All the different combinations of ways you can cascade down eh drop all the way down, drop one level, drop to the bottom after dropping one level etc. Think it was 7?

It was 3 because the electron didn’t have enough energy to reach the top two energy levels I think...?

Reply 509

akash3141

13

Isn't the value of the force in the first question wrong? And also the value of the force in the MCQ, i'm sure I did it an another way because i'm quite sure about my answer, although I could've done it wrong, lol

Reply 510

examjam

5

a capacitor of capacitance c has a charge of q stored on the plates. the pd between the plates is doubled. what is the change in energy stored by the capacitor?

Options: (0.5Q^2/C ; Q^2/C ; 1.5Q^2/C ; 2Q^2/C)

So when the pd is doubled, how do we know whether Q or C stay constant?

I know that if a particular dielectric is removed, leaving the plates separated only by air, therefore reducing permittivity, charge stays constant, pd increases.

But if pd is doubled, then what happens? does capacitance decreases and charge stays the same or charge increases but capacitance stays the same?

Reply 511

Tommy59375

9

Original post by examjam

a capacitor of capacitance c has a charge of q stored on the plates. the pd between the plates is doubled. what is the change in energy stored by the capacitor?

Options: (0.5Q^2/C ; Q^2/C ; 1.5Q^2/C ; 2Q^2/C)

So when the pd is doubled, how do we know whether Q or C stay constant?

I know that if a particular dielectric is removed, leaving the plates separated only by air, therefore reducing permittivity, charge stays constant, pd increases.

But if pd is doubled, then what happens? does capacitance decreases and charge stays the same or charge increases but capacitance stays the same?

Capacitance stays constant. There are three ways you can look at it:

1) The definition for capacitance is “charge per unit PD between the plates” and from this you can deduce that if the PD changes then the charge will also change (so the capacitance will stay constant).

2) The equation for capacitance is
C = (A * E_0 * E_r) / d
where A is the intersecting area of the plates, E_0 is the permittivity of free space, E_r is the relative permittivity of the dielectric and d is the distance between the plates.
This is on the formula sheet.
The formula does not have a Q (charge) or V (PD) term in it so you can deduce that capacitance is independent of charge or PD.

3) Capacitors in the real world have a number of Farrads printed on the side of them. This is the capacitance of the capacitor. The number is a constant, for example you might have a “4700 micro Farad capacitor”. The PD across the plates or the charge could change but at the end of the day you still have a 4700 micro Farad capacitor.