### Archive

Posts Tagged ‘Magnet’

## Sloooow Going

Posts have been slowing down a little as of late. This is due to a number of factors including my training for a 10 km run (6.21371192 miles for my American readers) which I completed in 1 hour and 2 minutes! I was hoping to get in under an hour, but considering a year ago I could barely run 2 km, this was a pretty big accomplishment for me!

Hurray for strengthening your cardiovascular system!

Posts have also slowed down because I’m actually get to do some physics-related stuff at work. We are developing a new toy for inspecting pipes and I get to look at data and read about Faraday’s law and play with magnets!

While completely fake, this explanation is still better than Insane Clown Posse's explanation of magnets working by "miracle"

But I could never forget about you guys. I still love reading and writing about science and that will never change, so I’ll be back to full form in no time.

But in the meantime, here’s 31 bad (and therefore, awesome) jokes for nerds. Enjoy!

Categories: Me

## Teaching Electricity and Magnetism: Part I

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

Categories: Physics

## The Physics of MRI

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.