What is an electric cell?
An electric cell is a small device that pushes electric charge round a circuit. Inside it, a chemical reaction happens. The cell changes chemical energy into electrical energy.
Every cell has two terminals: the positive terminal (+) and the negative terminal (−). In circuit diagrams a cell is drawn as a long thin line (+) and a short thick line (−).
A battery is two or more cells joined together. In daily life people call a single AA cell a "battery", but in science it is a cell.
How a cell makes electricity
A cell needs three things:
- Two different metals (or a metal and carbon), called electrodes.
- An electrolyte: a liquid or paste that conducts electricity, such as an acid, salt water or lemon juice.
- A wire to join the electrodes outside.
The more reactive metal (zinc) reacts with the electrolyte and slowly dissolves. Each zinc atom that goes into the liquid leaves 2 electrons behind on the strip. So electrons pile up on the zinc: it becomes the negative terminal. When the wire is joined, these electrons flow through the wire to the copper (the positive terminal). Inside the liquid, charged particles called ions move to complete the loop.
If both electrodes are the same metal, both react in the same way, so there is no push and no current.
Making a simple cell
You need: a lemon (or a potato, or a cup of vinegar or salt water), a zinc-coated (galvanised) nail, a copper coin or strip, wires with clips, an LED and a multimeter if you have one.
- Push the nail and the copper into the lemon about 2 cm apart. They must not touch.
- Measure: one lemon cell gives about 0.9 V.
- Make 2–4 cells and join them in series: copper of one to zinc of the next.
- Join the free copper end to the long leg of the LED and the free zinc end to the short leg.
Why it is weak: a lemon cell gives only a tiny current, so it can light an LED faintly but not a torch bulb. Never experiment with mains electricity or open real batteries.
Famous early cell: in 1800 Alessandro Volta stacked zinc and copper discs with cloth soaked in salt water. This "voltaic pile" was the first battery. The unit volt is named after him.
Cells in series: voltages add
When cells are joined + of one to − of the next, they are in series. Their voltages add: total V = V₁ + V₂ + V₃ …
- 2 AA cells: 1.5 + 1.5 = 3 V (a torch or remote).
- 6 lead-acid cells: 6 × 2 = 12 V (a car battery).
If one cell is put in the wrong way round, its voltage is subtracted, and the device may not work.
Types of cells
| Cell | Inside | Voltage | Rechargeable? |
|---|---|---|---|
| Simple (voltaic) cell | Zinc, copper, dilute acid or juice | about 1 V | No |
| Dry cell (zinc–carbon / alkaline) | Zinc case (−), moist paste, carbon rod (+) | 1.5 V | No (primary cell) |
| Button cell | Zinc, silver oxide or lithium | 1.5–3 V | Mostly no |
| Lead-acid cell | Lead plates in sulfuric acid | 2 V | Yes (secondary cell) |
| Lithium-ion cell | Lithium compounds | about 3.7 V | Yes |
A primary cell is used once; its chemicals get used up. A secondary cell can be recharged: a charger pushes current backwards and reverses the reaction.
Safe disposal: used cells contain metals that harm soil and water. Do not throw them in fire or ordinary waste; give them to an e-waste collection point.
Key formulas and definitions
- Cell: chemical energy → electrical energy
- Cell = 2 different electrodes + electrolyte
- Electrons flow in the wire from − (zinc) to + (copper)
- Cells in series: V_total = V₁ + V₂ + …
- Dry cell ≈ 1.5 V; lead-acid cell ≈ 2 V; lemon cell ≈ 0.9 V
- Battery = two or more cells joined together
Worked examples
1. A torch uses 3 dry cells of 1.5 V in series. What is the total voltage?
Voltages add in series: 1.5 + 1.5 + 1.5 = 4.5 V.
2. One lemon cell gives 0.9 V. An LED needs at least 1.8 V. How many lemon cells in series are needed?
1.8 ÷ 0.9 = 2. At least 2 lemon cells in series.
3. A car battery gives 12 V. Each lead-acid cell gives 2 V. How many cells are inside?
12 ÷ 2 = 6 cells in series.
4. A student puts two copper strips in a lemon. The meter shows 0 V. Why?
Both electrodes are the same metal, so they react in the same way. There is no difference to push electrons, so no voltage. One must be a different metal, such as zinc.
5. In a remote, 2 cells of 1.5 V are fitted, but one is put in the wrong way. What voltage does the remote get?
The reversed cell pushes the other way, so its voltage is subtracted: 1.5 − 1.5 = 0 V. The remote does not work.
6. Name the energy change and the terminals in a dry cell.
Chemical energy → electrical energy. The zinc case is the negative terminal; the carbon rod with a metal cap is the positive terminal.
Common mistakes
- Thinking a cell stores electricity like a tank of electrons. It stores chemicals; the reaction makes the push.
- Using two pieces of the same metal. A cell needs two different electrodes.
- Calling a single AA cell a battery in a science answer. A battery is two or more cells.
- Saying electrons flow from + to − in the wire. Electrons go from − to +; conventional current is drawn from + to −.