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Posts Tagged ‘electricity’

It’s F#*%kin’ Quantum Mechanics!

June 7, 2011 Leave a comment

Great commercial for the Powermat wireless charger.

I saw this on PhysicsBuzz, which also gives a brief explanation how inductive charging works. Coooool.

Teaching Electricity and Magnetism: Part II

March 30, 2011 2 comments

ResearchBlogging.org
This is the second part of my posts about teaching Electricity and Magnetism (EM). Part I can be found here, which dealt with the confusion of students in learning electricity and magnetism together. Part II deals with a paper looking at ways to help improve teaching methods for EM. The paper is entitled “Using multimedia learning modules in a hybrid-online course in electricity and magnetism“.

When I was still TAing (about 2 years ago) the University was starting to implement a new way of performing tutorial sessions. They were going to do it online. This was done by the students logging into a virtual classroom with the other students and were able to type out questions to the TA. The TA was in a computer lab somewhere and outfitted with an electronic on-screen writing tool (don’t ask me what it’s really called) and would work out problems on their screen by hand, which the students were able to see in the virtual classroom.

When asked if I wanted to participate in this type of tutorial, I refused. Call me a dinosaur (I’m only 26, but whatever) but I wanted to be in the room with the students when I taught them.

But do online and multimedia learning tools help? Or are they worse? That was the topic of this study.

A multimedia learning module (MLM) was developed by the Physics Education Research Group at the University of Illinois at Urbana Champaign and implemented as a pre-lecture assignment to students in an introductory physics course. MLMs are interactive online exercises which include flash animations which introduced physics concepts to the students. The MLMs were about 12-15 minutes long.

So the goal of the study was to determine if using MLMs prior to learning the concepts in class resulted in better grades for the students and a better student experience. The study tried them out in an introductory Electricity and Magnetism course in the Fall of 2008 at California State Polytechnic University at Pomona.

They used two different sections of the course as the control group and the experimental group. The control group (N = 48) had only the traditional coursework. The experimental group (N = 34) used traditional coursework in conjunction with the MLMs. To make sure any increase in performance was not simply due to increased time spent on the material in the experimental group (i.e. classtime + time spent on MLMs) the amount of time spent in the class was reduced by one-third for the experimental group.

Students in the experimental group viewed the MLMs prior to learning the material in class. Both groups were approximately equal in academic performance  prior to taking the course, as determined by a survey.

Student performance after the term was measured by a multiple choice test, as well as the results of answering questions in class using a personal response system called a  “clicker“. Students were also asked to fill out a questionnaire to rate the usefulness of different aspects of the course, such as the textbook or the MLM.

Students who used the MLM showed an 8% higher normalized gain than those in the control group (45% compared to 37%) in their multiple choice test. In addition, students who used the MLMs answered a slightly higher percentage of in-class clicker questions correctly (60 +/- 4.0%) compared to the control group (54 +/- 3.0%). This leads to an effect size of 0.25, which is considered a small effect.

Finally, students rated the usefulness of the different course material on a scale of 1 (not useful at all) to 5 (extremely useful). Students in the experimental group rated the MLMs higher (~2.5) than the course textbook (~1.3).

So does multimedia course material improve student performance? Well these results show that it is no worse than traditional coursework. One thing to note is that any increased improvement of the group which did MLMs compared to the control group is very small. With a sample size of about 40 students in each group, it is difficult to draw any firm conclusions.

In addition,

It is worth mentioning that the comparison of final exam scores between students in the control and those in hybrid-MLM group showed no significant differences.

So at the end of the day, students did roughly just as well in both groups.

But this is an interesting study nonetheless. Probably the best thing to do would be to offer the MLMs as an optional and additional resource to the students, without cutting out the in-class learning time. Everybody learns differently, whether it be through visual stimuli, auditory or simply repetitiveness. The important thing is to make resources available so people of all learning styles can benefit.

I felt I could teach my students best face-to-face, so I declined to use the new fangled technology for online tutorials. But I understand they are still being used, and some students actually prefer them. So I guess in the end, this study showed that no single manner of learning is better than any other. Do what works for you and stick with it.

Sadaghiani, H. (2011). Using multimedia learning modules in a hybrid-online course in electricity and magnetism Physical Review Special Topics – Physics Education Research, 7 (1) DOI: 10.1103/PhysRevSTPER.7.010102

Teaching Electricity and Magnetism: Part I

March 29, 2011 2 comments

ResearchBlogging.org

When I was a physics TA, there were two topics which always got the students easily mixed up. The first was Newton’s Laws; students had a hard time knowing which law to apply in what situation. But with a little practice and teaching, they soon found that you could follow a very specific procedure to solving any problem involving Newton’s Laws, which helped immensely.

Electricity and Magnetism (EM) was different, however. There really is no set procedure for solving an EM problem. There are strategies and guidelines, but no step-by-step ways of solving EM problems like there is for Newton’s Laws.

I’m not the only one who has noticed this either. Two papers were published this month in Physical Review Special Topics – Physics Education Research. The first was entitled “Interference between electric and magnetic concepts in introductory physics“.

This study looked at the difficulty students had in determining which direction the force on a charged particle would be, if it were in either an electric or a magnetic field.

For a positive charge in an electric field, the force is always in the same direction as the field. If the charge is negative, the force is in the opposite direction of the electric field. Students generally don’t have a problem with this rule; that is, until you introduce the concept of a magnetic field to them.

In a magnetic field, the force on a charged particle is always perpendicular to the magnetic field lines. So when you get to the end of the term and you ask an EM question, students often (understandably) get confused which rule they should use.

The main hypothesis of the study, therefore, was that students have trouble because they learn about electric fields first, and then apply those lessons to working with magnetic fields.

You can test this hypothesis by seeing if the opposite is true. Does learning about the magnetic field first negatively affect the way students answered questions about electric fields?

The subjects (I mean, ‘participants’ hehe) of the study were students in an introductory physics course at The Ohio State University. The students were asked to answer EM related physics questions. They were split up between groups which had learned i) nothing about EM, ii) electricity but not magnetism, iii) magnetism but not electricity, and iv) having learned both. The order in which the questions were asked and some other variables were randomized for better results. Below is an example of the type of question the students were asked.

Can you get the answer to this question? For the left hand side the answer is 'e - Into Page' and for the right hand side the answer is 'f - Out of Page'

There are actually several results from this study, so if you are interested in them all I encourage you to read it (it is free to read). But the main hypothesis turned out to be true:

directly after instruction about magnetism, many students answer that the direction of the force on a charged particle moving through an electric field is perpendicular to the electric field, presumably by employing the same right-hand rule that was learned for magnetic forces. Thus, despite the fact that directly before magnetic force instruction students were answering electric force questions correctly, up to two weeks (and possible longer) after they learn about magnetic force, they answer electric force questions as though they were magnetic force questions.

So the authors actually showed that it is not electric fields or magnetic fields alone that confuse students, but after learning both they get them mixed up, which makes sense. It doesn’t seem to matter, either, which they learn first. After learning both electric and magnetic fields they still get confused.

The authors suggest (and I agree) that to combat this the instructor must frequently point out the distinction between electric and magnetic forces. It is a difficult thing to get a feel for, kind of like learning the offside rule in hockey.

A good strategy is always visual demonstrations. Take for example this video of MIT professor Walter Lewin demonstrating the perpendicular magnetic force (jump to around 46:40 for the demonstration):

So what else could we do about students having trouble with EM? What about online and multimedia tools?

That will be the topic of Part II of this series.

Scaife, T., & Heckler, A. (2011). Interference between electric and magnetic concepts in introductory physics Physical Review Special Topics – Physics Education Research, 7 (1) DOI: 10.1103/PhysRevSTPER.7.010104

The Physics of MRI

February 9, 2011 3 comments

MRI being performed. Via Wikimeda Commons

Phew. I know, right?!

I’ve been saying I’ll write a post about the physics of MRI for months. Never got around to it. Mostly because I knew that since magnetic resonance imaging (MRI) was my field of research, that I would want to go into a lot of detail. I loved the time I spent doing MR research, and now I’ll finally share with all of you how it works.

Let’s start at the beginning.

We are all made of atoms. Atoms consist of a nucleus (made of neutrons and protons) orbited by electrons. The most common atom in our bodies is Hydrogen, which is a single proton orbited by a single electron. It is the simplest atom in nature, and it is quite fitting that it makes up the majority of our bodies.

We are mostly made of hydrogen because we are mostly water. Water has 2 hydrogen atoms and 1 oxygen atom (H20). When we do an MRI, what we are actually taking an image of is the hydrogen in our bodies.

How is that done? Well protons have charge, a positive charge. Any particle that has charge also has what is called a magnetic moment. A magnetic moment is essentially a measure of the strength and direction of the magnetic field of  particle. For a proton, the magnetic moment looks like this:

A proton (red circle) and it's magnetic moment (arrow)

Usually, all the magnetic moments in your body are jumbled about in all directions. That’s why you are not magnetic. When this is the case, we say that your net magnetic moment is zero.

Normally, all the magnetic moments in your body are jumbled around. Thus, your net magnetic moment is zero.

But if we apply a magnetic field from the outside, we can get the moments in your body to line up with the field, like so:

If we apply a magnetic field in a certain direction (blue arrow) the magnetic moments in your body will tend to align with the magnetic field.

In order to do that, we need BIG MAGNETS. That’s why MRI’s are so strong; you need that strong magnetic field to get the moments in your body to line up. (The diagram above is an exaggeration; in reality, only 1 in a million of the magnetic moments will line up with the field, on average).

How strong is an MRI magnet? Magnetic fields are measured in units of Tesla (in honour of Nikola Tesla). A typical MR scanner has a field of 1.5 Tesla. For comparison, this is about 30 000 times stronger than the Earth’s magnetic field, the field that makes a compass needle point north.

Now your body has what is called a net magnetization. This means that the magnetic moments have lined up and you are a little bit magnetic.

But that’s only half the battle. Next, we have to get some kind of signal from your body to make an image. How do we do that?

Well, have you ever swiped a credit card in a machine? What would happen if you just held it still? Would it still work?

The answer is no. Your credit card only works because the black strip in the back is magnetic. When you swipe it through the machine, it creates an electric current. This is due to Faraday’s Law, which says that a changing magnetic field in a loop of wire will create an electric current in that wire.

When you swipe a credit card through a credit card machine, you induce an electric current in the machine.

We can apply this concept to your body too now that its been magnetized by the MRI machine. You see, MRIs actually have a radio transmitter/receiver inside of them as well. How does THAT help?

Unlike a credit card, we can’t swipe YOU through the MRI machine. That’s not feasible. But we can manipulate the fact that you are now magnetized. This can be done using a radio-frequency pulse. Basically this is just a short, intense burst of radio waves at your body. This will actually make your net magnetic moment spin.

If your net magnetic moment is now spinning, it creates the exact same effect as if we were to swipe you through the MRI machine: it generates an electric current in a receiver in the MRI. Cool eh?

Ok so now we have a signal from your body by magnetizing it, and then making that net magnetization vector spin. How does that make an image?

So we made your net magnetization spin by using a radio-frequency pulse. How fast it spins depends on how strong the magnetic field is. So what MR scanners do is they make the magnetic field a bit different at each point in your body. That way, the magnetization from each part of your body spins at a different rate (or frequency) and we can then determine what part of your body is giving the signal.

Confused? Think of it this way:

Let’s say you had a piano. You hit a key on the left side of the piano and what happens? It makes a deep, low-frequency sound. Now you hit a key on the other end of the piano and you get a high-pitched, high frequency sound.

Different keys on the piano make different frequencies of sound. You can get an idea of what key someone hit from the frequency of the sound, without actually seeing the piano.

Now pretend you weren’t looking at the piano at all, and someone else hit a key. If you hear a low-frequency sound, you know they hit a key on the left side of the piano. If you hear a high frequency sound, you know it came from the right side. You were able to determine the position of the key by its frequency.

Its the exact same idea in MRI. Using the frequency of the signal we get, we can determine where in your body that signal came from. We then put all those signals from all parts of your body together, and we get an MRI image.

MRI of the head (Photo: NASA)

I know, there’s a lot of steps to MRI,  so I’ll recap:

  1. We magnetize your body by using a really big, strong magnet.
  2. We make your net magnetization spin using a radio-frequency pulse.
  3. This spinning magnetization generates an electric current in the radio receiver in the MRI machine.
  4. We can tell where in your body this signal came from by its frequency.
  5. Putting all the signals from all over your body together, we can make an image.

So there you go. Simple right?

If you want some more information you can check out the MRI article on HowStuffWorks.

Or if you want something a little more technical and goes a bit deeper into MR theory (e.g. relaxation, pulse sequences), check out this online book by J.P. Hornak, or the Wikipedia page on the physics of MRI.

Happy learning!

Feel The Heat! US Approves Thermal Solar Power Plant

September 2, 2010 2 comments

Solar power is the future. There are lots of ideas for renewable energy out there, but I am convinced that solar will be the only energy source truly strong enough to handle our power needs.

So we got some good news to that end! The first solar thermal power plant in the United States just got approval to be built in the great (sunny) state of California.

The approval is for a 250-megawatt solar thermal power plant by the Beacon Solar Energy Project. The approval came as a direct result of the green energy incentives granted by President Obama.

The plant will operate on over 2000 acres of land and will use abut 521 million gallons of water annually.

So, I wrote a post awhile ago on the Physics of Solar Power. That post described how semi-conductors are used to make the more common photovolataic cells used to generate electricity using the sun’s energy.

Thermal solar plants work differently. What happens is that a series of parabolic mirrors are installed in long rows surrounding one large tower in the centre. The mirrors will follow the sun throughout the day they are angled in such a way that the sunlight will be focused on water filled tubes.

Thermal Solar Power uses mirrors to direct sunlight onto a central tower, heating water into steam and driving turbines.

These tubes will then get extremely hot, several thousand degrees in fact, and the steam generated will be used to spin turbines, and thus generate electricity.

Its a bit less fancy than the semi-conducting solar panels, but it produces electricity just the same. And 250 megawatts is nothing to sneeze at either; thats enough to power 200,000 homes!

And lets not stop there! The final approval of a 1000 megawatt solar thermal power plant (The Blythe Solar Power Project) is nearing completion. This plant will also be built in California, be 7000 acres large and will generate enough electricity to power 800,000 homes!

The Physics of Solar Power

July 6, 2010 2 comments

In my previous post, I discussed how President Obama is helping to fund the development of Solar Energy. I thought I would then take the opportunity to explain a bit of the physics behind solar power.

Don’t worry, you won’t find any equations here :)

First, lets start with the sun. That big bright thing up in the sky.

The sun generates light, and light can be thought of as a bunch of tiny packets of energy. These packets are called “photons”. The different amounts of energy in a photon will correspond to the colour of the light that is emitted. For example, photons of the colour blue have more energy than photons of the colour red.

Energy increases from left to right (Source: Opensource Handbook of Nanoscience and Nanotechnology)

 

So how do we harness the energy in these photons? We can use Photovoltaic cells. Put simply, they convert solar energy into electricity. Let’s see how…

A photovoltaic cell is made of special materials called semiconductors, which are made of things like silicon (Yes, that stuff used to make fake boobs. Isn’t science awesome?).

Now, all atoms are made up of a nucleus (which is made of protons and neutrons) and electrons which circle around the nucleus.

Electrons can actually absorb the energy from a photon, but this happens only if the photon has a very specific amount of energy (a specific colour). When an electron does absorb a photon, it causes the electron to “jump”, and sometimes even break free of the entire atom! Electricity is a constant flow of electrons, which we refer to as an electric current.

Silicon structures like to hold onto their electrons. They don’t normally let them move around which makes silicon what we call an insulator. But in a Photovoltaic cell we add impurities, little bits of stuff that doesn’t belong there. The impurities will actually encourage the silicon to release its electrons and let them move around.

Now the magic happens. So a photon (those little packets of energy from the sun) hits the Photovoltaic cell. If the photon has just the right energy, it will knock loose one of the electrons in the silicon atoms. And, because of the impurities, that electron will move around.

If you get enough electrons moving around, you get an electric current which we can then use to power all of our awesome toys!

Thats it in a nutshell. If you want to read about this stuff in a bit more detail, check out the links spread all through this post, or some of the cool sites below.

Hooray for Physics!

Further Reading:

http://science.howstuffworks.com/solar-cell.htm

http://www.physlink.com/Education/AskExperts/ae451.cfm

http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/