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Physics SL
Physics SL
Sample Internal Assessment
Sample Internal Assessment

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Table of content
Background information
Hypothesis
Variables
Data processing
Conclusion

How does the terminal velocity of the metal ball in cm s-1 moving vertically downwards through glycerin depends on the radius (in cm) of the metal balls, determined by measuring the time taken for the ball to fall?

How does the terminal velocity of the metal ball in cm s-1 moving vertically downwards through glycerin depends on the radius (in cm) of the metal balls, determined by measuring the time taken for the ball to fall? Reading Time
11 mins Read
How does the terminal velocity of the metal ball in cm s-1 moving vertically downwards through glycerin depends on the radius (in cm) of the metal balls, determined by measuring the time taken for the ball to fall? Word Count
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Table of content

Rationale

Life has become so dependent on laptops, i-pad and mobile now, especially during this lockdown! Every day since morning all the plug points of the house have an electric component hanging which is called a charger. The chargers are of different shape and size! Being intrigued by the same I wondered “what if I directly connect my iPad or the phone to the plug point with wires without the charger?” “What is present inside the charger which makes it so useful? “In order to search for an answer to my question, I started searching the net and read across a number of journals and watched some youtube videos. I was fascinated with the thought that our supply current is ac but the phone charges on dc. I was more intrigued to know “how is this conversion possible?”  In order to quench my thirst, I read more articles and watched more videos? Thus, I came across the new terms like diode, capacitor, dielectric which are all dealing with electricity, which is a major component in IB syllabus in the chapter of electricity dealing with capacitance and dielectrics and in power transmission where we will learn about diodes and transformers and how they are used. Hence, I decided to take up a project on this interesting topic as to how an ac is converted to dc and thus what would be the best method of such conversion? My ever-enquiring mind further asked “Can I vary the ratio of ac to dc conversion rate?” Thus, I came across the concept of ripple factor which finally led me to my research question.

 

This project of mine might help in better understanding of dielectrics and help make better adaptors using the permittivity contempt to increase the capacitance and thus reduce the ripple factor.

Background information

What is a capacitor

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. A capacitor is much simpler than a battery, as it can't produce new electrons -- it only stores them. Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric.

What is permittivity

Permittivity is a constant of proportionality that relates the electric field in a material to the electric displacement in that material. It characterizes the tendency of the atomic charge in an insulating material to distort in the presence of an electric field. The larger the tendency for charge distortion (also called electric polarization), the larger the value of the permittivity. The permittivity of an insulating, or dielectric, material is commonly symbolized by the Greek letter epsilon, ε; the permittivity of a vacuum, or free space, is symbolized ε0; and their ratio ε/ε0, called the dielectric constant is symbolized by the Greek letter κ. The permittivity ε and the dielectric constant κ in the cgs system are identical; both of them are dimensionless numbers.

What is a bridge rectifier

The bridge rectifier is made up of four diodes namely D1, D2, D3 and D4. The input signal is applied across the two terminals A and B while the DC output is obtained across the load resistor connected between the terminals C and D.

What is the use of capacitor in the rectifier circuit

The pulsating DC output obtained across the load resistor RL contains small ripples. To reduce these ripples, we use a filter at the output. The filter normally used in the bridge rectifier is a capacitor filter. In the below circuit diagram, the capacitor filter is connected across the load resistor RL..

Figure 1 - Use Of Capacitor In The Rectifier Curcuit
Figure 1 - Use Of Capacitor In The Rectifier Curcuit

How does bridge rectifier works

When an input AC signal is applied, during the positive half cycle both diodes D1 and D3 are forward biased. At the same time, diodes D2 and D4 are reverse biased. On the other hand, during the negative half cycle, diodes D2 and D4 are forward biased. At the same time, diodes D1 and D3 are reverse biased. Thus, the bridge rectifier allows both positive and negative half cycles of the input AC signal.

 

The DC output produced by the bridge rectifier is not a pure DC but a pulsating DC. This pulsating DC contains both AC and DC components. The AC components fluctuate with respect to time while the DC components remain constant with respect to time. So, the AC components present in the pulsating DC is an unwanted signal. The capacitor filter present at the output removes the unwanted AC components. Thus, a pure DC is obtained at the load resistor RL.

Figure 2 - Working Of A Bridge Rectifier Circuit
Figure 2 - Working Of A Bridge Rectifier Circuit

Capacitance and dielectrics

In the parallel-plate capacitor in the above simulation if the potential difference between the plates is changed then

Q ∝ ΔV

⇒ Q = C Δ V

 

where C is called the capacitance of the capacitor.

The capacitance depends on the geometry of the capacitor, namely the distance d between the plates [∝ 1/d] to the area A of the plate  [Q Aand on the material between the plates. Hence Capacitance is given by: C = ε0 A/d where εis the permittivity of free space and has a value of 8.85 x 10-12  C2\Nm2In most capacitors, an electrically insulating material called a dielectric is inserted between the plates. This decreases the electric field inside the capacitor. This happens because the molecules inside the dielectric get polarized in the field and they align themselves in a way that sets up a field in the opposite direction. The ratio of the field without the dielectric, Eo, and the field with the dielectric, E, is known as the dielectric constant, k. That is: k = Eo/E where is dimensionless and always greater than or equal to one. For a given potential difference across the plates, a capacitor with a dielectric can store more charge than the one without. The capacitance of a capacitor with a dielectric is given by: C = ε0 A/d

 

What is ripple factor

Ripple factor is a measure of the AC component present in a rectified AC signal. More the Ripple factor, the less efficient the rectification methodology is. An ideal rectification circuit or an ideal AC to DC converter would have 0 as the magnitude of ripple factor as there should not be any AC component in the output after rectification. Ripple Factor is the ratio of input AC current and the output DC current.The amount of ripple in power supplies is often indicated by the ripple factor:

 

For practical purpose, it is defined by: γ = \(\frac{V_{AC}}{V_{DC}}\)

Theoretically, its form becomes:  γ =  \(\sqrt{\frac{V^2_{rms}}{V^2_{DC}}-1}\)

 

Where – V ripple (rms) is the rms value of ripple voltage at the output and Vdc is the absolute value of the power supply output dc value.

 

Theoretically, for a bridge rectifier with capacitor filter the ripple factor depends on the capacitance and is given by, γ =  \(\frac{1}{4\sqrt{3}f\,C\,R_{L}}\)

 

where, f = frequency of the AC supply, C is the capacitance of the capacitor and RL is the load resistance.

Cathode ray oscilloscope (CRO)

The cathode ray oscilloscope (CRO) is a type of electrical instrument which is used for showing the measurement and analysis of waveforms and other electronic and electrical phenomena. It is a very fast X-Y plotter that shows the input signal versus another signal or versus time. The CROs are used to analyse the waveforms, transients, phenomena, and other time-varying quantities from a very low-frequency range to the radio frequencies.

Experimental methodology

First the bridge rectifier circuit using four diodes with capacitive filter was prepared. A step-down transformer was used with household AC supply as input to the transformer and the bridge rectifier circuit was connected to the output of the transformer. The value of capacitance of the capacitive filter was varied throughout the experiment by varying the dielectric inside the capacitor and the AC voltage (Vrms) and (Vdc) as measured using a multimeter across the load. The ripple factor has been calculated for each trial and the variation of ripple factor with an increase in capacitance of the capacitive filter due to dielectric was studied.

 

Literature survey

In the paper by P. P. Sahu, M. Singh and A. Baishya, "A Novel Versatile Precision Full-Wave Rectifier," in IEEE Transactions on Instrumentation and Measurement, vol. 59, no. 10, pp. 2742-2746, Oct. 2010.

 

In this paper, it was proposed and realized a novel precision full-wave rectifier using an all-pass filter as a 90° phase shifter.

Hypothesis

Prediction

It was predicted that the dielectric introduced in the capacitor will increase the capacitance of the capacitive filter in the bridge rectifier circuit which would reduce the ripple factor. Thus, a negative correlation is expected between the permittivity of the medium and the magnitude of Ripple factor of the Bridge Wave Rectifier circuit.

 

Justification

The assumption was based on the fact that, with an introduction of dielectrics in the capacitor with permittivity “κ” the capacitance of the filter circuit would increase, hence more AC current will be bypassed to the output in which the capacitor was connected because capacitance offers extremely low resistance towards AC current. Since reactance is inversely proportional to capacitance, it would decrease with increase in capacitance and hence, more AC current will flow through the filter thus decreasing the amount of AC current component in the load.

 

The schematic graph shown below depicts a relationship between the Ripple factor and the permittivity of the medium represented graphically as a straight line sloping downwards and indicating a negative correlation.

Figure 3 - The Schematic Graph Shown Below Depicts A Relationship Between
Figure 3 - The Schematic Graph Shown Below Depicts A Relationship Between

Variables

Independent variable

Dielectric of the capacitive filter

The permittivity of the dielectric of the capacitive filter inserted between the capacitor connected across the load of the bridge rectifier circuit was the independent variable of this exploration. The magnitude of capacitance was varied over a range of 10 μF from 1 μF to 10 μF at an interval of multiple of permittivity value. The capacitance of the capacitive filter was increased by inserting dielectric inside the plates of the capacitor within the dielectric constant range of 1.0 to 9.0, connected parallel to the load resistor (output). The dielectric constant was increased over the above-mentioned range to understand the variation in AC component of voltage across the output over a wide range of capacitance to strengthen the correlation obtained.

 

The values of permittivity of the medium used have been taken from literature sources and to use more reliable data, the values have been taken from three different secondary sources and a mean value has been considered.

 

In certain cases, a range was observed instead of a specific value and as the investigation demands a specific value of permittivity along the x axes in the graphical analysis, those materials were intentionally avoided. For example, pyrex glass has been used as a medium which is reported to have a permittivity of the medium in the range of 4.0 to 6.0.

 

The range of permittivity has been kept within 1.0 to 9.0 as the materials required to have the permittivity within this range are all solids and can be easily procured. Materials having dielectric above this range are usually in the liquid state and will thus have problems while executing the experiment.

 

The table below shows the permittivity of the medium used. The data has been collected from three sources-

  • Source A- engineertoolbox.com – This is a website with physical data of electrical properties of various dielectric mediums.
  • Source-B - Foundations for microstrip circuit design published in onlinelibrary.wiley.com
  • Source-C- nvlpubs.nist.gov – a government approved official website of National Institute of Standards and Technology under US Department of Commerce.
Figure 4 - Table On Permittivity Of The Dielectric Used
Figure 4 - Table On Permittivity Of The Dielectric Used

Sample calculation:

For asbestos,

Mean permittivity = \(\frac{Source\,-A\,+\,Source B\,+\,Source\,-\,C}{3}= \frac{4.00\,±\,0.05 ,+\,4.10\,±\,0.05\,+\,4.00\,±\,0.05}{3}\)= 4.03 ± 0.05

 

Absolute uncertainty = \(\frac{Max value\,-\,Min value}{2}= \frac{4.80\,-\,4.70}{2}\) = 4.03 ± 0.05

Dependent variable

Ripple Factor

Ripple factor is the dependent variable of the exploration.

 

It is a measure of the AC component present in a rectified DC signal. More the Ripple factor, the less efficient the rectification methodology is. An ideal rectification circuit or an ideal AC to DC converter would have 0 as the magnitude of ripple factor as there should not be any AC component in the output after rectification. Ripple Factor is the ratio of output AC current and the output DC current. It is also represented by the following formula:

 

γ = \(\sqrt{\frac{v^2_{rms}}{V^2_{dc}} -1}\) or γ = \(\frac{V_{AC}}{V_{DC}}\)

γ = Ripple Factor

vrms = Output RMS Voltage

Vdc = Output DC Voltage

 

In the experiment, the rms output voltage and the dc output voltage has been obtained using a multimeter and by using the above-mentioned expression the ripple factor has been calculated.

Controlled variable

Variable
Why it has been controlled?
How is the variable controlled?
Load Resistance
If the load resistance was varied then the potential drop across the load at the output would change. Hence, AC output voltage and the DC output voltage would change, thus affecting the ripple factor.
A fixed resistance of 1000 Ω was connected at the output for every trial of the experiment.
Input Voltage
If the input supply voltage was varied, then the output voltage would change (both DC output voltage and AC output voltage) across the load. Hence, the ripple factor would change.
Input supply voltage was the household supply AC voltage of 220 V, 50 Hz throughout the experiment.
Ratio of Turns of transformer
Variation in the ratio of turns of primary and secondary coil would have changed the voltage across the rectifier circuit which further would result in variation in DC output voltage and AC output voltage. As a result, the ripple factor would have changed.
Same transformer has been used throughout the experiment.
Temperature
An increase in temperature would have increased resistance of the circuit affecting the output voltage.
The experiment has been done in a confined place with least possibility of change in temperature.
Figure 5 - Table On Controlled Variable

Apparatus and materials required

Apparatus
Specification
Quantity
Least Count
Uncertainty (±)
AC Power Supply
220 V, 50 Hz
1
-
-
Transformer
Step Down (Turn Ratio equals to 44)
-
-
-
Multimeter
-
1
0.001 V
0.001V
PN – Junction Diode
Silicon
4
-
-
Resistor
1kΩ
1
-
-
Capacitor
Polar, 10F, 10V
1
-
-
Multimeter Probe
-
2
-
-
Connecting Wires
Single stranded wire
As required
-
-
CRO
1MHz
1
-
-
Bread Board
-
1
-
-
CRO Probe
-
2
-
-
Figure 6 - Table On Apparatus Table

List of materials required

Materials
Mass (± 0.01 g)
air
-
asbestos
2.00
bakelite
2.00
calcium carbonate
2.00
cotton
2.00
mica
2.00
paper
2.00
Figure 7 - Table On List Of Materials Required

Safety precautions

  • Since AC current was used to avoid electric shock leather shoes and rubber gloves were worn during the experiment.
  • The transformer temperature was monitored as at very high temperatures, the transformer often bursts.
  • For emergency purposes a fire-extinguisher was kept right beside the experimental set up.

 

Ethical Considerations

  • Capacitors of 10 V have been purchased to cut down the experimental cost as the secondary coil of the transformer will supply a voltage of 5V to the rectifier circuit.
  • No External AC power supply has been purchased. Rather the household supply has been used to reduce the experimental cost.

 

Environmental Considerations

In this experimental procedure, not such steps have been encountered in which the environment will be harmed.

Experimental methodology

  • Part - A

Making of capacitor

Figure 8 - A Parallel Plate Capacitor Was Made With Two Conducting Aluminium Plates
Figure 8 - A Parallel Plate Capacitor Was Made With Two Conducting Aluminium Plates
  • A parallel plate capacitor was made with two conducting aluminium plates of area 102 m2 and distance 0.09 mm between the plates.
  • The aluminium plates were connected to the crocodile clips which acted as the two terminals.The air capacitance was thus made 10μF.
  • The dielectric capacitors were made by inserting dielectrics one at a time inside the parallel plate capacitor. The polarization thus increased the capacitance C’=κC.
  • After inserting the dielectric the experimental value of capacitance was verified with the calculated value of capacitance.

Verification of the capacitance of the capacitors

  • First connect the two ends of the air capacitor made, to the multimeter probe.The multimeter has an inbuilt knob, which was set to a calibrated value that shows results in 2dp of Capacitance.
  • Then on inserting the dielectrics one by one the capacitance was again measured.
  • This step was repeated with all the 6 dielectrics.

Calculated value of the capacitance of the capacitors based on the literature value of permittivity

Serial no.
Dielectric material used
Permittivity of the medium
Capacitance (10μF± 0.05)
1
Air
1.00
10.0
2
Cotton
1.27
12.7
3
Paper
3.47
34.7
4
Asbestos
4.03
40.3
5
Bakelite
4.73
47.3
6
Mica
6.83
68.3
7
calcium carbonate
8.67
86.7
Figure 9 - Table On Calculated Value Of The Capacitance
Figure 10 - The Bread Board Was Taken And As Per The Circuit Figure
Figure 10 - The Bread Board Was Taken And As Per The Circuit Figure
  • The bread board was taken and as per the circuit diagram the circuit was designed.
  • A transformer was taken to which the AC power supply which is the input, was connected to the primary circuit.
  • The output from the two terminals of the secondary circuit of the transformer was fed to the diode bridge between D1-D4 and D2-D3.
  • The diode bridge was then made. The diodes were connected such that the p junction of D1 was connected to the n junction of D4 and the p junction of D2 was connected to the n junction of D3.
  • Further the p junction of D3 and D4 was made common and the n junction D1 and D2 was made common.
  • The common terminals of the diodes were connected to the two terminals of the load resistance.
  • The capacitor was further connected in parallel to the load resistance which acted as the filter.
  • In the circuit, the air capacitor used was of 1pF,10V and the load resistor was 1kΩ.
  • A multimeter was connected in parallel to the load resistor to measure the output voltage using the Multimeter probe. The multimeter has an inbuilt knob, which was set to a calibrated value that shows results in 3dp of Voltage.
  • A CRO was also connected across the output.
  • ut load using CRO probes to check the dc output.
  • The AC power supply was turned on and the output DC voltage and output AC voltage across the load resistor was measured using the multimeter and noted. The AC supply was turned off.
  • The dc output was verified using the CRO. A straight line came as the waveform indicating the current is constant and in one direction.
  • Then cotton as the dielectric was introduced in the air capacitor, thus capacitance was increased. Steps 9 onwards was repeated.
  • One after the other the same steps were performed with all the other dielectric.
  • For each dielectric the experiment was repeated 5 times.
  • This process was thus performed for seven such dielectric where air is the base dielectric having permittivity 1.

Data processing

Raw data table

Figure 11 - Table On Raw Data For The AC Output Voltage Against Various Values Of Permittivity Of The Dielectric Medium
Figure 11 - Table On Raw Data For The AC Output Voltage Against Various Values Of Permittivity Of The Dielectric Medium
Figure 12 - Table On Raw Data For Output DC Voltage Against Various Values Of Permittivity Of The Dielectric Medium
Figure 12 - Table On Raw Data For Output DC Voltage Against Various Values Of Permittivity Of The Dielectric Medium
Dielectric medium
Permittivity of the medium
Mean AC output voltage ± 0.001 V
Mean DC output voltage ± 0.001 V
Ripple factor
air
1.00
2.80
12
0.23
cotton
1.27
2.542
14.26
0.178
paper
3.47
1.668
23.136
0.072
asbestos
4.03
1.324
25.36
0.052
bakelite
4.73
1.166
26.924
0.043
mica
6.83
0.982
29.21
0.033
calcium carbonate
8.67
0.852
31.272
0.027
Figure 13 - Table On Ripple Factor Versus Permittivity

Sample calculation

For the air medium,

Theoretically, the ripple factor is calculated as γ = \(\frac{1}{4\sqrt{3}\,f\,C\,R_{L}}\) = \(\frac{1}{4\sqrt{3}^{*}\,50^{*}\,10\,F^{*}\,1\,k\,Hz}\)

 

= 0.28

 

Practically, γ = \(\frac{V_{AC}}{V_{DC}}= \frac{2.80}{12.0}\) = 0.023

Error propagation

\(\frac{∆γ}{γ}=\frac{∆v_{AC}}{v_{AC}}+\frac{∆v_{DC}}{v_{DC}}\)

 

\(\frac{∆γ}{γ}=\frac{{0.001}}{{2.8}}+\frac{{0.001}}{{12}}\) = ± 4.4 × 10- 4

 

Percetage error : 0.04

Figure 14 - Ripple Factor Versus Permittivity
Figure 14 - Ripple Factor Versus Permittivity

In this graph we have considered Permittivity along the x-axis and the Ripple factor along y-axis since it depends on the permittivity. As the permittivity increases from 1.00 to 1.27 the ripple factor decreases from 0.23 to 0.178. It is not a gradual decrease since the data points are not equally spaced. We obtained two values of permittivity at 1.00 and at 8.67 which are outside the trendline.

 

We have also obtained an equation of the trendline: y = - 0.0246x + 0.196, where y is ripple factor and x is permittivity. If the permittivity is 0 the value of the ripple factor becomes 0.196 which gives the ripple factor for a medium with zero permittivity.

Scientific justification

The ripple factor of a bridge rectifier is inversely proportional with the capacitance of the filter capacitor. But the capacitance is directly proportional with the permittivity of the dielectric medium. With increase in capacitance, the capacitor filters out the AC ripples much more and the DC signal gets smoothened as a result. Hence the ripple factor is inversely proportional with permittivity and gradually decreases with its increase.

Evaluation of hypotheses

Null Hypothesis:

Permittivity and ripple factor are not corelated with each other.

 

Alternate Hypothesis:

Permittivity and ripple factor are corelated with each other. The equation we obtained from the graph: y = - 0.0246x + 0.196 establishes a negative gradient between permittivity(x-axis) and ripple factor (y-axis). The value of R2 obtained from the graph is 0.072. Hence there is 72% co-relation between permittivity and ripple factor and they are negatively co-related. Hence the null hypothesis has been rejected and alternate hypothesis has been established.

Conclusion

The aim of the research was to address the question of how the Ripple factor of an AC (alternating current) to DC (Direct current) bridge rectifier circuit depend on the permittivity of the material placed between the two parallel plates of the capacitor filter, within the permittivity range of 1.0 to 9.0. From the experiment we are able to determine that :

  • The ripple factor is inversely proportional with permittivity and capacitance. With increase in capacitance, the capacitor filters out the AC ripples much more and the DC signal gets smoothened as a result.
  • We have also obtained an equation of the trendline: y = - 0.0246x + 0.196, where y is ripple factor and x is permittivity. If the permittivity is 0 the value of the ripple factor becomes 0.196 which gives the ripple factor for a medium with zero permittivity
  • There is 72% co-relation between permittivity and ripple factor and they are negatively co-related.

Evaluation

Strengths:

  • The standard deviation of the AC and DC voltages are really less, so the values obtained are precise.
  • The error percentage is miniscule so the data obtained are highly accurate.
  • We have considered seven mediums for our experiment which makes the results rich and diverse.
Type of error
Source of error
Effect of error
Improvement
Random
Fluctuation of voltmeter
Precisement of the data is lost
We have taken five trials and calculated their mean
Systematic
Heating of the diodes and wires
Slight variation of actual data
Experiment done in an Air-conditioned room
Methodological
The capacitor used are of alumunium and parallel plated and are prone to more discharges with time.
More ripples within the DC voltage.
Ceramic capacitor could be used.
Figure 15 - Table On Limitations

Further scope of analysis:

We studied the variation of permittivity with ripple factor. We could also make the capacitance constant and vary the load resistance and study it’s effect on the ripple factor.

References

  • Jovanovic, Milan M., and David E. Crow. "Merits and limitations of full-bridge rectifier with LC filter in meeting IEC 1000-3-2 harmonic-limit specifications." IEEE Transactions on Industry Applications 33.2 (1997): 551-557.
  • Samsudin, Nor Azura, Dahaman Ishak, and Ashraf Bani Ahmad. "Design and experimental evaluation of a single-stage AC/DC converter with PFC and hybrid full-bridge rectifier." Engineering science and technology, an international journal 21.2 (2018): 189-200.
  • Sakui, Masaaki, and Hiroshi Fujita. "An analytical method for calculating harmonic currents of a three-phase diode-bridge rectifier with DC filter." IEEE transactions on Power Electronics 9.6 (1994): 631-637.