Answering Questions as a Teaching Assistant
Posted by Qrystal on February 24, 2009 at 2:12.
Category: Concepts. Tags: electromagnetism, light, physics, relativity, spacetime, teaching.
The following questions and answers are from a course called Physics and Society, offered at the University of Windsor. I am the teaching assistant for the course, and as such I answer the students’ questions on a private online discussion board. The current unit covers the basics of electromagnetism and relativity, and I have included two of the students’ questions below that lead up to a summary that I posted for them today.
“Can you please explain field, like electric field and magnetic field”
[My response:] A field is a region of space with a measurable physical property that may vary at different points in the region.
For example, an incandescent lightbulb glowing steadily is surrounded by a field of increased temperature. This is a scalar field, because each point in space can be associated with a temperature, which has a scalar value.
For the case of an electric field, at each point it is possible to measure how much electric force would act on a charged particle, and in which direction the force would act. This is a vector field, because each point in space has an associated vector magnitude and direction.
Similarly, a magnetic field is a region of space where a magnetic force would be able to be measured, and a gravitational field is a region of space where a gravitational force would be able to be measured.
I hope that helps! If not, I will gladly explain more.
“How does the current/charge form electromagnetic fields?”
[My response:] A charge could be defined as the source of an electromagnetic field, or the electromagnetic field could be defined as that force field that surrounds a charge. They define each other, and are defined in terms of each other. They each exist because the other exists.
Since a current is just made up of moving charges, the above description covers magnetic fields due to currents as well.
It’s a tricky question, but I hope my answer helped!
TODAY, February 23, 2009, is the day before the unit test… Nobody sent in any questions before my scheduled answer session, and so I came up with the following thoughts to summarize the subjects they’re studying.
I could take the description above one step further: a charge is something that is the source of an electromagnetic field (or the other way around: an electromagnetic field is a combination of electric field vectors and magnetic field vectors surrounding the thing we call a charge).
If you are moving at the same speed as the charge (that is, you are in the same inertial frame as it) you will only detect an electric field. If you and the charge are in relative motion, you will also be able to detect the magnetic part to the field. This is why a moving charge seems to “create” a magnetic field.
If a charge is moving back and forth in a regular pattern (relative to the observer), the electromagnetic field changes in a periodic way. The changes ripple outwards in waves we call electromagnetic waves — light, radio waves, microwaves, x-rays, depending on the frequency of the oscillations.
These waves move outwards in straight lines, unless something is bending the space the waves are travelling through — something like gravity. Note that gravity is bending spacetime itself, and light just naturally follows the curve of space, and that’s why it seems to bend.
Electromagnetic waves also move extremely fast, though not infinitely fast, and the actual speed depends on the material it’s passing through (usually slower than the speed of light in empty space, which we call c).
Just remember: the “speed of light” or “bending of light” is all about the transmission of electromagnetic field information. Thinking about light as rippling changes in an electromagnetic field might be useful in trying to see why there is a constant speed of light: it is electromagnetic field information that is travelling through space, not just our everyday notion of light (which just happens to be made of electromagnetic waves).
Isn’t it amazing that we can understand this, just from understanding what happens when charges are moving?
If you enjoyed these posts, stay tuned, there will be more as the class progresses! The next unit, starting after tomorrow’s test, is about quantum theory. OooOOooooOOoo~! —Qrystal