Every circuit needs a push — something to give the charge energy and send it round the loop. That push is called an electromotive force (EMF), and lots of different devices can provide it: batteries, solar panels, wind turbines, the mains. On this page we’ll meet the main sources, see what energy each one converts into electrical energy, and weigh up their strengths and weaknesses — a favourite of examiners.
📘 What you need to know
A source of EMF is any device that supplies electrical energy to a circuit
Each source converts some other form of energy into electrical energy
Chemical cells (batteries) convert chemical energy; they can be rechargeable or single-use
Solar (photovoltaic) cells convert light energy from the Sun into electrical energy
Other sources include mains electricity and wind generators
You should be able to compare the advantages and disadvantages of each source
What is a source of EMF?
When charge passes through a power supply, it gains energy — the supply gives it a boost so it can travel round the circuit and do useful work. The size of that boost, per unit of charge, is the electromotive force, or EMF. (Despite the name, it isn’t a force at all — it’s an energy-per-charge, measured in volts.)
The key idea for this page: a source of EMF doesn’t create energy from nothing. It converts some other store of energy into electrical energy. Different sources just start from different places:
Different starting stores — chemical, light, kinetic — all funnel into the same output: electrical energy for the circuit.
Chemical cells (batteries)
The most familiar source is the chemical cell — a battery. Inside, a chemical reaction takes place that separates charge and provides a potential difference. It converts chemical energy into electrical energy. Batteries come in two families:
A single-use cell runs down and gets thrown away. A rechargeable cell’s reaction can be reversed by a charger, so it’s used again and again.
Non-rechargeable (single-use)
These can only be used once. The chemicals inside gradually get used up, and when they’re gone the cell is flat — you throw it away and replace it. The everyday alkaline AA battery is the classic example, powering toys, remotes and torches.
Rechargeable
In a rechargeable cell, the chemical reaction can be reversed. Connect it to a charger and the reaction runs backwards, storing energy again so the cell can be reused many times. Examples include:
Lithium-ion batteries in phones and laptops
Lead-acid batteries in cars and other vehicles
A neat way to picture the difference: a single-use battery is like a box of matches — strike one and it’s gone for good. A rechargeable battery is like a refillable lighter — when it runs low, you top it back up and keep going. Both give you the same flame; one just resets and one doesn’t.
Solar (photovoltaic) cells
A solar cell — more precisely a photovoltaic cell — turns light energy from the Sun into electrical energy. Light arrives as tiny packets of energy called photons. When a photon hits the cell, it can give an electron on the surface enough energy to break free and start moving. Those freed electrons make up a current, which is delivered to the external circuit.
Sunlight knocks electrons free in the photovoltaic cell; those electrons flow round the circuit as an electric current.
Solar panels are just lots of these photovoltaic cells joined together. They’re a clean, renewable source — but as we’ll see, they only work when the Sun is shining.
Comparing the sources
The exam won’t ask you to explain the inner chemistry of every cell — but it will ask you to weigh up their pros and cons. Here’s a compact comparison of the main sources of EMF:
Source
Advantages
Disadvantages
Single-use battery (e.g. alkaline AA)
Cheap, portable, convenient; easy to carry many for a bigger voltage
Not rechargeable, so needs replacing; high internal resistance; disposal causes pollution
Lithium-ion battery (phones, laptops)
Rechargeable and long-lasting; very high energy density; low internal resistance; charges quickly
Loses capacity over time; internal resistance rises with age; expensive
Lead-acid battery (cars)
Cheap, rechargeable; can deliver very large currents quickly
Heavy for the energy it stores; limited number of charge cycles; uses toxic materials
Solar cell
Clean, renewable, no fuel needed; free energy everywhere; cheap to maintain
Output varies with weather; only works in daylight; low efficiency; large area and high upfront cost
Wind generator
Free fuel, renewable, no chemical pollution
Output is inconsistent; needs windy sites; noise and visual impact
Mains (fossil-fuel generated)
Reliable, always available; high energy density; well-established
Produces greenhouse gases and pollution; non-renewable
Don’t try to rote-learn the whole table — look for the patterns instead. Renewables (solar, wind) are clean but unreliable and weather-dependent. Chemical cells are portable and reliable but eventually run down or wear out. Fossil-fuel mains is reliable but polluting. If you can argue those trade-offs in your own words, you can tackle any comparison question they throw at you.
Other energy chemical / light / kinetic
source of EMF converts it
Electrical energy for the circuit
WE 1
A student is choosing a power source for a small portable sensor that must run for years in a remote, sunny location with no mains access. Compare a single-use battery with a solar cell for this job.
Single-use battery
Portable and reliable, but would run down and need replacing — hard in a remote spot
Solar cell
Needs no fuel and the location is sunny, so it can keep working for years
but its output drops at night / in poor weatherSolar cell suits a remote, sunny, long-term jobMatch the source to the situation: “sunny + long-term + remote” points straight to solar.
💡 Top tips
Every source converts energy — always name the starting store (chemical, light, kinetic) and the output (electrical).
Rechargeable = reversible reaction; single-use = chemicals used up once.
Solar cells need light — no Sun, no output. State this in comparison answers.
Learn the trade-offs, not the table: clean-but-unreliable vs reliable-but-polluting/running-down.
Read the scenario in comparison questions — the “best” source depends on the situation.
⚠ Common mistakes
Saying a source “creates” energy — it converts one form into electrical energy
Calling EMF a force — it’s an energy per unit charge, measured in volts
Confusing rechargeable and single-use — only rechargeable cells reverse their reaction
Forgetting that solar and wind are weather-dependent and intermittent
Giving one-sided answers — comparison questions want both pros and cons
Quick recap: A source of EMF supplies electrical energy by converting another form of energy. Chemical cells convert chemical energy and are either single-use or rechargeable (reversible reaction). Solar cells convert light into electricity but need daylight. Each source has trade-offs — renewables are clean but intermittent, chemical cells run down, and mains is reliable but polluting.
So far we’ve treated a cell’s EMF as the full voltage it delivers. But here’s the catch: a real battery wastes some of its own energy internally, so the voltage you actually get out is a little less than its EMF. In the next page, EMF & Internal Resistance, we’ll pin this down with the equation ε = I(R + r) and meet the idea of “lost volts”.
Want the energy sources to stick?
Book a free meeting and we’ll go through cells, solar and the comparison questions together.
