Category: Life Science

Teaching PHY1308 this semester has got me thinking about electricity, magnetism, and everyday things. It is Halloween season now, and Jodi and I took the time to construct a new decoration for our house: a graveyard of famous physicists (pictures to come when it’s done). The idea for this is that people may perish, but their ideas are eternal.

We needed to light the cemetery at night, so we bought some solar-charging LED floodlights at Lowe’s for about $30. I wasn’t prepared to be impressed by them – but I just wanted some lighting that charged off the sun and cast some useful light on the gravestones at night. We were actually pretty disappointed – they shone SO weakly, for so brief a period, that we figured something must be wrong.

The answer was Amp-hours. Put simply, the batteries sold with the lights are cheap and underpowered. Jodi did some tinkering on the solar charger today and found out that the batteries were just standard 1.2 Volt (V) off-the-shelf AA batteries – three of them, to be precise, arranged in series to deliver a total of 3.6V for three floodlights. Each floodlight consists of an array (in parallel, I assume) of 3 super-bright white LEDs.

LEDs are great, low-power devices that efficiently convert electricity into light. We discussed them in my Modern Physics class (PHY3305) last semester when we talked about solid-state physics. However, the problem with these store-bought lights was not the LEDs – it was the batteries provided for them. They were 1.2V AAs alright, but they were also weak: they can store a pitiful 900 mAh (milliAmp-hours).

Let’s discuss those units for a second. An Amp (A) is the amount of charge (in units called Coulombs, C) per unit time that can be moved by the battery. This is called “electric current,” and is denoted by the letter “I”. More Amps mean more current can be moved. 1A is equal to 1C/s (one Coulomb per second), and 1C is equal to 6.25×10^{18} electrons (that’s 625 with 16 zeroes afterward – a huge number).

An Amp-hour, therefore, is the amount of current a battery can deliver in one hour, before it can’t deliver anymore. Amp-hours are just Coulombs (Amps times second is equal to Coulombs), written in a different way. More Amp-hours means more charge, and more charge means more work. More work means more light.

So our batteries were a pitiful 900 mAh. I did the math on our floodlights. Assuming they contain typical super-bright white LEDs hooked up in parallel to a 3.6V power supply (the three AA batteries), and assuming that they can dissipate a power of 100 mW (milli-Watts, or 0.001 Joules each second), that means they each draw a current of about 30mA. You can figure this out using the simple formula,

P = IV (Power = Current multiplied times Voltage)

Since they are all in parallel, the total current that they draw is the sum of the current through each LED, for nine LEDs: 9x30mA = 270mA. Since the AA batteries were only capable of 900 mAh, they could power these floodlights for about 3 hours before they drained [2].

The lights go on once the sunlight in the yard drops to a certain level. However, being in a shady part of the front lawn, the floodlights come on long before they’re actually needed. By the time they are needed, they are greatly weakened. So we had the problem of a nice idea (solar-charging LED floodlights) coupled with weak batteries that left the lights too dim by the time they were really needed.

So we upgraded. We replaced the cheap-o 900 mAh batteries with 2500 mAh “heavy duty” AA batteries (also rechargeable). Now, for the same current draw we can run the lights for about 8-9 hours before the batteries run dry. This setup easily covers that period between dusk and dark that before was leading to significant drain on the system.

The results speak for themselves; tonight, when we walked out to look at the graveyard, it was easy to read Einstein’s and Maxwell’s headstones! A little physics will take you a long way.

[1] http://www.theledlight.com/technical1.html

[2] Since the floodlights did NOT include specs on the LEDs, I have to assume that these are 100 mW LEDs. They may, in fact, be more like 1W LEDs (those found in commercial flashlights). In that case, their current draw is more like I = (1W)/(3.6V) = 278mA, or about 300 mA – ten times more current! Under those conditions, 900 mAh batteries will last a mere third of an hour! Presumably, the truth lies somewhere in the middle, closer to P=0.1W.

This semester (and next), my teaching responsibility is the second half of our General Physics sequence for life sciences majors (PHY1308 [1]). The course is designed to cover many aspects of electricity and magnetism, starting with the definition and function of electric charge and moving forward into Gauss’ Law, circuits, magnetism, Maxwell’s Equations and light, and optics. In addition, I am trying to hold 1-2 periods at the very end for “special topics” – material that takes us a little beyond the course into the meaning and context of electricity and magnetism.

I take my mandate from the undergraduate catalogue very seriously – the course should be designed for life sciences majors. That doesn’t mean easing up on the math; I promised them at the beginning that I would treat them like physicists. But it does mean connecting the course to things they already know or ought to know. Relevance is extremely important to me as a teacher, but certainly never at the expense of rigor.

As an example, I’ve been setting the students loose on an exploration of the neuron as an electrical workhorse. For instance, you can model the axon – the long “tail” of the neuron that connects the soma (central cell) to other neurons – as a long coaxial cable whose myelin sheathe serves as an outer conductor and whose central filaments act like an internal conductor. You can use this as a model for computing the potential of the axon, or the electric field in the axon (knowing the action potential of the neuron). You can also treat this system as a cylindrical capacitor and compute its capacitance. In addition, the divisions of the axon – the nodes of Ranvier – serve as “leaky capacitors” in an RC circuit, and so the whole axon can be treated as a resistor-capacitor system.

The universe is quite cool, in that the laws of physics are never far from any particular manifestation. Whether it’s lightning striking the Texas desert prairie (charge, current, potential, force, fierld), or E-ink in a Kindle (potential, field, kinetics), or cell biology (the membrane as a capacitor, or as a resistive circuit), physics has all these beautiful little connections to the life sciences.

Ultimately, many of my students need to take and pass the MCAT. Studies have shown that by majoring in physics, you can do much better on the MCAT than just by majoring in a life science [2]. I hope that students taking my course will have some of the benefit of a full physics major conferred upon them. I intend to accomplish that by continuing to engage the students on life science issues as they interface with physics.

There is still about half the course to go. We’re just exiting circuits and entering magnetic fields. Soon, we will identify light as the unification of electricity and magnetism, and then we will study the behavior of light (optics). I hope then that we’ll have some time left for “extra cool” stuff – special topics – such as superconductivity, quantum mechanics, subatomic particles physics, the origin of the universe, and the possible fates of the universe. After all, what is life without deep, piecing questions, and what is physics without the hubris and the tools to pursue such questions?

[1] http://www.physics.smu.edu/sekula/phy1308/

[2] http://aip.org/statistics/trends/reports/mcat2003.pdf

Dallas hosts one of the largest science fairs in the country [1]. Last night, Jodi and I dressed up and headed down to Fair Park to meet the students and peruse the projects. Housed in Centennial Hall, long display tables filled the vast space, students buzzed around all over the place, and a devoted staff tending to all of the final events of the day.

Since it was so late in the day, most of the students were not attending to their projects. One of the Honorable Mention projects was attended by its investigator, a young man from one of the Plano High Schools who had studied Compton Scattering using a UV light source and a Strontium-90 beta emitting source. Many other projects spanned a space of topics, including the best storage conditions for popcorn (a freezer, not a pantry), wind conditions at a local school (for a turbine), a cellular-automata model for H1N1 spread with and without vaccination, the lead content of lipstick, and a hundred other studies.

Jodi and I were impressed with the breadth and quality of these studies. While we both wish we’d been able to meet more students, we were still happy to have time to spend a beautiful evening witnessing the future of science in this country.

[1] http://www.dallassciencefair.org/