Tuesday, April 21, 2015

Physics 9702 Doubts | Help Page 118

  • Physics 9702 Doubts | Help Page 118


Question 597: [Electric field strength]
Horizontal beam of electrons is passed between two horizontal parallel plates, 2.0 cm apart, as shown.

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 α-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 α-particle has the largest mass since it is the nucleus of a helium atom (42He). The α-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 α-particle and the proton, both the mass and the charge of the α-particle are greater. So, the momentum of the α-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 α-particle and the electron, both the mass and the charge of the α-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 ΔE = hc / λ
ΔE = (1.5 – 0.85) x 1.6x10-19 = 1.04x10-19J
Wavelength λ = hc / ΔE
λ = [(6.63x10-34)x(3.0x108)] / (1.04x10-19) = 1.9x10-6m

(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 3rd 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]




3 comments:

  1. "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." 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.

    ReplyDelete
    Replies
    1. 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).

      But 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.

      Delete

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