Topic B.5 — Current & CircuitsPaper 1 & 2Cells, solar, wind, mains~6 min read
Sources of EMF
Every circuit needs something to give charge its energy — something to push it around. That “push” is called electromotive force, or emf for short. Let’s look at the different things we can use to supply it, and weigh up their pros and cons in plain terms.
📘 What You Need to Know
A source of emf gives charge energy so it can flow around a circuit
Every source works by turning some other kind of energy into electrical energy
Common sources: chemical cells (batteries), solar cells, mains electricity, and wind generators
No source is perfect — each one has its own trade-offs in cost, lifespan, and impact on the environment
What do all these sources have in common?
Here’s the simple idea behind every single one: none of them create energy out of nothing. They all just take energy that already exists in some other form, and convert it into electrical energy that a circuit can use.
Whatever the source, the job is the same: take energy that already exists somewhere else, and turn it into electrical energy a circuit can use.
Chemical cells (batteries)
Inside a battery, a chemical reaction pushes charge around, giving it energy along the way. Some batteries can only be used once — the chemicals inside get used up and that’s it, like a simple AA battery. Others are rechargeable: plug them into a charger, and the chemical reaction runs in reverse, ready to be used all over again. Phone batteries and car batteries both work this way.
Solar cells
A solar cell is a small photovoltaic device that catches light from the Sun and turns it straight into electrical energy. It’s clean, it never runs out, and it needs almost no fuel — but it only works well when there’s plenty of sunlight, so cloudy days and night time are a real problem.
Wind generators & mains electricity
A wind turbine catches the kinetic energy of moving air and turns it into electricity — free fuel, but only when the wind is actually blowing hard enough. Mains electricity, on the other hand, is usually generated by burning fuel (like coal or gas) to spin a generator. It’s reliable and available any time you need it, but it isn’t kind to the environment.
Quick recap: every source of emf converts a different starting form of energy — chemical, light, kinetic, or fuel — into electrical energy that can drive a current.
Weighing up the pros and cons
🔧 Chemical Cells (e.g. AA battery, phone battery)
Good: portable, convenient, and can be used almost anywhere
Good: rechargeable types can be used again and again
Not so good: non-rechargeable ones need replacing, adding cost and waste
🔧 Solar Cells
Good: clean, free fuel, almost no ongoing running cost
Not so good: only works well in daylight, and output drops in poor weather
🔧 Wind Generators
Good: zero fuel cost, no chemical pollution while running
Not so good: needs a windy location, and output changes constantly with the weather
🔧 Mains Electricity (fossil fuel generator)
Good: reliable, available whenever you need it
Not so good: burns non-renewable fuel and produces greenhouse gases
WE 1
A remote weather station sits on a hillside, far from any mains cables, but the location is famous for being very windy. Which source of emf would suit it best, and why?
Rule out mains electricity:
There are no cables reaching the site, so that’s not an option.
Think about what’s actually available:
The location has plenty of wind, which is exactly what a wind generator needs.
A wind generator makes the most sense here — it uses the resource that’s already plentiful on site
WE 2
A solar-powered garden light works fine all summer, but stops turning on some nights in winter. Why might this be happening?
Think about what a solar cell needs:
It only converts light energy — no light means no electrical energy being generated.
Connect that to winter conditions:
Shorter, cloudier winter days mean less light reaches the panel during the day.
The light isn’t collecting enough energy during the day to store up for the night
💡 Top Tips
Always ask “what energy is going in?” first — it instantly tells you which type of source you’re dealing with
Weigh up more than one factor when comparing sources — cost, reliability, and environmental impact rarely all point the same way
Rechargeable does not mean “never wears out” — batteries do lose some of their capacity over many charging cycles
⚠ Common Mistakes
Thinking a source of emf creates energy from nothing — it always converts energy from another form
Assuming renewable sources are always the “better” choice in every situation, even when reliability matters most
Mixing up emf with potential difference — we’ll pin down exactly how they differ in the next lesson
Up next: Electromotive Force & Internal Resistance — now we know where the “push” comes from, let’s look closely at what emf really measures, and why a real cell never quite delivers its full potential.
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