IDF Bilkent Erzurum Laboratory High School

IDF Bilkent Erzurum Laboratory High School
2011 Summer School
PHYSICS LAB REPORT DESIGN
001
Aim:
To determine the internal resistance of a dry cell by changing the resistance in the
circuit and measuring the current.
Assessment Criteria
Criterion
Grade
Design
A1
A2
A3
Data Collection and
Processing
A1
A2
A3
Conclusion and
Evaluation
A1
A2
A3
Complete
Partial
None
Student’s:
Cooperators:
Teacher’s:
Name: M.Murat TEKİN
Duygu YALDIR
Name: Mehmet Ali
DOĞRU
Class: 11 FB/C
Number: 86
Department:
OBEL Science Department
M. Murat TEKİN
11 FB-C
15.08.2011
Physics Lab Report Design 001
Aim

To determine the internal resistance of a dry cell by changing the resistance in the circuit and
measuring the current.
Research Question

How could the internal resistance of a cell be found by changing the resistance in the circuit?
Variables



Independent Variables
o Resistance in the circuit (R)
Dependent Variables
o Current (I)
Controlled Variables
o The e.m.f. of the cell (E)
o The resistance of the circuit (leads, switch, etc.)
Procedure:

Apparatus
o Electronic ammeter
o 1.5 V cell in holder
o Four 10 Ω resistors
o Switch
o
o
o
o
Leads(1 red,3 black)
Two crocodile clips
Calculator
Graph Paper
During the experiment, the e.m.f of the cell and the resistance of the circuit were to be kept
constant. To achieve that, the cell used was not changed all through the experiment, and so the
e.m.f. was constant. Also the cables, crocodile clips, the switch and the measurement devices were
not changed to keep the resistance of the circuit the same all through the experiment.
To find the internal resistance of the cell, the resistance (R) in the circuit was to be changed and the
current (I) was to be measured for each value of resistance. Then a graph of
against
was to be
sketched. To sketch the graph at least 5 different readings with different resistance values were
needed to be taken. In this experiment 7 different values of resistance were decided to be used.
With the given four 10Ω resistors the resistance values that could be created vary from 2.5Ω to 40Ω,
which is a good range for the experiment. The resistance values chosen for this experiment were
2.5Ω, 5Ω, 10Ω, 15Ω, 25Ω, 30Ω and 40Ω.
In the beginning the apparatus were taken and checked if they worked. Then the circuit was to be
prepared. To do that one black lead was attached to the negative end of the cell in holder. The other
end of the lead was connected to the switch. The switch was connected to the first series of
1
M. Murat TEKİN
11 FB-C
15.08.2011
resistances by another black lead with the help of a crocodile clip. The resistance series was attached
to a black lead, whose other end was connected to the ammeter. Then the ammeter was connected
to the positive end of the battery by a red lead and the circuit was completed.
The first of the resistance series used was four 10Ω resistors connected in parallel to give a resultant
resistance of 2.5Ω. With this resistance connected, the switch was closed and the current value (I)
that the ammeter showed was recorded on Table 1. Then the switch was opened and closed after a
few seconds, twice. The values of current were again recorded on Table 1.
The second one of the resistance series was two 10Ω resistors connected in parallel. This produced a
resultant resistance of 5Ω. The procedure for 2.5Ω resistor series was repeated the same way with
the 5Ω series. The results were recorded on Table 1. Then the resistances were connected in
different combinations to create the given resistance values, and for each value of resistance the
procedure was repeated and the values of current were recorded on Table 1, a copy of which is given
below. Also the values of
Table 1
Resistance (R)
/Ω
2.5
5.0
10.0
15.0
25.0
30.0
40.0
were calculated and recorded for that they were needed for the graph.
Current (I) /A ±0.01
Trial 1
Trial 2
0.43
0.44
0.25
0.24
0.12
0.13
0.09
0.08
0.05
0.05
0.04
0.04
0.03
0.03
With respect to the results the graph of
Trial 3
0.44
0.24
0.12
0.09
0.05
0.04
0.03
against
Mean
0.44
0.24
0.12
0.09
0.05
0.04
0.03
2.29
4.11
8.11
11.54
20.00
25.00
33.33
could be sketched. A graph paper was used to
sketch the best fit line graph and it was handed out to the teacher. Graph 1 sketched by using
Microsoft Excel program will be given on the next page to demonstrate the relation between
resistance and the value of
on this report too.
2
M. Murat TEKİN
11 FB-C
15.08.2011
Graph 1
40,00
35,00
30,00
25,00
Current(I)-1/A
20,00
15,00
10,00
5,00
0,00
0
5
10
15
20
25
30
35
40
Resistance (R)/Ω
It is necessary to mention that this graph is not exactly the same as the one handed out to the
teacher. The gradient and the y-intercept values given in the following parts do not belong to this
graph, but to the one handed to the teacher, so there must not be confusion about them.
The formula
was used to calculate the internal resistance of the cell (r) by using the
gradient of the graph which was equal to . The gradient of the graph was found to be 0.82. From
this value the E value could be calculated as 1.21V.
The value was supposed to be equal to the y intercept of the graph. In the graph handed out to the
teacher, this value was found to be approximately 0.6. Knowing that E is equal to 1.21, the value of r
could be calculated as 0.73Ω. Since this value is small, the effect of the resistance of the circuit
equipment is probably significant, which decreases the reliability of the result. But as a maximum
number, 0.73 Ω could be accepted to be a sensible result.
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