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

1. If anyone has their nelson thornes book with them, can I get help with question 3 part b, on topic 8.2 asap? thanks
2. (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

Hope this helps
3. 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.
4. (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

Hope this helps
thanks a lot
5. (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.
"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."

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6. Can some one help me with question 4 on topic 8.3
7. (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."

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Thank you! I tried to give you rep but it won't let me yet
8. (Original post by _Caz_)
Thank you! I tried to give you rep but it won't let me yet
No problem! I didn't do it for the rep honest I wanted clarification myself tbh! Also I don't think you need to know in that much detail :L

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9. 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
10. (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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11. (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
12. (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."

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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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13. (Original post by _Caz_)
Thank you! I tried to give you rep but it won't let me yet
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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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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14. (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

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15. (Original post by Mehrdad jafari)
Thank you

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No problem! Interesting stuff

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16. (Original post by CD223)
No problem! Interesting stuff

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

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17. (Original post by AR_95)
Can some one help me with question 4 on topic 8.3

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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18. Hey could anyone explain questions 15 and 19?

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

The answers are B and C respectively.
19. (Original post by aprocrastinator)
Hey could anyone explain questions 15 and 19?

http://filestore.aqa.org.uk/subjects...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 = 0.5CV2 so this is also correct.
20. (Original post by aprocrastinator)
Hey could anyone explain questions 15 and 19?

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

The answers are B and C respectively.
Question 15:
• Energy is transferred when movement is parallel or perpendicular to field lines for gravitational and electric fields.

• The overall energy transferred only depends on the start and finish points of a particular path followed by a charge/mass. It is independent of the path followed.

• In other words, for example, you can move an object away from and towards Earth as many times as you want, but the overall work done only depends on the distance between the start and finish points. Each time you move the object away from Earth, work is done. Each time you move it back towards earth, energy is released. But the overall energy transferred is the only dependent on the distance moved between the start and finish points. Energy is always conserved but just transferred from one form to another.

Question 19:
• The capacitance, C, of a capacitor is defined as:

Or in words as the "charge stored per unit potential difference applied across two plates". So A is true.

• A uniform potential difference exists between the two plates. As such, a uniform electric field exists between the two plates. B is also true.

• The energy stored on a capacitor is:

So D is true.

• HOWEVER, rearranging the equation for capacitance:

So C is incorrect.

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