quiz 5

Question 1

4 Properties of magnets

Answer 1

§ Magnets will attract ferromagnetic substances.
§ Like poles of the magnet repel each other and unlike poles attract each other.
§ A suspended magnet always comes to rest in the north-south direction. § The poles of the magnet are in pairs.

Question 2

• Permanent magnets

Answer 2

are materials where the magnetic field is generated by the internal structure of the material itself.

Question 3

• Temporary magnets

Answer 3

are made of soft metals that are magnetised only when exposed to a permanent magnetic field or an electric current.

Question 4

• Electromagnet

Answer 4

is a type of magnet in which the magnetic field is produced by an electric current.

• Electromagnets usually consist of wire wound into a coil. A current through the wire creates a
  magnetic field which is concentrated in the hole, denoting the center of the coil.

Question 5

What are magnetic ceramics made of

Answer 5

Ferrites -> are crystalline minerals composed of iron oxide in combination with manganese (Mn2+), nickel (Ni2+), cobalt (Co2+), zinc (Zn2+), copper (Cu2+), or magnesium (Mg2+)

magnetite *

Question 6

Coercivity

Answer 6

Field strength required to demagnetize a material

Question 7

diamagnetic

Answer 7

very weak form of magnetism that is nonpermanent and persists only while an external field is being applied

The magnitude of the induced magnetic mo- ment is extremely small and in a direction opposite to that of the applied field

µ less than 1

Question 8

paramagnetic

Answer 8

atoms possess permanent dipole moments with random orientation

Aligns with external magnetic field along the directions of the magnetic field

Question 9

Ferromagnetic

Answer 9

permanent magnetic moment aligning over large domains

iron (as BCC 𝛼-ferrite), cobalt, nickel, and some rare earth metals such as gadolinium (Gd)

Overall, the domains may cancel out, but the material has the potential to stay magnetized, especially if you apply a field.

Question 10

Hard vs soft magnetics

Answer 10

Hard: high coercivity, used for permanent magnets. add particles/voids to
inhibit domain wall motion – example: tungsten steel –
Hc = 5900 amp-turn/m)

Soft: low coercivity, used in motors, easy to magnetize/demagnetize. commercial iron 99.95 Fe

Question 11

What effect do superconductors display

Answer 11

Meissner Effect

Superconductors expel magnetic fields.
This causes levitation over magnets.

Have zero resistivity below a critical temperature (Tc).

Question 12

Ferrimagnetic

Answer 12

permanently aligned magnetic moments, but…

Their atoms are arranged in opposing directions, and the opposing moments are unequal, so there’s still a net magnetization.

This happens often in ceramics like ferrites (e.g., Fe₃O₄).

Common in magnetic ceramics used in electronics and storage.

Question 13

How does increased applied magnetic field affect ferrimagnetic and ferromagnetic materials

Answer 13

Applying a magnetic field to ferromagnetic or ferrimagnetic materials causes magnetic domains to shift and grow, aligning more with the field. This increases the overall magnetization (B), until the material becomes saturated.

Question 14

Optical Property

Answer 14

A material’s response to electromagnetic radiation (especially visible light).

Question 15

Visible light range

Answer 15

electromagnetic radiation with wavelengths ranging from 0.39 to 0.77 μm.

Question 16

Photon

Answer 16

A quantum unit of light energy.

Question 17

Refraction

Answer 17

Bending of light as it passes from one medium to another.

Light slows down in materials compared to vacuum. The index of refraction (n) measures this slowdown.

Question 18

Index of refraction

Answer 18

n = c/v

(light speed in vacuum / in medium)

Question 19

Total Internal Reflection
Critical Angle (ϕc)

Answer 19

All light is reflected back into material.

Angle above which total internal reflection occurs.

Question 20

what are the three ways light interacts with a solid

Answer 20

Transmission, reflection, absorption

Question 21

Fermi Energy

Answer 21

The highest occupied electron energy level at absolute zero temperature.

Question 22

Electron Transition

Answer 22

Movement of an electron from a filled to an unfilled state when absorbing energy.

Produces a photon

Question 23

Skin Depth

Answer 23

The thin outer layer where light is absorbed in metals (less than 0.1 μm).

Question 24

Why are metals opaque to visible light

Answer 24

Metals are opaque to visible light because their electrons can absorb light across all visible frequencies due to available empty energy states above the Fermi level. However, this absorption is confined to a very thin surface layer. The absorbed light is often re-emitted, which is why metals appear shiny. Only extremely thin metal films (<0.1 μm) can transmit some light.

Question 25

reflectivity of metals

Answer 25

I_R/I_0 0.9-0.95 -> why metals appear shiny

Question 26

Optical properties of nonmetals

Answer 26

(glass, plastics and semiconductors.) Nonmetals show all four optical behaviors and may be transparent or colored depending on band structure.

Question 27

Absorption

Answer 27

§ The optical phenomena that occur within solid materials involve interactions between the electromagnetic radiation and atoms, ions, and/or electrons. if delta E> hv -> opaque

Question 28

color absorption in semiconductors

Answer 28

Larger gap → absorbs blue, appears red; hν=1.8eV smaller gap → absorbs red, appears blue. hν = 3.1 eV

Question 29

Luminescence

Answer 29

– reemission of light by a material – Material absorbs light at one frequency and reemits it at another (lower) frequency. – Trapped (donor/acceptor) states introduced by impurities/defects

Question 30

Phosphorescence vs fluorescence

Answer 30

If residence time in trapped state is relatively long (> 10-8 s) -- phosphorescence For short residence times (< 10E-8 fluorescence

Question 31

Metals vs non metals color

Answer 31

Nonmetals show color because they selectively absorb some wavelengths of light and transmit or re-emit the rest, based on their band gap and energy states. Metals, in contrast, mostly reflect visible light and have no real band gap, so their interaction with light is dominated by surface behavior.

Question 32

Opacity

Answer 32

single crystal transparent polycrystalline dense transluscent Polycrystalline porous is opaque

Question 33

Photoluminescence

Answer 33

Explanation: UV light excites electrons to higher energy levels (e.g., in mercury vapor). When electrons drop back down, they release energy as visible light. A phosphor coating on the inside of the lamp absorbs UV and reemits visible light.

Question 34

Cathodoluminescence

Answer 34

= Light emitted when electrons (not photons!) hit a phosphor. Phosphor material bombarded with electrons – Electrons in phosphor atoms excited to higher state – Photon (visible light) emitted as electrons drop back into ground states – Color of emitted light (i.e., photon wavelength) depends on composition of phosphor

Question 35

LASER acronym and conditions

Answer 35

LASER = Light Amplification by Stimulated Emission of Radiation A laser produces light that is: Coherent (waves in phase) Monochromatic (one wavelength) Directional (narrow beam) Requires population inversion — more electrons in excited states than ground state. Emission is stimulated, not spontaneous → sharp, clean beam.

Question 36

Solar Cell

Answer 36

= A p-n junction device that converts light into electricity. Light hits the junction → creates electron-hole pairs. Electrons and holes separate due to built-in electric field. Voltage develops across the junction (~0.5 V typical). Current flows if connected to a circuit → solar power.

Question 37

Optical Fiber

Answer 37

= A thin glass fiber that transmits light using total internal reflection.