A chemical cell turns chemical energy into electrical energy
Definition
Voltage
The potential difference between two points, measured in volts, that pushes charge around a circuit.
- A chemical cell contains chemicals that react and change chemical energy into electrical energy.
- The reaction produces a voltage between the two terminals of the cell.
- When the cell is part of a complete circuit, an electric current flows and can run a device.
- The cell keeps supplying electricity only while its reactants can carry on reacting.
Key Idea
- A chemical cell supplies electrical energy from a chemical reaction.
- It stops supplying electricity when the reaction can no longer continue.
The metals and the electrolyte set the voltage of a simple cell
Definition
Electrode
A conducting material through which electrical current enters or leaves a cell, with a simple cell using two different metal electrodes.
Definition
Electrolyte
A liquid or paste containing ions that can move and complete the internal circuit of a cell.
- You can build a simple cell by connecting two different metal electrodes and dipping both into an electrolyte.
- The electrodes do not touch, because the wire outside the cell and the electrolyte inside it complete the circuit.
- The voltage depends on the two metals chosen for the electrodes.
- Changing the electrolyte can also change the voltage.
- With the same electrolyte and conditions, metals that are further apart in reactivity usually give a bigger voltage.
- Only compare voltage results when everything except the metals has been kept the same.
Example
- With one electrolyte, magnesium and copper give 1.8 V1.8\ \text{V}1.8 V, zinc and copper give 1.1 V1.1\ \text{V}1.1 V and iron and copper give 0.7 V0.7\ \text{V}0.7 V.
- Magnesium and copper give the largest voltage, so this pair has the largest difference in reactivity.
- If copper is the least reactive metal in the set, the data support the order magnesium, zinc, iron, then copper.
Joining cells in series adds their voltages to make a battery
Definition
Series
A connection in which cells are placed one after another so their voltages add when they are aligned in the same direction.
Definition
Battery
Two or more cells connected together to provide electrical energy.
- A battery is two or more cells joined together in series.
- When the cells face the same way, their voltages add together.
- The total voltage is Vtotal=V1+V2+…V_{\text{total}} = V_1 + V_2 + \dotsVtotal=V1+V2+….
- A battery can therefore give a bigger voltage than any single one of its cells.
- The battery still has to match the voltage the device needs, because too big a voltage can be unsuitable.
Example
- Three 1.5 V1.5\ \text{V}1.5 V cells in series give a total of 1.5+1.5+1.5=4.5 V1.5 + 1.5 + 1.5 = 4.5\ \text{V}1.5+1.5+1.5=4.5 V.
- Four of the same cells would give 4×1.5=6.0 V4 \times 1.5 = 6.0\ \text{V}4×1.5=6.0 V.
Recharging reverses the reactions; a used-up non-rechargeable cell cannot
Definition
Electrical current
The rate of flow of electric charge through a circuit.
- Non-rechargeable cells and batteries cannot have their reactions reversed for reuse by an external current.
- They stop giving electricity once one of the reactants has been used up.
- Alkaline batteries are non-rechargeable.
- Rechargeable cells and batteries can be recharged because an external current reverses their reactions.
- During use, the reactions run in the direction that supplies electricity.
- During charging, the external current drives the reactions the opposite way and restores the reactants.
Common Mistake
- Do not say that charging fills a battery up with electricity.
- Charging supplies energy that reverses the chemical reactions and restores the cell's store of chemical energy.
- Do not try to recharge an alkaline battery, because it is non-rechargeable.
Choosing a cell weighs voltage, cost, reuse and waste against the device
- Voltage needed: the cell or battery must supply the voltage the device requires, adding cells in series if more is needed.
- How often it is used: rechargeable cells often suit a device used every day, such as a wireless games controller.
- A non-rechargeable battery can be convenient for a low-use device like a wall clock.
- Cost: rechargeable cells usually cost more to buy but can cost less per use if recharged many times.
- Resources and waste: reusing rechargeable cells means fewer cells are made and thrown away, though recharging still uses electricity.
- Convenience: non-rechargeable cells can be swapped straight away, while rechargeable cells need a charger and time to charge.
- A strong answer judges the cell against the needs of the named device rather than claiming one type is always better.
Exam technique
- From voltage data, only give a reactivity order if the electrolyte and conditions were kept the same and at least one metal's reactivity is known.
- For a series battery, add the individual cell voltages and give the total with the unit V\text{V}V.
- To evaluate rechargeable against non-rechargeable, judge each against the named device's needs and end with a clear, supported choice.
Self review
- What energy change happens inside a chemical cell?
- How do you make a simple cell?
- With conditions kept the same, how does the difference in reactivity of the two metals affect the voltage?
- Why does connecting cells in series give a bigger voltage?
- How do non-rechargeable and rechargeable cells differ?
