When a secondary source of e.m.f 2.0V is being charged by an external supply, is the terminal voltage of secondary cell is greater or less than 2.0V ?

When a secondary source of electromotive force (e.m.f) like a rechargeable battery is being charged, the terminal voltage can actually be greater than the nominal e.m.f of the battery, which in this case is 2.0V. This might seem counterintuitive at first, so let’s break it down to understand why this happens.

Understanding Terminal Voltage During Charging

The terminal voltage of a battery is the voltage measured across its terminals when it is connected to a circuit. This voltage can vary depending on whether the battery is being charged or discharged. When charging a secondary cell, the external supply must provide a voltage that is higher than the battery's e.m.f to push current into the battery.

Why Terminal Voltage Increases

When you connect a charger to the battery, the charger needs to overcome the internal resistance of the battery and the chemical processes occurring within it. Here’s a step-by-step breakdown:

• External Voltage Requirement: To charge the battery, the charger must supply a voltage greater than the battery’s e.m.f. For a 2.0V battery, the charger might need to provide around 2.4V or more, depending on the battery's state of charge and internal resistance.
• Overcoming Internal Resistance: As current flows into the battery, there is an internal resistance that causes a voltage drop. The terminal voltage must be higher than the e.m.f to account for this drop.
• Charging Process: During charging, the chemical reactions inside the battery are reversed, which can temporarily increase the terminal voltage above the nominal e.m.f.

Example Scenario

Imagine you have a 2.0V lead-acid battery. When you connect a charger that outputs 2.4V, the terminal voltage of the battery may rise to around 2.2V during charging. This is because the charger is providing enough voltage to not only overcome the e.m.f but also to push current into the battery, which can lead to a temporary increase in voltage.

Implications of Higher Terminal Voltage

It’s important to note that while the terminal voltage can exceed the nominal e.m.f during charging, this should be done carefully. If the voltage is too high, it can lead to overheating or damage to the battery. Therefore, chargers are designed to regulate the voltage appropriately to ensure safe charging.

In summary, when charging a secondary source of e.m.f like a 2.0V battery, the terminal voltage can indeed be greater than 2.0V due to the need to overcome internal resistance and facilitate the charging process. Understanding this concept is crucial for anyone working with rechargeable batteries, as it helps ensure safe and effective charging practices.