BGCSE Science Double Award Paper 3 (October/November 2015) Review

Speed-Time Graphs and Terminal Velocity Concepts

• Terminal Velocity Identification:     * Terminal velocity is the constant speed reached by an object falling through a fluid (like air) when the drag force equals the weight.     * On a speed-time graph, this is represented by a horizontal line where the gradient is zero.     * According to the Fig 1.1 sketch, an object falling through air reaches a terminal velocity of $30\,m/s$ at point A.     * The time taken to reach this terminal velocity is exactly $6\,s$.

• Distance Calculations from Graphs:     * The distance traveled by an object is represented by the area under the speed-time graph.     * For the interval between point A ($t = 6\,s$) and point B ($t = 11\,s$), the object is at a constant terminal velocity of $30\,m/s$.     * Calculation for distance: $\text{Distance} = \text{speed} \times \text{time} = 30\,m/s \times (11\,s - 6\,s) = 30\,m/s \times 5\,s = 150\,m$.

• Forces Acting on Falling Objects:     * Weight: In a standard gravitational field, the weight of a falling object remains constant throughout its descent.     * Air Resistance: As an object accelerates from rest towards point A, air resistance increases. This increase continues until the air resistance is equal in magnitude to the weight, resulting in zero net force and constant terminal velocity.

Hydroelectric Power Generation and Energy Conversions

• System Components:     * A hydroelectric power station utilizes a dam to store water, which then flows through a pipe to reach a turbine and generator set.     * The system uses transformers to regulate voltage for transmission to villages.

• Transformer Types and Voltage Regulation:     * Transformer B: Identified as a step-down transformer because it reduces high transmission voltage to a lower level suitable for household use ($25\,kV$ or less).     * Transmission Efficiency: Electricity is transmitted at high voltages (e.g., $230\,kV$) rather than lower voltages (e.g., $25\,kV$) to reduce energy loss as heat in the transmission cables. Higher voltage allows for lower current ($I$) for the same power, and according to $P = I^2R$, lower current significantly reduces power loss.     * Conversion factor: $1\,kV = 1000\,V$.

• Potential Energy and Velocity Calculations:     * Decrease in Potential Energy ($PE$): Calculated using the formula $\Delta PE = m \times g \times h$.     * Given parameters: mass ($m$) = $500\,kg$, height ($h$) = $15\,m$, and gravity ($g$) = $10\,N/kg$.     * Calculation: $500\,kg \times 10\,N/kg \times 15\,m = 75,000\,J$ (or $75\,kJ$).     * Theoretical Maximum Velocity: Assumes all potential energy is converted to kinetic energy ($KE$).     * Formula: $\frac{1}{2}mv^2 = mgh \rightarrow v = \sqrt{2gh}$.     * Calculation: $v = \sqrt{2 \times 10\,N/kg \times 15\,m} = \sqrt{300} \approx 17.32\,m/s$.     * Real-world Discrepancies: The actual velocity is always less than the theoretical maximum due to energy losses. Friction within the pipes and air resistance convert some kinetic energy into thermal energy (heat).

• Environmental Impact:     * Hydroelectric stations can lead to habitat destruction, displacement of local communities, and interference with fish migration patterns.

Electromagnetism and Relay Switching Circuits

• Electromagnet Design:     * An electromagnet consists of a coil of wire wound around a soft iron core.     * Core Material (Iron): Iron is used because it is a "soft" magnetic material, meaning it is easily magnetized when current flows and quickly demagnetized when the current is switched off.

• Circuit Operation Mechanism:     * The electromagnet acts as a relay to switch on a high-voltage ($340\,V$) circuit.     * Step-by-step process:         1. Current flows through the coil of wire.         2. The soft iron core becomes magnetized, creating a magnetic field.         3. The magnetic field attracts the soft iron armature.         4. The armature moves/rotates about a pivot point.         5. This movement closes the electrical contacts in the high-voltage circuit.         6. The high-voltage circuit is completed, powering the machine.     * The Spring: The fixed spring serves to pull the armature back to its original position once the current in the electromagnet is cut, ensuring the high-voltage circuit is safely broken.

Electrical Safety, Power, and Domestic Costing

• The Electric Iron Components:     * Consists of a metal sole, a heating element, and a plastic handle.     * Wiring follows standard color coding: Neutral, Live, and Earth wires.

• Energy and Material Principles:     * Energy Change: The primary change is from electrical energy to thermal energy (heat).     * Insulation: The handle is made of plastic because it is a poor conductor of heat and electricity (insulator), protecting the user from burns and electric shocks.     * Earthing Safety: The earth wire is connected directly to the metal sole. If a fault occurs and the live wire touches the metal casing, the current flows safely to the ground via the earth wire, preventing the user from receiving a fatal shock.

• Power and Cost Calculations:     * Input Power: Given energy ($E$) = $3\,kWh$ and time ($t$) = $2\,hours$.     * Formula: $\text{Power} = \frac{\text{Energy}}{\text{Time}}$.     * Calculation: $3\,kWh / 2\,h = 1.5\,kW$.     * Cost of Use: Given the cost rate is $P0.65$ per $kWh$.     * Calculation: $3\,kWh \times P0.65/kWh = P1.95$.

Thermal Physics and Heat Transfer

• Transfer Mechanisms in Water Heating:     * Conduction: The process by which heat energy passes through the metal of the boiler to reach the water inside.     * Convection: The process by which heat energy is transferred from the boiler to the water in the storage tank. In this process, heated water becomes less dense and rises, while cooler, denser water sinks, creating a circulation current.

• Efficiency:     * Energy loss from the storage tank can be reduced through insulation (e.g., adding a lagging jacket or foam around the tank).

Nuclear Physics: Radioactivity and Penetrating Power

• Experimentation Setup:     * Uses a radioactive source within a lead shield directed at a detector, with space (point P) for shielding materials.

• Background Radiation:     * A count rate detected in the absence of a radioactive source (e.g., $10\,counts/second$) is caused by background radiation from natural sources (cosmic rays, rocks, soil).

• Radiation Characteristics:     * Alpha Radiation: Effectively blocked by a single sheet of paper.     * Beta Radiation: Passes through paper but is blocked by a thin sheet of aluminum.     * Gamma Radiation: Highly penetrating; passes through paper and thin aluminum, requiring thick lead or concrete to stop.     * Data Analysis: If the count rate drops from $150$ to $40$ when paper is added, but remains at $40$ when aluminum is added, the source emits Alpha (stopped by paper) and Gamma (passing through both paper and aluminum).

• Safety Precautions:     * Handle sources with long-handled tongs.     * Store sources in lead-lined containers.     * Maintain maximum distance and minimize exposure time.

General Chemistry: Substances and Carbon Allotropes

• Substance Identification:     * Carbon Monoxide: Produced during the incomplete combustion of hydrocarbon fuels.     * Sodium: A reactive metal that reacts with water to produce an alkaline solution (sodium hydroxide) and hydrogen gas.     * Nitrogen and Hydrogen: React together (via the Haber process) to form ammonia gas ($NH_3$).     * Bromine: Used as a test reagent to distinguish alkanes from alkenes (alkenes decolorize bromine water).     * Vinegar (Ethanoic Acid) or Sodium Chloride: Used frequently as food preservatives.

• Allotropes of Carbon:     * Forms: Diamond and Graphite are the most common allotropes.     * Uses:         1. Diamond: Used in cutting tools/glass cutters due to extreme hardness; used in jewelry due to high refractive index.         2. Graphite: Used as a dry lubricant or in pencil leads due to layered structure; used as electrodes due to conductivity.

Quantitative Chemistry and Stoichiometry

• Reaction Equation: $Na_2CO_3 + H_2SO_4 \rightarrow Na_2SO_4 + CO_2 + H_2O$

• Molecular Mass and Molar Calculations:     * Relative Molecular Mass ($M_r$) of Sodium Carbonate ($Na_2CO_3$):         * $(2 \times 23) + 12 + (3 \times 16) = 46 + 12 + 48 = 106$.     * Moles of Substance: Mass = $21.2\,g$.         * $\text{Moles} = \frac{\text{Mass}}{M_r} = \frac{21.2}{106} = 0.2\,mol$.

• Solution Concentration:     * Volume = $500\,cm^3 = 0.5\,dm^3$.     * $\text{Concentration} = \frac{\text{Moles}}{\text{Volume}} = \frac{0.2\,mol}{0.5\,dm^3} = 0.4\,mol/dm^3$.

• Gas Stoichiometry:     * According to the balanced equation, $1\,mole$ of $Na_2CO_3$ produces $1\,mole$ of $CO_2$.     * Therefore, $0.2\,moles$ of $Na_2CO_3$ will produce $0.2\,moles$ of $CO_2$.     * Volume of Gas at r.t.p.: $1\,mole = 24\,dm^3$.     * $\text{Volume} = 0.2\,mol \times 24\,dm^3/mol = 4.8\,dm^3$.

• Bonding:     * The water ($H_2O$) molecule is formed by covalent bonding. Oxygen (6 outer electrons) shares one electron with each of two Hydrogen atoms (1 outer electron each), resulting in two single covalent bonds and two lone pairs on the Oxygen atom.

• Reaction Rates and Graphing:     * The rate of reaction is measured by the volume of $CO_2$ produced over time.     * As shown in Table 9.1, the reaction slows down over time and stops when the volume reaches a constant $60\,cm^3$ at $4\,minutes$.     * If half the mass of $Na_2CO_3$ is used, the total volume of gas produced will be half ($30\,cm^3$), and the graph (Graph Y) will plateau at this lower level.

Organic Chemistry: Esters and Bio-fuels

• Bio-petrol Synthesis:     * Bio-petrol is an ester produced from the reaction of vegetable oil with methanol ($CH_3OH$). The byproduct is glycerol.     * Pros of Bio-petrol: It is a renewable energy source compared to crude oil fuels and can be carbon-neutral.

• Glycerol and Methanol Structure:     * Glycerol belongs to the Alcohol (Alkanol) homologous series.     * Empirical formula of glycerol: Derived from its structure ($C_3H_8O_3$), the empirical formula remains $C_3H_8O_3$.     * Methanol Combustion: $\text{Methanol} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water}$.

• Esterification:     * Ethyl ethanoate is formed by the reaction of ethanol and ethanoic acid.

Biology: Plant Physiology and Cells

• Water Regulation in Plants:     * Transpiration: The process by which plants lose water vapor through their leaves.     * Wilting and Osmosis: When excessive fertilizer is added to soil, the water potential of the soil becomes lower than that of the plant's root cells. Water moves out of the plant cells and into the soil by osmosis. The cells lose turgor pressure (become flaccid), causing the plant to wilt.

• Diffusion:     * The movement of particles (fumes) from an area of higher concentration (the fertilizer) to an area of lower concentration (the farmer's nose).

• Cell Classification and Structure:     * Animal Cells: Identified by the absence of a cell wall and chloroplasts (Cells P and Q).     * Plant Cells: Identified by the presence of a cell wall and often chloroplasts (Cells R and S).     * Cell P: Likely a specialized animal cell like a red blood cell (no nucleus) or a sperm cell.     * Cell R: Distinguished by its cell wall or chloroplasts. Chloroplasts contain chlorophyll to trap light energy for photosynthesis.

Human Physiology: Endocrine and Reproductive Systems

• Blood Sugar Regulation:     * The Pancreas is the gland responsible for secreting insulin.     * Hypoglycaemia (Low Blood Sugar):         * Can occur during heavy exercise because muscle cells increase respiration to provide energy, consuming glucose from the blood.         * Excessive insulin injection causes the liver and muscles to convert too much glucose into glycogen for storage, lowering blood sugar levels.     * Insulin Administration: Insulin is a protein. It cannot be taken orally because it would be digested (broken down) by proteases in the stomach and small intestine before reaching the bloodstream.

• Reproductive System:     * P: Oviduct (Fallopian tube).     * Q: Uterus (Womb).     * R: Implantation (where the embryo embeds in the lining).     * S: Ovary. It secretes hormones like Oestrogen and Progesterone which influence the menstrual cycle and the uterus lining.     * Birth Control Methods:         1. Natural (e.g., rhythm method).         2. Chemical (e.g., the pill, spermicides).         3. Mechanical (e.g., condoms, IUD).         4. Surgical (e.g., vasectomy, tubal ligation).

Ecology and Conservation

• Food Chains:     * A typical chain: Mophane tree (Producer) $\rightarrow$ Mophane worm (Primary Consumer) $\rightarrow$ Human (Secondary Consumer).

• Conservation Management:     * Deforestation (cutting mophane trees) destroys the habitat and food source for the worms, leading to a population crash in the entire food chain.     * Methods for conservation: Setting harvest limits/quotas, establishing protected seasons, and reforestation (planting new trees).