# URGENT A Level Physics help

how did they get the values for position of max current, im struggling to visualise it?

QP: https://www.ocr.org.uk/Images/643600-question-paper-unified-physics.pdf
MS: https://www.ocr.org.uk/Images/643605-mark-scheme-unified-physics.pdf

Question 5d
you only care about the direction of oscillation of the electric vector of the incoming wave (not the magnetic vector)

the incoming wave has a vertical electric vector oscillation and you can think of the wave as "pushing" or "pulling" the electrons but only in the direction of the waves oscillation (ie vertically in this case)

so when the wave and the aerial is vertical you have maximum "pushing" or "pulling" of the electrons so max current
but when the aerial is horizontal the electrons have minimum "pushing" or "pulling" since theres not much room to push or pull the electrons vertically, so min current

hope that helps
(edited 1 month ago)
Original post by mosaurlodon
you only care about the direction of oscillation of the electric vector of the incoming wave (not the magnetic vector)
the incoming wave has a vertical electric vector oscillation and you can think of the wave as "pushing" or "pulling" the electrons but only in the direction of the waves oscillation (ie vertically in this case)
so when the wave and the aerial is vertical you have maximum "pushing" or "pulling" of the electrons so max current
but when the aerial is horizontal the electrons have minimum "pushing" or "pulling" since theres not much room to push or pull the electrons vertically, so min current

hope that helps

thanks, but im still not sure how you know at what position to place the sheet in order to get max oscillation / max signal or vice versa, could you help with that please?
The reflected waves from the sheet superpose with the incident wave creating standing waves

When the path difference is lambda*(2n+1)/2 then it will be destructive and n*lambda is constructive

But since it’s reflected it travels twice the distance so when the sheet is lambda*(2n+1)/4 away its destructive and if it’s n*lambda/2 away its constructive

At constructive interference it has the highest amplitude and this highest oscillation of electrons so highest current and vice versa
(edited 1 month ago)
Original post by mosaurlodon
The reflected waves from the sheet superpose with the incident wave creating standing waves
When the path difference is lambda*(2n+1)/2 then it will be destructive and n*lambda is constructive
But since it’s reflected it travels twice the distance so when the sheet is lambda*(2n+1)/4 away its destructive and if it’s n*lambda/2 away its constructive
At constructive interference it has the highest amplitude and this highest oscillation of electrons so highest current and vice versa
But since it’s reflected it travels twice the distance so when the sheet is lambda*(2n+1)/4 away its destructive and if it’s n*lambda/2 away its constructive, still dont get this, could you maybe draw a diagram please?
(edited 1 month ago)
tbh I dont really know how drawing a diagram for this would help.

Ill try to explain the entire process but I can help more if youre specific with what you dont get

I think youre happy with destrc/constr interference at lambda*(2n+1)/2 and n*lambda respectively?

If so, think about about how much distance the wave has travelled AFTER it passes the aerial till it reaches the metal sheet -call this distance d.

when the wave is reflected it would have to travel this distance AGAIN to reach the aerial, so it would travel 2d.

This reflected wave superposes with the incident wave which has NOT PASSED the aerial and both waves meet at the aerial.
the path difference is the difference in how far each wave has travelled.
The reflected wave has travelled 2d more than the incident wave.
So its path difference is 2d.

So for constructive interference the sheet has to be ____ away ... and for destructive interference...
(edited 1 month ago)
Original post by mosaurlodon
tbh I dont really know how drawing a diagram for this would help.
Ill try to explain the entire process but I can help more if youre specific with what you dont get
I think youre happy with destrc/constr interference at lambda*(2n+1)/2 and n*lambda respectively?
If so, think about about how much distance the wave has travelled AFTER it passes the aerial till it reaches the metal sheet -call this distance d.
when the wave is reflected it would have to travel this distance AGAIN to reach the aerial, so it would travel 2d.
This reflected wave superposes with the incident wave which has NOT PASSED the aerial and both waves meet at the aerial.
the path difference is the difference in how far each wave has travelled.
The reflected wave has travelled 2d more than the incident wave.
So its path difference is 2d.
So for constructive interference the sheet has to be ____ away ... and for destructive interference...

Yh i was going to say i was unsure about the effect of the reflection on the path difference.

So if 2d is the path diff, and im taking the 180* phase change upon reflection into account, then So for constructive interference the sheet has to be d away and 2d for destructive interference?
IMO Its easiest to understand if you just think about path diff rather than phase diff so ignore the 180 degree phase change.

d IS the distance between the aerial and the sheet so the sheet is always "d away"
and since d is a variable you have to measure d with lambda since lambda is a constant.

So for constructive interference, the required path diff = n*lambda
2d = path difference, so d = ___*lambda

do the same for destructive
Original post by mosaurlodon
IMO Its easiest to understand if you just think about path diff rather than phase diff so ignore the 180 degree phase change.
d IS the distance between the aerial and the sheet so the sheet is always "d away"
and since d is a variable you have to measure d with lambda since lambda is a constant.
So for constructive interference, the required path diff = n*lambda
2d = path difference, so d = ___*lambda
do the same for destructive

d = n/2*lambda, i see it now, thanks.