Electrostatics

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 🌐 Version en français : Électrostatique

📘 Electrostatics

Reference Textbook

📖 Reference Textbook:
University Physics Volume 2: Chapter 5

Theory

Charge, Conductors, and Insulators: Introduction to Electrostatics

Electric Charge (q)

• Fundamental property: Objects can have a positive or negative electric charge.
• Types: An object generally becomes positively charged when it loses electrons and negatively charged when it gains electrons.
• Conservation: Electric charge is neither created nor destroyed; it is only transferred.
• Quantization: $Q=n\cdot e$. Electric charge exists in multiples of the elementary charge $e=1.6\times 10^{-19}\text{C}$.

Conductors

• Definition: Materials in which electric charges, such as electrons or ions, can move freely.
• Examples: Metals such as copper and silver; salt solutions.

Insulators

• Definition: Materials in which electric charges cannot move freely.
• Examples: Glass, rubber, and plastic.

▶ Video: Charge, Conductors, and Insulators

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Coulomb's Law without Vectors

The magnitude of the electrostatic force $F_{\mathrm{t}\mathrm{s}}$ between two point charges $q_{\mathrm{s}}$ (source charge) and $q_{\mathrm{t}}$ (test charge), separated by a distance $r$, is given by:

$F_{\mathrm{t}\mathrm{s}}=k\cdot \frac{|q_{\mathrm{s}}|\cdot |q_{\mathrm{t}}|}{r^2}$

where $k\approx 8.99\times 10^9\mathrm{N}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}/{\mathrm{C}}^{\mathrm{2}}$ is Coulomb's constant.

▶ Video: Coulomb's Law without Vectors

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Coulomb's Law in Vector Form

The electrostatic force vector ${\overrightarrow{F}}_{\mathrm{t}\mathrm{s}}$ between two point charges $q_{\mathrm{s}}$ (source charge) and $q_{\mathrm{t}}$ (test charge), separated by a distance $r$, is given by:

${\overrightarrow{F}}_{\mathrm{t}\mathrm{s}}=k\frac{q_{\mathrm{s}}{q}_{\mathrm{t}}}{r^2}\hat{r}$

where:

• ${\overrightarrow{F}}_{\mathrm{t}\mathrm{s}}$: force exerted on the test charge by the source charge;
• $k$: Coulomb's constant, approximately $8.99\times 10^9\mathrm{N}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}/{\mathrm{C}}^{\mathrm{2}}$;
• $q_{\mathrm{s}}$: source charge, which exerts the force;
• $q_{\mathrm{t}}$: test charge, on which the force is exerted;
• $r$: distance between the charges;
• $\hat{r}$: unit vector pointing from the source charge toward the test charge.

▶ Video: Coulomb's Law in Vector Form

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Calculation Examples: Coulomb's Law in Vector Form

▶ Calculation Example 1

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▶ Calculation Example 2

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Triboelectric Effect and Triboelectric Series

The triboelectric effect is a phenomenon in which certain materials become electrically charged after coming into contact and then being separated. Friction often enhances this effect by repeatedly bringing the materials into contact and separating them. Electrons are transferred from one material to the other. Consequently, one material gains electrons and becomes negatively charged, while the other loses electrons and becomes positively charged.

Different materials have different tendencies to gain or lose electrons. These tendencies are organized into the triboelectric series. Materials higher in the series, such as glass or hair, tend to lose electrons and become positively charged. Materials lower in the series, such as rubber or Teflon, tend to gain electrons and become negatively charged.

The triboelectric effect is commonly observed in everyday life, for example:

• when a balloon rubbed against hair makes the hair stand up;
• when synthetic clothing produces static cling.

▶ Video: The Triboelectric Effect

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Demonstrations

Bending a Stream of Water

▶ Demonstration: Bending a Stream of Water

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Sticking a Balloon to a Wall

▶ Demonstration: Sticking a Balloon to a Wall

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Electrostatics Simulations

Explore electric charges with these simulations:

• Be careful, John Travolta!
• Charge a balloon

Additional resources:

• Static electricity resources
• Lightning animation

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