Flashcards · 9630 · A Level Topic 13

Energy sources (A Level) flashcards

21 cards on A Level Topic 13, subtopics 3.13.1 to 3.13.4. Pick a subtopic or study them all.

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Moment of inertia: definition and what it depends on: A measure of how hard it is to change an object's rotation about an axis: I = Σmr² (kg m²). It depends on the mass, how the mass is distributed about the axis, and the axis.
Torque and rotational Newton's second law: Torque T = Fr, with r the perpendicular distance from the axis to the line of the force. Resultant torque T = Iα, where α is the angular acceleration.
Rotational kinetic energy, work and power: Ek(rot) = ½Iω². Work done by a torque W = Tθ. Power P = Tω.
Conservation of angular momentum: Angular momentum Iω of a system stays constant if no external resultant torque acts on it.
A spinning diver tucks in, reducing I from 3.6 to 1.2 kg m². What happens to ω (1.5 rad s⁻¹) and Ek?: Iω is conserved: ω = 3.6 × 1.5/1.2 = 4.5 rad s⁻¹. Ek rises threefold (4.05 J to 12.15 J); the extra comes from work done by the diver's muscles.
Equations for uniform angular acceleration: ω = ω₀ + αt θ = (ω₀ + ω)t/2 θ = ω₀t + ½αt² ω² = ω₀² + 2αθ

Maximum power available from the wind: P = ½πr²ρv³, where r is the blade length and ρ the air density. Mass per second ρπr²v carries kinetic energy ½mv² per second.
Wind speed doubles. Effect on the power available?: P ∝ v³, so it rises by a factor of 8.
Why can a turbine not extract all the wind's kinetic energy?: The air must keep moving past the rotor, so it keeps some kinetic energy. There are also friction, turbulence, gearbox and generator losses.
Why are turbines in a wind farm spaced well apart?: Behind each turbine is a wind shadow of slower, turbulent air; spacing stops one turbine sitting in another's shadow.
Advantages and disadvantages of wind power: For: no fuel, no carbon dioxide in operation, low running cost. Against: intermittent, noise, visual impact, risk to birds and bats, needs a lot of land or sea.

Intensity at distance r from a point source: I = P/(4πr²), in W m⁻². I ∝ 1/r², so tripling the distance cuts intensity to one ninth.
Solar cell V–I characteristic: where is the maximum power?: Current is nearly constant at low p.d., then falls steeply to zero at the open-circuit voltage. Power IV is zero at both ends and greatest near the knee: calculate IV for each point.
Solar cells in series and in parallel: n cells in series: n × the voltage, same current. m branches in parallel: same voltage, m × the current. A series string is limited by its weakest (e.g. shaded) cell.
Panel of area 1.6 m², 800 W m⁻² at right angles, 17% efficient. Output power?: Incident power = 800 × 1.6 = 1280 W. Output = 0.17 × 1280 = 218 W.
Required practical: testing the inverse square law with an LDR: Small lamp and LDR on a rule in a darkened room; vary d and record resistance, subtracting background. A distance offset to the filament is a systematic error; the LDR may not respond in proportion to intensity.

Power from water falling through a head h: P = ρQgh, where Q is the volume flow rate (m³ s⁻¹) and ρ the density of water.
Energy changes in a hydroelectric station: Gravitational potential energy of water → kinetic energy of water → kinetic energy of the turbine → electrical energy from the generator.
Pumped storage: how it works: Water is pumped to an upper reservoir when demand is low and released through turbines when demand is high. It is an energy store, not a source, and returns less energy than it uses.
Why do hydroelectric and pumped storage stations suit back-up supply?: They can start up and reach full output within minutes when demand rises, unlike base-load stations that run continuously.
Round-trip efficiency of pumped storage with pumping and generating each 85% efficient: 0.85 × 0.85 = 0.72 (72%).