AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread]

If anyone has their nelson thornes book with them, can I get help with question 3 part b, on topic 8.2 asap? thanks

Reply 2401

_Caz_

Original post by AR_95

If anyone has their nelson thornes book with them, can I get help with question 3 part b, on topic 8.2 asap? thanks

Because the coil is reversed, the sign on the magnetic flux density changes. This means that the flux linkage goes from 1.54x10-3 to -1.54x10-3. SO the change in flux linkage is ~3.1x10-3 (which is the difference between both values).

For part ii) you work out emf. emf is equal to change in flux linkage over time so you divade the previous answer by 95x10-3 which gives you 0.0326v :smile:

Hope this helps

Reply 2402

_Caz_

Does anyone have any good resources explaining eddy currents? I just cant understand them properly? Anything would be great thanks or if any of you know it would be great if you could explain it to me. :smile:

Reply 2403

Original post by _Caz_

Because the coil is reversed, the sign on the magnetic flux density changes. This means that the flux linkage goes from 1.54x10-3 to -1.54x10-3. SO the change in flux linkage is ~3.1x10-3 (which is the difference between both values).

For part ii) you work out emf. emf is equal to change in flux linkage over time so you divade the previous answer by 95x10-3 which gives you 0.0326v :smile:

Hope this helps

thanks a lot

Reply 2404

OP

Original post by _Caz_

Does anyone have any good resources explaining eddy currents? I just cant understand them properly? Anything would be great thanks or if any of you know it would be great if you could explain it to me. :smile:

"Eddy currents are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction."

"If the induced currents are created by a changing magnetic field then the Eddy currents will be perpendicular to the magnetic field and flowing in circles if the B-field is uniform."

"Eddy currents are undesirable in transformers. A metal core is used in the transformer to increase the flux. Unfortunately, the eddy currents produced in the metal core increase the energy lost. By constructing the metal core of alternating layers of conducting and nonconducting materials, the size of the induced loops are reduced thereby reducing the energy lost. The buzz you often hear when you are near a transformer substation is the due to alternating layers vibrating against each other."

ImageUploadedByStudent Room1433344024.794383.jpg

ImageUploadedByStudent Room1433344024.794383.jpg

Reply 2405

Can some one help me with question 4 on topic 8.3

Reply 2406

_Caz_

Original post by CD223

"Eddy currents are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction."

"If the induced currents are created by a changing magnetic field then the Eddy currents will be perpendicular to the magnetic field and flowing in circles if the B-field is uniform."

"Eddy currents are undesirable in transformers. A metal core is used in the transformer to increase the flux. Unfortunately, the eddy currents produced in the metal core increase the energy lost. By constructing the metal core of alternating layers of conducting and nonconducting materials, the size of the induced loops are reduced thereby reducing the energy lost. The buzz you often hear when you are near a transformer substation is the due to alternating layers vibrating against each other."

ImageUploadedByStudent Room1433344024.794383.jpg

Thank you! :h:

I tried to give you rep but it won't let me yet

Reply 2407

OP

Original post by _Caz_

Thank you! :h:

I tried to give you rep but it won't let me yet

No problem! I didn't do it for the rep honest :wink:

I wanted clarification myself tbh! Also I don't think you need to know in that much detail :L

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Reply 2408

Are we expected to know about the grid system part in detail, or just that electrical power is more efficient at high voltages and why

Reply 2409

OP

Original post by AR_95

Are we expected to know about the grid system part in detail, or just that electrical power is more efficient at high voltages and why

Yeah just why high voltage transmission is used between the power station and households then obviously that it's lowered for safety.

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Reply 2410

Original post by CD223

Yeah just why high voltage transmission is used between the power station and households then obviously that it's lowered for safety.

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Awesome. Genuinely hate the whole EMF part of Unit 4 so much. Makes my head hurt

Reply 2411

Absent Agent

Original post by CD223

"Eddy currents are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction."

"If the induced currents are created by a changing magnetic field then the Eddy currents will be perpendicular to the magnetic field and flowing in circles if the B-field is uniform."

"Eddy currents are undesirable in transformers. A metal core is used in the transformer to increase the flux. Unfortunately, the eddy currents produced in the metal core increase the energy lost. By constructing the metal core of alternating layers of conducting and nonconducting materials, the size of the induced loops are reduced thereby reducing the energy lost. The buzz you often hear when you are near a transformer substation is the due to alternating layers vibrating against each other."

ImageUploadedByStudent Room1433344024.794383.jpg

Nicely done!
So could you have eddy current not circular. I understand a circular current is induced in a "closed" conductor as in a metal ball, for example, given the conditions satisfied, but can a current induced in a generator be considered as an eddy current because the magnetic field produced by the current interacts with the magnetic field producing the the current to retard the motion of the generator?

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Reply 2412

OP

Original post by _Caz_

Thank you! :h:

I tried to give you rep but it won't let me yet

Original post by Mehrdad jafari

Nicely done!
So could you have eddy current not circular. I understand a circular current is induced in a "closed" conductor as in a metal ball, for example, given the conditions satisfied, but can a current induced in a generator be considered as an eddy current because the magnetic field produced by the current interacts with the magnetic field producing the the current to retard the motion of the generator?

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ImageUploadedByStudent Room1433345896.227160.jpg

I believe they're only circular due to the specific conditions mentioned. The induced current in a generator I believe is simply an application of Lenz's law - the induced current's field acts against the change that causes it by retarding the motion of the generator.

"Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material."

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Reply 2413

Absent Agent

Original post by CD223

I believe they're only circular due to the specific conditions mentioned. The induced current in a generator I believe is simply an application of Lenz's law - the induced current's field acts against the change that causes it by retarding the motion of the generator.

"Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material."

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Thank you

Reply 2414

OP

Original post by Mehrdad jafari

Thank you

No problem! Interesting stuff :tongue:

Reply 2415

Absent Agent

Original post by CD223

No problem! Interesting stuff :tongue:

True man, but the examiners will make them more interesting lol

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Reply 2416

OP

Original post by AR_95

Can some one help me with question 4 on topic 8.3

ImageUploadedByStudent Room1433347178.247294.jpg

a) The motor current is very small when the load is zero because the motor may turn at high speeds when the load is low. At high speeds, the back emf is very large, reducing the induced current.

b) The motor current increases when the load is increased because the motor turns at a lower speed when the load is high. At lower speeds, the back emf is reduced, increasing the induced current.

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Reply 2417

aprocrastinator

Hey could anyone explain questions 15 and 19?

http://filestore.aqa.org.uk/subjects/AQA-PHYA4-1-QP-JAN13.PDF

The answers are B and C respectively.

Reply 2418

Lau14

11

Original post by aprocrastinator

Hey could anyone explain questions 15 and 19?

http://filestore.aqa.org.uk/subjects/AQA-PHYA4-1-QP-JAN13.PDF

The answers are B and C respectively.

19 - this is mostly a look at your formula sheet question!
A. C = Q/V, so this is correct
B. There are two charged parallel plates, so there is a uniform field - correct.
C. Q = CV - it is directly proportional, so this is incorrect and the answer you want!
D. E = 0.5CV²so this is also correct.

Reply 2419

OP