Sources of Emf and Circuit Diagrams
- You’re holding a flashlight.
- You press the switch, and the bulb glows, instantly lighting up the space around you.
- But have you ever wondered how this happens? What makes the electrons in the wire move in such a coordinated way to produce light?
The answer lies in something called electromotive force (emf), the driving energy that sets electrons in motion.
What is Electromotive Force (emf)?
Definition
Electromotive force
The electromotive force (emf) is the work done per unit charge to move a charge completely around a circuit, including through the battery.
- Despite its name, emf is not a "force" in the traditional sense.
- Instead, it’s the energy provided per unit charge, measured in volts (V).
How Cells Provide emf
Chemical Cells:
- In chemical cells (like those in a typical battery), chemical reactions create a potential difference between the positive and negative terminals.
- This potential difference drives the movement of electrons through the circuit.
In an alkaline battery, chemical reactions involving zinc and manganese dioxide generate the emf.
Solar Cells:
- Solar cells convert light energy into electrical energy.
- When sunlight strikes the surface of a solar cell, it excites electrons, creating a flow of charge.
In a solar-powered calculator, the solar cell generates enough emf to power the device by converting light into electricity.
Advantages and Disadvantages of Chemical and Solar Cells
| Type of Cell | Advantages | Disadvantages |
|---|---|---|
| Chemical Cells (Batteries) | Portable and compact. | Limited lifespan; requires replacement or recharging. |
| Can provide a stable voltage output. | Can contain harmful chemicals, leading to environmental concerns. | |
| Works in the absence of sunlight. | Efficiency decreases over time due to internal resistance. | |
| Solar Cells | Renewable and environmentally friendly. | Dependent on sunlight, making them ineffective at night or in low-light conditions. |
| No emissions or harmful byproducts. | Initial installation cost is high. | |
| Long lifespan with minimal maintenance. | Energy storage (e.g., batteries) is required for continuous power supply. |
Key Formula
The emf can be expressed by the formula:
$$
\text{emf} = \frac{W}{q}
$$
where $ W $ represents the work done (in joules) and $ q $ is the charge (in coulombs).
