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Flashcards · J249 · Topic 8

Global challenges flashcards

32 cards on Topic 8, subtopics P8.1 to P8.3. Pick a subtopic or study them all.

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P8.1 Physics on the move

Typical speeds: walking, cycling, car in town, car on a motorway, sound in air
Walking ~1.5 m/s, cycling ~6 m/s, car in town ~13 m/s, motorway ~30 m/s, sound in air ~340 m/s.
Typical accelerations: car pulling away, emergency braking, free fall
Car pulling away: 2 to 3 m/s². Emergency braking: 6 to 8 m/s². Free fall: about 10 m/s².
How do you measure reaction time with a ruler drop?
Drop the ruler without warning between the person’s thumb and finger, read the distance fallen when caught, convert it to a time, repeat and take the mean. A typical value is about 0.2 s.
Thinking distance and braking distance: what affects each?
Thinking (speed × reaction time): speed, tiredness, alcohol, drugs, distraction. Braking: speed, wet or icy roads, worn tyres or brakes, greater mass.
A driver at 90 km/h has a reaction time of 0.60 s. Thinking distance?
90 ÷ 3.6 = 25 m/s, so 25 × 0.60 = 15 m
Why is a large deceleration dangerous?
F = ma, so a large deceleration means a large force on the body, which can cause broken bones and internal injuries.
How do crumple zones, seat belts and airbags reduce injury?
They make the stop take a longer time (or distance), so the deceleration and the force on the person are smaller. Airbags also spread the force over a larger area.
Estimate the stopping distance at 25 m/s (reaction time 0.7 s, deceleration 5.0 m/s²).
Thinking: 25 × 0.7 = 17.5 m Braking: 25² ÷ (2 × 5.0) = 62.5 m Total ≈ 80 m
A 1000 kg car at 20 m/s stops in 40 m. Estimate the braking force.Higher
Kinetic energy = ½ × 1000 × 20² = 200 000 J. Force = 200 000 ÷ 40 = 5000 N.

P8.2 Powering Earth

Renewable vs non-renewable energy resource
Renewable: replaced as fast as it is used (wind, tides, the Sun, hydroelectricity, biofuel). Non-renewable: used faster than it forms, so it will run out (coal, oil, gas, nuclear fuel).
How do fossil-fuel and nuclear power stations generate electricity?
The fuel’s energy heats water to make steam, the steam turns a turbine, and the turbine turns a generator.
Compare gas and wind for generating electricity
Gas: reliable on demand, but burning it releases carbon dioxide and it needs a continuing fuel supply. Wind: no fuel and no carbon dioxide, but output depends on the weather.
National grid
The network of cables and transformers linking power stations to users.
Step-up transformer
Raises the potential difference, at the power station end of the grid.
Step-down transformer
Lowers the potential difference near where the power is used, so homes get a safe, usable supply.
Why is high-voltage transmission efficient?
For a fixed power, P = VI, so a high pd means a low current. Wasted power is I²R, so a lower current means much less heating in the cables.
Does a transformer increase power?
No. A transformer changes the potential difference. It does not increase power.
How do the turns on a step-up transformer compare?Higher
More turns on the secondary coil than the primary (Vp / Vs = Np / Ns), so the secondary pd is higher and, for the same power, the current is lower.
UK domestic electricity supply
Alternating current, 50 Hz, about 230 V. One cycle lasts 1 ÷ 50 = 0.02 s.
Live, neutral and earth wires: colours and potential differences
Live: brown, about 230 V. Neutral: blue, about 0 V, completes the circuit. Earth: green and yellow, 0 V, safety wire joined to the metal case.
What does the earth wire do if the live wire touches a metal case?
A large current flows through the earth wire instead of through the user. Without an earth wire the case would be at about 230 V and could give a shock.

P8.3 Beyond Earth

How does the radiation from a body change as it gets hotter?
The intensity (energy emitted per second per square metre) increases, and the strongest emission moves to a shorter wavelength.
Order of the planets from the Sun
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Geostationary vs polar orbit
Geostationary: about 36 000 km up, period 24 hours, above the equator, appears fixed (communications). Polar: low, period around 1.5 hours, passes over both poles (mapping, monitoring, weather).
In a circular orbit, which stay constant: speed, velocity, size of the resultant force?Higher
Speed and the size of the resultant force stay constant. The velocity changes because the direction changes; gravity acts towards the centre.
How are orbit radius and speed linked for a stable orbit?Higher
A lower orbit needs a faster speed (and has a shorter period). If the speed changes, the radius must change.
What determines whether a body warms, cools or stays at a constant temperature?Higher
Absorbed more than emitted: it warms. Emitted more than absorbed: it cools. Equal: constant temperature (equilibrium).
Red-shift
Light from a receding source is observed at a longer wavelength and lower frequency than it was emitted. A bigger shift means a faster recession.
How does red-shift data support an expanding universe?
Nearly every galaxy is red-shifted, and more distant galaxies have bigger red-shifts, so they move away faster: space itself is expanding.
How does the CMBR support the Big Bang model?
The early universe was very hot and dense and full of radiation. Expansion stretched its wavelengths, so it is now seen from all directions as microwaves.
How does a star form and become stable?
Gravity pulls a cloud of dust and gas together; it heats up until hydrogen nuclei fuse to helium, releasing energy. Inward gravitational collapse is then balanced by outward expansion from the fusion energy.
Life cycle of a star much more massive than the Sun
Cloud → protostar → stable star → red supergiant → supernova → neutron star or black hole.

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