Physics

Energy stores and transfers

Energy is stored in stores: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear. It moves between stores in four ways: mechanically (a force does work), electrically (a current does work), by heating, and by radiation (light, sound).

A system is an object or group of objects. When a system changes, energy moves between stores. Example: a ball thrown upwards: kinetic store → gravitational potential store.

Specific heat capacity (c): the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water has a high c (4200 J/kg °C), so it heats up and cools down slowly.

Power and efficiency

Power is the rate of energy transfer, or the rate of doing work. 1 watt = 1 joule per second.

Efficiency is a decimal or a percentage (× 100). It can never be more than 1 (100%). Wasted energy is usually dissipated to the thermal store of the surroundings.

Reducing wasted energy

• Lubrication reduces friction between moving parts.
• Thermal insulation (loft insulation, cavity walls, double glazing) reduces heat loss. Materials with a low thermal conductivity transfer energy more slowly.
• Thicker walls and walls made of lower thermal conductivity material mean a slower rate of cooling for a building.

Energy resources

Trend: the UK is using less coal and more renewables, but fossil fuels are still used because they're reliable and existing power stations are already built. Changing costs money and needs political will.

Current, charge and potential difference

• Current (I, amps): the rate of flow of charge. Q = It (charge in coulombs, time in seconds).
• Potential difference (V, volts): the energy transferred per unit charge. Measured with a voltmeter in parallel.
• Resistance (R, ohms Ω): how much a component opposes the current. V = IR
• Ammeters go in series; voltmeters go in parallel.

Components and I–V graphs

Series and parallel

Mains electricity

• UK mains is a.c. at 230 V and 50 Hz. Batteries give d.c. (one direction).
• Live (brown): carries the 230 V. Neutral (blue): completes the circuit, around 0 V. Earth (green and yellow): safety wire, only carries current if there's a fault.
• Touching the live wire is dangerous even if the switch is off: your body is at 0 V, so there's a large p.d. and a current flows through you.

Power and energy in circuits

The National Grid

A system of cables and transformers linking power stations to consumers. Step-up transformers increase the p.d. (to around 400 000 V) so the current is lower. Lower current means less heating of the cables, so less energy is wasted and transmission is more efficient. Step-down transformers reduce the p.d. to 230 V for homes.

Static electricity Triple

• Rubbing two insulators together transfers electrons from one to the other. The one that gains electrons becomes negative; the one that loses them becomes positive.
• Like charges repel; unlike charges attract. A charged object can attract small uncharged objects.
• If enough charge builds up, the p.d. gets high enough for a spark to jump across the gap.
• A charged object creates an electric field around it. The field is strongest close to the object. Field lines point away from positive and towards negative charges.

Density

Solids are usually densest (particles close together in a regular pattern), liquids slightly less, gases much less dense (particles far apart).

Internal energy and changes of state

• Internal energy = total kinetic + potential energy of all the particles in a system.
• Heating raises the temperature or changes the state, not both at once.
• Changes of state are physical changes: mass is conserved and the change can be reversed.
• On a heating curve the flat parts are changes of state: energy is breaking the bonds between particles (increasing potential energy), so the temperature doesn't rise.

Specific latent heat

Specific latent heat (L): the energy needed to change the state of 1 kg of a substance with no change in temperature. Latent heat of fusion: solid ↔ liquid. Latent heat of vaporisation: liquid ↔ gas.

Gas pressure

Gas particles move randomly and collide with the container walls, exerting a force. Pressure = force per unit area. Heating a gas at constant volume gives particles more kinetic energy, so they hit the walls harder and more often, and the pressure increases.

Triple At constant temperature, pV = constant. Halve the volume and the pressure doubles (particles hit the walls more often). HT Doing work on a gas (e.g. a bike pump) transfers energy to it, so its temperature rises.

The atom

• The atom has a radius of about 1 × 10⁻¹⁰ m. The nucleus is about 10 000 times smaller, but contains almost all the mass.
• Protons (+1) and neutrons (0) are in the nucleus. Electrons (−1) are in energy levels (shells). Electrons move to a higher level by absorbing EM radiation and drop back by emitting it.
• Isotopes: same number of protons, different number of neutrons. ¹⁴₆C has 6 protons and 8 neutrons.

Model of the atom over time

1. Dalton: atoms are tiny solid spheres that can't be divided.
2. Thomson (plum pudding): found the electron. A ball of positive charge with negative electrons in it.
3. Rutherford (alpha scattering): most alpha particles went straight through gold foil (atom is mostly empty space); a few bounced back (tiny, dense, positive nucleus). Gave the nuclear model.
4. Bohr: electrons orbit the nucleus at fixed distances (energy levels). Agreed with experiments.
5. Chadwick: discovered the neutron, about 20 years later.

Nuclear radiation

Decay equations

Gamma emission doesn't change the mass or atomic number. Top numbers and bottom numbers must balance on both sides.

Half-life and activity

• Radioactive decay is random: you can't predict which nucleus will decay next.
• Activity: the rate at which a source decays, in becquerels (Bq). Count rate is what a Geiger-Müller tube detects.
• Half-life: the time for the number of unstable nuclei (or the activity) to halve.
• Example: 800 Bq, half-life 2 days. After 6 days (3 half-lives): 800 → 400 → 200 → 100 Bq.
• HT Net decline as a ratio: after 3 half-lives, the activity is ⅛ of the start.

Contamination and irradiation

Irradiation: being exposed to radiation from an outside source. It stops when you move away; you don't become radioactive. Contamination: radioactive atoms get onto or into something. It's more dangerous because the exposure carries on. Alpha is most dangerous inside the body (highly ionising); outside the body, gamma and beta are more dangerous because they can reach your organs.

Peer review matters: findings on radiation's effects are checked by other scientists before being accepted.

Background radiation and uses Triple

• Background radiation comes from natural sources (radon gas, rocks, cosmic rays) and man-made sources (nuclear weapons testing fallout, nuclear accidents). Dose is measured in sieverts (Sv); usually millisieverts (mSv).
• Medical uses: tracers (gamma emitters with short half-lives, so they don't stay active in the body) and radiotherapy (gamma beams or implants to kill cancer cells).
• Choose a source by its half-life and penetration: a tracer must reach a detector outside the body, so it's gamma.

Fission and fusion Triple

• Fission: a large unstable nucleus (uranium-235 or plutonium-239) absorbs a neutron, then splits into two smaller nuclei, releasing 2 or 3 neutrons, gamma rays and lots of energy. The neutrons can cause more fissions: a chain reaction. In a reactor, control rods absorb neutrons to keep it steady.
• Fusion: two light nuclei (e.g. hydrogen) join to make a heavier nucleus. Some mass is converted to energy. It powers the Sun. It needs extremely high temperature and pressure, so it's hard to do on Earth.