Global Challenges | OCR GCSE Physics A, Higher tier (J249)

Global Challenges

239 questions
13 subtopics
Paper 4
Paper 4

Global Challenges is examined in Physics A Paper 4.

It covers typical speeds and everyday accelerations, reaction time, stopping distance and the dangers of large decelerations, estimating stopping distances and the forces in a collision, energy resources, and trends in their use, the national grid, and step-up and step-down transformers, why the national grid is efficient, and the transformer equation, mains electricity: a.c., d.c. and the domestic supply, live, neutral and earth wires, and electrical danger, red-shift and the Big Bang, the formation of the Sun, and radiation from bodies, the solar system, circular orbits and stable orbits and radiation and the temperature of a body, and seismic waves and sonar.

Sample questions from Global Challenges

Answer each one closed book first, then open the answer.

Typical speeds and everyday accelerations

What does an acceleration of 2 m/s² mean in words?

Show the answer
The speed increases by 2 m/s during every second.
Reaction time, stopping distance and the dangers of large decelerations

Why can braking too hard cause a vehicle to skid?

Show the answer
If the braking force demanded is greater than the friction the tyres can provide, the wheels lock and the vehicle slides, so the driver loses steering control and the stopping distance increases.
Energy resources, and trends in their use

Why is biofuel counted as renewable when burning it releases carbon dioxide?

Show the answer
The crops or waste it is made from can be regrown, and growing plants absorb carbon dioxide, so the resource is replaced as it is used.
The national grid, and step-up and step-down transformers

Where in the grid is a step-down transformer found?

Show the answer
At substations, between the transmission cables and the local supply to consumers.
Mains electricity: a.c., d.c. and the domestic supply

Why is the mains supply dangerous to touch?

Show the answer
About 230 V is large enough to drive a current through a person that can stop the heart or cause burns.
Live, neutral and earth wires, and electrical danger

How does insulation around a wire protect the user?

Show the answer
The plastic covering has a very high resistance, so almost no current can pass from the wire into anything that touches it.
The formation of the Sun, and radiation from bodies

What are the stages in the life of a star before it becomes stable?

Show the answer
A cloud of dust and gas is pulled together by gravity to form a protostar, which heats until fusion starts and it becomes a main sequence star.
The solar system

Describe a polar orbit and what it is used for.

Show the answer
The satellite orbits at low altitude passing close to both poles and completes an orbit in a couple of hours, so the Earth turns beneath it and the whole surface is scanned, which suits weather monitoring and imaging.

The 13 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Typical speeds and everyday accelerations	Typical speeds for walking, running, cycling, sound, wind, cars, trains and aeroplanes, what an acceleration in m/s² means, estimating the accelerations of cars, sprinters, cyclists and buses, and converting kilometres per hour, kilometres, tonnes and minutes for speed calculations.	21
Reaction time, stopping distance and the dangers of large decelerations	Typical reaction times, measuring them by ruler drop or computer and what lengthens them, thinking and braking distance and what increases each, why large decelerations injure and how crumple zones, seat belts and air bags help, overheated brakes and skids, and handling reaction-time data with averages, histograms, scatter diagrams and sampling.	32
Estimating stopping distances and the forces in a collision	Estimated stopping distances at typical speeds, how thinking and braking distance change when speed doubles and why, calculating overall stopping and thinking distances, estimating the braking, driving and collision forces on cars, lorries and passengers, and why collision forces are so much larger than in ordinary driving.	24
Energy resources, and trends in their use	Renewable and non-renewable resources, how wind, hydroelectricity, tides, the Sun and biofuel are used, what energy resources are used for, trends in coal, gas, nuclear and renewables and the reasons behind them, and ethical questions about nuclear waste, dams and wind farms that science alone cannot settle.	23
The national grid, and step-up and step-down transformers	What the national grid is, the potential differences used for transmission and for homes, what substations do, and where step-up and step-down transformers sit and which coil has more turns.	16
Why the national grid is efficient, and the transformer equation	Why a high potential difference cuts the energy wasted in cables, how loss depends on current, why thick cables help, the turns ratio equation with calculations, and the power relationship for an ideal transformer.	17
Mains electricity: a.c., d.c. and the domestic supply	The UK mains supply as alternating current at 50 Hz and about 230 V, the length of one cycle, why the mains is dangerous, direct supplies from cells and adapters, how direct and alternating potential differences and their traces differ, and why a transformer needs an alternating supply.	16
Live, neutral and earth wires, and electrical danger	What the live, neutral and earth wires do, the potential differences between them and their colours, why a live wire is dangerous even with the switch open, how insulation, double insulation, the earth wire and a fuse protect the user, and why wet hands raise the danger.	16
Red-shift and the Big Bang	What red-shift is and what it shows about galaxies moving away, why greater distance means greater red-shift and an expanding universe, the Big Bang model, the cosmic microwave background and other supporting evidence, and why the model stays open to change.	16
The formation of the Sun, and radiation from bodies	How a collapsing cloud of dust and gas formed the Sun, what fusion needs and does, why a star stays stable, how stars end, and how emitted radiation depends on temperature.	16
The solar system	The eight planets in order and how the inner and outer planets differ, the gravity that holds them in orbit, minor planets, asteroids and comets, natural and artificial satellites and how they compare, and what geostationary and polar orbits are used for.	10
Circular orbits and stable orbits	Where gravity acts on an orbiting body, why a circular orbit has constant speed but changing velocity, why the Sun does no work on a planet, and how orbital speed and radius are linked.	16
Radiation and the temperature of a body, and seismic waves and sonar	How the balance of absorbed and emitted radiation sets the temperature of a road, a white surface and the Earth, how the atmosphere affects incoming and outgoing radiation, P and S seismic waves and what they reveal about the Earth's core, and finding the depth of water by sonar.	16

Global Challenges is 239 of the 1,584 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.