(b) In an x-ray tube, an electron is accelerated from rest towards a metal target by a 30 kV source. Calculate the kinetic energy of the electron. [e=1.6 x \(10^{-19}\) C]
(c) The table below shows the frequencies of radiations incident on a certain metal and the corresponding kinetic energies of the photoelectrons.
(i) Plot a graph of kinetic energy, K.E, on the vertical axis and frequency, f, on the horizontal axis starting both axes from the origin (0,0).
i. Planck's constant;
ii. Threshold frequency of radiations;
iii. Work function of the metal.
(a)() Meaning of artificial radioactivity;
The process by which a stable nucleus is bombarded with a neutron to make it unstable and so disintegrates/decays with the emission of particles/radiation and. energy.
| Emission |
Nature |
Charge |
Ionizing |
| Beta (\(\beta\)) |
High speed electron |
Negative |
Moderately ionizing |
| Gamma (\(\gamma\)) |
Electro-magnetic radiation |
Neutral |
Negligible ionizing ability |
| Alpha particles |
Helium nucleus |
Positive |
Highly ionizing |
(b) The kinetic energy of the electron can be calculated using the formula: KE = qV, where q is the charge of the electron and V is the potential difference. Substituting the given values, we get:
K.E = eV
KE = (1.6 x 10^-19 C)(30,000 V)
KE = 4.8 x 10^-15 J (c)
(i)
To calculate the slope of the graph, you need to determine the change in the dependent variable (kinetic energy) divided by the change in the independent variable (frequency). In this case, you can choose any two points on the graph and calculate the slope using the following formula:
slope = (kinetic_energy2 - kinetic_energy1) / (frequency2 - frequency1)
Let's take two points from the given data, for example:
Point 1: (frequency1, kinetic_energy1) = (6.8 x 10^14 Hz, 0.8 x 10^-19 J)
Point 2: (frequency2, kinetic_energy2) = (8.0 x 10^14 Hz, 1.6 x 10^-19 J)
Now, we can calculate the slope:
slope = (1.6 x 10^-19 J - 0.8 x 10^-19 J) / (8.0 x 10^14 Hz - 6.8 x 10^14 Hz)
slope = 1 x 10^-5 J Hz^(-1).
To determine Planck's constant from the given graph and slope, we can use the equation:
slope = h / e
where h is Planck's constant and e is the elementary charge (1.602176634 x 10^-19 C).
From the previous calculation, the slope of the graph is 1 x 10^-5 J Hz^(-1).
Let's substitute the values into the equation to solve for Planck's constant:
1 x 10^-5 J Hz^(-1) = h / (1.602176634 x 10^-19 C)
To isolate h, we can rearrange the equation:
h = slope * e
Substituting the values:
h = (1 x 10^-5 J Hz^(-1)) * (1.602176634 x 10^-19 C)
Evaluating the expression:
h ≈ 1.602176634 x 10^-24 J·s
Therefore, from the given graph and slope, the approximate value of Planck's constant is 1.602176634 x 10^-24 J·s.
(ii) To determine the threshold frequency of radiation from the given information, we need to use the concept of the photoelectric effect and the relationship between the kinetic energy of photoelectrons and the frequency of incident radiation.
According to the photoelectric effect, electrons are ejected from a metal surface when illuminated by electromagnetic radiation of sufficient energy. The minimum frequency of radiation required to eject electrons is known as the threshold frequency.
The relationship between the kinetic energy of photoelectrons and the frequency of incident radiation is given by the equation:
K.E. = h * (frequency - threshold_frequency)
where K.E. is the kinetic energy of the photoelectrons, h is Planck's constant, frequency is the frequency of incident radiation, and threshold_frequency is the threshold frequency.
From the graph, we have the slope, which is equal to h, and the kinetic energy corresponding to each frequency. We can select any point on the graph where the kinetic energy is non-zero and solve for the threshold frequency.
Let's choose the point (frequency, kinetic energy) = (6.8 x 10^14 Hz, 0.8 x 10^-19 J) from the given data.
0.8 x 10^-19 J = slope * (6.8 x 10^14 Hz - threshold_frequency)
Substituting the slope value:
0.8 x 10^-19 J = 1.602176634 x 10^-24 J·s * (6.8 x 10^14 Hz - threshold_frequency)
To solve for the threshold frequency, we can rearrange the equation:
threshold_frequency = 6.8 x 10^14 Hz - (0.8 x 10^-19 J / (1.602176634 x 10^-24 J·s))
Calculating the threshold frequency:
threshold_frequency = 6.8 x 10^14 Hz - 4.992706701 x 10^4 Hz
threshold_frequency ≈ 6.799500729 x 10^14 Hz
Therefore, the threshold frequency of radiation is approximately 6.799500729 x 10^14 Hz.
(iii)
To determine the work function of the metal, we can use the equation:
Work function = h * threshold_frequency
where h is Planck's constant and threshold_frequency is the threshold frequency of radiation.
From the previous calculations, the approximate value of Planck's constant is 1.602176634 x 10^-24 J·s and the threshold frequency is approximately 6.799500729 x 10^14 Hz.
Substituting these values into the equation, we can calculate the work function:
Work function = (1.602176634 x 10^-24 J·s) * (6.799500729 x 10^14 Hz)
Work function ≈ 1.090589631 x 10^-9 J
Therefore, based on the given information, the approximate value of the work function of the metal is 1.090589631 x 10^-9 J.
