OCR GCSE Physics: Energy and Global Challenges (P7–P8)
OCR GCSE Physics: Energy and Global Challenges (P7–P8)
Topics P7 Energy and P8 Global challenges of OCR Gateway Science A GCSE Physics (J249) close Paper 2 — and close the course. P7 is the unifying idea of physics: energy is stored, transferred and conserved, never created or destroyed. P8 is OCR's applied, outward-looking topic, taking the physics of the whole course to real-world problems — the safety of transport, the momentum of collisions, how we generate and distribute electricity, and the physics of the solar system and the wider universe. This guide works through both topics with the definitions, equations and worked examples that earn marks, and it forms part of our complete OCR GCSE Physics revision guide.
P7 — Energy
Energy Stores and Transfers
Modern GCSE physics describes energy in terms of stores and transfers. Energy can be held in a number of stores — kinetic (movement), gravitational potential (height), elastic potential (stretched or compressed), thermal (internal energy), chemical, nuclear, magnetic and electrostatic. Energy is transferred between these stores by four pathways: mechanically (a force doing work), electrically (a current doing work), by heating, and by radiation (including light and sound).
The bedrock principle is the conservation of energy: energy cannot be created or destroyed, only transferred from one store to another. In any real process, some energy is transferred to less useful stores — usually the thermal store of the surroundings — and is said to be "dissipated" or wasted.
Kinetic and Potential Energy
Two energy equations recur constantly. The kinetic energy of a moving object is
Ek=21mv2
where m is mass and v is speed. Because the speed is squared, doubling an object's speed quadruples its kinetic energy — a fact with major consequences for stopping distances in P8.
The gravitational potential energy gained when an object of mass m is raised through a height h is
Ep=mgh
where g is the gravitational field strength. When an object falls, its gravitational potential store is transferred to its kinetic store, so (ignoring air resistance) mgh becomes 21mv2.
Worked example. A ball of mass 0.5 kg is raised to a height of 3 m (g=9.8 N/kg). Its gain in gravitational potential energy is
Ep=mgh=0.5×9.8×3=14.7 J
Elastic Potential Energy and Work Done
Stretching or compressing a spring stores elastic potential energy. Work done is the energy transferred when a force moves an object through a distance, W=Fd, measured in joules — and work done against friction transfers energy to thermal stores. This links directly back to forces in P2.
Power and Efficiency
Power is the rate of energy transfer — the energy transferred per second — measured in watts:
P=tE
A more powerful device transfers the same energy in less time, or more energy in the same time.
Efficiency measures how much of the energy supplied to a device ends up in the useful output store rather than being wasted:
efficiency=total energy inputuseful energy output
Efficiency is a fraction or percentage and can never exceed 1 (or 100%), because some energy is always dissipated. Reducing wasteful transfers — through lubrication, insulation or better design — raises efficiency.
Worked example. A motor is supplied with 500 J of electrical energy and does 350 J of useful work. Its efficiency is
efficiency=500350=0.70=70%
Thermal Energy Transfer
Thermal energy moves by three mechanisms. Conduction transfers energy through solids as vibrating particles pass energy to their neighbours — metals conduct well because of their free electrons. Convection transfers energy through fluids as warmer, less dense regions rise and cooler regions sink, setting up a convection current. Radiation transfers energy as infrared electromagnetic waves and needs no medium — it is how the Sun's energy reaches us. Reducing unwanted transfers — loft insulation, cavity walls, double glazing — is a common exam context, and investigating thermal insulation is one of the required practicals.
P8 — Global Challenges
Transport and Safety: Stopping Distances
The total stopping distance of a vehicle is the sum of the thinking distance (the distance travelled during the driver's reaction time, before the brakes are applied) and the braking distance (the distance travelled while the vehicle decelerates to a stop):
total stopping distance = thinking distance + braking distance.
Thinking distance increases with speed and with anything that lengthens reaction time — tiredness, alcohol, drugs or distraction. Braking distance increases with speed, and because kinetic energy is proportional to the square of the speed (Ek=21mv2), doubling the speed quadruples the energy the brakes must remove, so braking distance rises steeply. It also increases on wet or icy roads, or with worn brakes or tyres, which reduce friction. During braking, work done by friction transfers the vehicle's kinetic energy to the thermal store of the brakes, which get hot — a large amount of energy transferred quickly, which is why brakes can overheat.
Momentum
Momentum is a property of a moving object, defined as
p=mv
where m is mass and v is velocity; it is a vector, measured in kgm/s. In a closed system, momentum is conserved — the total momentum before an event (such as a collision or explosion) equals the total momentum afterwards. On Higher tier, force is related to the rate of change of momentum, which explains why safety features that increase the time of a collision — crumple zones, airbags, seatbelts — reduce the force on the occupants and so reduce injury.
Worked example. A trolley of mass 2 kg moves at 3 m/s. Its momentum is
p=mv=2×3=6 kgm/s
Energy Resources
Electricity is generated from a range of energy resources, split into non-renewable and renewable:
- Non-renewable: fossil fuels (coal, oil, gas) and nuclear fuel. Reliable and high-output, but fossil fuels release carbon dioxide and other pollutants, and their supply is finite.
- Renewable: wind, solar, hydroelectric, tidal, wave, geothermal and biofuel. They will not run out and generally produce far less pollution, but many are intermittent (wind and solar depend on the weather) or limited to particular locations.
Exam questions frequently ask you to evaluate a resource, weighing reliability, cost, environmental impact and output — so practise giving a balanced, two-sided answer with a conclusion rather than a one-sided list.
The National Grid
The national grid distributes electrical energy from power stations to consumers. It uses transformers to do so efficiently: a step-up transformer raises the voltage to very high values for transmission, which lowers the current for a given power. Since the energy wasted heating the cables depends on I2R, a lower current means far less energy lost, so high-voltage transmission is much more efficient. A step-down transformer then reduces the voltage to safe levels for homes and businesses. This ties directly back to the transformers and power equations of P3 and P4.
The Solar System and Orbits
Our solar system consists of the Sun at the centre, orbited by eight planets, along with dwarf planets, moons, asteroids and comets, held in orbit by the Sun's gravity. For a stable orbit, the gravitational force provides the centripetal force that keeps a body moving in a curved path; a body closer to the Sun must move faster to stay in orbit. Moons orbit planets, and artificial satellites orbit the Earth for communication, observation and navigation.
The Life Cycle of Stars
A star forms from a cloud of gas and dust (a nebula) pulled together by gravity into a protostar, which heats until nuclear fusion begins, forming a main-sequence star like our Sun. Fusion in the core balances the inward pull of gravity, keeping the star stable for billions of years. What happens next depends on the star's mass: a Sun-sized star swells into a red giant, then sheds its outer layers to leave a white dwarf; a much more massive star becomes a red supergiant, explodes as a supernova, and leaves behind a neutron star or, for the greatest masses, a black hole. The fusion of light elements in stars, and the supernova explosions, are where the chemical elements of the universe are made.
Red-Shift and the Expanding Universe
Light from distant galaxies is shifted towards the red (longer-wavelength) end of the spectrum — red-shift — and the more distant the galaxy, the greater its red-shift. This is interpreted as evidence that distant galaxies are moving away from us, and that the more distant they are, the faster they recede. The conclusion is that the universe is expanding, which in turn supports the Big Bang theory — the idea that the universe began from a single, extremely hot and dense point and has been expanding ever since. This is a favourite topic for questions that ask you to explain how observation (red-shift) supports a scientific theory.
How These Topics Connect
P7 and P8 are where the whole course comes together. The energy stores and conservation of P7 explain the kinetic-energy behaviour behind stopping distances and the efficiency of energy resources in P8. The forces and motion of P2 return in momentum and safety; the transformers of P4 return in the national grid; the waves of P5 return in red-shift; and the nuclear physics of P6 returns in nuclear power and in fusion inside stars. Physics is a connected subject, and these final topics reward students who can see those links and reason across them.
To drill these topics interactively, work through the Energy course and the Global Challenges course, each taking you from the foundations to exam-level questions with an AI tutor on hand. For calculation and six-mark technique, see the exam technique guide.