H2 Physics Definitions

7/24/2019 H2 Physics Definitions

1/16

Shiv H2 Physics Definitions

1: Measurements

Base Unit: A base unit i s one of seven base units of the SI system related to a base quantity. Its

magnitude is defined arbitrarily and not by combinations of other units

Derived Unit:

Derived units are units of derived quantities and are expressed as products and/or

quotients of base units

Random Errors: Random errors produce readings that scatter about a mean value. These errors have

an equal chance of being positive (making the readings too large) or negative (making readings too

small). They can be reduced by taking more readings and averaging.

Systematic Errors: A systematic error will result in all the readings taken differing from the true value

by a fixed positive or negative amount. A systematic error can be eliminated only if the source of the

error is known and accounted for. It cannot be eliminated by averaging but by correct laboratory

practice.

Precision: Precision is the degree of agreement of repeated measurements of the same quantity.

Accuracy: Accuracy is the degree of closeness of repeated measurements of the same quantity to the

actual true value.

Scalar:

A scalar quantity is a quantity which only has magnitude.

Vector: A vector quantity is a quantity which has both magnitude and direction.

2: Kinematics

Distance: The length of a path followed by an object.

Displacement: The distance moved by an object in a specified direction from some reference point.

Instantaneous Speed: The rate of change of distance travelled with respect to time.

Average Speed: Distance travelled over time taken.

Instantaneous Velocity: The rate of change of displacement with respect to time.

Average Velocity: The change in displacement over time taken.


2/16

Instantaneous Acceleration: The rate of change of velocity with respect to time.

Average Acceleration: The change in velocity over time taken.

3: Dynamics

Newton's First Law:

A body continues in its state of rest or uniform motion in a straight line unless a

resultant force acts on it.

Newtons Second Law:The rate of change of momentum of a body is proportional to the resultant

force acting on the body and takes place in the direction of the force.

Newtons Third Law:

If body A exerts a force on body B, then body B exerts a force of the same type

that is equal in magnitude and opposite in direction on body A.

Linear Momentum: The product of the mass of an object and its velocity.

Impulse: Impulse is the change in momentum, defined as the area under a force-time graph.

Force:

Force is the rate of change of momentum.

Principle of Conservation of Momentum:

When bodies in a system interact, the total momentum of a

system remains constant unless a resultant external force acts on the system.

Thrust: The force that is exerted on an object by the expulsion or acceleration of mass in one

direction.

4: Forces

Hookes Law:

The extension of a spring (or wire) is proportional to the applied load if the limit of

proportionality is not exceeded.

Friction: The force that exists between any two surfaces in contact and is parallel to the surfaces.

Density: The density of a substance is defined as its mass per unit volume.

Pressure: The force per unit area, where the force is acting at right angles to the area.

Upthrust: The vertical upward force exerted on a body by a fluid when it is fully or partially

submerged in the fluid, due to the difference in fluid pressure.

Archimedes Principle:The buoyant force (upthrust) on a submerged object is equal to the weight of

the fluid that is displaced by the object.

Principle of Floatation:If a body floats, the upthrust acting on it is equal to its weight.


3/16

Translational Equilibrium:An object is in translational equilibrium if the resultant force (vector sum of

forces) acting on it is zero.

Rotational Equilibrium: An object is in rotational equilibrium if the resultant torque on it about any

point is zero.

Principle of Moments:For a body to be in equilibrium, the sum of all the clockwise moments about

any point must equal the sum of the all the anticlockwise moments about the same point.

Turning Effect of a Force: The moment or torque of a force about a point is defined as the product of

the force and the perpendicular distance from the point to the line of action of the force.

Couple:

A couple consists of a pair of equal and opposite parallel forces whose lines of action do not

coincide.

Torque of a Couple: The product of one force and the perpendicular distance between the two forces.

The resultant force is zero, as the two forces are equal and opposite.

Centre of Gravity:

The Centre of Gravity of a body is the point at which its weight or the resultant of

the distributed gravitational attraction on the body appears to act.

5: Work, Energy and Power

Work Done: The product of the force and the displacement in the direction of the force.

Work Done by a Variable Force: The area under a force-displacement graph where F and s are in the

same direction.

Law of Conservation of Energy: Energy cannot be created or destroyed, but it can be converted or

transferred from one form to another.

Power:

The rate of work done or rate of transfer of energy.

6: Circular Motion

Radian: The angle subtended by an arc length equal to the radius of the arc.

Angular Displacement: The angle the object moves through during a specified time interval,

expressed in radians.

Angular Velocity: The angular velocityof an object is the rate of change of its angular

displacement.

Period: The period of an object in circular motion is the time taken for it to make one complete

revolution.


4/16

Frequency: Frequency of an object in circular motion is the number of complete revolutions made per

unit time.

7: Gravitation

Newtons Law of Gravitation: It states that the force of attraction between two point masses is

directly proportional to the product of their masses and inversely proportional to the square of their

distance apart.

Gravitational Field: A gravitational field due to a mass is a region in space in which another mass

placed in the region experiences a force of attraction by the first mass.

Gravitational Field Strength: The gravitational field strength at a point is defined as the gravitational

force experienced per unit mass placed at that point.

Gravitational Potential Energy: The gravitational potential energy of a mass is defined as the work

done by an external agent in bringing the mass from infinity to its present location (without any

change in kinetic energy).

Gravitational Potential: The gravitational potentialat a point is defined as the work done per unit

mass by an external agent in bringing a small mass from infinity to that point.

Escape Velocity: The minimum velocity that an object requires to escape from a source of

gravitational field.

8: Oscillations

Periodic Motion: A periodic motion is one in which an object continually retraces its path at equal

time intervals.

Simple Harmonic Motion: The motion of the body whose acceleration is directly proportional to its

displacement from a fixed point (equilibrium position) and is always directed towards that fixed

position.

Angular Frequency: The angular frequencyis defined as the rate of change in the phase of a

sinusoidal waveform.

Free Oscillations: An oscillating system is said to be undergoing free oscillations if its oscillatory

motion is not subjected to an external periodic driving force. The system oscillates at its natural

frequency about the equilibrium position.

Forced Oscillations:

A system is said to be in forced oscillations if its forced to oscillate by an external

periodic force. [1m] The system is being driven by the external force which provides a means of

supplying energy to the system to maintain the oscillations. [2m] The external periodic force is usually


5/16

called a driving force or driver. The frequency of a forced oscillation is the same as the frequency of

the driving force. [3m]

Damping: The process whereby energy is taken from the oscillating system.

Resonance: Resonance occurs when a system responds at maximum amplitude to an external driving

force. This occurs when the frequency of the driving force is equal to the natural frequency of the

driven system.

9.1: Thermal Physics (Part 1)

Thermal Equilibrium:

When an object gains thermal energy from its surrounding objects at the same

rate as it loses thermal energy to them, then there will be no net thermal energy transfer between the

object and its surrounding objects. They are all now in a state called thermal equilibrium.

Kelvin:

1/273.16 of the temperature difference between absolute zero and the triple point of water.

Evaporation:

A change of state from liquid to gas that takes place at the surface of a liquid at any

temperature.

Heat Capacity: The quantity of thermal energy required to produce a unit rise in the temperature of a

body.

Specific Heat Capacity:

The quantity of thermal energy required per unit mass of the substance to

produce a unit rise in its temperature.

Specific Latent Heat of Fusion: The amount of thermal energy required to change the state of a unit

mass of the substance from solid to liquid without change of temperature.

Specific Latent Heat of Vaporisation:

The amount of thermal energy required to change the state of a

unit mass of the substance from liquid to gas without change of temperature.

9.1: Thermal Physics (Part 2)

Internal Energy:The internal energy U of a system is determined by the state of the system. It can be

expressed as the sum of all the microscopic kinetic (translational, rotational, and vibrational) and

potential energies of the atoms/molecules within the system. The kinetic energy is associated with

the random motion of the atoms/molecules while the potential energy is associated with the

intermolecular forces between molecules.

Ideal Gas: A gas which obeys the equation of state pV=nRT where p is the pressure of the gas, V is the

volume of the gas, and T is the thermodynamic temperature of the gas. It obeys this relationship for

all values of pressure, volume and temperature.

First Law of Thermodynamics: The increase in the internal energy of a system is equal to the sum of

the heat supplied to the system and the work done on the system and that the internal energy of a

system depends only on its state.


6/16

10: Wave Motion

Displacement: The distance in a specific direction of a particle of a wave from its equilibrium position.

Amplitude A: The magnitude of the maximum displacement.

Period T: The time taken for a particle of a wave to complete one oscillation. In the case of a

progressive wave, period is also the time required for the waveform to travel one wavelength.

Frequency f: The number of oscillations made by a particle of a wave per unit time.

Waveform: Curve showing the displacement of a wave at a given time.

Wavelength : The distance between two consecutive points which are in phase, such as two

successive crests or two successive troughs.

Speed v

: The speed at which the wave shape/profile moves.

Phase Difference :

The difference in stages of oscillation cycles between two waves at a point or

two points on a wave.

Wavefront: Line or surface joining points on a wave that are in phase. The wave travels in a direction

perpendicular to the wavefront.

Intensity:

The rate of transfer of energy per unit area normal to the direction of propagation of the

wave.

Transverse Wave: A wave in which its particles oscillate in a direction perpendicular to the direction

of propagation of the wave.

Longitudinal Wave:

A wave in which its particles oscillate parallel to the direction of propagation of

the wave.

Polarisation: Polarisation is a phenomenon whereby the oscillations of waves are restricted to a single

plane.

Polarised Wave:

A polarised wave is one whereby the particles vibrate in the same plane at all times.

Unpolarised Wave: An unpolarised wave is one whereby the vibration of the particles can take place

in any direction.

11: Superposition

Principle of Superposition: When two waves of the same kind meet at a point in space, the resultant

displacement at that point is given by the vector sum of the displacements due to each of the waves

at that point.


7/16

Diffraction: The bending of waves through an aperture or round an obstacle. It is a phenomenon of

waves.

Coherence: Wave sources are said to be coherent if they have a constant phase difference. This

implies that the sources must have the same frequency or wavelength. Velocities of the waves are

assumed to be identical.

Interference: The superposition of two or more coherent waves to give a resultant wave whose

resultant amplitude is given by the principle of superposition.

Path Difference: The difference in the distances that each wave travels from its source to the point

where the two waves meet.

12: Electric Fields

Coulombs Law: It states that the force of attraction between two point charges is proportional to

each of the charges and inversely proportional to the square of their distance apart.

Electric Field Strength: The electric field strength E at a point is defined as the force per unit charge

acting on a small positive test charge placed at that point.

Electric Potential: The electric potential of a point in an electric field is the work done per unit charge

by an external force in moving a small positive charge from infinity to that point (without a change in

kinetic energy).

13: Current of Electricity

Electric Current:

Rate of flow of charge.

Average Electric Current: Net charge that passes through a surface area A per unit time.

Electric Charge (Q): When a constant current I flows through a cross-section of a conductor for a

duration t, the amount of electric charge Q that passes through it is given by Q=It.

Coulomb (C): The amount of electric charge that passes through a point in a circuit in one second

when there is a constant current of one ampere (1 C = 1 A s)

Potential Difference (p.d.): The p.d. between two points in a circuit is defined as the amount of

electrical energy that is converted to other forms of energy when unit charge passes from one pointto the other.

Volt: One volt is defined as the potential difference between two points in a circuit when one joule of

electrical energy is converted to other forms when one coulomb of charge passes from one point to

the other (1 V = 1 J C

-1 ).


8/16

Electromotive Force (e.m.f.):

The e.m.f. of a source is defined as the amount of electrical energy that

is converted from other forms of energy when the source drives unit charge through a complete

circuit.

Resistance:

The resistance of a conductor is defined as the ratio of the p.d. across it to the current

flowing through it.

Ohm: One ohm is defined as the resistance of a conductor when a p.d. of one volt across it causes a

current of one ampere to flow through it. (1= 1 V A -1).

Ohm's Law:

Ohm's law states that the p.d. across a conductor is directly proportional to the electric

current passing through it, provided that its temperature remains constant.

14: DC Circuits

Potential Divider: An arrangement of two or more resistors connected in series, across an applied

potential difference (p.d.) with the output p.d. V0 being taken from across a resistor. The potential

divider is used to produce an output voltage that is a fraction of the voltage supply.

15: Electromagnetism

Magnetic Flux Density: The force per unit length that acts on a wire carrying unit current lying

perpendicular to the magnetic field.

Tesla T: The magnetic flux density of a uniform magnetic field when a wire of length 1m, carrying a

current of 1A, placed perpendicular to the field, experiences a force of 1N in a direction perpendicular

to both the field and the current.

Field of Force: A region of space within which a force is experienced by a body placed in the field

16: Electromagnetic Induction

Faraday's Law of Electromagnetic Induction: States that the induced emf is directly proportional to

the rate of change of magnetic flux linkage or to the rate of cutting of magnetic flux.

Magnetic Flux: The magnetic flux through an area is defined as the product of the area and the

magnetic flux density that is perpendicular to the area.

Magnetic Flux Linkage: The magnetic flux linkage through the coil is the magnetic flux through each

turn multiplied by the number of turns of the coil.

Weber (Wb): One weber is defined as the magnetic flux through an area of 1m2when the magnetic

flux density perpendicular to the area is 1T.


9/16

Lenz's Law:

States that the induced current, if any, always flows in a direction so as to oppose the

change that produces it.

17: Alternating Currents

Period (T):

Period of AC is the time taken for one complete cycle.

Frequency (f): Frequency of AC is the number of complete cycles per second.

Peak Current (I0):Peak current of AC is the maximum value of the current.

Root-Mean-Square Current: The rms current is defined as the value of the steady current which

would deliver the same power to a given resistance as the alternating current. (For rms voltage,

replace the word current with voltage)

Rectification: Rectification is the process of changing an alternating current to a direct current.

18.1: Quantum Physics (Part 1)

Photon: A photon is a packet or quantum of electromagnetic energy.

Photoelectric Effect: Photoelectric effect is the phenomenon in which electrons are emitted from a

metal surface when light of sufficiently high frequency is incident on the surface.

Threshold Frequency: The threshold frequency is the minimum frequency of incident radiation that

gives rise to the ejection of electrons from a particular surface, below which no electrons are emitted

regardless of how intense the light is. Every metal has a characteristic threshold frequency.

Work Function: The work functionof a metal is the minimum amount of the energy necessary to

remove a free electron from the surface of the metal. This is related to the threshold frequency f, by

=hf0

Stopping Potential: The stopping potential Vs is the potential of the collector with respect to the

emitter (the target metal) that is just sufficient to stop the most energetic photoelectrons from

reaching the collector in a photoelectric experiment.

Wave-Particle Duality: Wave-particle duality describes the behaviour of electromagnetic radiation or

electrons as they have a dual nature, exhibiting both wave-like as well as particle-like characteristics.

Distinguish between emission and absorption line spectra

Emission line spectrum consists of a series of discrete bright lines of definite wavelength on a dark

background, whereas absorption line spectrum is a series of discrete dark lines on a continuous visible

spectrum (bright background).

Explain the X-Ray Spectrum


10/16

When high speed electrons decelerate due to collision, some of the loss in KE emerges as energy of

X-Ray photons emitted. Different electrons are slowed down by different extents over multiple

collisions. As braking is a continuous process, X-rays of continuous spectrum is obtained.

When an electron collides into an electron of a target atom orbiting in the K shell, the orbital electron

is ejected from the atom if sufficient energy is transferred (enough to completely remove an electron

from that shell).When the vacancy in the K shell is filled by an electron dropping from the L shell, the electron's loss in

energy is emitted as an X-ray photon of characteristic wavelength K-alpha. When filled by e from M

shell, it is K-beta.

Minimum wavelength corresponds to an electron losing all its kinetic energy in a single collision with a

target atom, emitting a photon of maximum energy with minimum wavelength.

Why is the KE of a photoelectron independent of intensity of incident light?

The intensity of the incident light determines the number of photons incident on a target metal per

unit time. However, the energy of each incident photon depends only on the frequency of the light.

Each photon is absorbed as a discrete quantity. Thus more photons incident on the target metal does

not affect the energy absorbed by each photoelectron. Hence, kinetic energy of a photoelectron is

independent of the intensity of the light.

Observations from Photoelectric effect experiment which prove particulate nature of

electromagnetic radiation

1. Existence of a threshold frequency below which no photoelectric emission occurs.

2. Maximum kinetic energy of the emitted photoelectrons is independent of the intensity of the

radiation.

3. Maximum kinetic energy of the emitted photoelectrons is dependent on the frequency of the

radiation.

4. Photoelectric emission takes place instantaneously

According to the classical wave theory,

1. Photoelectric effect should occur for any frequency of light, provided that the light intensity is

sufficient to supply energy to eject photoelectrons.

2. Kinetic energy of the photoelectrons should increase as the light beam is made more intense

3. Electrons should absorb energy continuously from the electromagnetic waves. As the light intensity

increases, energy should be transferred into the metal at a higher rate and the photoelectrons should

be ejected with more kinetic energy

4. There should be a measurable time lag for the electrons to absorb enough energy in order to

escapeThe above prediction by classical wave theory is in contradiction with the observed experimental

results.

Hence, the observation provides evidence for the particulate nature of electromagnetic radiation.

18.2: Quantum Physics (Part 2)


11/16

Heisenberg Uncertainty Principle:

If a measurement of the position of a particle is made with

uncertaintyx and a simultaneous measurement of its x-component of momentum is made with

uncertaintypx, the product of the two uncertainties can never be smaller than h/4, i.e.xpx

h/4.

Wave function:

A wave function(x) is the solution of the Schrdinger Equation and can be used to

describe the quantum state of a particle.

Do the uncertainty principles arise because of imprecision in measuring instruments?

No. Heisenberg was careful to point out that the uncertainty is not due to the measuring instruments

but due to the fundamental, intrinsic nature of matter.

Does the position-momentum uncertainty principle imply that a precise measurement of a

particle's position is impossible?

No. It just states that the more precisely the position of a particle is determined, the less precisely themomentum is known in this instant, and vice versa.

How can one get the wave functionof a body?

In quantum mechanics, the wave function can be obtained by solving the Schrdinger's equation for

the specified conditions.

Does the wave functionhave any physical meaning?

No. It is just a mathematical representation that scientists use to describe the state of the quantum

particle and it can have real and complex parts. However, the absolute square of the wave function||2 is always real and has a real physical meaning.

What is the physical meaning of ||2

||2 is the probability density function of the position of the particle it describes. That is,|(x)|2

dx gives the probability of finding the particle between x and x + dx. NOTE: in H2 syllabus, ||2is

taken to give the probability of finding a particle at that point.

What is the meaning of normalizing a wave function?

A wave function that obeys||2dx = 1 (from -to +) is said to be normalized, which implies

that the particle exists at some point in space.

What is a potential barrier?

A potential barrier refers to a potential energy distribution whereby classically a particle of lower

energy is unable to pass through, but quantum mechanically there is a probability that the same

particle can pass through the barrier and appear on the other side.


12/16

What is quantum tunneling?

Quantum tunneling is the quantum mechanical phenomenon where a particle tunnels through a

potential energy barrier that it classically would not be able to overcome. In classical physics, there

are regions in space that a particle is unable to enter due to insufficient energy, whereby the total

energy of the particle is less than the potential energy of the particle if it were to enter the region. In

quantum mechnaics, however, if two regions of low potential energies are separated by a classically

fobidden region (potential barrier), there is a non-zero probability that a particle can cross over the

forbidden region as long as its wave function is non-zero in the two low potential regions. This is

known as quantum tunnelling.

How does a Scanning Tunnelling Microscope work?

The gap between the STM probe tip and the sample surface acts like a potential barrier to the

electrons. When a p.d. is applied between the probe and the sample, there is a finite probability that

electrons can tunnel through this potential barrier due to the wave nature of the electrons eventhough the electron does not have sufficient kinetic energy. The probability of tunneling is Te-2kd

where d is the width of the potential barrier. Thus tunneling current varies exponentially with the

distance of gap d between the probe and the sample surface. As probe scans across the surface, the

distance d changes and the variation in tunneling current can be detected and used to plot the

topography of the sample surface.

What are the parameters that affect the transmission coefficient in a quantum tunneling process?

The height of the potential barrier, the width of the barrier, and the kinetic energy of the particle.

19: Lasers and Semiconductors

What are the characteristics of laser light?

Laser light is highly coherent, unidirectional (or collimated) and monochromatic.

Metastable State: The state in which the electrons remain longer than usual so that the transition to

the lower state occurs more likely by stimulated emission than by spontaneous emission.

Explain the difference between spontaneous emission and stimulated emission of radiation from

atoms.

In spontaneous emission, a photon is emitted by an atom randomly and in any direction, without any

external stimulation, whereas in the case of stimulated emission, an incoming photon, whose energy

is exactly equal to the excitation energy of the atom, induces the excited atom to fall to a lower

energy level and releases a photon in the process. This photon released is similar to the one which

induces its emission. The two photons are emitted at the same time and in the same direction.

Laser Principle


13/16

Atoms in a laser medium. Are excited to a metastable state. This continues until number of excited

atoms in the metastable state is more than that in the lower energy state, achieving population

inversion. One of the atoms in the metastable state de-excites to the lower energy level, producing a

photon, which then causes other atoms in the metastable state to be de-excited to the same lower

energy level, and produce photons of the same phase, frequency and direction as the photon causingstimulated emission. These photons are reflected back and forth to cause more stimulated emission in

other metastable atoms.

What is meant by a "population inversion" of atoms ?

Population inversion is a situation in which for a given sample of atoms of the same element, there

are more atoms in the excited state than in the ground state.

Why is a "population inversion" necessary in the construction of a laser?

"Population inversion" of atoms in the metastable state is necessary in order that the photons

produced by the first stimulated emission events can further trigger more excited atoms to produce

identical photons. In this way, a chain reaction of stimulated emissions can occur to build up a laser

beam of a sufficiently high intensity.

Explain the use of the two mirrors placed at the ends of a laser.

The mirrors at the ends of the laser face each other and are parallel. They reflect the photons

produced by stimulated emission back into the lasing medium and along the axis of the laser cavity so

that more stimulated emissions can occur to produce the laser beam. One mirror is totally reflecting

while the other is partially reflecting in order to allow the laser beam to emerge.

Distinguish between intrinsic and extrinsic semiconductors.

Intrinsic semiconductors are pure semiconductors, whereas extrinsic semiconductors are those which

are doped with other elements.

P-N Junction

Due to the concentration gradient, electrons diffuse from the n junction to the p junction while holes

diffuse from p to n junction. The electrons and holes in the region tend to recombine and the region

becomes depleted of mobile charge carriers. This region is the depleted region. The n side is positive

now while the p side is negatively charged, so the immobile charged ions create an electric field

across the region. At equilibrium, the electric field set up in depletion region is strong enough to

prevent further diffusion of electrons and holes. This potential difference results in a potential energy

barrier/contact potential.

In forward bias, the p side is now made positive while the n side is made negative. Electrons from the

n side can cross over to the p side and vice versa for holes. The depletion region is narrowed, the

potential energy barrier reduced, resistance falls, and there is high current conduction.


14/16

In reverse bias, the p side is now even more negative while the n side is more positive. Electrons and

holes are pulled away from the junction, widening the depletion region, increasing potential energy

barrier, resistance is very high and there is negligible current.

N-Semiconductor

In this semiconductor, the Si atoms are doped with a small amount of element of valency 5. When

bonded to 4 surrounding Si atoms, the extra electron is only loosely bound to the P ion core. It

occupies a localised energy state that lies within the energy gap, just below the conduction band.

Electrons here only require 0.05eV to reach conduction band, thus this readily occurs at r.t.. It is now

mobile and carrying charge. Electrons are the majority charge carriers.

P-Semiconductor

In this semiconductor, the Si atoms are doped by a small amount of element of valency 3. It can only

bond with 3 surrounding Si atoms, so there is a hole in the last Al-Si bond. An electron can be torn

from a neighbouring bond to fill the hole, with a small expenditure of energy, allowing the hole to

migrate. This electron occupies a localised energy state within the energy gap. Valence electrons are

easily bumped up across this small energy interval to the acceptor level, leaving holes in the valence

band. Holes are the majority charge carriers.

20.1: Nuclear Physics

Mass Defect: The amount by which the mass of a nucleus is less than the sum of the masses of its

constituent nucleons.

Binding Energy of a Nucleus:

The amount of energy needed to break a nucleus into its constituent

nucleons.

Nuclear Fusion: The combining of two or more lighter nuclei with very high energies to form a heavier

nucleus.

20.2: Nuclear Physics (Radioactivity)

Radioactivity

: The random and spontaneous decay of an unstable nucleus to a more stable nucleus by

the emission of particles and/or radiation.

Random:Radioactive decay is random because it is completely unpredictable which nucleus or when

a particular nucleus will decay in a certain time interval.

Spontaneous: Radioactive decay is spontaneous because it is not affected by any environmental

factor such as temperature and pressure.

Decay Constant: The decay constant of a radioactive substance is the probability of decay of a nucleus

per unit time.


15/16

Activity A:

The rate of radioactive decay.

Half-Life:The half-life of a radioactive substance is the time it takes for half of a given number of

radioactive nuclei to decay.

Extra: More Detailed Semiconductors Explanations:

What is meant by "doping" of a semiconductor ? Hence how may i) a p type semiconductor be

obtained ? ii) an n type semiconductor be obtained ? Give one example for each of the doped

semiconductors.

Doping is a process in which a small number of atoms of an element of valency 3 or 5 are added into

the lattice of silicon (or germanium) with the purpose of increasing the number density of free charge

carriers which will ultimately increase the electrical conductivity of the semiconductor. Example : An

impurity concentration of 1 part in 108 increases the conductivity of germanium by a factor of 12 at

room temperature.

A p-type semiconductor would be formed if silicon is doped with aluminium (valency 3) and an n-type

semiconductor would be obtained if silicon is doped with phosphorous (valency 5). In the p-type

material, the number density of holes is significantly increased, thus making holes the majority

carriers. In the n-type material, the number density of electrons is increased and these are the

majority carriers.

One way of understanding how hole number density outnumbers electron number density (as is the

case for p-type material) is that since aluminium atoms have only 3 valence electrons to form covalent

bonds with 4 neighbouring silicon atoms, there will be a missing electron in one of the 4 covalent

bonds. This missing electron is a hole which can accept an electron from another covalent bond. Thus

compared with pure silicon, there are now more holes in the lattice. Another way of understanding

this effect is by using the band theory --- see S9.

Compare the electrical conductivities of metals and semiconductors using the "band-theory" of

energy levels.

In a metal, the highest occupied energy band is not full. Hence, electrons in this band can easily gain

just a little energy to occupy other empty levels within the same band. In other words, the electrons

can easily move through the lattice when a potential difference is applied. However, in a

semiconductor such as silicon, the highest occupied band (called the valence band) is completely filled

at low temperatures. Thus electrons within the same band cannot move. The next available band,

called the conduction band has plenty of empty levels and is separated from the VB by an energy gap

of only 1.2 eV. Electrons at the top of the VB can easily gain 1.2 eV of energy to cross the gap over to

the CB. In doing so, an equal number of holes will be created in the VB. Thus both electrons in the CB

and holes in the VB contribute to current flow if a potential difference is applied across thesemiconductor. The conductivity of a semiconductor therefore depends very much on the number

density of charge carriers (includes both holes and electrons) and this number naturally increases with

temperature. As the number density of charge carriers (electrons) in a metal is very high (~1029 m3

for copper) compared with that in an intrinsic semiconductor (~1016 m3 for silicon), the electrical

conductivity of metals is much higher than that of semiconductors.


16/16

In terms of "band theory", why is the electrical conductivity of a doped semiconductor much higher

than that of a pure semiconductor ?

In the case of n-type silicon, the presence of dopant atoms introduces donor states within the energy

gap lying very close to the bottom of the conduction band. Electrons from the donor states easily gain

the small amount of energy required to go into the CB. (example: Ed is 0.05 eV below the CB for

silicon doped with phosphorous). At room temperature, typically all the electrons from the donorstates are in the CB. Thus the number density of charge carriers is significantly increased compared

with that of the intrinsic semiconductor.

Similarly in the case of p-type semiconductor, the presence of dopant atoms introduces acceptor

states into the energy gap and they lie very close just above the top of the valence band. Electrons

from the top of the VB readily gain the small amount of energy required to go into the acceptor

states, thus creating holes in the VB. As a result, the number density of charge carriers in the p-type

material increases and this explains why the conductivity increases.

What is meant by the "depletion layer (or zone)" in a p-n junction ? Explain its formation.

When a p-type semiconductor is jammed against an n-type semiconductor forming a plane junction,

electrons from the n-side close to the junction will diffuse over to the p-side because there are fewelectrons there. In so doing, positive charge accumulates on the n-side. Similarly, holes from the

p-side close to the junction will also diffuse to the n-side, causing negative charge to appear on the

p-side. The electrons diffusing from the n-side can recombine with holes on the p-side and results in a

loss of free electrons and free holes (the charge carriers in both types of materials) over a certain

width at the p-n junction. The appearance of positive charge on the n-side and negative charge on the

p-side produces an electric field directed from the n-side to p-side and we can associate this electric

field with a potential difference across the junction. The diffusion of charge carriers across the

junction will eventually stop when the potential difference is high enough to prevent further

movement of charge carriers across the junction. The region over the p-n junction in which there are

no free charge carriers is called the depletion layer (or zone).Why does a p-n junction conduct electricity predominantly in only one direction?

In forward bias, since the p-side is made more positive than the n-side, the height of the contact

potential V0 is reduced and hence more of the majority charge-carriers can now surmount this

smaller barrier. Thus the diffusion current increases markedly and this explains why current flows

from the p-side to the n-side during forward bias. In reverse bias, the p-side is made negative with

respect to the n-side. This causes the contact potential to increase and no electrons can move from

the n-side to the p-side. In other words the majority charge carriers cannot flow through the junction.

The minority charge carriers can flow across the junction, but because their number is very small, the

resulting current is negligible. Thus the p-n junction only conducts current predominantly in one

direction, that is; from the p-side to the n-side during forward bias.

Documents

H2 Physics Definitions