatus- A potentiometer, a battery or battery eliminator, a rheostat, two one-way keys, a Leclanche cell, a

resistance bo %?-/? 9&, a high resistance bo, an ammeter, voltmeter, galvanometer, :ockey, connecting wires and a

piece of sand paper.

Circuit Diaram

!ormu"a use$ -The internal resistance of a cell, using potentiometer is given by r " ( l1l2l2

)Rwhere # " hunt resistance in parallel with the cell.

l1 " 5alancing length of potentiometer wire without shunt.

l2 " 5alancing length of potentiometer wire with shunt

%&ser'atio(s- Step-1+. To determine the potential difference across battery$cell.

Least count of the voltmeter " .. .. . . . . . !

otential difference across the battery " ......... !

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C.C.H.S. PHYSICS PRACTICALS CLASS-XIIotential difference across the cell "......... !

Step ) -. Ta&"e ,or ,i($i( te i(ter(a" resista(ce o, a ce"".

l.o

.

#esistance

# %9&

osition of null point %cm& nternal resistance

r " ( l1l2l2

)R %ohm&Cithout shunt,l1 %key E/ is open&

. Cith shunt,

l2 %key E/ is

open&+.

/.

0.

1.

2.

Mea( i(ter(a" resista(ce; r 3 .

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Section a Practicals