LEDs in Water: Hands-on Electric Field Lines and Electric Potential
Leoš Dvořák a, Gorazd Planinšič b
a
Dept. of Physics Education, Faculty of Mathematics and Physics,
Charles University in Prague, Czech Republic
e-mail: leos.dvorak@gmail.com
b
Faculty for Mathematics and Physics, University of Ljubljana, Slovenia
e-mail: gorazd.planinsic@fmf.uni-lj.si
Abstract:
It is well known that concept of electric field and electric potential and relation between them
are one of the hardest to grasp at secondary level and even at university level. We propose a
simple experiment using light emitting diodes (LED) that can be used at various levels to
elucidate these concepts and demonstrate their behaviour. The experiment can be considered
as an example of “multilayered simple experiments” that the authors introduced at GIREP
2009 conference [1]. Some details of the proposed experiments represent interesting problems
on their own. Suggestions how to treat such problems theoretically and to compare the results
with real experiments are presented.
Introduction

Concepts of electric field E and electric potential φ are not easy for both high school and
university students, at least at introductory university level. Visualization of equipotential
lines in lab experiments often uses voltmeters or sensors connected to computers to measure
potential either in electrolytic tank or at weakly conducting carbon paper. However, as it is
pointed out e.g. by Knight [2], research showed strong evidence that most students gain little
or no knowledge of electric potential from such lab experience. He explicitly stated that “The
majority of students did not know what the voltmeter measured. When asked questions about
the electric field, nearly all students answered by referring to the electrodes and to the
terminals of the power supply, rather than to any measurements they were making with
voltmeter.” ([2], p.221-222)
We are suggesting that experiments using simple “local indicator” in a fish tank with water
can help students to visualize the electric field and equipotential lines in clearer way and offer
interesting examples students can discuss and use when learning these concepts.
These experiments can be treated at various levels, from primary to university, and can be
seen as an example of “multilayered simple experiments”, the concept we introduced at
GIREP 2009 [1].
Experimental setup
The basic experiment is not a new one. It was presented by V. Cigánik at the conference
“Šoltésove dni 2003“ in Bratislava, Slovakia. V. Cigánik was then Ph.D. student at Faculty of
Mathematics, Physics and Informatics, Comenius University, Bratislava. The experiment was
probably originally invented by him and/or some other people at that Faculty. We are not
aware of any previous work describing the experiment; according to our knowledge it also
was not published later. It is a pity that such a great idea is unknown - that is the reason why
we now (after years of using it by one of us mainly just as a “wow experiment” at some
seminars for teachers) decided to present it to wider audience, suggesting some new didactical
applications and analyze several of its interesting details.
The basic idea of the experiment is shown at Fig. 1. LED is put in a fish tank filled with
water. An electric field (due to the electrodes at the sides of the fish tank) causes a current to
flow through the water. Part of the current goes though the LED and the LED glows. The
whole situation is similar to a “normal” electrolytic tank, we just use LED as a “local
indicator” of the field. Fig. 2 shows the real experiment.
Fig. 1. The basic idea of the experiment
Fig. 2. LEDs in water – the real experiment
LEDs in water as a multilayered experiment for primary and secondary level
How such an experiment can be used in teaching and learning physics? We can utilize it at
different school levels, for slightly different purposes and, of course, discuss and try to
understand it at various depths. It may serve as an example of the approach with increasing
cognitive demands (see [1]).
At primary school level (either at schools or at physics clubs for kids) it can be used merely
to create excitement. The beauty of glowing LEDs of various colours such as shown at Fig. 3
can attract kids and increase their curiosity.
Fig. 3. Young children can just enjoy in observing glowing LEDs
Of course, even at this level the kids can learn something. They can see that:
• “Electricity” goes through the water (an important issue for the safety!).
• Orientation of LED matters.
(Note: At a primary school level we would use the vocabulary like that. There is no need to
speak about electric field etc.)
There is one safety warning to keep in mind when performing the experiment (not only at
primary level). The required voltage between the electrodes for a 20 cm fish tank is about
30 V (certainly more than about 15-20 V, see the experiments later in this text). And the
environment is more than wet, it is water itself! So kids should not put their fingers into water.
At lower secondary school level (for pupils of age 12-15) the experiment could be used in
physics or science classes. One important reason why to use it is the motivation. The
experiment is perhaps even more surprising for pupils who already learnt that for a light bulb
or LED it is necessary to connect both leads to the battery or to some other voltage source.
Here LEDs glow without any visible wires connected to them. Pupils can learn that:
• Electric current can flow through water. (It is basically the same observation as at the
primary level but now the concept of electric current can be used in the description
and explanation of the experiment.)
• Orientation of LED matters - again the point mentioned above but now pupils can
purposefully experiment as Fig. 4 shows. By this they can see that:
• If LED’s leads are oriented in the direction connecting the electrodes at sides of the
tank the LED glows brightly. If we tilt the LED it glows dimmer, as if connected to a
“weak battery”. This may be a first step towards building a concept of voltage (voltage
as a potential difference). Hopefully at least some pupils can grasp the idea.
Fig. 4. At lower secondary level the brightness of LED can be interpreted to tell something
about a voltage between different points
At high school level the experiment can help to elucidate the following additional points:
• The concept of potential difference. This can be investigated and discussed in greater
details using LED as a local indicator. (Greatest brightness of the LED corresponds to
the orientation in the direction of the electric field, if LED’s leads are in places where
the potentials are the same the LED does not glow at all, etc.)
• The field need not be only homogeneous. Shape of an electric field in more
complicated situations can be studied placing the electrodes connected to a battery into
different positions as it is shown at Fig. 5.
Fig. 5. LEDs indicate that changing the position of electrodes changes the shape of the electric
filed
At high school level such experiments can be part of either normal teaching or labs or
students’ projects. Of course, further piloting and research will be needed to develop concrete
teaching-learning sequences in physics or science classes at various levels of schools and to
find out their impact on students’ understanding of electric field concepts. Nevertheless, the
simplicity of the overall arrangement of the experiment enables its use as a cheap and easily
adaptable alternative to more cumbersome and perhaps less clear experiments traditionally
used in this area.
LED’s in water as problems for university level: the field near the electrodes
At an introductory university level the experiment, apart from its “wow aspect”, can provide
motivation for more detailed investigation. As we shall see it provides several interesting
problems. Their solution and results illustrate some concepts from the areas of electrostatics
and stationary currents and can be useful for lectures on Electricity and Magnetism or
Classical Electrodynamics.
One example of such question is: Should we isolate parts of LED’s leads? Intuitively it seems
it should be useful to isolate inner parts of leads to prevent “short-circuiting” the current
through the water near the LED or in other words, to achieve larger potential difference
between the LED leads (see Fig. 6).
Fig. 6. Should inner parts of LED be isolated as indicated in order to increase LED’s
brightness?
The experiment shows that the brightness of LED with isolated inner parts of leads is
practically the same as in the case of LED with bare leads (at least if the voltage between
outer electrodes is not too low). Let’s investigate what is going on.
For solving the above mentioned problem it is necessary to explore the electric field in the
vicinity of LED’s leads.
This can be a good starting point motivating us (and also students) to calculate the field
theoretically by solving the Laplace equations. We will not repeat here the explanation why
the field in a water tank fulfils Laplace equation. Instead, we will just shortly comment on
how the problem can be solved and present some results. We would like to note that a
problem like this provides a good motivation for students to solve Laplace equations - perhaps
better than standard, sometimes artificial end of chapter problems.
Of course, to be able to solve the problem easily, we should simplify the situation. We will
consider cylindrically symmetric situation described by Fig. 7. The leads of the LED are at the
axis, the outer electrodes with potentials ϕ = −U 0 and ϕ = +U 0 form the lower and upper base
of our cylinder. So the total battery voltage is U bat = 2 ⋅ U 0 . There is no electric flux through
the vertical surface R = Rmax .
Fig. 7. LED in water – simplified situation for the calculation
A simple reasoning can help us to determine the potentials at LED’s leads 1 and 2. The
voltage on the LED is approximately constant and does not depend (nearly at all) on the
current through the LED. We considered this voltage to be 2 V. (This is roughly right for a
yellow LED. It is a good approximation except for the case of very low current through the
LED.) For a symmetrical situation considered here it follows that the potentials of the leads
are ±1V . (For LEDs of other colours the voltage would be slightly different.)
Now we can solve the Laplace equation numerically. To do this we have used successive
overrelaxation method ([3], [4]). It is sufficiently simple so that its idea can be easily
explained to students and yet it converges reasonably fast. We have used meshes from 40x40
to 200x200 points. We will not present here any details concerning the numerical solution.
Instead, we will show and interpret some results.
Fig. 8 shows typical equipotential surfaces. (This case corresponds to the voltage between
electrodes equal to 32 V for a tank of length 20 cm; only part of water of size 10x10 cm is
shown at the figure.)
Fig. 8. The overall view of equipotential lines for LED (only leads are shown) in a “model
fish tank”
It can be clearly seen that the electric field is strongest near the tips of leads. Therefore also
the current density to the lead is strongest near the ends. Fig. 9 shows this in a greater detail.
The right graph at Fig. 9 presents the total current into the LED’s lead from a given point (z is
the distance from the centre of the LED) to the end (i.e. the tip) of the lead. It can be seen that
the “last centimetre” of the lead contributes to more than 2/3 of the total current. At the inner
part the current either into or out of the lead is small and does not influence the total current
very much. This is the reason why insulating the inner part of the leads practically does not
affect the total current and therefore the brightness of the LED.
Fig. 9. Radial component of electric field near the LED´s lead and total current flowing into
the LED´s lead from given z to the end of the lead (z2 = 3 cm).
Theoretical solution should be compared to real experimental results. One possibility is to
compare total current through the LED computed by numerical solution with a current really
measured. We did it for several values of battery voltage; the results are summarized in Fig.
10. According to a theoretical simulation the current increases linearly with the battery
voltage. (We can discuss with students that this is a natural consequence of the fact that the
Laplace equation is linear and we postulated constant voltage at LED’s leads.) The
experimental results show the same behaviour and also the “threshold” is nearly the same.
(The real behaviour near the threshold is caused by nonlinearity of LED, particularly by the
fact that the voltage at LED for low currents is no more constant.) The measured current is
slightly less than calculated one but surely we cannot expect a perfect agreement: in reality
LED is not in an infinite ocean of water or in a centre a cylinder filled with water but lies at
the bottom of the tank. Taking all this into account the agreements seems to be surprisingly
good.
Fig. 10. Total current through the LED as a function of the battery voltage:
results of simulations (left ) and measurements (right )
We omit here further results concerning the behaviour of the field near leads for low battery
voltage close to the threshold where LED stops glowing. In this case a real “short-circuiting”
of inner parts of leads occurs and insulating these parts may increase the current through the
LED. These results will be published elsewhere.
We conclude our comparison of theory and experiment by two qualitative observations. The
current going into LED’s lead is “visualised” by bubbles (of hydrogen) rising from the lead see left photo at Fig. 11. It can be clearly seen that most of the bubbles rise near the end of the
lead. At the other lead the oxidation of the surface of the lead causes the surface to become
dark (see right photo at Fig. 11). Again, the part of the lead close to the LED stays shiny
indicating that the current “prefers” the part closer to the end.
Fig. 11. The presence and even the relative size of the current going into LED’s anode can be
inferred by the bubbles of hydrogen that form due to electrolysis of water (left); the part of
the (other) lead through which the current goes is also identifiable by a change of colour of
the surface of the lead due to oxidation.
Conclusions
We have shown that LEDs can be used to visualize electric field and potential in more direct
and hopefully more effective way than other well known methods. We have, at least partly,
explained the behaviour of the field and electric current near the LED in water. Such deeper
understanding is useful also for designing experiments with LEDs in water for high school
and lower secondary level. Note that just some investigations and results were presented in
this short article. More detailed treatment, which we intend to publish elsewhere, would
include for example the behaviour of field and current in cases when the voltage is lower (and
the LED stops to glow) or the problem to what distance the field in water is substantially
distorted by the presence of a LED.
As we already stated above, further development and research would be also useful
concerning the use of these types of experiment in teaching and learning at various levels of
schools and its influence of students’ understanding of concepts related to electric field.
Part of this study was supported by bilateral Czech-Slovenian project MEB090907 “Support of scientific
reasoning development of students and teachers in physics classes through active learning”.
References
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Leicester, 2010) ISBN 978-1-4461-6219-4.
[2] Knight R D: Five Easy Lessons - strategies for successful physics teaching, Addison Wesley, 2004
[3] DiStasio M., McHarris W.C.: Electrostatic problems? Relax! Am.J.Phys. 47 (1979), No.5, 440-444.
[4] Press W.H., Flannery B.P., Teukolsky S.A., Vetterling T.W.: Numerical Recipes. The art of scientific
computing. Cambridge Univ.Press Cambridge, London, N.Y. 1986.