# Physics 9702 Doubts | Help Page 118

__Question 597: [Electric field strength]__Upper plate has an electrical potential of +4.0 V, and lower plate has an electrical potential of – 4.0 V.

What is the force on each electron when between the plates?

A 3.2 × 10

^{–17}N downwards

B 3.2 × 10

^{–19}N upwards

C 6.4 × 10

^{–19}N downwards

D 6.4 × 10

^{–17}N upwards

**Reference:**

*Past Exam Paper – June 2014 Paper 11 Q28*

__Solution 597:__**Answer: D.**

Electric Force, F = Eq

Electric field strength, E = V / d

Potential difference, V = 4 – (-4) =
8V

The electron, which has a negative
charge, would be attracted upwards towards the positive plate.

Electric Force F = (V/d) q = [8 /
(2x10

^{-2})] x (1.6x10^{-19}) = 6.4x10^{-17 }N

__Question 598: [Electric field strength]__
The following particles are each accelerated
from rest through same potential difference.

Which one completes the acceleration
with

**greatest**momentum?
A Î±-particle

B electron

C neutron

D proton

**Reference:**

*Past Exam Paper – November 2007 Paper 1 Q40*

__Solution 598:__**Answer: A.**

Momentum = mv where m is the mass and v is the velocity

Of the 4 particles given, the Î±-particle has the largest mass since it is the
nucleus of a helium atom (

^{4}_{2}He). The Î±-particle is also doubly-charged.
Electric force = Eq = (V/d) q

This force provides an acceleration
[F = ma]. V and d may be taken to be the same for all cases. But, the higher
the charge, the greater is the force and hence, the larger is the acceleration
on that particle. A greater acceleration causes the final speed to be higher,
and so the momentum would be higher.

Comparing the Î±-particle and the
proton, both the mass and the charge of the Î±-particle are greater. So, the
momentum of the Î±-particle is greater than that of the proton. [D is incorrect]

The neutron is neutral and will not
even be accelerated by the potential difference. [C
is incorrect]

Again comparing the Î±-particle and
the electron, both the mass and the charge of the Î±-particle are greater. [B is incorrect]

__Question 599: [Quantum Physics]__**(a)**Explain how line spectrum of hydrogen provides evidence for the existence of discrete electron energy levels in atoms

**(b)**Some electron energy levels in atomic hydrogen are illustrated in Fig.1.

Two possible electron transitions
giving rise to emission spectrum are shown.

These electron transitions cause
light of wavelengths 654nm and 488nm to be emitted.

(i) On Fig.1, draw arrow to show a
third possible transition.

(ii) Calculate wavelength of emitted
light for transition in (i)

**(c)**Light in a beam has a continuous spectrum of wavelengths from 400nm to 700nm. The light is incident on some cool hydrogen gas, as illustrated in Fig.2.

Using values of wavelength in (b),
explain the appearance of the spectrum of emergent light

**Reference:**

*Past Exam Paper – November 2011 Paper 41 & 42 Q7*

__Solution 599:__**(a)**Each line represents a photon of specific energy. A photon is emitted as a result of energy change of an electron. The specific energy changes arise from changes between discrete energy levels.

**(b)**

(i) An arrow from the – 0.85eV level
to the –1.5eV level

(ii)

Change in energy Î”E = hc / Î»

Î”E = (1.5 – 0.85) x 1.6x10

^{-19}= 1.04x10^{-19}J
Wavelength Î» = hc / Î”E

Î» = [(6.63x10

^{-34})x(3.0x10^{8})] / (1.04x10^{-19}) = 1.9x10^{-6}m**(c)**The spectrum appears as a continuous spectrum crossed by dark lines. Only two dark lines are seen.

{Those 2 would correspond
to 654nm and 488nm. The 3

^{rd}one: 1900nm is not within the range: 400-700nm}
The electrons in the cool gas absorb
photons with energies equal to their excitation energies. Light photons are
re-emitted in all directions.

__Question 600: [Current of Electricity]__
In deriving formula for the combined
resistance of three different resistors in series, Kirchhoff’s laws are used.

Which physics principle is involved
in this derivation?

A the conservation of charge

B the direction of the flow of
charge is from negative to positive

C the potential difference across
each resistor is the same

D the current varies in each
resistor, in proportion to the resistor value

**Reference:**

*Past Exam Paper – June 2010 Paper 11 Q34 & Paper 12 Q35 & Paper 13 Q36*

__Solution 600:__**Answer: A.**

The physics principle involved in
deriving Kirchhoff’s laws for 3 different resistors connected in series is the

**conservation of charge**.
The flow of charge in a circuit is
from the positive terminal of the battery to the negative terminal. [B is incorrect]

Ohm’s law: V = IR. In series
combination, the p.d. across different resistors is not the same. This is
because the same current would flow through them as they are connected in
series [D is incorrect], but since their
resistance are different, the p.d. across each of them will be different from
Ohm’s law. [C is incorrect]

__Question 601: [Current of Electricity]__
Safety on railways is increased by
using several electrical switches.

In diagram, switches P, Q, R, S and
T control the current through a green lamp.

Which row does

**not**allow the green lamp to light?
P Q R S T

A closed
closed
closed
open closed

B closed
open closed closed open

C closed
open open closed closed

D open
open closed
open closed

**Reference:**

*Past Exam Paper – June 2011 Paper 12 Q36*

__Solution 601:__**Answer: C.**

We are asked which row does

**NOT**allow the green lamp to light.
When R is opened, current does not
flow through R, S and T. For the lamp to light, current should flow through P
and Q – that is, both P and Q should be closed. [A
is incorrect] For choice C, P is closed but Q is opened. So, current
does not flow and the lamp will not light. Thus, this is the correct choice.

When P is opened, for the lamp to
light, R should be closed, along with either S or T. Choice D involves this
situation, so the lamp will light. [D is incorrect]
Even if P is closed and Q opened, if R and either S or T are closed, the lamp
will light. [B is incorrect]

"Electric force = Eq = (V/d) q

ReplyDeleteThis force provides an acceleration [F = ma]. V and d may be taken to be the same for all cases. But, the higher the charge, the greater is the force and hence, the larger is the acceleration on that particle. A greater acceleration causes the final speed to be higher, and so the momentum would be higher." Quoted Q.598

Im curious about the F=ma, since charge is proportional to force, when charge increase because Alpha particles contains higher charge, hence force is greater. You stated that acceleration increase because of that, but don't you forget that mass changed as well? Alpha particles has 2n,2p,2e which will make your statement questionable.

Note that when talking about charge, I was referring to the force without considering the effect of mass. A greater charge would give a greater force, and thus a greater acceleration (without taking any account mass).

DeleteBut the momentum also depends on mass, so a greater mass would actually have a greater momentum than a smaller one.

Consider the following example where both the mass and charge are taken into account together.

Let the mass of alpha be 4u (it is a helium nucleus, so it contains 2 protons + 2 neutrons. Note that it does not have electrons). The mass of a proton would be u.

Acceleration a = F / m = Vq / md

For alpha: a = 2V / 4ud = 0.5 (V / ud)

For proton: a = V / ud

Speed v = u + at = 0 + at. For comparison, time t should be the same for both particles – so it can be neglected. The final speed is proportional to the acceleration.

For alpha: Momentum = mv = (4u) × 0.5 (V / ud) = 2 (V / d)

For proton: Momentum = mv = u × V / ud = V / d

So, momentum is greater for the alpha.