Edexcel International GCSE Physics (Grade 9-1) - Complete Comprehensive Study Notes

Section 1: Forces and Motion - Velocity and Acceleration

• Speed vs. Velocity:

  • Speed: A scalar quantity describing how fast an object is moving (e.g., $30\,mph$ or $20\,m/s$). It disregards direction.

  • Velocity: A vector quantity that specifies both speed and direction (e.g., $30\,mph$ North or $20\,m/s, 060\degree$ ).

  • Constant Speed, Changing Velocity: An object can have a constant speed but a changing velocity if it is changing its direction (e.g., moving in a circle).

  • Average Speed Formula: $\text{Average speed} = \frac{\text{Distance moved}}{\text{Time taken}}$ ($v = s/t$).

  • Example: A cat skulks $20\,m$ in $35\,s$. Average speed is $\frac{20}{35} = 0.57\,m/s$. Time taken to skulk $75\,m$ is $\frac{75}{0.57} = 132\,s$ (or $2\,min\,12\,s$).

• Acceleration:

  • Definition: Acceleration is how quickly velocity is changing. This change can be in speed, direction, or both.

  • Unit: Measured in $\text{m/s}^2$.

  • Formula 1 (Constant Acceleration): $\text{Acceleration} = \frac{\text{Change in Velocity}}{\text{Time taken}}$ ($a = \frac{v - u}{t}$), where $v$ is final velocity and $u$ is initial velocity.

  • Deceleration: A negative value for acceleration indicates the object is slowing down.

  • Formula 2 (Velocity-Distance Relationship): $v^2 = u^2 + (2 \times a \times s)$, where $s$ is distance traveled while accelerating.

  • Example 1: A cat accelerates from $2\,m/s$ to $6\,m/s$ in $5.6\,s$. Acceleration $a = \frac{6-2}{5.6} = 0.71\,m/s^2$.

  • Example 2: A van at $23\,m/s$ decelerates uniformly at $2.0\,m/s^2$ for $112\,m$. Final velocity $v = \sqrt{23^2 + (2 \times -2.0 \times 112)} = \sqrt{81} = 9\,m/s$.

Section 1: Distance-Time and Velocity-Time Graphs

• Distance-Time Graphs:

  • Gradient (Slope): Represents the speed of the object.

  • Flat Sections: Indicate the object has stopped.

  • Steeper Gradient: Indicates a faster speed.

  • Curves: Represent acceleration (getting steeper) or deceleration (levelling off).

  • Calculating Average Speed: Divide the total distance by the total time taken ($v = s/t$).

• Velocity-Time Graphs:

  • Gradient (Slope): Represents the acceleration ($a$).

  • Flat Sections: Represent steady/constant speed.

  • Uphill (/) vs. Downhill (): Uphill is acceleration; downhill is deceleration.

  • Area Under the Graph: Represents the total distance traveled in that time interval.

  • Curves: Indicate changing acceleration.

  • Example Calculation: Acceleration of a section moving from $0$ to $30\,m/s$ in $20\,s$ is $\frac{30}{20} = 1.5\,m/s^2$. Shaded area between $80\,s$ and $100\,s$ at $50\,m/s$ equals $50\,m/s \times 20\,s = 1000\,m$.

Section 1: Mass, Weight, and Gravity

• Gravity Basics:

  • Gravity is the force of attraction between all masses. It is mostly noticeable near large masses like planets.

  • Three Effects: Accelerated fall towards Earth (at $g \approx 10\,m/s^2$), giving objects weight, and keeping planets/satellites in orbit.

• Mass vs. Weight:

  • Mass: The amount of "stuff" in an object, measured in kilograms ($kg$). It is constant throughout the universe.

  • Weight: The force of gravity pulling on a mass, measured in newtons ($N$). It varies depending on the gravitational field strength ($g$).

  • Measurement: Weight is measured with a spring balance (newton meter); mass is measured with a mass balance.

  • Relationship Formula: $\text{Weight} = \text{Mass} \times \text{Gravitational Field Strength}$ ($W = m \times g$).

  • Values of g: Earth $g \approx 10\,N/kg$. Moon $g \approx 1.6\,N/kg$.

  • Example: A $5\,kg$ mass weighs $50\,N$ on Earth ($5 \times 10$) and $8\,N$ on the Moon ($5 \times 1.6$).

Section 1: Forces and Friction

• Types of Force:

  • Gravity/Weight: Acts straight downwards.

  • Reaction Force: Acts perpendicular to and away from a surface.

  • Electrostatic Force: Between charged objects (Like repel, opposites attract).

  • Thrust: Push/pull from engines or rockets.

  • Drag/Air Resistance/Friction: Opposes motion.

  • Lift: Upward force from wings.

  • Tension: Pulling force in ropes or cables.

• Friction:

  • Force that opposes motion. Occurs in three ways: Static friction (gripping solids), Sliding friction (sliding solids), and Fluid friction (drag in liquids/gases).

  • Reducing Friction: Use lubricants (oil/grease) for solids; streamline shapes for fluids (e.g., sports cars, boat hulls).

  • Speed Dependency: In fluids, friction always increases as speed increases.

Section 1: Practical - Investigating Motion

• Toy Car Experiment:

  • Set-up: Ramp with light gates connected to a computer. Mark a start line and measure the distance between gates.

  • Procedure: Release car without pushing it. Light gates record timing as the car breaks the light beam.

  • Variables to Investigate: Load weights to change mass, use different materials on the ramp for friction, change ramp angle, or start the car higher up for gravity acceleration.

  • Reliability Tip: Use light gates instead of stopwatches to eliminate human reaction time errors.

Section 1: Newton's Three Laws of Motion

• First Law: If forces are balanced, an object remains stationary or continues moving at a constant velocity. A steady speed indicates zero resultant force.

• Second Law: If there is an unbalanced (resultant) force, the object will accelerate in that direction. Acceleration can be starting, stopping, speeding up, slowing down, or changing direction.

  • Formula: $\text{Resultant Force} = \text{Mass} \times \text{Acceleration}$ ($F = m \times a$).

  • Example: A car of $1750\,kg$ with a resultant force of $5200\,N$ has acceleration $a = \frac{5200}{1750} = 3.0\,m/s^2$.

• Third Law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. These forces act on different objects.

  • Example: In swimming, pushing the water back causes the water to push the body forward.

Section 1: Vectors and Scalar Quantities

• Vectors: Have both size (magnitude) and direction (e.g., Force, Velocity, Acceleration, Momentum).

• Scalars: Have size only (e.g., Mass, Temperature, Time, Length).

• Combining Vectors: Use direction positive (e.g., North = positive). Combine arrows end-to-end to find Resultant Force.

  • Example: $220\,N$ North, $180\,N$ South, and $90\,N$ South yields $220 - 180 - 90 = -50\,N$, which is $50\,N$ South.

Section 1: Terminal Velocity

• Process of Falling:

  1. Falling object starts with high acceleration (gravity > drag).

  2. Velocity increases, building up resistance/drag.

  3. Resistance reduces acceleration until gravity = resistance.

  4. Object reaches terminal velocity (steady speed).

• Shape and Area: Drag depends on shape and area. A skydiver without a parachute has small area and high terminal velocity ($120\,mph$). With a parachute, area increases significantly, lowering terminal velocity to a safe $15\,mph$.

Section 1: Hooke's Law

• Definition: Extension of a stretched wire/spring is proportional to the applied load (Force), provided the elastic limit is not exceeded.

• Formula Logic: $\text{Force} \propto \text{Extension}$.

• Elastic Limit (Point E): If force exceeds this, the material will be permanently stretched (plastic deformation).

• Elastic Behaviour: The ability of a material to return to its original shape after removing the force.

• Practical: Measure natural spring length, add masses, record new lengths. Extension = $\text{New length} - \text{Original length}$. Plot Force-Extension graph; Hooke's Law is shown by a straight line through the origin.

Section 1: Stopping Distances

• Formula: $\text{Stopping Distance} = \text{Thinking Distance} + \text{Braking Distance}$.

• Thinking Distance Factors: Speed, Reaction time (tiredness, alcohol, drugs, age, experience).

• Braking Distance Factors: Speed, Mass of vehicle, Brake condition, Grip (road surface, weather, tyres).

• Specific Ranges: At $70\,mph$, typical stopping distance is $96\,m$ ($21\,m$ thinking + $75\,m$ braking).

Section 1: Momentum and Collisions

• Momentum Formula: $\text{Momentum} = \text{Mass} \times \text{Velocity}$ ($p = m \times v$). Unit: $kg\,m/s$.

• Conservation of Momentum: Total momentum before a collision = Total momentum after a collision (if no external forces act).

• Force and Momentum: $\text{Force} = \frac{\text{Change in momentum}}{\text{Time taken}}$ ($F = \frac{mv - mu}{t}$).

• Car Safety: Crumple zones, seat belts (stretch), and air bags increase the time taken for a crash, thereby reducing the force ($F$) on the body to minimize injury.

Section 1: Turning Effects and Moments

• Moment Definition: The turning effect of a force.

• Formula: $\text{Moment} = \text{Force} \times \text{perpendicular distance from pivot}$. Unit: $Nm$.

• Max Moment: Achieved by pushing at right angles to the pivot.

• Centre of Gravity: The point through which the weight of a body acts. A suspended object rests with its centre of gravity vertically below the pivot.

• Principle of Moments: For a balanced object, Total Anticlockwise Moments = Total Clockwise Moments.

• Support Force: If an object is placed on a rod supported at both ends, the support closest to the object provides a larger force.

Section 2: Electricity - Circuits and Basics

• Current ($I$): Rate of flow of charge (electrons) in a circuit. Unit: Amperes ($A$).

• Voltage ($V$): Potential difference that drives current. Unit: Volts ($V$).

• Resistance ($R$): Opposes flow. Unit: Ohms ($\Omega$). Addition of components increases resistance.

• Ohmic Relationship: $V = I \times R$. Gradient of an $I-V$ graph shows inverse resistance behaviour (steeper = lower resistance).

• Test Circuit: Ammeter in series, Voltmeter in parallel with the component. Variable resistor used to change the current.

• Standard Supplies: UK Mains is approx $230\,V$, alternating current ($a.c.$). Batteries supply direct current ($d.c.$).

Section 2: Component Characteristics

• Current-Voltage Graphs:

  • Fixed Resistor/Wire: Straight line through origin (Current proportional to Voltage at constant temp).

  • Filament Lamp: Curved "S" shape (resistance increases as filament gets hot).

  • Diode: Current only flows in one direction; high resistance in reverse.

• Light-Dependent Resistor (LDR): Resistance falls in bright light, rises in darkness. Used for burglar detectors.

• Thermistor: Resistance drops in hot conditions, rises in cool conditions. Used for car engine sensors and fire alarms.

• Light-Emitting Diodes (LEDs): Emit light when current flows in forward direction. Used as indicator lights or in remote controls.

Section 2: Series and Parallel Circuits

• Series Circuits:

  • Single path; disconnected component breaks the whole circuit.

  • Total Voltage is shared: $V_{\text{total}} = V_1 + V_2 + ...$

  • Current is the same everywhere: $I_1 = I_2 = ...$

  • Total Resistance: Sum of all resistances ($R_{\text{total}} = R_1 + R_2 + ...$).

• Parallel Circuits:

  • Independent branches; removing one does not affect others. Standard for home electrics.

  • Voltage is the same across all branches: $V_1 = V_2 = ...$

  • Current is shared between branches: $I_{\text{total}} = I_1 + I_2 + ...$

  • Junctions: Total current entering a junction equals total current leaving it.

  • Adding a second resistor in parallel decreases total resistance.

Section 2: Charge, Energy, and Safety

• Charge Formula: $\text{Charge} = \text{Current} \times \text{Time}$ ($Q = I \times t$). Unit: Coulombs ($C$).

• Work/Energy Formula: $\text{Energy transferred} = \text{Charge} \times \text{Voltage}$ ($E = Q \times V$). Unit: Joules ($J$).

• Voltage Definition: One volt is one joule per coulomb ($1\,V = 1\,J/C$).

• Energy and Resistance: $E = Q \times I \times R$.

• Safety Features:

  • Three Wires: Live (Brown), Neutral (Blue), Earth (Green/Yellow).

  • Fuse: Melts and breaks the circuit if current exceeds rating.

  • Earth Wire: Attached to metal case; provides a low-resistance path if live wire touches case, blowing the fuse.

  • Double Insulation: Plastic casing requires no earth wire.

  • Circuit Breaker (RCCB): Detects current difference between live and neutral. Faster than fuses; can be reset.

• Power and Energy:

  • $\text{Power} = \text{Current} \times \text{Voltage}$ ($P = I \times V$).

  • $\text{Energy Transferred} = \text{Power} \times \text{Time}$ ($E = I \times V \times t$).

Section 2: Static Electricity

• Nature of Static: Caused by the build-up of charges that aren't free to move. Most common in insulators.

• Friction: Rubbing insulators causes electrons to be scraped off one and dumped on the other. Positive static is the absence of electrons.

• Interactions: Like charges repel; opposite charges attract.

• Examples:

  • Polythene rod: Electrons move from duster to rod (Rod -ve).

  • Acetate rod: Electrons move from rod to duster (Rod +ve).

• Detection: Gold-leaf electroscope (foil rises due to repulsion).

• Risks/Uses:

  • Lightning: Huge spark between cloud and earth.

  • Fuel Filling: Static build-up in pipes can cause sparks and explosions; avoided with metal nozzles and earthing straps.

  • Inkjet Printer: Droplets deflected by charged plates.

  • Photocopier: Charged image plate attracts black powder (toner).

Section 3: Waves - Basics

• Key Terms:

  • Wavelength ($\lambda$): Distance from peak to peak ($m$).

  • Frequency ($f$): Number of waves per second ($Hz$).

  • Amplitude: Height from rest position to crest ($m$).

  • Period ($T$): Time for one complete wave ($s$). Formula: $f = 1/T$.

  • Wave Speed: $\text{Speed} = \text{Frequency} \times \text{Wavelength}$ ($v = f \times \lambda$).

• Wave Types:

  • Transverse: Vibrations are at $90\degree$ to energy transfer direction (e.g., light, EM waves, water ripples).

  • Longitudinal: Vibrations are along the same direction as energy transfer (e.g., sound, ultrasound, shock waves). Consist of compressions and rarefactions.

• Doppler Effect: Observed change in frequency/wavelength when a source is moving relative to an observer. Source moving towards sounds higher pitched (short $\lambda$); moving away sounds lower pitched (long $\lambda$).

Section 3: Electromagnetic (EM) Spectrum

• Properties: All EM waves are transverse and travel at $3 \times 10^8\,m/s$ in a vacuum.

• Order (Long $\lambda$ / Low $f$ to Short $\lambda$ / High $f$): Radio, Microwaves, Infrared, Visible Light (ROYGBIV), UV, X-Rays, Gamma Rays.

• Uses:

  • Radio: Communication (Long-wave bends around Earth; Short-wave reflects off ionosphere).

  • Microwaves: Satellite TV/phones, heating food (absorbed by water molecules).

  • Infrared: Heaters, night vision equipment, grills.

  • Visible Light: Optical fibres (total internal reflection), photography.

  • Ultraviolet: Fluorescent lamps (efficiency).

  • X-Rays: Medicine (view bones; pass through flesh, not bone).

  • Gamma Rays: Sterilising medical equipment and food (kills microbes).

• Dangers:

  • Microwaves: Internal heating of body tissue.

  • Infrared: Skin burns.

  • Ultraviolet: Skin cell damage, blindness, cell mutation/cancer (ionising).

  • Gamma Rays: High energy ionising radiation; cell mutation and tissue damage.

Section 3: Light Reflection and Refraction

• Reflection: Law of Reflection states Angle of Incidence ($i$) = Angle of Reflection ($r$). Normal is the dashed line perpendicular to the surface.

• Refraction: Change in wave direction when entering a medium of different density. Entering denser medium: Bends towards the Normal. Entering less dense medium: Bends away from the Normal.

• Snell’s Law: $\text{Refractive Index} (n) = \frac{\sin(i)}{\sin(r)}$.

• Refractive Index Definition: $n = \frac{\text{speed of light in vacuum (c)}}{\text{speed of light in material (v)}}$. Typical glass $n \approx 1.5$, water $n \approx 1.33$.

• Total Internal Reflection (TIR): Occurs when light travels from dense to less dense medium and the angle of incidence is greater than the Critical Angle ($C$).

  • Critical Angle Formula: $\sin(C) = 1/n$.

  • Applications: Optical fibres (narrow core), Periscopes (using prisms).

Section 3: Sound Waves

• Nature: Longitudinal waves requiring a medium. Cannot travel in a vacuum. Speed increases with material density (solid > liquid > gas).

• Human Hearing Range: $20\,Hz$ to $20\,000\,Hz$.

• Pitch vs. Loudness:

  • Pitch depends on frequency (High $f$ = high pitch).

  • Loudness depends on amplitude (High amplitude = loud).

• Oscilloscope Display: Displays vibration patterns. Period ($T$) is time for one wave; $\text{frequency} = 1/T$.

• Sound Speed Practical: Use two microphones and an oscilloscope. Measure distance between microphones when waves align to find $\lambda$. Calculate $v = f \times \lambda$.

Section 4: Energy Resources and Transfer

• Energy Stores: Kinetic, Thermal, Chemical, Gravitational Potential, Elastic Potential, Electrostatic, Magnetic, Nuclear.

• Transfer Mechanisms: Mechanically (forces), Electrically (moving charge), By Heating (hot to cold), By Radiation (light/sound).

• Conservation of Energy: Energy cannot be created or destroyed, only transferred. Total input = Useful output + Wasted ($mostly thermal/dissipated$).

• Efficiency Formula: $(\text{Useful energy output} / \text{Total energy output}) \times 100\%$.

• Maths of Work and Power:

  • $\text{Work Done} (W) = \text{Force} \times \text{Distance moved}$ ($Unit: J$).

  • $\text{Power} (P) = \text{Work Done} / \text{Time taken}$ ($Unit: W; 1\,W = 1\,J/s$).

• Potential and Kinetic Energy Equations:

  • $KE = 1/2 \times m \times v^2$

  • $GPE = m \times g \times h$

  • Ideal Fall: $\text{GPE lost} = \text{KE gained}$.

Section 4: Thermal Transfer Mechanisms

• Conduction: Main heating in solids. Vibrating particles transfer kinetic energy to neighbors.

• Convection: Heating in liquids/gases. Hot fluid becomes less dense and rises; cold fluid sinks (Convection current).

• Radiation: Transfer by IR (Infrared) waves. Does not require particles (can travel in vacuum). Hotter objects emit more IR than they absorb.

• Surfaces and IR: Matt Black is the best absorber and emitter. Shiny/White are the worst absorbers (best reflectors) and worst emitters.

• Insulation: Traps air in small pockets to stop convection and reduce conduction (air at a low thermal conductivity).

Section 4: Energy Resources - Renewables vs. Non-Renewables

• Non-Renewables: Fossil Fuels (Coal, Oil, Natural Gas) and Nuclear. Reliable but pollutive (CO2, SO2, radioactive waste) and will run out.

• Typical Power Station: Boiler (heat water) -> Steam turns Turbine -> Generator (produces current).

• Renewables:

  • Wind: Kinetic energy of air turns blades. Renewable, no pollution, but noisy and weather-dependent.

  • Geothermal: Heat from radioactive decay in rocks. High set-up cost but free and reliable.

  • Solar Cells: Direct conversion of sunlight to $d.c.$ expensive set-up, weather dependent.

  • Wave: Coastal generators driven by water motion. High cost, unreliable.

  • Tidal Barrages: Estuary dams using tides. Reliable periodicity but affects habitats.

  • Hydroelectric: Flooding valleys; GPE to KE. Reliable, immediate response to demand.

  • Pumped Storage: Uses night surplus electricity to pump water up; used for peak demand.

Section 5: Solids, Liquids, and Gases

• Density Formula: $\rho = \text{mass} / \text{volume}$. Unit: $g/cm^3$ or $kg/m^3$.

• Pressure Formula: $P = \text{Force} / \text{Area}$. Unit: Pascals ($Pa$).

• Pressure in Liquids/Gases: $P = \text{height} \times \text{density} \times g$. Pressure acts equally in all directions.

• States of Matter:

  • Solid: Strong forces, fixed regular arrangement, vibratory motion.

  • Liquid: Weaker forces, close together but move past each other, irregular.

  • Gas: Almost no forces, free moving, high speed, random directions.

• Evaporation: Faster particles escape surface, decreasing average kinetic energy and cooling the remaining liquid.

• Temperature Scales: Absolute zero is $-273\degree C$ ($0\,K$). Particles have minimal energy. Conversion: $K = \degree C + 273$.

• Ideals Gas Laws:

  • $P \times V = \text{Constant}$ (at constant Temp).

  • $P / T = \text{Constant}$ (at constant Volume; Temp in Kelvin).

• Specific Heat Capacity: Change in thermal energy ($\Delta Q$) = $m \times c \times \Delta T$. Quantity needed to raise $1\,kg$ by $1\degree C$.

Section 6: Magnetism and Electromagnetism

• Magnetic Fields: Move from North to South. Seen with compasses or iron filings.

• Soft vs. Hard Magnetic Materials: Soft (e.g., Iron) loses magnetism easily; Hard (e.g., Steel) keeps it temporarily. Iron cores used in electromagnets.

• Electromagnets: Current flowing in a solenoid creates a bar-magnet field pattern. Induced magnetism increased by adding an iron core.

• The Motor Effect: Current-carrying wire in a magnetic field experiences a force (Fleming’s Left Hand Rule: thumb = Motion, first finger = Field, second finger = Current).

• Simple D.C. Motor: Coil on a spindle rotates; split-ring commutator swaps contacts to keep rotation continuous.

• Electromagnetic Induction: Creating voltage in a wire experiencing change in magnetic field (move wire or magnet). Generator (a.c.) uses slip rings and brushes.

• Transformers: Change alternating voltage using two coils on an iron core.

  • Equation: $V_p / V_s = N_p / N_s$.

  • Power Efficiency: $V_p I_p = V_s I_s$.

  • Transmission: Step-up transformer used at power stations to reduce heat loss in grid by boosting voltage (lowering current).

Section 7: Radioactivity

• Structure: Nucleus (protons + neutrons), Electrons (outside). Mass number (nucleons), Atomic number (protons).

• Isotopes: Same proton count, different neutron count. Unstable isotopes emit radiation.

• Radiation Types:

  1. Alpha ($\alpha$): $2p + 2n$ (Helium nucleus). Heavy, slow, high ionising, low penetration (stopped by paper).

  2. Beta ($\beta$): Electron from nucleus. Fast, small, moderate ionising, moderate penetration (stopped by thin metal).

  3. Gamma ($\gamma$): EM wave. No mass/charge, weakly ionising, high penetration (stopped by thick lead).

• Half-Life: Time taken for half of the radioactive atoms to decay or activity to fall by half. Used for isotope dating and medical tracing.

• Uses: Tracers (Gamma/Beta), Medicine (Check organs with iodine-123), Treating Cancer (Radiotherapy), Sterilising (Gamma rays), Industry (Thickness control using Beta).

• Nuclear Fission: Splitting heavy nuclei (U-235). Uses control rods (absorb neutrons) and moderators (slow neutrons).

• Nuclear Fusion: Joining light nuclei (Hydrogen) at high T/P ($10,000,000\degree C$) to overcome electrostatic repulsion.

Section 8: Astrophysics

• Hierarchy: Universe contains billions of Galaxies. Galaxy contains billions of Stars. Solar System contains one Star (Sun) + Planets, Dwarf Planets, Moons, Comets.

• Orbits: Gravity (gravitational force) provides centripetal force for orbit. Faster speed requires smaller radius.

• Orbital speed formula: $v = (2 \times \pi \times r) / T$.

• Stellar Evolution:

  1. Nebula -> Protostar -> Main Sequence Star.

  2. Small Stars: Red Giant -> White Dwarf.

  3. Large Stars: Red Supergiant -> Supernova -> Neutron Star or Black Hole.

• Classification: By color (Blue hottest, Red coolest). Absolute magnitude measures brightness at fixed distance.

• Hertzsprung-Russell: Graph of absolute magnitude vs. temperature showing main sequence, giants, and white dwarfs.

• Red-shift: Expansion of the universe causes light from distant galaxies to shift toward longer wavelengths.

  • Formula: $(\lambda - \lambda_0) / \lambda_0 = v / c$.

• Big Bang Proof: Observed red-shift of all distant galaxies and Cosmic Microwave Background (CMB) radiation.

Questions & Discussion (Interactive Practical Insights)

• Q: Why use light gates instead of a stopwatch for toy cars?

  • A: To eliminate human reaction time errors, which are significant over short intervals.

• Q: Which wire is a fuse connected to?

  • A: The Live wire.

• Q: What is absolute zero?

  • A: $-273\degree C$ ($0\,K$), where particles have as little kinetic energy as possible.

• Q: Explain the difference between contamination and irradiation.

  • A: Irradiation is exposure to radiation; contamination is the presence of radioactive particles on/in an object.

• Q: How do control rods work in a nuclear reactor?

  • A: They absorb excess neutrons to prevent the chain reaction from going out of control.

• Q: Why do comets travel faster near the Sun?

  • A: The increased pull of gravity at a closer distance causes the comet to accelerate speeds up to maintain its elliptical orbit.