L07: Electron Interactions with Matter

Question 1

The two types of charges

Answer 1

Positive and negative

• Like charges repel, unlike charges attract

Question 2

Proton

Answer 2

Nature’s basic carrier of positive charge

Question 3

Law of conservation of energy

Answer 3

Electric charge is always conserved

• Charge is not created, only exchanged

Objects become charged because negative charge is transferred from one object to another

Question 4

Unit of charge

Answer 4

Symbolized by e

• Electrons have a charge of –e
• Protons have a charge of +e

The SI unit of charge is the Coulomb (C)

• e = 1.6 x 10^-19 C

Question 5

Properties of Coulomb’s law

Answer 5

Electrical force has the following properties…

• It is along the line joining the two particles and inversely proportional to the square of the separation distance, r, between them
• It is proportional to the product of the magnitudes of the charges, |q1|and |q2|on the two particles
• It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs

Question 6

Voltage

Answer 6

The force or push of electricity

• Measured in volts (V)

Question 7

Electro-Motive Force (EMF)

Answer 7

Amount of work or energy potential (joules/charge)

Question 8

Electron volt

Answer 8

The energy that an electron gains when accelerated through a potential difference of 1 V

• Electrons in normal atoms have energies of 10’s of eV
• Excited electrons have energies of 1000’s of eV
• High energy gamma rays have energies of millions of eV

1 eV = 1.6 x 10-19 J

Question 9

Electric current

Answer 9

Whenever electric charges of like signs move, an electric current is said to exist

Question 10

Current

Answer 10

The rate at which the charge flows through a surface

The SI unit of current is Ampere (A)

• Algebraic symbol – I = intensity
• Graphic symbol – A = 1 coulomb/s

Question 11

Relationship between current time and charge

Answer 11

Q = charge (coulombs)
I = current (amperes)
t = time (seconds)

Question 12

Conventional current direction

Answer 12

The direction of the current is the direction positive charge would flow

• In a common conductor, such as copper, the current is due to the motion of the negatively charged electrons
• It is common to refer to a moving charge as a mobile charge carrier
• A charge carrier can be positive or negative

Question 13

A 60-Watt light bulb carries a current of 0.5 A. The total charge passing through it in one hour is?

Answer 13

I = 0.5 A
t = 60 x 60 = 3,600s
Q = 0.5 x 3600 = 1800 C

Question 14

Resistance

Answer 14

The constant of proportionality is the resistance of the conductor

• In a conductor, the voltage applied across the ends of the conductor is proportional to the current through the conductor

Units of resistance are ohms (Ω)

• 1 Ω = 1 V / A

Question 15

Why does resistance occur?

Answer 15

Resistance in a circuit arises due to collisions between the electrons carrying the current with the fixed atoms inside the conductor

Question 16

Ohm’s Law

Answer 16

For many materials, including most metals, the resistance remains constant over a wide range of applied voltages or currents

ΔV = IR

Ohm’s Law is an empirical relationship that is valid only for certain materials

Materials that obey Ohm’s Law are said to be ohmic

Question 17

Ohmic materials

Answer 17

The resistance is constant over a wide range of voltages

The relationship between current and voltage is linear

The slope is related to the resistance

Question 18

Non-ohmic materials

Answer 18

Those whose resistance changes with voltage or current

The current-voltage relationship is nonlinear

A diode is a common example of a non-ohmic device

Question 19

A lightbulb has a resistance of 240 Ω when operating at a voltage of 120 V. What is the current in the bulb?

Answer 19

I = V/R
I = 120/240
I = 0.5 A

Question 20

What happens to the electrical potential energy of a system when a charge moves through the battery within a circuit?

Answer 20

Electrical potential energy of the system is increased by ΔQΔV

The chemical potential energy of the battery decreases by the same amount

As the charge moves through a resistor, it loses this potential energy during collisions with atoms in the resistor

• The temperature of the resistor will increase

Question 21

Power

Answer 21

The rate at which energy is lost

Essentially
Power = Energy / Time
= V x Q / time
= V x I (Watts)
= I x R x I
= I^2 x R
= (V/R)^2 x R
= V^2 x R

Q = electric charge (coulombs)

Total energy dissipated = Power x time [kW hours]

Resistors are made with a power rating.

Question 22

What are the 2 types of current

Answer 22

• Alternating Current (AC)
• Direct Current (DC)

Question 23

Alternating Current (AC)

Answer 23

Electrical current flows in both directions; positive and negative terminals continuously trade places (polarity)

Example: Electricity provided by Vectren

Question 24

Direct Current (DC)

Answer 24

Electrical current flows in one direction; negative to positive

Example: Electricity provided by batteries

Question 25

Electron-sea model

Answer 25

Simple depiction of a metal as an array of positive ions surrounded by delocalised valence electrons. Metals are good conductors of electricity because of the mobility of these delocalised valence electrons. A metal also conducts heat well because the mobile electrons can carry additional kinetic energy

Question 26

Molecular orbital theory

Answer 26

Gives a more detailed picture of the bonding in metals. Because the energy levels in a metal crowd together into bands, this picture of metal bonding is called band theory. - According to band theory, the electrons in a crystal become free to move when they are excited to the unoccupied orbitals of a band

Question 27

Bands of energy levels

Answer 27

The highest energy band that is completely filled is the Valence Band The next higher energy band, which is at most partially filled, is the Conduction Band. - Electrons in the conduction band are “free” and can move through the solid. - Electrons in the valence band may be excited and receive enough energy to transfer across the forbidden energy gap into the conduction band Any energy can easily be absorbed by an electron in the partially filled conduction band, allowing the electron to move through the solid as a free electron.

Question 28

Resistivity in ohmic conductors

Answer 28

Is proportional to its length, L, and inversely proportional to its cross-sectional area, A ρ is the constant of proportionality and is called the resistivity of the material

Question 29

Increase in temperature affect on resistivity

Answer 29

For most metals, resistivity increases with higher temp - The metal’s constituent atoms vibrate with increasing amplitude - The electrons find it more difficult to pass through the atoms

Question 30

Temperature affect on resistivity formula

Answer 30

For most metals, resistivity increases approximately linearly with temperature over a limited temperature range ρ is the resistivity at some temperature T ρo is the resistivity at some reference temperature To To is usually taken to be 20° C α is the temperature coefficient of resistivity

Question 31

Temperature affect on resistance formula

Answer 31

Since the resistance of a conductor with uniform cross-sectional area is proportional to the resistivity, you can find the effect of temperature on resistance

Question 32

Calculate the diameter of a 2.0-cm length of tungsten filament in a small light bulb if its resistance is 0.050 Ω. [Resistance and resistivity relation : resistivity of tungsten = 5.6 X 10^-8 ohm-meter]

Answer 32

R = ρL/A R = 0.05 Ω ρ = 5.6 x 10^-8 Ω m L = 2 cm = 0.02 m A = πr^2 is the cross-sectional area Rearrange equation for area and sub A = ρL/R A = (5.6 x 10^-8)(0.02)/0.05 A = 2.24 x 10^-8 m^2 Now solve for diameter d = 2r 2r = 2sqrt(A/π) 2r = 2sqrt((2.24 x 10^-8)/π) 2r = 0.169mm = 0.169 mm or 169 μm

Question 33

Conductor properties

Answer 33

Applying EMF (Electro-Motive Force) creates electric currents, increasing R as temp increased due to the increased collisions between electrons and the vibrating +ve ion cores Incident light absorbed

Question 34

Superconductors

Answer 34

A class of materials and compounds whose resistances fall to virtually zero below a certain temperature, Tc (critical temperature) Once a current is set up in a superconductor, it persists without any applied voltage Since R = 0

Question 35

What affects the value of Tc (critical temperature)?

Answer 35

- Chemical composition - Pressure - Crystalline structure

Question 36

Insulators

Answer 36

Resist the flow of electric current because their electrons are tightly bound The forbidden energy gap is large (10 eV), and the conduction band is empty. Thus, electrons can only move to an empty state in the conduction band if the supplied energy > 10 eV

Question 37

Insulator properties

Answer 37

- Applied EMF doesn't result in electric current - Incident light is not absorbed - Poor thermal conductors

Question 38

Semiconductors

Answer 38

Gap between valence and conduction band is intermediate in size Metalloids: semiconducting elements - low electrical conductivity at room temperature - Electrical conductivity increases with temp Form the basis of solid state electronic devices - conductivities increase markedly when they are doped with small quantities of other element

Question 39

Properties of semiconductors

Answer 39

- Few electrons in conduction band -> small electrical conductivity - As temp increase, R decrease since the thermal energy increases the number of electrons in the conduction band. - Incident light is absorbed, as the small energy gap, visible light energy (E = hf)

Question 40

Intrinsic semi-conductors

Answer 40

Characterized by an empty conduction band with a small forbidden energy gap (~1 eV) At room temperature there is some tunnelling through the forbidden energy gap by a few electrons about > 1eV

Question 41

Nature of intrinsic silicon

Answer 41

Silicon that is free of doping impurities is called intrinsic Silicon has a valence of 4 and forms covalent bonds with four other neighbouring silicon atoms

Question 42

Extrinsic semiconductors

Answer 42

Additional energy states are produced within the forbidden energy gap by addition of appropriate impurities. - N-type - P-type

Question 43

N-type semiconductor

Answer 43

When silicon is doped with phosphorus, it becomes an n-type semiconductor, in which electric current is carried by electrons The impurity provides an extra energy level just below the conduction band – DONOR STATES - Donor state electrons are easily excited into the conduction band

Question 44

P-type semiconductor

Answer 44

When silicon is doped with boron, it becomes a p-type semiconductor, in which an electrical current is carried by positively charged holes The impurity provides an extra energy level just above the valence band – ACCEPTOR STATES - Electrons are easily excited into the acceptor energy level, leaving a +ve hole behind, which can “move” through the medium

Question 45

Rectifiers

Answer 45

A rectifier is a device that allows current to flow in one direction, but not the other. Joining a p-type semiconductor to an n-type semiconductor produces a p-n junction, which can function as a rectifier.