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Photo-electricity in Physics

Posted by on May 27, 2012 in Physics | 0 comments

Photo-electricity in Physics is the emission of electrons from metal surfaces when exposed to electromagnetic radiations of high enough frequency. Photo-electricity in Physics is governed by certain laws. Engineering in Kenya has more information.

Laws of Photo-electricity in Physics

• The number of electrons emitted per second is proportional to the intensity of the incident beam of radiation.
• The photoelectrons are emitted with a range of electric energy from zero to maximum, which increases as the frequency of radiation increases and is independent of the intensity of radiation.
• For a given metal, there is a certain minimum frequency called threshold frequency (fo) in Photo-electricity in Physics below which no emission occurs irrespective of the intensity of radiation.

The intensity according to Plank occurs in integral multiples of quantum energy. The quantum energy (E) for radiation of frequency (f) is given by E = hf, where h is the plank’s constant.

To liberate an electron from the surface of a metal, a quantity called the work function in Photo-electricity in Physics which is the characteristic of the metal has to be supplied hence for emission to occur; the energy supplied must be greater than the work function i.e. hf > wo. Wo = hfo = occurs when a photon has just enough energy to liberate an electron.

Einstein’s Equation in Photo-electricity in Physics

Let hf be the quantum energy of light incident on a metal surface whose work function is wo. The maximum energy of the liberated electrons = hf – wo. So;

½mv2 = hf – wo;

hf = wo + ½mv2

Stopping Potential in Photo-electricity in Physics

When light falls on a cathode (c) and electrons are emitted, the potential difference between the cathode and the anode can be increased so that no electrons reach the anode (a). Such a potential difference is called the stopping potential (Vs) in Photo-electricity in Physics.    hf = wo + eVs, so Vs = (hf – wo)/ e. When Vs is plotted on a graph against f, the value of the gradient gives the plank’s constant and the x-intercept gives the threshold frequency.

X-rays for Photo-electricity in Physics

In the x-ray tube, cathode rays are produced by thermal emission (emission due to heating). The process takes place in a vacuum and the beam is focused on the target by a focusing cylinder. The target is made of tungsten which has a high melting point. The tungsten is fixed on a copper bar with external cooling fins. The maximum emission occurs at approximately 90o to the cathode ray beam in Photo-electricity in Physics. In all other directions, the tube is shielded with sheets of lead to avoid them leaking into the air since they are harmful.

The penetrating power of x-rays can be regulated by varying the potential difference across the tube. With a high potential difference the rate of penetrating of radiation is high. The intensity of the beam depends on the number of electrons striking the target per second i.e. current through the tube if V is the potential difference between the anode and the cathode, the electrons acquire energy equal to eV;

E = eV, where e = 1.6 ×10-19J

The energy of 1 x-ray photon is given by hf = eV.

Properties of x-rays in Photo-electricity in Physics

X-rays are electromagnetic radiations hence also posses the following properties;

• Rectilinear propagation and casting of shadows of obstacles.
• They travel with the speed of light.
• They obey the laws of reflection, refraction, diffraction and interference.
• They have a very short wavelength.
• They are not deviated by electric or magnetic fields i.e. they have no charge.
• They cause fluorescence in certain chemical components.
• They ionize the gas in which they pass.
• They penetrate through solids; I = Ioe-mx, where Io is the original intensity and I is the intensity after absorption through a material of absorbing power and thickness x.
• They have harmful effects on human tissue. When they fall on group 1 elements, they liberate electrons from the surfaces.
• They have both continuous and characteristic spectrum depending on nature of the target.

Conclusion on Photo-electricity in Physics

X-rays can be soft or hard. Hard x-rays carry a higher charge, and have a smaller wavelength thus are more penetrating.

 Wavelength ( cmג) E Soft 10-7 103 eV hard 10-10 106 eV

Values like e, plank’s constant, are constants for Photo-electricity in Physics. the other values like work function, threshold frequency, and stopping voltage depend on the materials being used in Photo-electricity in Physics.