Energy Stores and Transfers

Energy stores

• Energy quantifies the effects of object interactions in a system.
• Total energy is conserved in a closed system; it cannot be created or destroyed, but can be converted into matter.
• Energy exists in various forms: kinetic energy (KE), gravitational potential energy (GPE), elastic potential energy, thermal energy, and chemical potential energy.
• KE is the energy of motion, calculated as E = ½mv², where 'm' is mass and 'v' is velocity.
• GPE is energy due to an object's position, calculated as E = mgh, where 'm' is mass, 'g' is acceleration due to gravity, and 'h' is height.
• Elastic potential energy is stored in stretched or compressed objects: E = ½ke², where 'k' is the spring constant and 'e' is the extension.
• Thermal energy is energy from particle random motion: E = mcΔT, where 'm' is mass, 'c' is specific heat capacity, and ΔT is the temperature change.
• Chemical potential energy is stored in molecular bonds.

Energy transfers

• Energy transfer between objects or energy stores is necessary for system change.
• Energy loss to the surroundings signifies an open system.
• Work is another term for energy used.
• A roller coaster at the top of a hill has GPE, no KE. As it falls, GPE converts to KE.
• At the bottom, GPE lost equals KE gained.
• Calculating the roller coaster's speed at the bottom using KE: v = √(2KE/m).
• Alternatively, using GPE and KE directly: v = √(2gh).

Specific heat capacity & Power

• Specific heat capacity (SHC) shows energy to raise 1 kg of a substance by 1°C.
• Power is the rate of energy transfer, calculated as P = E/t, where 'E' is energy and 't' is time.
• Power units are Watts (W), equivalent to Joules per second (J/s).
• Efficiency measures useful energy output compared to total input: Efficiency = Useful energy out / Total energy in, and Efficiency = Useful power out / Total power in.

Electricity

• Electricity is electron flow.
• Charge movement is current.
• Conventional current flows positive-to-negative, opposite electron flow.
• Potential difference (PD), or voltage, shows energy transferred per Coulomb of charge.
• Current is charge flow rate, measured in Amperes (A).
• Resistance opposes current flow.
• A resistor has constant resistance, and is "ohmic" (PD and current are directly proportional).
• A filament bulb has changing resistance, "non-ohmic" (PD and current are not proportional).
• Ohm's Law: V = IR.
• Circuit components can be wired in series or parallel.

Series circuits

• In a series circuit, the total PD is split among components.
• Current is the same through all components.
• Total resistance is the sum of all individual resistances.

Parallel circuits

• In a parallel circuit, PD is the same across each branch.
• Current is split between branches.
• Adding more resistors in parallel lowers the total resistance.

Resistor types

• Thermistors change resistance with temperature.
• LDRs change resistance with light intensity.

Power in electrical circuits

• Electrical circuit power: P = VI.
• Alternative power equations from Ohm's Law: P = I²R and P = V²/R.

Direct and alternating current

• Direct current (DC) flows in one direction.
• Alternating current (AC) flows back and forth, used in household power.
• UK AC frequency is 50 Hz.

Safety in electrical circuits

• Live wire (brown) carries fluctuating voltage, average 230V.
• Neutral wire (blue) has 0V potential.
• Earth wire (yellow/green) connects to plug's ground pin for safety during faults.
• Fuses (in live wire) melt if current exceeds limit.
• Appliance current: I = P/V.

National grid

• A network of power stations, cables, and transformers delivering electricity.
• Transformers change voltage.
• Step-up transformers increase voltage, reduce current, decrease energy loss.
• Step-down transformers decrease voltage, making it suitable for homes.

Atomic structure

• Atoms contain a positive nucleus, with orbiting negative electrons.
• The nucleus has protons and neutrons.
• Atomic number is the number of protons.
• Mass number is the total protons and neutrons.
• Isotopes are atoms of the same element with different neutron counts.

Radiation

• Radiation is emitted particles or waves.
• The electromagnetic spectrum includes all types of radiation.
• Gamma radiation comes from the nucleus's excess energy.
• Alpha radiation is a particle of two protons and two neutrons (Helium nucleus).
• Beta radiation is a high-speed electron from neutron decay.

Radioactive decay

• Unstable nuclei decay to become stable.
• Alpha, beta, and gamma are decay types.
• Alpha has high ionization but low penetration.
• Beta has moderate ionization and penetration.
• Gamma has low ionization but high penetration.

Half-life

• Half-life is decay time for half the activity of a radioactive source.
• Half-life is also the time for half the unstable nuclei to decay, and half the sample's mass.

Nuclear fission

• Fission splits a heavy nucleus (e.g., uranium-235) into lighter nuclei.
• Fission releases heat and kinetic energy.

Nuclear fusion

• Fusion combines light nuclei (e.g., hydrogen) into a heavier nucleus.
• Fusion releases more energy than fission.
• Fusion needs high temps and pressures.

Key concepts

• Energy is conserved.
• Power is energy transfer rate, and efficiency measures useful energy.
• Electricity is charge flow, and PD is energy per Coulomb.
• Resistance opposes flow, and Ohm's Law relates PD, current, and resistance.
• Circuits have series and parallel connections.
• Atoms have protons, neutrons, and electrons.
• Radioactive decay changes unstable nuclei.
• Nuclear fission splits heavy nuclei, and nuclear fusion joins light ones.