StudentQns' physics questions

StudentQns SG

Hi, my previous thread ended so I’m writing this new one :-)

1) when a current carrying wire is placed in a magnetic field such as a motor, a force is produced using a force. Then how does having more turns of the coil increase the force?

Also how does inserting a soft iron rod into the coil increase the force? I understand that it concentrates the field lines, but how does this increase the force?

Thanks for the help!

wee_ws SG

Hi,

For Q1, please read:

http://www.electronics-tutorials.ws/electromagnetism/electromagnets.html

For Q2, please read the bottom of:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

Thanks.

Cheers,
Wen Shih

eagle SG

1) Each coil produces a force. More coils produce more force. They vector summed

2) Magnetic force intensity and closeness of field lines are related. The closer the field lines are together, the stronger the force.

StudentQns SG

But how does it look like? I’m quite unsure one 2) but u cleared 1) up for me Smile

eagle SG

How (2) looks like should be in your textbook, else it will not be in your syllabus. I cannot remember if closeness of field lines are in your syllabus.

StudentQns SG

Oh ok. I think that how it looks like is out of o level cos no diagram given. I think we just need to know it only…

StudentQns SG

Hi I have another physics question to ask. For example when a steel is placed in between a solenoid and a current is turned on, it attracts 10 iron tacks. When I turn it off will it have like 10 still or less? I know that steel is a hard magnetic material which retains its magnetism for a long time. So in theory do I consider it to retain all of its magnetism? Thanks for all the help!

StudentQns SG

Here’s what I think:
When they just say a ‘current’ flows through the solenoid. then this means that the magnet induced is not strong enough to completely over right the earth’s magnetic. so when I turn off the current it will deviate slightly back into its original position.

if they say ‘large current’ it will totally over right the earth magnetic field. and when I remove it it still retains a while bunch of magnetism so it doesn’t deviate at all.

this is something like what my teacher said. but I’m not sure as even hard magnetic material loses some of its magnetism once current is turned off. just that it retains it longer as compared to iron

thanks for all the help

eagle SG

For O levels, consider steel to retain all its magnetism. I believe you will only be tested on whether it retains its magnetism well, of which the answer is yes.

eagle SG

if they say ‘large current’ it will totally over right the earth magnetic field. and when I remove it it still retains a while bunch of magnetism so it doesn’t deviate at all.

thanks for all the help

Is your first paragraph referring to the behaviour of a compass situated just outside the solenoid?

StudentQns SG

Yup. Thanks for the help!

Also, when I place a coil in between two magnetic fields, without the commutator the current would not be reversed to one of the armature will also be up and the other down. So if I leave it on, will the coil just continuously flip from anticlockwise then clockwise until the battery has no more current?

StudentQns SG

Is your first paragraph referring to the behaviour of a compass situated just outside the solenoid?

Yup! Thanks for the help. My previous post was not clear. In my TYS under electromagnetism, a rectangular coil is placed in between two magnets. So a force will be generated on the two armature which are in opposite directions. Then even as it changes direction the current will still flow from eg ABCD. So the forces will change in direction. IN the answer key, it will eventually stop in a vertical position. but why is this so? i thought that the force will keep on being generated so it will keep on flip like a roti prata :). thanks for the help!

StudentQns SG

Hi in this post i will type out questions regarding electromagnetism
1) when a solenoid is connected to a dc current and a steel rod is placed in between it, over time it will turn into a permanent magnet right? So while it is still interchanging into becoming a magnet the magnetic fields lines will still penetrate the steel rod and start to saturate it. so after time it will become magnetically saturated and become a permanent magnet. Is this correct?

2) When a solenoid is connected to a dc current and an iron rod is placed in between it, it becomes a ‘electromagnet’ right? it becomes temporary magnet immediately with the north and south pole which is determined using the right-hand grip rule. when i turn off the current it loses all of its magnetism immediately. Is this correct?

Now, the harder part comes in which i have a lot of trouble in

3) When a solenoid connected to a ac current and a steel rod is placed in between it, it will NOT get magnetised right? But, the magnetic field will still pass through the rod but the direction of it keeps changing. then, since its a hard magnetic material, then it wont get magnetised so will there be any poles at the solenoid? (drawn in diagram)

4) when a solenoid is connect to a ac current and an iron rod is placed in between it, it will get magnetised right? but, it will keep changing direction. so meaning that at t=1, its NS at t=2 its SN. so it can still attract stuff right?

i found out that i didnt understand these from this question:
5)An iron rod and steel rod is placed inside a solenoid with an ac current. describe and explain what happens.

im not sure what happens as i dont know what happens during 3) and 4). So i need your help on these questions thanks!

picture: http://imageshack.us/photo/my-images/856/soleno…

eagle SG

Was too busy with work

I saw your questions in physicsforum.com too. I hope you realise that by posting part of a question, or paraphrasing it in your own words, you could potentially be changing the whole question into an entirely different one, confusing yourself even more in the process. You could also be imposing impossible limtations or scenarios that cannot be solved, things that teachers would have considered before setting the question. And that is not the way to ask for help online.

Also, when seeking for help online, you need to let others know which level you are at so that help can be pitched at the right level. Whatever is being taught at O level physics is so grossly insufficient and inadequate that a number of concepts are overly simplified and have severe limitations when we go on to higher level physics. i.e. Newton's 2nd law is force times acceleration in O levels, but it has a different definition at A levels, and an even more complicated usage in Uni.

Please see our Guidelines for homework forum

Please post the exact wording of the question, as well as what you have attempted or found out.

Thanks.

StudentQns SG

Oh ok Smile
Nvm I got the answer for that question. However, I’m unsure about the relationship between energy and force. I know the link between them is work done, but the distance variable might change when more/less energy is converted to work. So a 100J energy might have more force than a 200J energy because of this changed variable-distance.

So I’m not sure how to apply “increase the energy”. Like can i say increase the energy so that the hammer nail will go in further. Or when I kick a ball. I’m unsure about this. Thanks for the help!

eagle SG

Concept is totally wrong

Force and energy cannot be linked like this. One is a vector, and one is a scalar. They are totally different.

"So a 100J energy might have more force than a 200J energy because of this changed variable-distance." -- this is totally wrong!

Energy is the ability to do work. You cannot say 100J has more force --> This is horrendously wrong!

You can only say that 100J of energy has lesser ability to do work as compared to 200J of energy.

StudentQns SG

oh then what is the link between force and energy? is it only that energy can be changed into work done which is equal to force X distance.

Thanks for the help! You rock!

eagle SG

Force is a vector. A force can do work.

When a force does work, you get work done. However, a force can do zero work as well.

Apart from that, there's actually not much linkage... Do try not to link these two.