BTEC Applied Science Unit 5 Physics: Exam Notes

Energy Efficiency Basics

Ever wondered why your phone gets warm when charging or why car engines aren't 100% efficient? Energy efficiency is all about how much useful energy you get out compared to what you put in.

The key thing to remember is that thermal energy (heat) is the "graveyard" of energy conversion. When energy gets converted to heat, it's really hard to turn it back into anything useful - like trying to power your phone with the warmth from your morning tea!

Heat engines always need a temperature difference to work. They take heat from something hot and dump some heat to something cooler. The formula for efficiency is: Efficiency = Work Output ÷ Heat Input. This is always less than 1 (or 100%) because some energy always gets wasted as heat.

Quick Tip: The bigger the temperature difference in a heat engine, the more efficient it can be!

Maximum Efficiency Limits

There's a hard limit to how efficient any heat engine can be, and it's all about temperature differences. The maximum theoretical efficiency formula is: Maximum efficiency = 1 - (T_cold ÷ T_hot).

Remember to always use Kelvin temperatures (not Celsius) in this calculation! T_cold is your "sink" temperature and T_hot is your hot reservoir temperature.

This means you can never get 100% efficiency - that would mean getting more energy out than you put in, which breaks the laws of physics. The bigger the temperature gap, the better your engine could theoretically be.

Real World: This is why power stations use really hot steam and try to cool their exhaust as much as possible!

The Carnot Cycle

The Carnot cycle is like the perfect theoretical engine that shows us the absolute best performance possible. It's got four stages that work like a perfectly choreographed dance.

A-B: Isothermal expansion - gas expands at constant temperature, absorbing heat. B-C: Adiabatic expansion - gas continues expanding but with no heat transfer, so temperature drops. C-D: Isothermal compression - gas gets squished at constant (cold) temperature, rejecting heat. D-A: Adiabatic compression - final squeeze with no heat transfer, bringing temperature back up.

The work done is the area inside the pressure-volume loop. When the cycle goes clockwise, you've got a heat engine doing useful work. The Carnot efficiency formula is: (T_hot - T_cold) ÷ T_hot × 100%.

Key Point: Real engines try to copy this ideal cycle but never quite achieve it due to friction and other real-world limitations.

Real Heat Engines

Your car engine and power station turbines are examples of heat engines in action. Internal combustion engines work in four strokes: intake (fuel in), compression (squash it), ignition (boom!), and exhaust (waste out).

Steam turbines work differently - they pump water near a furnace, heat it until it becomes high-pressure steam, use that steam to spin turbines (generating electricity), then cool the steam back to water in cooling towers.

Internal combustion engines are compact because they burn fuel right inside the cylinder where it expands. Steam turbines need big, heavy boilers but can get closer to ideal cycle efficiency using large heat exchangers.

Fun Fact: Steam turbines in power stations can be as big as buses and spin at thousands of RPM!

Refrigerators and Heat Pumps

Your fridge is basically a heat engine running backwards - it moves heat from cold (inside) to warm (your kitchen), which seems to break physics but actually just needs extra work input!

Here's how it works: 1) Compressor squashes gas into liquid 2) Warm liquid flows through coils at the back, losing heat to your kitchen 3) Liquid goes through an expansion valve and evaporates 4) Cold gas absorbs heat from inside your fridge 5) Cycle repeats.

Heat pumps use exactly the same process but focus on warming a space rather than cooling one. They can have a reversing valve to work as both heating and cooling systems.

The clever bit is using latent heat of vaporisation - when liquids evaporate, they absorb loads of energy, making the process much more effective than just moving hot air around.

Energy Saving: Heat pumps can move 3-4 times more heat energy than the electrical energy they consume!

Heat Pump Operation Details

Heat pumps have four main components working together: evaporator, compressor, condenser, and expansion valve. In the evaporator, refrigerant absorbs heat from the source and turns to gas.

The compressor squashes this gas, making it hot and high-pressure. In the condenser, this hot gas releases heat to warm your house and turns back to liquid. The expansion valve drops the pressure, preparing for the next cycle.

Reversing valve systems can switch between heating and cooling modes by changing the refrigerant flow direction. In heating mode, heat comes from outside and goes inside. In cooling mode, it's the opposite.

Both isothermal processes (constant temperature heat absorption) and adiabatic processes (compression with no heat exchange) happen in refrigerators, just like in the ideal cycles you studied.

Smart Design: The evaporator coils are where the magic happens - this is where heat gets "stolen" from the cold environment.

Coefficient of Performance

Coefficient of Performance (COP) is like efficiency but for heat pumps and fridges - and it can be greater than 1! This means you can move more heat energy than the electrical energy you use.

For heat pumps: COP = T_hot ÷ (T_hot - T_cold) For refrigerators: COP = T_cold ÷ (T_hot - T_cold). Remember to use Kelvin temperatures!

When the temperature difference is small, COP can be really high - that's why heat pumps work best in moderate climates. Ground source heat pumps use underground temperatures (around 8°C year-round) for better performance.

To maximise COP: use oversized heat exchangers, ensure good air circulation, choose steady heat sources like underground water, and minimise the temperature difference the system works across.

Pro Tip: Water-cooled systems often work better than air-cooled ones because water transfers heat more effectively.

Optimising Heat Pump Systems

Location and design matter hugely for heat pump efficiency. If your fridge's condenser coils are crammed against a wall with no airflow, the system works much harder and might even fail on hot days.

Underground coils provide a steady 8°C heat reservoir all year - perfect for cooling in summer and heating in winter. This beats trying to extract heat from freezing winter air!

Key strategies for better COP: use oversized heat exchangers to reduce temperature differences, tap into large steady heat sources, compress liquids rather than gases (less work needed), reduce friction losses, and keep compression work low.

Water cooling towers help large refrigeration plants by evaporating some water as it falls through an air stream - the latent heat of evaporation cools the remaining water even on hot summer days.

Design Rule: Keep the temperature gap (T_hot - T_cold) as small as possible for maximum efficiency.

Understanding Material Elasticity

Materials around you - from phone screens to bridge cables - all respond to forces in predictable ways. Elasticity is a material's ability to spring back to its original shape after being stretched, squashed, or twisted.

Stress is the force applied per unit area (like pressure), while strain is how much the material changes shape proportionally. Think of stress as the "push" and strain as the "response."

Every material has an elastic limit - stretch it beyond this point and it won't bounce back completely. Push even further and it'll either snap or permanently deform. This is why engineers need to know exactly how materials behave under different loads.

Stress-strain curves are like fingerprints for materials - each tells a unique story about the material's internal structure and how it'll behave in real applications.

Real World: This knowledge helps engineers design everything from smartphone cases to skyscraper frames.