1
Q
Separation technology is used when the pollutant is separated from the mixture. Describe the use of scrubbers and electrostatic precipitators. [4]
A
- Scrubbers
- These are air pollution control devices that use a liquid (often water or a chemical reagent) to remove particulate matter and/or gaseous pollutants from an industrial exhaust stream
- The polluted gas stream is brought into contact with the liquid, which captures the pollutants
- The contaminated liquid is then treated or disposed of, and the cleaned gas is released
- Electrostatic Precipitators (ESPs)
- These devices remove particulate matter from exhaust gases using electrostatic forces
- The gas stream passes through a chamber where wires apply a high negative voltage, charging the particles
- These charged particles are then attracted to and collected on positively charged plates (collecting electrodes)
- The collected particles are periodically removed by rapping the plates
2
Q
Contrast “Reserves” (proven reserves) and “Resources base” (Recoverable resources). In each of the two categories, will you include occurrences? [3]
A
- Reserves (Proven Reserves)
- The portion of the total resource that has been discovered, can be recovered with existing technology, and is conomically viable to extract under current market conditions
- Occurrences are NOT included; reserves are a strictly defined, commercially viable subset
- Resources Base (Recoverable Resources)
- The total quantity of a resource that is estimated to be ultimately recoverable, including known and undiscovered deposits, regardless of current economic or technological feasibility
- Occurrences ARE included in the resource base, as it encompasses all that may ever be recoverable
3
Q
List the three components of a battery and state briefly their purpose in the operation of the battery. [3]
A
- Anode (Negative Electrode): The electrode where oxidation occurs, releasing electrons to the external circuit
- Cathode (Positive Electrode): The electrode where reduction occurs, accepting electrons from the external circuit
- Electrolyte: A medium (often liquid or paste) that allows the flow of ionic charge between the anode and cathode, completing the internal circuit, while preventing the direct flow of electrons
4
Q
Define in words “Dielectric displacement D” in a capacitor
A
A vector field that represents the density of free electric charge in a material. It describes how an electric field influences the organization of electric charges in a material, and is related to the free charges only
5
Q
Define in words “Polarization P” in a capacitor
A
- A vector field that represents the density of permanent or induced electric dipole moments in a dielectric material
- It measures the extent to which the dielectric material becomes polarized (i.e., the alignment of molecular dipoles) in response to an external electric field
6
Q
Which maximum energy gap value (in eV) must a material have so that all photons in the visible range are absorbed
A
- To absorb all visible photons, the material’s energy gap, Eg, must be less than or equal to the energy of the lowest-energy (longest wavelength) visible photon
- This ensures even the weakest visible photon has enough energy to be absorbed
- Eg ≤1.77 eV. The maximum Eg for this condition is 1.77 eV
7
Q
Which minimum energy gap value must a semiconductor material have so that no photon in the visible range will be absorbed? Explain briefly your choice.
A
- To absorb no visible photons, the material’s energy gap, Eg , must be greater than the energy of the highest-energy (shortest wavelength) visible photon
- This ensures that no visible photon has sufficient energy to excite an electron across the band gap
- Eg >3.10 eV. The minimum Eg for this condition is just above 3.10 eV
8
Q
Depletion region in intrinsic Si doped semiconductor
A
- In a p-n junction, p-side has excess holes, n-side has excess electrons
- Diffusion causes holes to move to n-side and electrons to p-side near the junction
- This leaves fixed ionized donors (positive) on n-side and fixed ionized acceptors (negative) on p-side → creates an electric field (built-in potential) across the depletion region, which opposes further diffusion
9
Q
Thermalisation in photovoltaics
A
- In a semiconductor, photons with energy greater than the bandgap excite electrons from valence to conduction band
- The excess energy above the bandgap is quickly lost as heat (thermalisation) as the electron relaxes to the bottom of the conduction band
- This heat loss reduces the efficiency
10
Q
What are the criteria fpr a glazing cover of a flat plate solar collector
A
- High transmissitivty for shortwave radiation
- Should allow as much incoming solar radiation as possible to pass through absorber plate
- While being opaque to longwave infrared radiation to trap the heat re-radiated by absorber
11
Q
Name the selectivity criteria for a absorber plate ( metal ) in a solar collector
A
- High absorptivity for shortwave radiation
- Low emissitivity for long wave radiation
- Plate should be covered by a very good absorber of the incoming light spectrum to maximize heat gain
12
Q
Explian Forward Biasing
A
- The p-type is connected to the positive terminal and the n-type to the negative terminal
- The applied voltage reduces the built-in potential barrier of the depletion region
- This allows the majority charge carriers (holes in p-side, electrons in n-side) to flow easily across the junction
- The current is primarily due to this diffusion of majority carriers and is large
- The flow of minority carriers is negligible
13
Q
Explain Reverse Biasing
A
- The p-type is connected to the negative terminal and the n-type to the positive terminal
- The applied voltage increases the built-in potential barrier and widens the depletion region
- This prevents the flow of majority carriers
- The only current is a very small drift current due to minority charge carriers (electrons in p-side, holes in n side) being pulled across the junction
- This saturation current is small and constant for a given temperature
14
Q
A bar magnet is forced in motion relative to a stationary conductor loop. What is the direction of the induced current in the loop if the North of the bar magnet is approaching the loop?
A
- The induced current will flow in a direction to oppose the approach of the North pole
- Using the right-hand grip rule, to oppose the approaching North pole, the face of the loop towards the magnet must become a North pole
- This requires a current flowing counter-clockwise (as viewed from the magnet)
15
Q
A bar magnet is forced in motion relative to a stationary conductor loop. What is the direction of the induced current in the loop if The North of the bar magnet is pulled away from the loop conducto
A
- The induced current will flow in a direction to oppose the receding of the North pole
- To oppose the receding North pole, the face of the loop towards the magnet must become a South pole (to attract it back)
- This requires a current flowing clockwise (as viewed from the magnet)
16
Q
Answer the same question as above if the conductor loop is falling into a stationary bar magnet facing up from South to North. Explain briefly your choice
A
- The scenario is the same as the magnet moving relative to the loop
- As the loop falls towards the magnet’s North pole, the relative motion is the North pole approaching the loop
- The induced current will be in a direction to oppose this change
- Thus, the face of the loop facing the North pole must itself become a North pole to repel the magnet
- Using the right-hand rule, this requires a counter-clockwise current (as viewed from above, looking down on the loop falling towards the magnet’s North pole)
17
Q
You learnt that the conductivity of a doped semiconductor depends strongly on the charge carrier density and the mobility of those carriers. Fig. 2 shows how the electron concentration varies with the temperature in
n-type Si. Three regions are clearly distinguishable on the figure. Discuss in detail the meaning of the labels allocated with these regions and explain the Physics behind the evolution of the curve ( 2019 exam )
A
- Ionization / Freeze-out Region (Low Temperature)
- Extrinsic / Saturation Region (Room Temperature)
- Intrinsic Region (High Temperature)
18
Q
Use the law of electromagnetic induction to discuss how a current is induced in an AC generator; use the same law to discuss how an AC voltage fed to the primary induces a voltage in the secondary circuit of a step-up transformer
A
- AC Generator:
A coil is rotated in a magnetic field. The changing magnetic flux (Φ = BAcosθ) through the coil, due to the varying angle θ, induces an EMF according to Faraday’s Law (EMF = -dΦ/dt). The continuous, periodic rotation produces a sinusoidal flux change, resulting in an alternating current (AC). - Step-up Transformer:
An alternating voltage in the primary coil creates a changing magnetic flux in the iron core. This changing flux is coupled to the secondary coil, inducing an EMF via Faraday’s Law. Since the induced EMF is proportional to the number of turns (EMF = -N dΦ/dt), a secondary coil with more turns (N_s > N_p) produces a higher output voltage
19
Q
Explain 1. Ionization / Freeze-out Region (Low Temperature)
A
Physics
- At very low temperatures, the donor atoms (e.g., Phosphorus in Si) are neutral because the donated electrons are bound to them and do not have enough thermal energy to escape into the conduction band
Evolution of the Curve
- As temperature increases, the thermal energy becomes sufficient to “ionize” the donor atoms, releasing their electrons into the conduction band
- This causes a sharp exponential rise in the electron concentration
20
Q
Explain 2. Extrinsic / Saturation Region (Room Temperature)
A
Physics
- At this moderate temperature range, all the available donor atoms have been ionized
- The electron concentration is approximately equal to the donor concentration (n ≈ N_d) and is essentially constant, as there are no more donors to ionize
Evolution of the Curve
- The curve forms a plateau
- This is the useful operating region for most semiconductor devices, where conductivity is stable and predictable
21
Q
Explain 3. Intrinsic Region (High Temperature)
A
Physics
- At very high temperatures, the thermal energy becomes large enough to break the covalent bonds within the semiconductor lattice itself, generating electron-hole pairs
- This process is called intrinsic excitation
Evolution of the Curve
- The concentration of these thermally generated intrinsic carriers eventually surpasses the concentration from the dopant atoms (n ≈ n_i, the intrinsic carrier concentration)
- The electron concentration rises sharply again, following the exponential temperature dependence of intrinsic carriers, and the semiconductor loses its n-type characteristics
22
Q
During the conversion of solar radiation into electric power; two major processes are involved: generation of electron-hole pairs and separation of electrons and holes. Explain in details how these steps occur.
A
- Generation of Electron-Hole Pairs
- Separation of Electrons and Holes
23
Q
Explain 1. Generation of Electron-Hole Pairs
A
- When a photon with energy greater than or equal to the semiconductor’s band gap (Eg) strikes the material, it can be absorbed
- This absorption provides enough energy to excite an electron from the valence band into the conduction band
- This process leaves behind a vacant state in the valence band called a “hole.” Together, the free electron and the hole form an electron-hole pair
24
Q
Explain 2. Separation of Electrons and Holes
A
- This separation is driven by an internal electric field. In a typical p-n junction solar cell, this field is created at the junction between p-type and n-type semiconductor materials
- The built-in electric field sweeps the negatively charged electrons towards the n-type side and the positively charged holes towards the p-type side
- This spatial separation of charges creates a voltage across the device
- When an external circuit is connected, electrons can flow from the n-side through the circuit to the p-side to recombine with holes, thereby doing useful work
25
Explain why two conductors of different electrode potentials are required on the anode and cathode
- This is the inherent tendency of a material to either lose electrons (oxidation, low potential) or gain electrons (reduction, high potential)
e
- If both electrodes were made of the same material, they would have the same electrode potential
- As a result, there would be no driving force (voltage) for a net electron flow, and no useful electrical work could be done
- By using two different materials with different electrode potentials, one electrode (the anode) has a stronger tendency to oxidize and lose electrons, while the other (the cathode) has a stronger tendency to reduce and gain electrons
- This difference in tendency creates an electrical potential difference between them
- Electrons naturally flow from the anode (lower potential) through the external circuit to the cathode (higher potential), generating an electric current
26
A battery is one of the devices used for energy storage; discuss the electrochemical reactions involved during “discharge”
1. At the Anode (Negative Electrode)
- Oxidation occurs
- The active material at the anode loses electrons. For example, in a simple zinc-carbon battery, the reaction is:
Zn(s) → Zn²⁺(aq) + 2e⁻
The electrons released flow through the external circuit, providing electric current
2. At the Cathode (Positive Electrode)
- Reduction occurs
- The active material at the cathode gains the electrons that have traveled through the external circuit
- In the same zinc-carbon battery, the reaction at the cathode (a carbon rod surrounded by MnO₂) is:
2MnO₂(s) + 2NH₄⁺(aq) + 2e⁻ → Mn₂O₃(s) + 2NH₃(aq) + H₂O(l)
27
Zn, Fe and Cu have electric potential values of -0.76 V; -0.45 V and +0.34 V respectively. State which values of voltage a voltmeter will measure if they are used as electrodes of a battery as:
(a) Zn/Fe couple?
- The cell voltage (EMF) is calculated as: E°cell = E°cathode - E°anode
- Zn (-0.76 V) is more negative than Fe (-0.45 V), so Zn will be the anode (oxidation) and Fe will be the cathode (reduction).
EMF = (-0.45 V) - (-0.76 V) = +0.31 V
28
Zn, Fe and Cu have electric potential values of -0.76 V; -0.45 V and +0.34 V respectively. State which values of voltage a voltmeter will measure if they are used as electrodes of a battery as: Fe/Cu couple?
- Fe (-0.45 V) is more negative than Cu (+0.34 V), so Fe will be the anode and Cu will be the cathode
EMF = (+0.34 V) - (-0.45 V) = +0.79 V
29
Consider the emission spectrum of a black body say at 1000 K and compare it to the spectrum of another black body say at 6000 K,
a) Identify and explain the main remarkable similarity between them
- Both spectra have the same characteristic continuous and smooth shape
- This shape is universal and is described by Planck's law
- All black bodies, regardless of their temperature or composition, emit radiation with this same broad, continuous distribution of wavelengths, from long-wave infrared to short-wave ultraviolet, depending on their temperature
30
Consider the emission spectrum of a black body say at 1000 K and compare it to the spectrum of another black body say at 6000 K, Identify and explain the two main remarkable differences between them?
1. Total Power Emitted (Stefan-Boltzmann Law)
- The black body at 6000 K emits vastly more total power per unit area than the one at 1000 K
- The total power radiated is proportional to T⁴ (Stefan-Boltzmann Law), so the hotter object (6000 K) radiates (6000/1000)⁴ = 6⁴ = 1296 times more energy
2. Peak Wavelength (Wien's Displacement Law)
- The peak of the emission spectrum (the wavelength at which the most intense radiation is emitted) is at a much shorter wavelength for the 6000 K body
- According to Wien's Law (λmax ∝ 1/T), as temperature increases, the peak wavelength shifts toward the blue/UV part of the spectrum
- The 6000 K spectrum would peak in the visible range (yellow), while the 1000 K spectrum would peak in the infrared
31
Si has a higher bandgap than Ge. Contrast their contributions towards the spectral conversion efficiency (current & voltage) when they are used as the absorber layer in a solar cell
1. Silicon (Higher Bandgap ~1.1 eV):
Voltage: Produces a higher output voltage. A larger bandgap means a larger built-in potential, which is the maximum possible voltage a solar cell can generate.
Current: Produces a lower short-circuit current. A high bandgap means it only absorbs and converts photons with higher energy (shorter wavelengths), missing a large portion of the lower-energy photons in the solar spectrum (especially in the infrared). This results in fewer electron-hole pairs and thus a lower current
2. Germanium (Lower Bandgap ~0.67 eV):
Voltage: Produces a lower output voltage due to its smaller built-in potential.
Current: Produces a higher short-circuit current. Its low bandgap allows it to absorb a much wider range of the solar spectrum, including more infrared photons. This generates a larger number of electron-hole pairs, leading to a higher current.
32
What is Thermal Efficiency
- The actual efficiency of a real heat engine, defined as the ratio of the net work output to the total heat input (n = Wnet / Qh )
- It accounts for all real-world losses (friction, heat loss, etc.)
33
What is Carnot Efficiency
- The maximum theoretical efficiency possible for a heat engine operating between two thermal reservoirs at temperatures, Th and Tc
- It represents an ideal, reversible process that no real engine can surpass
34
What are the Carbon Stores on Earth with %
1. Sediments and Rocks (including fossil fuels): ~66% of total carbon
2. Oceans: ~27% of total carbon (mostly as dissolved inorganic carbon)
3. Terrestrial Biosphere (plants, soil, detritus): ~4% of total carbon
35
What is Radioactive decay
The spontaneous disintegration of an unstable atomic nucleus, emitting radiation
36
Electric generator working
- A generator converts mechanical energy to electrical energy via electromagnetic induction
- A coil rotates in a magnetic field, changing magnetic flux through the coil, inducing an EMF (Faraday’s law)
37
Transformer working:
- Two coils (primary and secondary) wound on a common iron core
- AC in primary produces a changing magnetic flux in the core, inducing AC in the secondary
38
Solar heating collectors ( Non-concentrating collectors & Concentrating collectors )
Non-concentrating collectors
- Absorb solar energy without focusing; e.g., flat-plate collectors. Absorber plate with transparent cover, insulated backing
- Heat transfer fluid circulates
Concentrating collectors
- Use mirrors/lenses to focus sunlight onto a small receiver, achieving higher temperatures; e.g., parabolic troughs, power towers
39
Generation vs. recombination currents
Generation current
- Minority carriers generated in depletion region are swept by built-in field → small reverse-bias current
Recombination current
- Majority carriers crossing junction recombine in neutral regions → dominant in forward bias
