Primary and Secondary Cells
The Science Behind Batteries: From a Simple Torch to Your Smartphone
Understand the concept. Discover the logic. Challenge your thinking.
1. Imagine This!
You switch on a torch. The bulb glows immediately. After several days, the torch becomes dim and eventually stops glowing.
Now consider your mobile phone. Its battery becomes discharged, but you connect it to a charger and use it again.
The answer lies in the nature of the chemical reaction taking place inside the cell.
2. What Is an Electric Cell?
An electric cell is a device that converts chemical energy into electrical energy through a chemical reaction.
A basic cell contains two electrodes and an electrolyte. The electrodes are connected to an external circuit. Chemical processes within the cell establish a potential difference that can drive current through a complete circuit.
Chemical energy → Electrical energy
Cells are broadly classified into two types according to whether their chemical reactions can be reversed sufficiently for practical recharging.
3. Primary Cells: Use and Replace
A primary cell produces electrical energy through a chemical reaction that is not practically reversible under ordinary charging conditions.
Once the cell's useful chemical reactants are consumed, it cannot normally be recharged and reused as intended.
Examples
- Dry cell used in many torches and remote controls
- Simple voltaic cell
- Leclanché cell
- Daniell cell
Think of a typical non-rechargeable household battery.
Where are primary cells useful?
- Remote controls
- Wall clocks
- Some torches
- Devices that consume little power and are used intermittently
No. Chemical changes and self-discharge can occur even when a cell is not powering an external device.
4. Secondary Cells: Recharge and Reuse
A secondary cell uses chemical reactions that can be reversed sufficiently by supplying electrical energy. Therefore, it can be recharged and used repeatedly.
During discharge, chemical energy is converted into electrical energy. During charging, electrical energy is used to restore the cell's chemical state.
Chemical energy → Electrical energy
During charging:
Electrical energy → Stored chemical energy
Examples
- Lead-acid accumulator
- Nickel-iron cell
- Lithium-ion rechargeable cell
A rechargeable battery can be used repeatedly, although its capacity and performance gradually deteriorate with age and use.
Where are secondary cells useful?
- Mobile phones
- Laptops
- Car batteries
- Rechargeable emergency lights
- Power banks
5. Primary vs Secondary Cells
| Feature | Primary Cell | Secondary Cell |
|---|---|---|
| Chemical reaction | Not practically reversible during ordinary use | Reversible sufficiently for recharging |
| Recharging | Generally not rechargeable | Rechargeable |
| Energy during discharge | Chemical to electrical | Chemical to electrical |
| Energy during charging | Not designed for charging | Electrical to chemical |
| Examples | Typical dry cell, Leclanché cell | Lead-acid, nickel-iron, lithium-ion |
| Common applications | Clocks, remotes, some torches | Phones, laptops, vehicles |
Note: Actual performance, capacity, internal resistance and cost depend on the particular cell chemistry and design. Not every cell within a broad category has identical characteristics.
6. The Most Important Conceptual Difference
Why does a secondary cell recharge?
During discharge, chemical reactions produce electrical energy. When an appropriate external voltage is applied during charging, the reactions can proceed in the reverse direction, restoring much of the original chemical state.
In a primary cell, the chemical changes are not practically reversible under normal charging conditions. Attempting to recharge an ordinary non-rechargeable cell can cause leakage, overheating or rupture.
7. Connect Cells with Electric Current
A cell can produce a current when its terminals are connected through a suitable conducting path that forms a complete circuit.
Electric current is the rate of flow of electric charge through a cross-section of a conductor.
Where:
- I = current in amperes (A)
- Q = charge in coulombs (C)
- t = time in seconds (s)
Charge = Current × Time
Q = It
One ampere means that one coulomb of charge passes through a cross-section every second.
Cell B transfers twice as much charge because Q = It.
8. Numericals: Learn the Method
Numerical 1: Calculate current
A charge of 0.5 C passes through a conductor in 5 seconds. Find the current.
Step 1: Write the given quantities.
Q = 0.5 C, t = 5 s
Step 2: Select the formula.
I = Q / t
Step 3: Substitute.
I = 0.5 / 5 = 0.1 A
Numerical 2: Calculate charge
A current of 1.5 A flows through a circuit for 20 seconds. Calculate the charge transferred.
Given: I = 1.5 A, t = 20 s
Q = It = 1.5 × 20
Numerical 3: Calculate time
A charge of 24 C passes through a conductor carrying a current of 2 A. Calculate the time taken.
Given: Q = 24 C, I = 2 A
t = Q / I = 24 / 2
If current is missing, use I = Q/t.
If charge is missing, use Q = It.
If time is missing, use t = Q/I.
9. Real-Life Thinking Lab
A remote control uses ordinary non-rechargeable cells. The cells become exhausted. Why should you not attempt to recharge them using a phone charger?
Think: Consider the cell's chemical design and the risks of charging it improperly.
A phone battery becomes discharged and is then connected to a suitable charger. Which energy conversion occurs during charging?
Think: Is electrical energy being produced or supplied?
A torch is left switched on inside a cupboard. Its cells become exhausted even though nobody uses the torch. Explain why.
Think: Is the circuit complete when the switch is on?
A family uses rechargeable batteries for a device that supports rechargeable cells. What potential advantage does this offer over repeatedly replacing disposable cells?
Think: Consider the number of batteries discarded over time, along with electricity use, battery lifespan and recycling.
10. Rainbow Revision: Seven Steps
Read the definitions of primary and secondary cells.
Identify the type of cell from its example and intended use.
Understand why a secondary cell can be recharged.
Connect the concept to torches, phones and car batteries.
Remember the classification: primary cells and secondary cells.
Apply I = Q/t to numerical problems.
Explain the role of reversible and irreversible chemical reactions.
11. Interactive Physics Challenge
Test your conceptual understanding, application skills and numerical ability.
There are 25 questions. Each correct answer earns 1 point. Choose one answer for each question and click Submit Answers. Your score, correct answers and explanations will appear automatically.
Your Physics Report Card
12. Five Questions to Ask Yourself
- Why is reversibility of a chemical reaction important in a rechargeable cell?
- Can two different types of cells provide the same current? Explain why cell type alone does not determine current.
- Why does a cell not continue supplying current indefinitely?
- How does increasing the time affect the charge transferred when current remains constant?
- Why is it important to dispose of used batteries responsibly?
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