Physics 8463.

4.1.1Energy stores and systems

• A system is an object or a group of objects; describe how the energy stored changes when a system changes (a ball thrown up, a car braking, a kettle boiling).
• Calculate energy changes caused by heating, by forces doing work and by an electric current.
• Kinetic, elastic potential and gravitational potential energy.
• Specific heat capacity: the energy to raise 1 kg by 1 °C.
• Power as the rate of transferring energy or doing work; 1 W is 1 J/s.

• \(E_{\mathrm{k}} = \tfrac{1}{2}mv^2\)
• \(E_{\mathrm{e}} = \tfrac{1}{2}ke^2\)
• \(E_{\mathrm{p}} = mgh\)
• \(\Delta E = mc\Delta\theta\)
• \(P = \dfrac{E}{t}\)
• \(P = \dfrac{W}{t}\)

Required practical 1 · specific heat capacity

4.1.2Conservation and dissipation

• Energy is never created or destroyed; in a closed system the total stays the same.
• Some energy is always dissipated into less useful stores (“wasted”); reduce it with lubrication and thermal insulation.
• Higher thermal conductivity means faster conduction; how wall thickness and conductivity affect how fast a building cools.
• Efficiency as a decimal or a percentage.
• HT Ways to make an energy transfer more efficient.

• \(\text{efficiency} = \dfrac{\text{useful output energy}}{\text{total input energy}}\)
• \(\text{efficiency} = \dfrac{\text{useful power output}}{\text{total power input}}\)

Required practical 2 Physics only · thermal insulators

4.1.3National and global energy resources

• Fossil fuels, nuclear fuel, bio-fuel, wind, hydroelectricity, geothermal, tides, the Sun and water waves.
• Renewable against non-renewable; reliability; uses in transport, electricity generation and heating.
• Environmental impact, and trends in how resources are used.
• Science can identify the problems but political, social, ethical and economic factors limit what is done.
• How each resource generates electricity is not needed.

4.2.1Current, potential difference and resistance

• Draw and read circuit diagrams with standard symbols.
• Current is the rate of flow of charge, and is the same all round a single loop.
• Ohmic conductors at constant temperature; non-constant resistance of filament lamps, diodes, thermistors and LDRs, and their uses (thermostats, lights that switch on at dusk).
• Measuring resistance with an ammeter and voltmeter; reading I–V graphs.

• \(Q = It\)
• \(V = IR\)

Required practicals 3 · resistance, and 4 · I–V characteristics

4.2.2Series and parallel circuits

• Series: same current, p.d. shared, resistances add.
• Parallel: same p.d., currents add, total resistance less than the smallest resistor.
• Explain why adding resistors in series raises the total and in parallel lowers it.
• Calculations in series circuits; the total of two resistors in parallel is not calculated.

• \(R_{\mathrm{total}} = R_1 + R_2\) (not on the sheet)

4.2.3Domestic uses and safety

• Direct and alternating p.d.; UK mains is ac at 50 Hz and about 230 V.
• Three-core cable: live (brown), neutral (blue), earth (green and yellow).
• Why a live wire is dangerous even with the switch open, and why any connection between live and earth is dangerous.

4.2.4Energy transfers

• Power of a device from p.d. and current, or from current and resistance.
• Energy transferred depends on power and time switched on; appliances transfer energy to motors and heaters.
• The National Grid: step-up transformers for transmission, step-down for homes, and why this is efficient.

• \(P = VI\)
• \(P = I^2R\)
• \(E = Pt\)
• \(E = QV\)

4.3.1Changes of state and density

• Particle diagrams of solids, liquids and gases, and how they explain differences in density.
• Melting, freezing, boiling, evaporating, condensing and subliming conserve mass; they are physical changes, reversed to get the original material back.

• \(\rho = \dfrac{m}{V}\)

Required practical 5 · density

4.3.2Internal energy and energy transfers

• Internal energy is the total kinetic and potential energy of all the particles.
• Heating raises the temperature or changes the state.
• Specific latent heat of fusion and of vaporisation: energy for a change of state with no change in temperature.
• Heating and cooling graphs; telling specific heat capacity and latent heat apart.

• \(\Delta E = mc\Delta\theta\)
• \(E = mL\)

4.3.3Particle model and pressure

• Gas molecules move randomly; temperature relates to their average kinetic energy.
• At constant volume, a hotter gas exerts a higher pressure.
• Physics only Pressure gives a force at right angles to the walls; at constant temperature a larger volume means a lower pressure.
• Physics only HT Doing work on a gas (a bicycle pump) raises its temperature.

• \(pV = \text{constant}\) Physics only

4.4.1Atoms and isotopes

• Atom radius about \(1 \times 10^{-10}\) m; nucleus under \(\dfrac{1}{10\,000}\) of that but holding almost all the mass.
• Electrons in energy levels move up by absorbing and down by emitting electromagnetic radiation.
• Atomic number, mass number, isotopes and positive ions.
• How the model changed: solid spheres, plum pudding, the nuclear model from alpha scattering, Bohr’s orbits, protons, Chadwick’s neutron.

4.4.2Atoms and nuclear radiation

• Unstable nuclei decay at random; activity in becquerels; count rate.
• Alpha, beta, gamma and neutron emission; penetration, range in air and ionising power, and choosing a source for a job.
• Balanced nuclear equations for single alpha and beta decays.
• Half-life, found from data or a graph. HT The fraction left after a number of half-lives, as a ratio.
• Contamination against irradiation; why studies of radiation effects are published for peer review.

4.5.1Forces and their interactions

• Scalars and vectors; forces as arrows.
• Contact forces (friction, air resistance, tension, normal contact) and non-contact forces (gravitational, electrostatic, magnetic).
• Weight, centre of mass, and weight in proportion to mass.
• The resultant of forces along a line.
• HT Free body diagrams; resolving a force into two components; scale vector diagrams for resultants and equilibrium.

• \(W = mg\)

4.5.2Work done and energy transfer

• A force does work when it moves an object; 1 J = 1 N m.
• Work against friction raises the temperature.

• \(W = Fs\)

4.5.3Forces and elasticity

• Stretching, bending and compressing need more than one force; elastic against inelastic deformation.
• Extension is proportional to force up to the limit of proportionality; linear and non-linear graphs.
• Work done on a spring is stored as elastic potential energy.

• \(F = ke\)
• \(E_{\mathrm{e}} = \tfrac{1}{2}ke^2\)

Required practical 6 · force and extension

4.5.6.1Describing motion

• Distance and displacement, speed and velocity; typical speeds (walk 1.5, run 3, cycle 6, sound 330 m/s).
• Speed from the gradient of a distance–time graph; acceleration from the gradient of a velocity–time graph.
• HT Circular motion has constant speed but changing velocity; tangents to curved distance–time graphs; distance from the area under a velocity–time graph.
• Free fall at about 9.8 m/s²; terminal velocity. Physics only Velocity–time graphs for terminal velocity, explained by the forces.

• \(s = vt\)
• \(a = \dfrac{\Delta v}{t}\)
• \(v^2 - u^2 = 2as\)

4.5.6.2Newton’s laws

• First law: velocity changes only if there is a resultant force. HT Inertia.
• Second law: acceleration is proportional to resultant force and inversely proportional to mass. HT Inertial mass as \(\dfrac{\text{force}}{\text{acceleration}}\).
• Estimating speeds, accelerations and forces for road vehicles.
• Third law: interacting objects exert equal and opposite forces.

• \(F = ma\)

Required practical 7 · force, mass and acceleration

4.5.6.3Forces and braking

• \(\text{stopping distance} = \text{thinking distance} + \text{braking distance}\).
• Reaction times of 0.2 to 0.9 s, lengthened by tiredness, drugs, alcohol and distractions; measuring them.
• Road, weather, brakes and tyres affect braking distance; brakes do work that heats them; dangers of large decelerations.
• Estimate how emergency stopping distance changes over typical speeds. Physics only Read graphs of stopping distance against speed for different vehicles.
• HT Estimate braking forces for road vehicles.

4.5.7Momentum HT

• Momentum is conserved in a closed system; describe and explain examples such as collisions.
• Physics only Calculations for events such as collisions.
• Physics only Force is the rate of change of momentum; air bags, seat belts, crash mats, helmets and soft playground surfaces.

• \(p = mv\)
• \(F = \dfrac{m\Delta v}{\Delta t}\) Physics only

4.6.1Waves in air, fluids and solids

• Transverse and longitudinal waves (compressions and rarefactions); the wave moves, not the medium.
• Amplitude, wavelength, frequency and period; measuring the speed of sound and of ripples.
• Physics only How speed, frequency and wavelength change when sound enters a new medium.

• \(T = \dfrac{1}{f}\)
• \(v = f\lambda\)

Required practical 8 · waves in a ripple tank and a solid

4.6.2Electromagnetic waves

• The spectrum from radio to gamma; all at the same speed in a vacuum; transverse waves that transfer energy.
• Ray diagrams for refraction. HT Wavefront diagrams; absorption, transmission, refraction and reflection depending on wavelength.
• HT Radio waves made by, and inducing, oscillations in circuits.
• Hazards of ultraviolet, X-rays and gamma rays; dose in sieverts.
• Uses of each type. HT Why each type suits its use.

Required practical 10 · infrared from different surfaces

4.7.1Magnets and magnetic fields

• Poles; like poles repel, unlike attract; permanent and induced magnets.
• Field lines run north to south and are densest at the poles; plotting them with a compass.
• The Earth’s magnetic field as evidence of a magnetic core.

4.7.2The motor effect

• The field around a wire and a solenoid; an iron core makes an electromagnet.
• Physics only Explaining electromagnetic devices from diagrams.
• HT Fleming’s left-hand rule; the force on a wire; how a coil turns in a motor.
• Physics only HT Loudspeakers and headphones.

• \(F = BIl\) HT