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Physics Induced EMF exam question help

box A conducting rod is held horizontally in an east–west direction.
The magnetic flux density of the Earth’s magnetic field is 4.9 × 10−5 T and is directed
at an angle of 68° to the ground.

0 3 . 1 Figure 5 shows the arrangement. The rod has a length of 2.0 m.

Figure 5

The rod is released and falls 8.0 m to the ground. It remains in a horizontal
east–west direction as it falls.
Determine the average emf across the rod during its fall to the ground.
Assume that air resistance is negligible.

[3 marks]

So the diagram shows the rod falling vertically down and the magnetic field is at an angle of 68 degrees. So with the values given i know it has to be e=blvcos(68). however I do not understand why we use the final velocity before it hits the ground as v in the emf equation. Because in the question it states average emf, but if we use the max value of velocity would that not give us the max emf.
Note : the correct answer is 4.5x10^-4v, which is obtained by using the max/final velocity v which is 12.5

Reply 1

Original post
by head-merchandise
box A conducting rod is held horizontally in an east–west direction.
The magnetic flux density of the Earth’s magnetic field is 4.9 × 10−5 T and is directed
at an angle of 68° to the ground.
0 3 . 1 Figure 5 shows the arrangement. The rod has a length of 2.0 m.
Figure 5
The rod is released and falls 8.0 m to the ground. It remains in a horizontal
east–west direction as it falls.
Determine the average emf across the rod during its fall to the ground.
Assume that air resistance is negligible.
[3 marks]
So the diagram shows the rod falling vertically down and the magnetic field is at an angle of 68 degrees. So with the values given i know it has to be e=blvcos(68). however I do not understand why we use the final velocity before it hits the ground as v in the emf equation. Because in the question it states average emf, but if we use the max value of velocity would that not give us the max emf.
Note : the correct answer is 4.5x10^-4v, which is obtained by using the max/final velocity v which is 12.5

I agree with you
I got 2.70V

Reply 2

Original post
by Drummy
I agree with you
I got 2.70V

Honestly I'm baffled, its from the 2024 aqa physics paper 2 and in the only walkthrough i can find on youtube the person got it wrong aswell. Praying aqa dont give such badly worded questions tomorrow

Reply 3

I did it a couppe of days ago and was super confused too. So ur supposed to use suvat to fin the time taken to reach ground and the calculate average speed. Divide distance by time taken for v

Reply 4

For average speed, v.

Reply 5

Original post
by Kiwi27496
I did it a couppe of days ago and was super confused too. So ur supposed to use suvat to fin the time taken to reach ground and the calculate average speed. Divide distance by time taken for v

i just tried that, i did suvat to work out time and got time=1.27s. Then distance over time to work out average speed and i got V=6.3. But that doesnt work, as i plugged it back into the equation e=blvcos68 and i got a value that is not near the correct answer. also, in the mark scheme it states that v is 12.5 which is the value for final velocity

Reply 6

Really because when i put it into my calculator i get the correct answer. Are u perhaps on radians mode instead of degrees?

Reply 7

I just checked on radian mode when u input everything ive said then u get the answer that u are saying ur getting so i think that may be the problem

Reply 8

Original post
by Kiwi27496
Really because when i put it into my calculator i get the correct answer. Are u perhaps on radians mode instead of degrees?
I defo plugged it in right. What was your value for V, because in the mark scheme it explicitly states that v is 12.5, which is found by doing suvat for the value of v (final velocity)

Reply 9

Okay so E=Blv, i multiplied together: 2 x cos68 x 6.26 x 4.9x10^-5 and that gives me 2.3x10^-4. My v=6.26 which u get by dividing 8 by 1.27

Reply 10

Hope that helps

Reply 11

Original post
by Kiwi27496
Okay so E=Blv, i multiplied together: 2 x cos68 x 6.26 x 4.9x10^-5 and that gives me 2.3x10^-4. My v=6.26 which u get by dividing 8 by 1.27

thanks, but thats wrong the mark scheme states that the final answer is 4.5x10^-4volts

Reply 12

?? Kind of baffled rn cuz my answer is what the markscheme is saying is correct. 😭😭 are u looking at the official one?

Reply 13

Original post
by Kiwi27496
?? Kind of baffled rn cuz my answer is what the markscheme is saying is correct. 😭😭 are u looking at the official one?

Yep, my teacher sent it to me and he got it from the aqa website. Where is your mark scheme from?

Reply 14

Original post
by head-merchandise
Yep, my teacher sent it to me and he got it from the aqa website. Where is your mark scheme from?


My tutor. Its also the official one 😂😂

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