Electricity in the Home

The electricity that powers your home is much more complex and potentially dangerous than the batteries you use in school experiments.

UK mains electricity supplies alternating current (AC) at 230 volts and a frequency of 50Hz. This means the direction of current changes 50 times per second, unlike direct current (DC) which flows in one direction. The peak voltage in mains electricity is actually higher than 230V - this is the effective value.

A standard UK plug contains three wires: the live wire (brown) alternates between positive and negative, the neutral wire (blue) completes the circuit at 0 volts, and the earth wire (green and yellow) provides safety by connecting to the metal case of appliances.

Safety first: The fuse in a plug is a crucial safety device that melts when too much current flows, breaking the circuit and preventing fires. Always use the correct fuse rating for appliances!

The power of an electrical appliance tells us the rate at which it transfers energy:

• Power = current × potential difference
• Power = current² × resistance
• Energy transferred = power × time

The efficiency of appliances affects your electricity bills. Energy-efficient appliances transfer more of the electrical energy to useful forms and less to wasted heat.

States of Matter and Particles

Everything around us exists as a solid, liquid, or gas - and understanding these states helps explain countless everyday phenomena.

The particle model explains these states: in solids, particles are closely packed in a regular arrangement; in liquids, they're still in contact but move more freely; in gases, they move randomly with large spaces between them. When substances are heated, they gain internal energy, which increases the kinetic energy of their particles.

Density tells us the mass per unit volume of a substance $density = mass ÷ volume$. Objects float in water if their density is less than water's. You can measure density of regular objects by measuring mass and volume directly, or for irregular objects using displacement of water.

Try this: Next time you're in the bath, notice how you float more easily when your lungs are full of air. This is because the air decreases your overall density!

Changes between states require energy but don't change temperature. These latent heats appear as flat sections on temperature-time graphs. Evaporation occurs at the surface of a liquid below its boiling point, while boiling happens throughout the liquid at a specific temperature. Pure substances have precise melting and boiling points, unlike mixtures which melt and boil over a range of temperatures.

Energy Resources

Our modern world runs on energy, but where does it come from? We have many options, each with their own advantages and disadvantages.

Renewable energy sources include solar power (using cells to convert sunlight directly to electricity), wind power (using turbines), hydroelectric power (from falling water), tidal power (from ocean tides), and wave power (from ocean waves). These never run out and produce minimal greenhouse gases, but they can be unreliable and expensive to install.

Non-renewable sources include fossil fuels (coal, oil, gas) and nuclear power. Fossil fuels contribute to global warming but can meet high energy demands. Nuclear power doesn't emit greenhouse gases and produces massive amounts of energy, but generates radioactive waste that remains dangerous for thousands of years.

Important! Energy demand continues to grow globally, making the transition to sustainable energy sources one of the greatest challenges of our time.

Biofuels offer a carbon-neutral alternative as they only release the carbon they absorbed during growth. While geothermal energy taps into the Earth's internal heat by pumping water down to produce steam that turns turbines, generating clean electricity with minimal environmental impact.

Atoms and Radiation

Deep within atoms, fascinating processes can release tremendous energy. Radioactive substances have unstable nuclei that emit radiation to become stable.

There are three main types of radiation: Alpha particles (helium nuclei) are highly ionising but stopped by paper; beta particles (electrons) can penetrate further but are stopped by aluminium; and gamma rays (electromagnetic waves) can pass through most materials and are stopped only by thick lead.

Nuclear fission occurs when a heavy nucleus splits into smaller nuclei, releasing energy. This happens in nuclear power stations where controlled chain reactions produce electricity. In contrast, nuclear fusion joins light nuclei to form heavier ones, releasing even more energy - this is what powers our sun!

Mind-blowing fact: The Rutherford scattering experiment revealed the atomic nucleus by firing alpha particles at gold foil. When some particles bounced back, it showed that most of an atom's mass is concentrated in a tiny central nucleus!

The half-life of a radioactive isotope is the average time taken for half of the unstable nuclei to decay. This property makes radioactive isotopes useful in medicine for imaging and treating cancer. However, nuclear waste remains radioactive for thousands of years, creating significant storage and safety challenges.

Energy Stores and Transfers

Energy is constantly changing form all around us, but it can never be created or destroyed. When an object moves upward, its gravitational potential energy increases as work is done against gravity.

Energy can be stored in various ways - as kinetic energy (movement), chemical energy (food and batteries), gravitational potential energy (position), elastic potential energy (stretching), or thermal energy (temperature). These energy stores can be transferred through heating, waves, electrical currents, or by forces.

In a closed system, energy transfers take place but the total amount remains constant. Even when energy seems to "disappear," it's actually being transferred to less useful forms - often as thermal energy or sound. This is why machines are never 100% efficient.

Did you know? Every time you use a machine, some energy is always "wasted" through friction, heat or sound. That's why perpetual motion machines are impossible!

The efficiency of a device tells us how much of the input energy is transferred to useful output energy. We can calculate it using:

• Work done = force applied × distance
• Change in GPE = mass × g × height change
• Kinetic energy = ½ × mass × speed²
• Efficiency = (useful output energy ÷ total input energy) × 100%

Energy Transfer by Heating

Heat is energy in transit, and understanding how it moves helps us design everything from buildings to cooking appliances.

Heat transfers through conduction (particle vibrations in solids), convection (movement of particles in liquids and gases), and radiation (electromagnetic waves). Metals are excellent conductors while non-metals are typically good insulators. The energy transferred through a material depends on temperature difference, thickness, and its thermal conductivity.

All objects emit infrared radiation, with hotter objects emitting more energy per second. When an object's temperature remains constant, it's absorbing exactly the same amount of radiation as it's emitting. This is the principle behind black body radiation - a perfect absorber and emitter of radiation.

Practical application: Double-glazed windows trap air between panes, reducing heat transfer because air is a poor conductor. This same principle applies to cavity wall insulation in homes!

The specific heat capacity of a material tells us how much energy is needed to raise the temperature of 1kg by 1°C. Materials with higher specific heat capacity heat up (and cool down) more slowly. We can calculate energy transferred using:

Energy transferred (J) = mass (kg) × specific heat capacity $J/kg°C$ × temperature change (°C)

Electric Circuits

Electric circuits allow us to control and use electrical energy. The way we connect components affects how electricity flows through them.

In a series circuit, components are connected in a single loop. The current is the same throughout, but the potential difference (voltage) is shared among components. Adding more resistors increases the total resistance, reducing current flow. Total resistance is calculated as R₁ + R₂ + R₃...

In parallel circuits, components are connected across different branches. The potential difference is the same across each branch, but the current divides between paths. Adding more resistors actually decreases total resistance, as there are more paths for current to flow.

Think about it: Your home uses parallel circuits - that's why when one light bulb fails, the others stay on. Imagine if all your home appliances were wired in series!

Ohm's Law states that the current through a resistor at constant temperature is proportional to the potential difference (voltage) across it. Special components like thermistors (whose resistance decreases as temperature increases) and diodes (which allow current to flow in only one direction) have non-linear behavior.

Key equations to remember:

• Potential difference = energy ÷ charge
• Resistance = potential difference ÷ current
• Charge = current × time