Energy
Energy tests the forms of energy, how energy is transferred and conserved, how power, cost and efficiency are calculated, and how electricity behaves in circuits. Questions ask you to trace energy transfers in a device, compare renewable and non-renewable resources, explain heat transfer, and solve numerical problems using simple formulae.
Forms, transfer and conservation
Forms include kinetic, gravitational potential, elastic, chemical, thermal, light, sound, electrical and nuclear. Energy is transferred, not used up; in every transfer some spreads to the surroundings as thermal energy, which is why no device is 100 per cent efficient. Renewable resources such as solar, wind, hydroelectric, tidal, wave, geothermal and biomass are replenished naturally. Non-renewable resources such as coal, oil, gas and nuclear fuel are finite.
Power, energy and efficiency
Power = energy transferred divided by time, in watts (joules per second). Home energy = power in kW multiplied by time in hours, in kWh; cost = kWh multiplied by price per kWh. Efficiency = useful energy output divided by total energy input, multiplied by 100 per cent.
Heat transfer
Conduction occurs in solids as vibrating particles pass energy on, best in metals with free electrons. Convection occurs in fluids: heated fluid expands, becomes less dense and rises while cooler fluid sinks, forming a convection current. Radiation is infrared and needs no medium; dark matt surfaces absorb and emit best, shiny light surfaces reflect.
Electricity
Current in amperes is measured with an ammeter in series; potential difference in volts with a voltmeter in parallel; resistance in ohms. Ohm's law: V = I x R. In series the current is the same everywhere, voltage is shared and resistances add. In parallel the voltage is the same across each branch and current is shared.
Worked example
A toy car's motor is supplied with 500 J of electrical energy and transfers 150 J as kinetic energy. Efficiency = 150 / 500 x 100 = 30 per cent. Reasoning: divide the useful output by the total input, then multiply by 100 to express the answer as a percentage. The remaining 350 J is dissipated mainly as heat and sound in the motor and gears. An answer of 0.3 per cent, or 30 with no unit, loses the mark.
Traps that cost marks
Confusing energy (J) with power (W); forgetting to convert watts to kilowatts and minutes to hours; quoting efficiency above 100 per cent; describing convection in a solid; swapping the series and parallel rules; assuming renewable means no environmental cost.
Forces and Earth in Space
Forces and Earth in Space tests how forces change motion and shape, how weight and pressure are calculated, and how Earth's movements produce day and night, the seasons, moon phases, eclipses and tides. Questions mix short definitions with calculations and ordered explanations of astronomical cycles.
Forces and motion
A force is a push or a pull measured in newtons. Contact forces include friction, air resistance, tension and normal reaction; non-contact forces include gravity, magnetism and electrostatic force. The resultant force is the sum of the forces, taking direction into account. If the resultant is zero the forces are balanced: a stationary object stays still and a moving object continues at constant speed in a straight line. An unbalanced force causes acceleration, deceleration or a change of direction. Friction opposes motion and generates heat; it is reduced by lubricants, streamlining and bearings, but is needed for braking and walking.
Mass, weight and pressure
Mass is the quantity of matter, in kilograms, and does not change with location. Weight is the force of gravity on a mass, in newtons: W = m x g, with g about 10 N/kg on Earth. Pressure = force divided by area, in pascals; a sharp edge concentrates force on a small area, giving high pressure. Liquid pressure increases with depth.
Speed and graphs
Speed = distance divided by time; convert units before substituting. On a distance-time graph the gradient is the speed: a horizontal line means at rest, a straight slope means constant speed, a curve means acceleration.
Earth in space
Earth rotates on its axis once in about 24 hours from west to east, so the Sun appears to cross the sky and day alternates with night. Earth orbits the Sun once in about 365 and a quarter days; because the axis is tilted about 23.5 degrees, each hemisphere receives more direct sunlight at different points in the orbit, producing the seasons. The Moon orbits Earth about every 28 days, showing phases as the lit portion we see changes. In a solar eclipse the Moon lies between Sun and Earth; in a lunar eclipse Earth lies between Sun and Moon. Tides are caused mainly by the Moon's gravitational pull.
Worked example
An astronaut has a mass of 60 kg. Weight on Earth = 60 x 10 = 600 N. On the Moon, where g = 1.6 N/kg, weight = 60 x 1.6 = 96 N. Reasoning: mass stays at 60 kg because it measures the amount of matter; only the gravitational field strength changes. Standing on both feet with a total contact area of 0.03 m squared, the pressure on Earth's surface would be 600 / 0.03 = 20 000 Pa.
Traps that cost marks
Giving weight in kilograms; explaining seasons by distance from the Sun rather than axial tilt; explaining night by Earth's orbit; swapping solar and lunar eclipses; substituting speed without converting metres and seconds.