Force acting on an electric charge in a homogenous
electric field
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Objective
The objective of this experiment is to demonstrate the relationship between electric field
strength E and the force acting on a charge Q. The experimental equipment has been set
up for you but you have to operate it yourselves. After the experiment you will write a
report that describes the results as well as possible error sources in the experiment.
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Experimental Equipment
Never switch on the equipment without the assistants explicit authorization!
Experimental setup is depicted in figure 1. It consists of 4 parts:
1. Current balance. It can be used to measure horizontal forces. The force applied on
the balance lever is measured from the dynamometer (1b) reading
2. Capacitor plates with adjustable separation between the plates.
3. High-voltage power supply. Don’t turn on the power supply without assistants
permission!
4. Laser-screen system that can be used to check very accurately if the balance stays
level
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Figure 1: Measurement setup
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Before the experiment
When you’re preparing the measurement, think and if needed write down, the following:
• Give a mathematical expression for the relationship you plan to test.
• What will you measure as function of what?
• How does the separation between the capacitor plates influence the measurement?
• What’s the electric field inside the capacitor like? How would this affect the measurement?
• How would you determine the charge on the charge plate? How to estimate the
error of this measurement?
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During the experiment
Before using the equipment, explain to the assistants your setup and what you are planning to measure. With their permission, you can carry out your measurements. After
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you are done, check with one of the assistants that you have performed all the necessary
experiments. You can also ask questions about the report at this point.
NOTE: the dynamometer is placed at a shorter distance from the pivot of the balance
than the probe plate. How does this influence the measured forces?
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After the experiment
In the report, explain clearly but shortly
• What physical laws you intended to demonstrate with your experiment
• How your experimental setup works
• What possible error sources are there? Compare the influences of different sources?
• Based on your results, estimate the charge on the charge plate and its error.
Use graphical representations. Try to represent the most relevant data in a single graph.
Make sure your report doesn’t exceed the 7 page limit (excluding title page and references).
During the experiment you study capacitance and the charge accumulation in the
plates of a capacitor. Write a short essay (about 1 page) about the effect of supercapacity
and the possible applications of this effect.
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