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11.3 Capacitance
11. Electromagnetic Induction HL
What is capacitance?
Background
“
Capacitance is the ratio of the amount of electric charge stored on a conductor to a difference in electric
potential.
T
he electric potential difference is measured between the object and ground. A material with a
large self capacitance holds more electric charge at a given potential difference than one with low
capacitance. In a typical capacitor, two conductors are used to separate electric charge, with one
conductor being positively charged and the other negatively charged, but the system having a total charge
of zero. The ratio in this case is the magnitude of the electric charge on either conductor and the potential
difference is that measured between the two conductors.
The capacitance is a function only of the
geometry of the design (e.g. area of the plates and the distance between them) and the permittivity of the
dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and
thus the capacitance, is independent of the potential difference between the conductors and the total
charge on them.
The SI unit of capacitance is the farad (symbol: F), named after the English physicist
Michael Faraday. A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential
difference of 1 volt between its plates.
”
Source:
https://en.wikipedia.org/wiki/Capacitance
Experiment
You are going to investigate the relationship between the voltage of a capacitor and the charge
distribution across it.
This will allow you to plot a graph of the data obtained and therefor you will able to determine the
relationship between the two variables and the value of the capacitance of the capacitor.
Method
1.
Go to the website:
https://phet.colorado.edu/sims/html/capacitor-lab-basics/latest/capacitor-
lab-basics_en.html
2.
Check all the boxes.
3.
Set up a table to record your data: you will be plotting voltage / V against current / pC.
4.
Leave the separation and plate area constant.
5.
Determine the maximum and minimum voltages available.
6.
Obtain at least fi ve data points covering this range and add to the data table below.
7.
Observe the visual change in electric fi eld strength and charge distribution as you change the
voltage.
Data
Analysis
1. Plot a graph to voltage against charge. I suggest you do this physically as this is best practise
for your examinations.
Voltage / V
Charge / pC
11.3 Capacitance
11. Electromagnetic Induction HL
2. Draw a line of best fi t for your data.
3. Annotate the graph to show how to determine the gradient of the line including the unit. This is
the capacitance, C.
4. Write down the equation of the line in symbols (Q for charge, V for voltage and C for
capacitance.
5. Re-write this equation to make C the subject.
Conclusion and evaluation
What happens to the electric fi eld as you increases the voltage?
What happens to the charge distribution as you increase the voltage?
Why were error bars not added to this graph?
