Physics Test - 37

Photoelectric cell or photocell, device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material.

The photoelectric effect is the observation that many metals emit electrons when light shines upon them. Electrons emitted in this manner can be called photoelectrons.

A particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.

The maximum velocity of an electron emitted from the photocell is given as 3.75×10⁸ cm/s. To find the stopping potential, follow these steps:

• Use the formula for kinetic energy: KE = 0.5mv².

• Convert the velocity into metres per second: 3.75×10⁶ m/s.

• Calculate the kinetic energy of the electron: 
  KE = 0.5 × 9.11×10^-31 kg × (3.75×10⁶ m/s)².

• Convert the kinetic energy into electron volts (eV): 1 eV = 1.6×10^-19 J.

• The stopping potential, V, is given by the kinetic energy in eV, which is approximately 40 volts.

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can be seen only when exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

In the alpha-particle scattering experiment, if a thin sheet of solid hydrogen is used in place of a gold foil, then the scattering angle would not be large enough. This is because the mass of hydrogen (1.67×10−27) is less than the mass of incident α−particles (6.64×10−27). Thus, the mass of the scattering particle is more than the target nucleus (hydrogen). As a result, the α−particles would not bounce back if solid hydrogen is used in the α−particle scattering experiment.

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.

To find activity of the sample --->which is the rate of disintegration.

Since radioactivity comes under 1^o kinetics.

[R]=k[A]     [A]-->amount of initial sample 10¹⁸ atoms

Given,

Half-life = 2days

K = 0.693/2x24x60x60 sec

R = (0.693/2x24x60x60) x 10¹⁸

R≈3.5x10¹² Bq

The force on a straight current-carrying conductor in a uniform magnetic field is F = I(L × B), where L is the length vector. The definition of the ampere is based on the force between two parallel current-carrying conductors (attractive when currents are in the same direction). For currents perpendicular to each other, the force is not necessarily zero (depends on the angle between I and B).

The magnetic dipole moment of a current-carrying loop is m = I A (vector perpendicular to the plane, magnitude I × area). Torque on a magnetic dipole in a uniform field is τ = m × B. Potential energy of a magnetic dipole is U = −m · B. The magnitude depends on the area enclosed, so it is not independent of loop shape if the area changes.

A moving coil galvanometer is used to detect and measure small electric currents. Its current sensitivity is defined as deflection per unit current (θ/I). To convert it into an ammeter (measure larger currents), a low-resistance shunt is connected in parallel. To convert it into a voltmeter, a high resistance is connected in series (not parallel).