Table of Contents
What Are Excess Electrons?
When an object becomes electrically charged, it has gained or lost electrons relative to its neutral state. A negatively charged object has excess electrons beyond the number of protons; a positively charged object has a deficit of electrons. The number of excess (or missing) electrons directly determines the object's net charge.
Electric charge is quantized, meaning it always comes in integer multiples of the elementary charge e = 1.602 × 10⁻¹⁹ C. You cannot have a fraction of an electron. This quantization was first demonstrated by Robert Millikan in his famous oil drop experiment in 1909.
Formula
Where n is the number of excess electrons, Q is the net charge in coulombs, and e = 1.602 × 10⁻¹⁹ C is the elementary charge. A negative Q means excess electrons; positive Q means electron deficit.
Examples
| Object | Typical Charge | Excess Electrons |
|---|---|---|
| Rubbed balloon | -10 nC | 6.2 × 10⁹° |
| Van de Graaff | -5 μC | 3.1 × 10¹³ |
| Lightning bolt | -5 C | 3.1 × 10¹⁹ |
| Static shock | -20 nC | 1.2 × 10¹¹ |
Frequently Asked Questions
Can you count individual electrons?
Yes, with specialized equipment. Single-electron transistors and quantum dot devices can detect and count individual electrons. Millikan's oil drop experiment measured charges of individual electrons by observing oil drops in an electric field.
Does gaining electrons change an object's mass?
Technically yes, but negligibly. Each electron has mass 9.109 × 10⁻³¹ kg. Even 10¹³ excess electrons add only about 10⁻¹⁷ kg, which is far too small to measure with any conventional scale.
Why do objects become charged through rubbing?
Triboelectric charging occurs when two materials are rubbed together. Electrons transfer from the material with weaker electron affinity to the one with stronger affinity. The triboelectric series ranks materials by their tendency to gain or lose electrons.