<p>I think the electron microsopes use a magnetic field to focus the image so that was the answer, even though there are electron guns in the microscopes. A particle accelerator is used for other things…like creating high speed collisions of basic particles to break them into parts. Not related to the micrsocope…and too big to fit in one I think lol. </p>
<p>I put there was counterclockwise current in the square wire, but I got it wrong. Pretty sure no change in magnetic flux, bc no change in area or theta.</p>
<p>but i’m 100% sure there was a current!! it didn’t ask for change in flux
look at the link i posted from princeton review. same question. loop of wire moving perpendicularly through a magnetic field. no change in area or angle but there is still a current. </p>
<p>i can’t find anything about magnetic fields and electron microscopes from google.</p>
<p>Yeah but there has to be a change in flux to induce current. </p>
<p>(This is wikipedia, but still)
“The electron microscope uses electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it at a specific plane relative to the specimen in a manner similar to how a light microscope uses glass lenses to focus light on or through a specimen to form an image.”
So, light microsope uses glass to focus the image. Electron microscope uses electromagnetic fields.</p>
<p>cause by drawing it from outside the magnetic field into the magnetic field, there is a change in influx (for the question in TPR review book)
but the question on the test, the conditions show that it stayed in the magnetic field</p>
<p>^yeah, i get what you’re saying
but as the loop moves through the whole magnetic field. the current doesn’t just stop midway. the current doesn’t just flow for like 2 seconds as it gets a little bit into the field and then just disappear.
even if it starts in teh middle and moves to the end, the current still flows.</p>
<p>and since it stays in the field, there is still flux coming one direction due to the direction of the magnetic field (perp to the wire) and therefore, you apply lenz’s law</p>
<p>if the wire just stayed still and didn’t have a velocity, then there is no current. but it moves.</p>
<p>well I guess i was wrong.
Does the question say that there was a current already flowing?
then why did they ask you in what direction the induced current is flowing?</p>
<p>it just had a picture of a magnetic field going into the page, and a square loop of wire moving to the right perp to the magnetic field.
they wanted to see if you knew how to use lenz’s law or the right hand rule to the find the direction that the current would flow.</p>
<p>No offense, but a general note about many posters on this thread is the fact that Princeton/Barron/whatever are no use if you want to know physics…You guys are discussing what would be called in the Indian educational system ‘fundamentals’. I regret even having purchased that Barrons. Go out and learn from some REAL books instead of wiki and guides for high-scoring a test!</p>
<p>Yeah, you’re probably right. Though I did read most of a 600 page text, I’ve only done a year of physics. Also, do you recall the early two or three questions regarding a circular loop in a magnetic field? I think I have an 800(only 4 confirmed mistakes and one omit) but those questions are nagging me.</p>
<p>yeah. for a particle moving relative to an equipotential surface, work is only done if the particle’s distance from the surface changes. Therefore, if the surface is circular, no work is done if the particle simply moves around the surface (one of the questions). If both are of the same sign, potential energy is maximized when the charges are closer together, and if a particle simply revolves around the surface, its potential energy does not change either (another of the questions). and i forgot the last one.</p>
<p>yeah, i think its pointless that were all arguing about these even though theres a distinct possibility that well all get 800 (so far i have 5 confirmed wrong and i honestly cant think id have many more than that, and id say that regardless of the curve you can miss at least 10, since on the practice collegeboard one you could miss 14).</p>
<p>^ that was a different question than I was thinking of. this question had a loop of wire in a magnetic field. and asked us about rotating the loop about a dotted line, enlarging it, etc.</p>
<p>For the question where you move the loop in a magnetic field, there is no INDUCED current. Current and induced current are different. For there to be an induced current, there has to be a change in flux. Since the area of the wire is constant and it is not rotating producing a different angle between the field and the normal vector, there is no change in flux and thus no induced current.
From wiki:
“In practice, this means that an electrical current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes.”
Thus, as the magnetic flux is not changing, there is no induced current.
End of discussion.</p>
<p>“there is a difference between current and induced current.”</p>
<p>that sounds ridiculous. first of all, there is no distinction between current and induced current, if there is a current, then there is a current. current always flows in a direction or else it wouldn’t be called a current. the question on the test said, when this square loop is pulled through this field, in what direction does the current flow? </p>
<p>geez just give it up on this question please. there are MANY ways in which flux can change, not just with area and theta, physics C covers change in flux with velocity. the equation ABcostheta does not account for moving things. If the loop didn’t move at a velocity v, and just stayed still perpendicular to the field, THEN there would be no current.</p>
<p>think about it like this. as it keeps moving, there is ALWAYS new flux IN to balance the magnetic field. </p>
<p>the current flows counterclockwise in the square loop when it moves through a magnetic field going into the page at constant velocity. right hand rule. look back at the princeton review question.
can you guys just admit that you’re wrong?</p>
<p>well I asked my physics teacher today who is very well-loved and highly respected although popularity and respect might not correspond to having correct answer.
Yet, the example that you continue to cite (current flowing wire moving) is a case of 'electromotive force) and even with the right hand rule, there was no force (since it was moving constant) and there was only magnetic field, which means that current could not have been induced. Moreover, the reason we can apply BVLsinx is because it is in essence the way to measure the change in magnetic influx
Now i am getting really confused. Can anyone more sagacious and articulate explain this.
???</p>
<p>Hey, I thought I’d just compile a list of answers</p>
<p>electric field max work - moves inside the circle
electric field no work - the two where radius is still same
quassar - since the beginning of earth ?
heat engine - 25% efficient, 100J work
spaceship/light q - 3x10^8
compass- wire perp. to page
half life mummy - 5 x 10^-3 ?
irrelevant one in mag of force - Dir. of current
light/slit - more than that of all sorrounding points?
the resistors in series - +/- 0.5
electron microscope - magnets? particle acc?
bohrs model - energy levels equally spaced
sound light string waves - all interfere, all carry energy, light and string are transverse
induced current - no induced current?
consrvation of momentum - its conserved unless no ext. force
spring scale - 10N
mass spring vertical system - force of gravity is conserved
- velocity changes at the bottom most point.</p>