PHYD72H3 Study Guide - Potentiometer, Internal Resistance, Ammeter


Department
Physics and Astrophysics
Course Code
PHYD72H3
Professor
P H Y B20

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The following experiment deals with the working relationship of the d’Arsonval meter and
different components of an electrical circuit. The experiment also analyzes the power
dissipated in a circuit of unknown resistance and power supply.
The experiment takes into account the internal resistance of a simple d’Arosonval meter
and using the latter I have deduced how the d’Arosnonval meter relates to resistors of
known resistances, by using Kirchoffs and Ohms laws.
The experiment also shows how half and full-scale deflection relate in a ratio of 2:1, when
the d’Arosnvol meter setup to measure current and voltage respectively.
The experimental reading for the D’Arsonvol meter was hence calculated to be 4.3e4
ohms. The experiment also illustrates how deflection (θ) is directly proportional to current
(I).
Using the various equations and result, I have shown how d’Arsonval meter can be used as
a voltmeter and how it can be used to deduce the components of a black box, using
Thevenin’s theorem. The calculated resistance of the black box using d’Arsonvol meter
was 14919 ohms, which was close enough to the actual value of 1500 ohms.
My experiment also deals with the relationship of power across a resistance load connected
to a black box circuit. The experiment shows how thw
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Introduction
A d’Arsonval meter
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Experiment and Result
The experiment can be divided in four parts for easier understanding. The experiments
consists of :
Part A
In this part I tried to find a value for the internal resistance of a d’Arosonval meter. Two
circuits were constructed for this part of the experiment.
The apparatus for to find the internatl resistance of a a’Arsonvol meter is as follows:
1. 1 1.2 VOLT Power supply
2. 1 Variable reisistor of a maxmmum value of 10kohm.
3. 1 Variable resistor of a maximum value of 10kohm.
4. 1 d’Arosonvol meter with minimum value of 0 and maximum value of 25. The
meter pointer deflected from left to right.
It was very important that I measured my readings on a flat plane so as not to affect the
deflection of the sensitive meter. The meter, in general, is very sensitive and it was very
important that I handled it accordingly.
The galvo can be setup both as an ammeter and a voltmeter. When a galvo is setup as an
ammeter, a resistor or a shunt resistor is connected with it in parallel, to prevent the galvo
from burning out.
A galvo can also be setup as a voltmeter by connecting a resistor in series with a resistor,
which is also called a multiplier. This increases the resistance of the circuit hence voltage
can be measured accurately.
Another property of a galvo that helped me in my calculation was the how the FSD
remains unchanged for any particular circuit connection. Which implies that the current at
which FSD occurs is constant.
I used these applications and properties of the galvo to relate internal resistance (Ri) of the
galvo and the resistances connected with it in series and parallel respectively.
As mentioned earlier, due to the meters high sensitivity, it was important that I connected a
resistor to the galvo in order to calculate its full scale deflection ( FSD). As I did not had no
formal data to calculate the FSD I had to connect a variable resistor to deduce the FSD. The
resistor was connected in series with the galvo as shown in figure (Fig A.1). This setup
effectively the converts the meter into a voltmeter.
The following setup did demonstrate a random error. This is mainly because due to the
mechanical friction in the galvo. This is something that could not e helped.
Another kind of error that can occur is the variation in deflection due to change in
temperature. To avoid this I, took regular breaks between measurements.
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