Download Definitions IB Physics All Topics 2015-17

85 Definitions
Topic 2
1. Distance is a scalar ( magnitude only) quantity and is the total
distance of the path taken and depends on the path taken. Displacement
is vector quantity ( direction and magnitude) and is a measure of the net
distance traveled and does not depend on the path taken.
Scalar quantities only measure magnitude. For example: You are going
40 km/ h.
Vector quantities have both magnitude (“how much” or “how big”) and
direction .
2. Speed is a scalar quantity and equal to total distance over time : v = s
t
3. Velocity is a vector quantity and equals total displacement over time :
v= Δs
Δt
4. Free Fall
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5. Newton’s Laws of Motion
NEWTON’S FIRST LAW OF MOTION
An object at rest tends to stay at rest and an object in motion tends to remain in motion
with constant velocity, unless acted on by an unbalanced force (a non-zero net force).
OR
If the net force acting on a body is equal to zero, the body will move with constant
velocity and zero acceleration.
NEWTON’S SECOND LAW
The acceleration of an object is directly proportional to the net external
force acting on it and is inversely proportional to its mass. The direction of
acceleration is in the same direction as the net force acting on the object
NEWTON’S THIRD LAW
Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first.
OR
If body A exerts a force on body B (an “action”) then body B exerts an equal and opposite
force on body A (a “reaction”). These two forces have the same magnitude but opposite
direction.
2
6. Translational equilibrium occurs when a body is not accelerating. That
is, the net force acting on the body is zero.
7. Inertia : The tendency of a body to maintain its state of rest or in uniform
motion in a straight line is called inertia. Hence Newton’s First Law is often called
the law of inertia.
8. The contact force that acts perpendicular to a common surface of contact is
usually referred to as the Normal Force (‘normal’ means perpendicular) and
labeled F or
FN
in diagrams.
9. Change in momentum based on force and time is called impulse.
Impulse changes momentum in much the same way that force changes velocity
causing acceleration.
Formulas - Units
Linear momentum ( p) is defined as the product of an object’s mass and its
velocity.
p  mv
(Linear momentum defined)
10. ELASTIC COLLISIONS
Net momentum before collision = Net momentum after collision
Total Kinetic Energy is conserved.
Law of Conservation of Momentum and Collisions
The total momentum of an isolated system of bodies remains constant (i.e. when
no external forces act on a system, the total momentum of the system stays the
same).
pbefore  p after
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11. Inelastic Collisions
In an inelastic collision KE is conserved. The law of conservation of energy still
holds true.
Completely Inelastic Collision ( v
 v A  vB )
A completely inelastic collision is a collision where the two bodies stick and move
together as one after the collision. e.g. a bullet imbedding itself in wood.
mAu A  mBuB  mA  mB v
(Completely inelastic collision)
NOTE: FORMULA NOT IN DATA
12. Work in physics is given a very specific meaning to describe what is
accomplished by the action of a force when it acts on an object as the object
moves through a distance.
Work is defined to be the product of displacement and the component of the
force parallel to the displacement.
When a constant force acts in the same direction as displacement the work done
by the force is:
Work = Force x Displacement
W  Fs
13. Work due to gravity is independent of path followed = mgh
Work done BY gravity on an object that is moving horizontally ( by another force),
is zero because the angle at which gravity acts is completely vertical with no
horizontal component i e perpendicular to the displacement, ( 90 0 ) .
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14. Energy is one of the most important concepts in science. By definition ,
energy is the ability to do work. Kinetic energy is the energy of motion. The
work – energy theorem relates the work done on an object to the kinetic
energy ( K) of that object:
The net work done on an object by all forces acting on it is equal to the
change in kinetic energy of the object.
W = ΔEk CHANGE in Ek
( unit: Joules J = N m)
15. Kinetic Energy
* Brainpop:
Kinetic Energy
Kinetic energy is the energy associated with a body in motion.
Kinetic Energy is:
E
1 2 p2
mv 
2
2m
( E k Kinetic Energy defined)
Note:
16. Potential Energy (Stored Energy)
*
Potential Energy is energy associated with the position of a system (not its
motion).
There are three types of potential energy:
Gravitational potential energy (PE of a diver is converted into KE as she falls).
Elastic potential energy (energy stored in the diving board as the diver jumps on
it or energy stored in a compressed spring).
Electrical potential energy (covered later).
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17. Gravitational Potential Energy
Gravitational potential energy is defined as the energy a body has because of its
height relative to a given point.
EP = mgh ( mgy )
(Gravitational potential energy defined)
Gravitational Potential Energy Difference
Experimentally we can only quantify changes in potential energy. Gravitational
potential energy difference is defined as the work that must be done by an
external force to move an object through a vertical displacement Δh. The work
done by the external force is stored as potential energy.
Δ EP = mg Δh (Gravitational potential energy difference)
Conservation of Energy is one of the most important laws in Physics.:
Energy can not be created or destroyed; it may be transformed from one
form into another, but the total amount of energy of a system never changes.
18. The work- energy theorem applies to changes in kinetic and
potential energy that can also be used to explain changes in thermal,
mechanical , electrical and nuclear energy as well. It is important to
understand how energy changes or transforms form one form to another or
from one location to another.
Drawing:
Conservation of Mechanical Energy is:
E  Ek  Ep = Constant
(Mechanical energy is conserved when only gravity does
work)
For example, when a ball is thrown vertically Ek is converted to Ep; and on the
way down Ep is converted back to Ek. But E is always constant (provided only
gravity does work i.e. no air resistance
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19. Power
* Brainpop: Power
Definition of work has no reference to time but it is often necessary to know how
quickly work can be done. Power is defined as the rate at which work is
performed.
Power = work (or energy)  time
P=W
t
Note:
Power is a scalar quantity.
20. Efficiency
Efficiency is defined as the amount of useful work performed per amount of
available energy.
Efficiency 
Usefuloutput
Efficiency = Power output
Totalinput
21. The centripetal acceleration ( ac ) of an object in uniform
circular motion is NOT in the same direction as the tangential velocity vector.
If it were, the object would accelerate and motion would not be uniform.
Therefore, centripetal acceleration is a good example of how you can cause
acceleration just by changing direction without changing speed.
As a matter of fact, the centripetal acceleration is directed towards the center.
This centripetal force ( Fc causing the acceleration) causes the velocity vector to
continuously change direction thereby maintaining uniform circular motion. The
centripetal force is always perpendicular to the direction of motion and does no
work. Therefore, there is no change in kinetic energy and no change in speed
(energy-work theorem) but a change in velocity.
If there were no centripetal force the object would move in a straight line.
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22. Newton’s Universal Law of Gravitation
Every point mass of matter in the universe attracts
every other point mass with a force that is proportional
to the product of the masses of the two particles and
inversely proportional to the square of the distance
between them.
Translating this into an equation, we have:
Fg 
GMm
r2
(Newton’s Universal Law of Gravitation)

Point mass assumption: If the separation between
the two objects is large compared to their radii we
can treat spherical objects as point masses particles with all their mass concentrated at the
center (Figure 12-2 Young and Freedman 2000 p.359) and r =
distance between the two centers of the
spheres.

Gravitational forces always act along a line joining
the two particles (Figure 12-1 Young and freedman 2000; 359).

Even when the masses of the two particles are different, the two
interacting forces have equal magnitude (and form an action-reaction pair
Newton’s 3rd Law).
Don’t confuse g with G. ^ CP DVD - Jolly’s Method of measuring of G

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23. Gravitational field strength ( g) and gravitational force (F)
definition :
Physicists wondered how a mass knows the presence of another mass nearby
that will attract it. They developed the idea of a gravitational field. A mass M is
said to create a gravitational field in the space around it. This means that when
another mass ( m) is placed at some point near M it feels the gravitational field.
The gravitational field strength ( g) at a certain point is the force per unit
mass ( F/m) experienced by a small point mass m…. at that point :
g= F
m
=
g = GM
R2
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TOPIC 3 THERMAL
24. Temperature is a measure of the ‘hotness’ (or ‘coldness’) of a
substance. More specifically, it is a measure of the average kinetic energy
of individual atoms and/or molecules. To measure temperature we need a
property that changes as the ‘hotness’ changes. The most commonly used
property is linear expansion e.g. mercury (or alcohol) thermometer.
25. Internal Energy
The internal energy is the total kinetic energy of the molecules, plus any
potential energy between the molecules. The kinetic energy of the molecules
arises from their random translational (linear) and rotational motion. The
potential energy arises from the forces between the molecules. Discuss pushing
a block along a rough horizontal surface at constant velocity (Hamper and Ord
2007; 52-53). Is any work being done on the block? Is the KE or PE increasing?
Where is the energy going?
26. Thermal Energy (Heat)
*
Brainpop: Heat
Thermal energy (or heat) is energy that is transferred from one body to
another because of a difference in temperature. Heat always flows from hot
to cold, never in the reverse. Heat continues to flow from the warmer object to
the cooler object until they reach thermal equilibrium. Heat is transferred by three
methods: conduction, convection, and radiation.
Thermal equilibrium is reached when two bodies are at the same
temperature
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27. Specific Heat Capacity
Thermal Capacity
If heat is added to an object its temperature rises, but the actual increase in
temperature depends on the materials that make up the object. Thermal
capacity (C) is defined as the amount of thermal energy (heat) required to raise
the temperature of an object by 1K (1Cº). Units: [J K-1 = J Cº -1].
C
Q
T
(Thermal capacity )
Calorimetry I: Specific Heat Capacity
The amount of heat Q required to change the temperature of a given material is
proportional to the mass m of the material and to the temperature change T.
Q  mcT
Specific heat capacity ( c) is defined as the
amount of thermal energy (heat) required to raise the
temperature of 1kg of a substance by 1K (1Cº). Specific heat
capacity c is an intrinsic/characteristic property of the substance.
SI unit: [J kg-1 K-1]
c
Q
mT
( Specific Heat Capacity )
Note:
12
28. Phase change
During a phase change additional heat energy is used to overcome forces
between the molecules. When matter changes state energy is needed to
enable the molecules to move more freely and thus molecules gain potential
energy. The kinetic energy, and hence the temperature, remains the same.
After all the substance has been melted/vaporized the temperature will rise again
with additional heat input.
Calorimetry II: Specific Latent Heat
Specific Latent Heat of Fusion (Lf) is the heat required to change 1kg of a
substance from solid to liquid. Specific Latent Heat of Vaporization (Lv) is the
heat required to change 1kg of a substance from liquid to vapor. Specific latent
heat is an intrinsic property of the substance
29. Evaporation
Molecules of a liquid move about with various random speeds (and kinetic
energies). The faster, and hence more energetic molecules, can escape the
surface of the liquid and become a gas. This phenomenon is known as
evaporation. While boiling occurs throughout the liquid, evaporation only occurs
at the surface. The average kinetic energy of the molecules left behind, and
hence the temperature of the liquid, is reduced.
Evaporation occurs at temperatures lower than the boiling point and the values of
latent heat increases slightly with a decrease in temperature. For example at
20°C, Levap for water is 2.45 x 106 J/kg compared to 2.26 x 106 J/kg for Lv at
100°C.
The rate of evaporation (i.e. the number of molecules escaping per unit time) can
be increased by:

increasing the surface area (more molecules near the surface increases
the chance to escape);

increasing the temperature of the liquid (more KE enables more molecules
to escape);

increasing airflow above the liquid (removing escaped molecules allows
more to escape).
Does not depend on depth of liquid because evaporation basically occurs
at the surface
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30. The Kinetic Model for an Ideal Gas
This model makes assumptions about the molecules of a gas, which reflect a
simple view of a gas, but nonetheless correspond well (i.e. allows for accurate, verifiable
predictions) to the essential features of a real gas at low pressure and far from
liquefaction (condensation) point. The assumptions are summarized by the
following four points:

There are large numbers of molecules N, each with mass m, moving in
random directions at a variety of speeds;

The molecules are spread out, such that their average separation is
much greater than the average diameter of each molecule, hence the
volume of the molecules is negligible compared with the volume of the gas
itself;

The molecules only interact with and exert forces on each other when
they collide, therefore molecules move with constant velocity between
collisions;
: the forces are only important when the molecules are colliding

Collisions with each other and the walls of the container are assumed to
be perfectly elastic (i.e. momentum and kinetic energy are both
conserved).
31. Gas Pressure is defined as the force per unit area exerted on the walls
of the container ( note : not between gas particles) as a result of particle
collisions with the walls.
The force exerted by molecules is equal to the rate of change in momentum F =
p/Δt (true form of Newton’s 2nd Law) [Figure 13-6(b) (Giancoli 2005; 368)].
P 
F p t mv


A
A
At (Gas P is proportional to v and inversely proportional t)
Hence, the pressure exerted by a gas is proportional to the speed of gas
molecules and the frequency of collisions. Hence, Gas pressure is
inversely proportional to the time between collisions.
Note: Collisions between gas particles do not affect the pressure.
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TOPIC 4 and Topic 9 ( HL)
Oscillations and Waves
32. Oscillations are essentially any motion that moves back and forth
repetitively. Oscillations can also be referred to as waves, cycles or vibrations.
33. SIMPLE HARMONIC MOTION takes place when an object ( particle) that is
disturbed away from its fixed equilibrium position experiences an acceleration
and force that is proportional and OPPOSITE to its displacement.
In general to check whether SHM will take place we must check that :
1. There is a fixed equilibrium position
2. Acceleration and force must be opposite and directly
proportional to displacement : a ~ -x
34. Period (T) is the amount of time it takes to complete one cycle or one
oscillation , wave or vibration . [SI Unit: s].
Frequency (f) = number of cycles , oscillations, vibrations or waves per
second [SI unit: Hz = s-1].
35. Displacement (x) and Amplitude (A or x0)
Displacement (x) is the net distance from the equilibrium point. It can be positive or
negative depending on the chosen coordinate system.
Amplitude (A or x0) is the maximum displacement from equilibrium
36. Angular frequency (ω) is a scalar measure of the angular rate of
rotation. Angular frequency is found by multiplying the frequency by 2π [SI unit:
s-1].

2
T
 2 f
Angular frequency (ω)
add s-1 = Hz
15
37. Damping HL ONLY
SHM is an important motion to study when considering oscillations but it does
not consider friction and other resistance forces. The effect of these forces on an
oscillating system is that the oscillations will eventually stop and the energy of the
system will be dissipated mainly as thermal energy to the environment and the
system itself.
Oscillations taking place in the presence of resistance forces are called damped
oscillations. The behavior of the system depends on the degree of damping.
There are three different types of damping:
2. Under-damping
3. Critical damping
4. Over –damping
38. Resonance - frequency of external force is equal to natural frequency of
the system. This results in oscillations with large amplitudes. Fig 1.20 p.208
Bridge video
16
39. Definition of a wave : a wave is a disturbance that travels in a medium
transferring energy and momentum from one place to another. The direction of
energy transfer is the direction of propagation of the wave.
Examples:
Sound is a type of wave called a longitudinal wave that travels through the
medium air. The energy vibrates parallel to the direction of propagation of the
wave.
Light is a type of electromagnetic wave called a transverse wave, that travels
through a vacuum . The energy vibrates perpendicular to the
40. Refraction
When waves strike a transparent surface part of it is reflected and part of it is
transmitted where it is refracted. Refraction is the bending of waves when they pass
from one medium to another. It occurs because the wave speed changes as it passes
through different mediums (analogous to driving a car along the edge of the road: if one
tire moves off the edge of the road into sand or thick mud it slows down, pulling the car
off to the side of the road)
In the diagrams below the wave is passing from a less dense medium to a more dense
medium and so the wave slows down and bends or refracts.
ADD: As wave travel to more dens medium wavelength, velocity and
amplitude DECREASE BUT FREQUENCY DOESN’T CHANGE
Normal Line of reference
In diagram (b) the RAY is said to bend toward the normal when going from a less dense
to a more dense material. add: The WAVE bends UP.
In diagram( b )below light is passing from water to air or more dense to less dense and
the RAY bends away from the normal. The WAVE bends
17
41. Snell’s Law
n1 sin θ1 = n2 sinθ2
Snell’s law explains the relationship between the angle of refraction ( θ ) of a
light ray as it passes from one media to another . It allows you to determine the
angle if the density of the substance is known as related to the index of
refraction ( n).
42. Diffraction
43. Interference
Open the following link http://www.physicsclassroom.com/Class/waves/
Define wave interference.
Wave interference is the phenomenon that occurs when two waves meet while traveling along the
same medium. The interference of waves causes the medium to take on a shape that results from
the net effect of the two individual waves upon the particles of the medium
Distinguish between constructive and destructive interference.
Constructive interference : the displacement of the two waves is in the same
direction and is additive resulting in one over all larger displacement.
Destructive interference : the displacement of the two waves is in the opposite
direction and is subtractive resulting in one over all smaller displacement or
sometime zero displacement.
18
44. Polarization – property of transverse waves: a wave is polarized if the
displacement of the wave always lies in the same plane.
45. Malus’s Law
Malus’s Law describes the intensity of the polarized light after it passes through
the polarizer . Basically the intensity of light decreases as the orientation –
transmission axis of the polarizer changes. Light intensity is maximum when θ =
0º ( parallel) and zero when θ = 90º ( perpendicular).
Figure 11.5-1 shows a graph of the relative light intensity of polarized light after it
is transmitted through an analyzer that is oriented at different angles. According
to Malus’s Law, light intensity follows a cos2θ shape as the axis of the analyzer is
rotated.
Light intensity is maximum when θ = 0º and zero when θ = 90º. See below :
19
46. Brewster's Law
In 1812, Sir David Brewster found experimentally that, the reflected ray is 100%
polarized when the angle between the refracted ray and the reflected ray is 90º.
The angle of incidence, called the Brewster's or Polarizing angle θp, required for
100% polarization is determined by the refractive index of the two materials
(using Snell's Law).
θp + θr = 900
θr = 900 - θp
If the ray is incident from air (n1 = 1.00), then:
n  tan  p
(Brewster's Law, Ray incident
20
TOPIC 5, 10 – 11(HL only)
Electromagnetism
47. Electromagnetism is one of the 4 fundamental forces of the universe
along with gravity, the strong and weak nuclear forces. Electric force is much
stronger than gravity.
48. Electricity is the phenomena associated with interaction between
electrical charges. Electrostatics is the study of charged objects at rest.
Electric charge is the fundamental property of matter based on the atom: e- , p
+ , and n 0 . Electric force is the force that keeps e- , p + , and n 0 together. The
fundamental law of electricity is that opposite charges attract and like charges
repel.
49. The electric field is a force field or vector field that allows one to
determine the force (F) acting on a charge ( q):
E=F
q
units : NC-1
50. Electric Potential Difference or Electric Potential – Voltage ( V) :
Voltage also called emf ( electromotive force) ε – is the amount of work done
per unit charge. Also defined as the change in electric potential energy per unit
charge.
51. E = V / d
Parallel Plates
n the case of parallel plates the electric field is determined by E = V/ d where d is
the distance between the plates. The electric field is uniform and has the same
value at all points between the plates. Its direction is from high potential to low
potential.
Work done to move a charge
is not uniform and depends
on distance the charge
moves : W = Eqd
21
52. The Electron Volt: A Unit of Energy
The electron volt ( eV) is a unit of energy that can also be converted to joules ( J)
1 eV = 1.6 x 10-19 J
NOT IN DATA BOOKLET BUT REMEMBER AS
RELATED TO COULOMB ( C) :
The joule is a very large unit for dealing with electrons, atoms or molecules. For
this purpose we use the electron volt (eV). One electron volt is defined as the
energy acquired by a particle carrying a charge equal to one electron as a result
of moving through a potential difference of 1V.
1eV  1.60 1019 C  1V 
1eV  1.60 1019 J
(Electron volt)
53. Voltage also called
emf ( electromotive force) ε and also called
Electric Potential Difference ( V) is the amount of work done per unit charge.
Also defined as the change in electric potential energy per unit charge.
54. Current ( I) is the flow of “electricity” or charge ( q) and what causes
current is an electric potential difference – voltage ( V) or commonly called a
battery.
Current (I) is the rate at which charge ( Q) flows. More specifically current is
rate at which electrons or charges random drift from one region to another.
I = ∆Q
∆t
unit : ampere ( commonly called amps) = A = 1 coulomb per second
A = Cs-1
55. Ohm’s Law and Resistance – Voltage - Current
22
OHM´S Law R = V :
I
This is known as Ohm’s Law, although it is not really a law (as it does not apply
to all materials or devices) but rather a definition of resistance.
Thus the definition of resistance is: R = V/I.
S.I. unit of resistance is the ohm (Ω) [1Ω= 1V/A].
Devices that follow ‘Ohm’s Law’ are said to be ohmic and display a linear
relationship between V and I
56. Resistivity ρ
The resistance of a material is partly determined by intrinsic atomic properties
described by the material’s resistivity ( ρ ) . The resistivity of a material depends
on the following length, material, cross-sectional area and temperature.
Remember A = π r2 when considering cross-sectional area.
Formula
R=ρ L
A
57. INTERNAL RESISTANCE
Batteries have internal resistance that causes a V drop inside the battery. This
is part of the total V drop of the battery.
58. Carrier Drift
23
Charge Carrier Drift - Charges moving in a conductor
The lower the value of the current the slower the rate at which ions ( charges) are
moving in a conductor.
I = nAvq
Current = number of charge carriers x cross sectional area x velocity x charge
Example
answer : 0.055 mm s-1
24
59. Kirchhoff’s First and Second Laws
First Law – Conservation of Charge
Second Law – Conservation of Energy
Data booklet:
25
60. HL ONLY Capacitance – Stored Charge : The arrangement of
parallel plates in which two plates are separated by an insulator is
called a capacitor. The insulator , also called dielectric material,
could be a vacuum ( basically empty space with nothing) , air, or any
non-conducting material such as plastic. The main function of a
capacitor is to store charge ( capacity – ability to store).
C=Q
V
NOTE IN Data booklet : Q = q
Units: Coulomb = Farad (F)
Volt
F = CV-1
26
TOPIC 7 – 12 ( HL ONLY) Nuclear Physics/ Quantum Mechanics
61. The Electromagnetic Spectrum
Visible light is part of the electromagnetic spectrum which are waves of energy
that are produced by one of the major forces in the universe called
electromagnetism
62. The Dual Nature of Light
The wave-particle nature of light states that light can travel as waves or as tiny
packets of energy called photons or quanta. So , energy can be quantitized.
The energy of a photon is given by E = hf where h is Planck’s constant =
6.626x10-34 Js
and v is the frequency . Einstein relates the energy of a photon to mass : E =
mc2 .
63. ELECTRON ABSORPTION – EMISSION
As atoms absorb energy the electrons can get further from the nucleus with this
gained energy. n= 1is called the ground state and represents the atom and its
electrons in the lowest possible energy state. The excited state or states n= 2,
n= 3 etc. are different energy states that the atom can exist when absorbing
energy.
64. Isotopes: Two isotopes of one element have same number of protons
but different number of neutrons
65. Nuclear Forces : Strong Nuclear Force vs. Coulomb Force
The strong nuclear force is defined as a short range force that holds neutrons
and protons together in the nucleus. The other nuclear force is called the
Coulomb force . It only involves protons that repel each other (remember the
fundamental law of electricity: like charges repel , opposite charges attract). The
Coulomb force causes an electromagnetic repulsion force.
66. The half-life ( t1/2) of a radioactive sample is defined as the time it takes
for the amount of the substance to decay to ½ its original value.
27
67. The unified atomic mass unit is defined as being 1/12th the mass of
a carbon-12 atom.
68. The mass defect ( denoted as ᵟ )states that the mass of the nucleus
of an atom is less than the protons and neutrons that make up the nucleus ( you
can ignore the electrons since they are not part of the nucleus).
69. So, the missing mass , or mass defect, was converted to energy and
this energy is stored inside the nucleus to help keep it together. This energy is
called the binding energy of the nucleus and is denoted by EB. This binding
energy is the energy released in nuclear reactions when the atom is split.
Unfortunately this led to the nuclear bomb.
70. MeV - electron volts : unit needed when calculating binding
energy
1 MeV = 1 million electron volts
1 u = 1 atomic mass unit and is equivalent to 931.5 MeV
To calculate the binding energy get the mass defect (ᵟ ) and multiply by 931.5
Binding energy = mass defect x 931.5 MeV
71. The wave theorists studied polarization and the interference/diffraction
patterns produced when light is passed through small slits to support their theory.
The followers of Einstein's particle (photon) theory, believed electromagnetic
radiation is emitted and absorbed by matter as if it existed in individual ‘packets’
of energy called photons. The behavior of light acting as a stream of photons is
illustrated by the photoelectric effect.
72 - 75 HL ONLY
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72. The Photoelectric Effect
The photoelectric effect describes the emission of electrons when light strikes a
metal surface.
73. Work function (Φ )
A certain amount of energy had to be imparted to an electron on the metal
surface in order to liberate it. This was known as the metal’s work function, or
Φ . If an electron absorbed energy E it would leave the metal as a photoelectron
with a maximum kinetic energy of Emax = E – Φ ( i.e you have to subtract the
energy it took to liberate the electron from the metal ).
Threshold frequency - The puzzling feature of this graph is that there exists a
frequency, called the critical or threshold frequency fc , such that no electrons
at all are emitted if the frequency of the light source is less than fc.
REGARDLESS of the intensity of the light
29
74. de Broglie wavelength – hypothesis :
Since an electromagnetic wave can behave like a particle, can a particle of
matter behave like a wave? In 1923, the French physicist Louis de Broglie said
yes. His hypothesis, which has been supported by experiment, is that a particle
of mass m and speed v and thus linear momentum p = mv, has an associated
wavelength. This wavelength is called the de Broglie wavelength.:
λ=h
p
p = momentum = mv
h = Planck’s constant = 6.63 x
10-34
J.s


1eV


 1.6010 19 J 
= 4.14 x 10-15 eV.s
Particles in motion can display wave characteristics and behave as if they had a
wavelength.
75. The Heisenberg uncertainty principle states that the simultaneous
measurement of position and momentum will always have some uncertainty. In
fact the more certain we are about position the less certain we are about
momentum, and vice versa.
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TOPIC 8 Energy , Power and Climate Change
76. The Kilowatt hour: Defined as the energy used in kilowatts in one hour.
77. Sankey diagram – diagram that represents energy flows
78. Energy degradation – excess energy lost and is “ less useful” and can
not be used to perform mechanical work
79. Non- renewable energy source – finite sources that will run out e.g :
fossil fuels, nuclear
80. Renewable energy source - energy that can renew itself eg. : solar,
wind, wave - tidal, geothermal, hydroelectricity
81. Specific energy or Energy density– energy that can be obtained
from one unit mass. Jkg-1 . High energy density = high power output
82. Efficiency = output energy
input energy
83. The black body is a theoretical body that is a “ perfect” emitter of
radiation Any body in the universe will radiate energy in the form of
electromagnetic radiation.
The Stefan – Boltzmann law states that the amount of energy per
second or power (P) radiated by a body depends on the surface area
(A), absolute temperature (T), and the properties of the surface called
emissivity ( e) :
84. The Albedo - α
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The albedo of a body is defined as the ratio of power of radiation reflected
or scattered from the body to the total power incident on the body:
α = total scattered or reflected power
total incident power
total reflected radiation = total incident – total absorbed
85. Greenhouse effect – Basics
The greenhouse effect is the warming of the earth caused by infrared
radiation, emitted by the earth’s surface, which is absorbed by various
gases in the earth’s atmosphere. The radiation is then partly radiated back
towards the surface of the earth. In other words the energy from the sun
can get in to the earth’s atmosphere but can not get out.
The gases primarily responsible for this effect are : water vapor, carbon
dioxide , methane and nitrous oxide.
OPTION D ASTROPHYSICS – SEE PACKET / NOTES
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