AS Physics Question

[kratos009]

Hey guys,
I got two physics question that i don't understand and i would appreciate it if i could get the answer ASAP, i got my Paper 1 tomorrow...

1. Each of the nuclei below is accelerated from rest through the same potential difference.
Which one completes the acceleration with the lowest speed?

A. 1 H       B  4 He   C 7 Li    D. 9  Be
    1             2           3          4

The answer was C. Could someone please explain to me why the answer was C? i thought that acceleration is affected by Force which in turn is affected by mass, so shouldn't the answer technically be D because it has the greatest mass?

2. A plane wave of amplitude A is incident on a surface of area S placed so that it is perpendicular
to the direction of travel of the wave. The energy per unit time reaching the surface is E.
The amplitude of the wave is increased to 2 A and the area of the surface is reduced to 1/2 S
How much energy per unit time reaches this smaller surface?

A 4E   B 2E   C E    D 1/2E

the answer was B. I was wondering which formula did they use to figure out this question, or is this question meant to be figured out using logic...either way i couldn't figure it out so if anyone knows how you do it please post. Thank you very much

thanks for answering.

[astarmathsandphysics]

Will answer when i get up.

[astarmathsandphysics]

1. I think it must be that the charges on all the ions were not the same. For instance if the charge on Be were +2 it would be faster than Li with a charge of +1. It is really the charge to mass ratio that matters. Is there something else in the question.
2>E is proportional to A^2 so if A is doubled the energy is multiplied by 4. If the area is halved, the energy is halved. Hence B

[Tyserius]

Hey guys,
I got two physics question that i don't understand and i would appreciate it if i could get the answer ASAP, i got my Paper 1 tomorrow...

1. Each of the nuclei below is accelerated from rest through the same potential difference.
Which one completes the acceleration with the lowest speed?

A. 1 H       B  4 He   C 7 Li    D. 9  Be
    1             2           3          4

The answer was C. Could someone please explain to me why the answer was C? i thought that acceleration is affected by Force which in turn is affected by mass, so shouldn't the answer technically be D because it has the greatest mass?

2. A plane wave of amplitude A is incident on a surface of area S placed so that it is perpendicular
to the direction of travel of the wave. The energy per unit time reaching the surface is E.
The amplitude of the wave is increased to 2 A and the area of the surface is reduced to 1/2 S
How much energy per unit time reaches this smaller surface?

A 4E   B 2E   C E    D 1/2E

the answer was B. I was wondering which formula did they use to figure out this question, or is this question meant to be figured out using logic...either way i couldn't figure it out so if anyone knows how you do it please post. Thank you very much

thanks for answering.

For 1,

F = qE
ma = qE
a = (q/m)E
As E is constant since they all experience the same potential difference, so what determines the acceleration is the q/m ratio which shows that Lithium has the least with 3/7.

For 2,
Intensity = Power / Perpendicular Area
So, Power = Intensity x Perpendicular Area
As Intensity is directly proportional to Amplitude^2,
I = ka^2
So, Power = k (A^2) x S
Since kSA^2 = E,
A' = 2A
S' = 1/2 S
So, P' = kS'A'^2 = k (1/2 S) (2A)^2 = 2kSA^2 = 2E (By substituting E = kSA^2)

[Tyserius]

1. I think it must be that the charges on all the ions were not the same. For instance if the charge on Be were +2 it would be faster than Li with a charge of +1. It is really the charge to mass ratio that matters. Is there something else in the question.
2>E is proportional to A^2 so if A is doubled the energy is multiplied by 4. If the area is halved, the energy is halved. Hence B

Lol, Mr. Paul is always fast to solve things

[astarmathsandphysics]

nuclei. I read atoms. Anyway it is e/m

[kratos009]

Oh right i understand it now. Thanks a lot both of you for answering the question so quickly  .

[Tyserius]

Oh right i understand it now. Thanks a lot both of you for answering the question so quickly  .

Lol. All the best tomorrow for us I'm having the paper tomorrow too

[kratos009]

Good Luck!

[Tyserius]

Good Luck!

Haha. Still studying?