CAIE A-Level Physics 22.1 Photon Energy and Momentum

Practise analysing photon quanta, calculating energy, wavelength, momentum and radiation force, and using power to find photon emission rate.

Exam points

distinguish particulate nature and photon quanta in electromagnetic radiation
calculate photon energy with E = hf or E = hc/? and convert between joules and electronvolts
use photon energy and power to determine photon emission rate
calculate photon momentum and radiation force using p = E/c and momentum change

Covered syllabus points

22.1.1—Electromagnetic radiation has a particulate nature 22.1.2—A photon is a quantum of electromagnetic energy 22.1.3—E = hf 22.1.4—The electronvolt (eV) as a unit of energy 22.1.5—A photon has momentum and that the momentum is given by p = E / c

[Maximum number: 2]

Define magnetic flux.

product of (magnetic) flux density and (cross-sectional) area

M1

direction of flux normal to (plane of the) area

A1

or

(magnetic) flux density × area × $\sin \theta$

(M1)

where $\theta$ is angle between direction of flux and (plane of the) area

(A1)

[Maximum number: 7]

(a)

State what is meant by a photon.

[ 2 ]

(b)

For an accelerating voltage of 32 kV in Fig. 8.1, determine:

[ 5 ]

(i)

the maximum momentum of an X-ray photon produced at the target

maximum photon momentum =

[ 2 ]

momentum =E / c

C1

momentum $=\left(0.032 \times 1.60 \times 10^{-13}\right) /\left(3.00 \times 10^{8}\right)$ $=1.7 \times 10^{-23} \mathrm{~N} \mathrm{~s}$

A1

(ii)

the minimum wavelength of X-rays produced at the target.

minimum wavelength =m [3]

[ 3 ]

E=h f and $\lambda=c / f$

C1

$\begin{aligned} \lambda =h c / E =\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(0.032 \times 1.60 \times 10^{-13}\right) \end{aligned}$

C1

$\lambda=3.9 \times 10^{-11} \mathrm{~m}$

A1

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Covered syllabus points

22.1 Energy and momentum of a photon question 1

22.1 Energy and momentum of a photon question 2

Question (a)

Question (b)

Question (i)

Question (ii)