Welcome to "How much energy do we use?"
In this chapter, we are going to explore how energy moves from one place to another, how we measure it, and why some of it always seems to "go missing." Understanding energy is the first step toward a sustainable future. By the end of these notes, you'll be able to calculate energy like a pro and understand why your house needs good insulation!
Don't worry if some of the formulas look scary at first—we will break them down step-by-step.1. Energy Stores: Where is the Energy Kept?
Think of energy like money. It can be stored in different "accounts." The syllabus mentions several key ways energy is stored:
• Chemical (in batteries or fuel like coal and gas)
• Kinetic (anything that is moving)
• Gravitational (anything held up high)
• Elastic (stretched or squashed things)
• Thermal (the heat in an object)
• Nuclear (stored in the middle of atoms)
• Electrostatic and Electromagnetic
How does it move?
Energy doesn't just stay in one place. It moves through processes called working (moving things) or heating. For example, in a power station, energy in a chemical store (fuel) is transferred by an electric current to do work on devices in your home, like a toaster or a motor.
Quick Review: The Golden Rule
The Law of Conservation of Energy states that in a closed system, the total amount of energy never changes. It can't be created or destroyed—only moved between stores. If you start with 100 Joules, you must end with 100 Joules somewhere!
2. Calculating Energy Use
To know how much energy an appliance uses, we need to look at its Power rating. Power is just a fancy way of saying "how fast energy is transferred every second."
The Energy Equation
To calculate the energy transferred, use this formula:
\( \text{energy transferred (J)} = \text{power (W)} \times \text{time (s)} \)
Units Matter!
• Energy is usually measured in Joules (J).
• Power is measured in Watts (W).
• Time must be in seconds (s).
Did you know? In your home, Joules are too small to be practical. Electricity companies use kilowatt-hours (kWh) instead. In that case, the formula is:
\( \text{energy (kWh)} = \text{power (kW)} \times \text{time (h)} \)
Common Mistake to Avoid:
Always check your units! If a question gives you time in minutes, you must multiply by 60 to get seconds before using the Joules formula.
3. Wasted Energy and Efficiency
Whenever energy is transferred, some of it is dissipated. This is a scientific word for "wasted." This energy usually spreads out into the surroundings as thermal energy (heat) that we can't use again.
Analogy: The Leaky Bucket
Imagine carrying water (energy) in a bucket to a garden. If the bucket has a small hole, some water leaks out on the way. The water on the ground isn't "gone" from the universe, but it's not helping your plants anymore. That's dissipation!
Calculating Efficiency
We want our machines to be as efficient as possible. We calculate efficiency using this ratio:
\( \text{efficiency} = \frac{\text{useful energy transferred (J)}}{\text{total energy transferred (J)}} \)
How to increase efficiency:
1. Lubrication: Using oil on moving parts to reduce friction (which stops energy being wasted as heat).
2. Thermal Insulation: Stopping heat from escaping buildings.
4. Sankey Diagrams
A Sankey diagram is a visual way to show energy transfers.
• The width of the arrow represents the amount of energy.
• The main arrow points to the useful energy.
• Arrows branching off (usually pointing down) show the wasted energy.
Key Takeaway: Because energy is conserved, the width of the "input" arrow must equal the width of all "output" arrows (useful + wasted) combined!
5. Keeping the Heat In: Insulation
In the context of sustainable energy, we want to stop wasting heat in our homes. The rate at which a building cools down depends on its walls.
Factors affecting cooling:
1. Thickness: Thicker walls slow down the rate of cooling.
2. Thermal Conductivity: This is a measure of how easily heat travels through a material. To keep a house warm, we want materials with low thermal conductivity (like fiberglass insulation or air gaps in double glazing).
Memory Aid: High conductivity means the heat Hi-tails it out of there! Low conductivity keeps it slow.
Chapter Summary
• Energy is stored in different ways and transferred by working, heating, or electricity.
• Power is the rate of energy transfer \( (P = E / t) \).
• Conservation means energy isn't lost, just moved—often to less useful thermal stores (dissipation).
• Efficiency measures how much energy is actually used for its intended purpose.
• Insulation works by using thick materials with low thermal conductivity to save energy.