HELP please v confused - magnetic fields

For question 3cii;

http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/OCR-Old/Unit-5/June%202013%20QP%20-%20Unit%205%20OCR%20Physics%20A-level.pdf

"The magnetic flux density is increased further. The electric field strength is unchanged.
Describe and explain what happens to the path of the electrons."

obviously the magnetic field is greater than the electric field now but why would the electrons move DOWNWARDS?? if anything thats the way the electric field strength has direction, no??

MS: http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/OCR-Old/Unit-5/June%202013%20MS%20-%20Unit%205%20OCR%20Physics%20A-level.pdf

Many thanks

Reply 1

Pangol

15

Original post by MrToodles4

obviously the magnetic field is greater than the electric field now but why would the electrons move DOWNWARDS?? if anything thats the way the electric field strength has direction, no??

The electric field is pointing down, but the convention is that this is the direction of the force that would be felt by a positive charge. The electrons would therefore feel an upwards force due to this field, and if they are travelling in a straight line, then the force they feel due to the magnetic field must be downwards.

Reply 2

MrToodles4

OP

10

Original post by Pangol

The electric field is pointing down, but the convention is that this is the direction of the force that would be felt by a positive charge. The electrons would therefore feel an upwards force due to this field, and if they are travelling in a straight line, then the force they feel due to the magnetic field must be downwards.

Ohhh, i see. that makes perfect sense thank you so much :smile:

I also got a bit confused with question 5ii2 - would you mind explaining the answer to me please? On the same paper.

Reply 3

Pangol

15

Original post by MrToodles4

Ohhh, i see. that makes perfect sense thank you so much :smile:

I also got a bit confused with question 5ii2 - would you mind explaining the answer to me please? On the same paper.

What do you know about the mass of an object that is moving at speeds comparable to the speed of light?

Reply 4

MrToodles4

OP

10

Original post by Pangol

What do you know about the mass of an object that is moving at speeds comparable to the speed of light?

wouldn't the mass decrease?? I thought it would but the MS says mass increases :s

Reply 5

Pangol

15

Original post by MrToodles4

wouldn't the mass decrease?? I thought it would but the MS says mass increases :s

The mark scheme is correct - mass increases as speed increases, although this increase is negligible until you get to an appreciable fraction of the speed of light.

Reply 6

MrToodles4

OP

10

Original post by Pangol

The mark scheme is correct - mass increases as speed increases, although this increase is negligible until you get to an appreciable fraction of the speed of light.

Oh alright, I guess thats just something to remember for me now then.
If you have time could you check this out; https://www.thestudentroom.co.uk/showthread.php?t=5337176

Original post by MrToodles4

For question 3cii;

http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/OCR-Old/Unit-5/June%202013%20QP%20-%20Unit%205%20OCR%20Physics%20A-level.pdf

"The magnetic flux density is increased further. The electric field strength is unchanged.
Describe and explain what happens to the path of the electrons."

obviously the magnetic field is greater than the electric field now but why would the electrons move DOWNWARDS?? if anything thats the way the electric field strength has direction, no??

MS: http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/OCR-Old/Unit-5/June%202013%20MS%20-%20Unit%205%20OCR%20Physics%20A-level.pdf

Many thanks

I understand that your query had been addressed however, I would like to add a few point(s).

The saying of “the magnetic field is greater than the electric field” is wrong and is common among students. The physical quantity of magnetic field strength is different from the electric field strength.

Original post by MrToodles4

wouldn't the mass decrease?? I thought it would but the MS says mass increases :s

Original post by MrToodles4

Oh alright, I guess thats just something to remember for me now then.
If you have time could you check this out; https://www.thestudentroom.co.uk/showthread.php?t=5337176

I recommend that you don’t remember this if you intend to study physics in the future. To answer 5(a)(ii)2, I would recommend that you use the alternate answer suggested by MS instead of answer from the MS which says “The mass of the electron is greater than the rest mass of the electron”.

The following article may help to explain why I discourage the usage of “mass increases when the speed of the object increases to the speed of light”.
http://www.desy.de/pub/www/projects/Physics/Relativity/SR/light_mass.html

Within the article, there is a link to an interesting article which addresses "Does mass change with speed?"

When an object of mass m is moving at a speed of u which is near the speed of light, we need to do a relativistic correction for the linear momentum p = mu formula according to special relativity:

p = \dfrac{mu}{\sqrt{ 1 – u^2 / c^2} }

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/relmom.html

HELP please v confused - magnetic fields

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