What is Coulomb's law, and how can it be used to describe the relationship between attractive and repulsive forces?

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**Coulomb's Law** is a fundamental principle in electromagnetism that describes the force between two charged particles. Named after the French physicist Charles-Augustin de Coulomb, who first published it in 1785, Coulomb's Law quantifies the electric force between charged objects and explains whether this force is attractive or repulsive.

### **Coulomb's Law Formula**

Mathematically, Coulomb's Law is expressed as:

$$
F = k_e \cdot \frac{|q_1 \cdot q_2|}{r^2}
$$

Where:
- $$ F $$ is the magnitude of the electrostatic force between the two charges.
- $$ k_e $$ is Coulomb's constant, approximately equal to $$ 8.988 \times 10^9 \, \text{N·m²/C²} $$.
- $$ q_1 $$ and $$ q_2 $$ are the amounts of the two charges, measured in coulombs (C).
- $$ r $$ is the distance between the centers of the two charges, measured in meters (m).

### **Direction of the Force: Attractive vs. Repulsive**

Coulomb's Law not only quantifies the magnitude of the force but also its direction:

- **Repulsive Force:** If both charges have the **same sign** (both positive or both negative), the force between them is **repulsive**. This means the charges push each other apart.
  
  - *Example:* Two positively charged protons repel each other, as do two negatively charged electrons.

- **Attractive Force:** If the charges have **opposite signs** (one positive and one negative), the force between them is **attractive**. This means the charges pull toward each other.
  
  - *Example:* A proton (positive charge) attracts an electron (negative charge).

### **Vector Form of Coulomb's Law**

To fully describe both the magnitude and direction of the force, Coulomb's Law is often expressed in vector form:

$$
\vec{F}_{12} = k_e \cdot \frac{q_1 q_2}{|\vec{r}_{12}|^3} \cdot \vec{r}_{12}
$$

Where:
- $$ \vec{F}_{12} $$ is the force exerted on charge $$ q_1 $$ by charge $$ q_2 $$.
- $$ \vec{r}_{12} $$ is the displacement vector from $$ q_1 $$ to $$ q_2 $$.

In this vector form:
- If $$ q_1 $$ and $$ q_2 $$ have the same sign, $$ \vec{F}_{12} $$ points away from $$ q_2 $$, indicating repulsion.
- If $$ q_1 $$ and $$ q_2 $$ have opposite signs, $$ \vec{F}_{12} $$ points toward $$ q_2 $$, indicating attraction.

### **Applications of Coulomb's Law in Describing Force Relationships**

1. **Atomic Structure:**
   - **Electrons and Nuclei:** Electrons (negative charges) are attracted to the positively charged nucleus (protons) due to Coulomb's Law. This attractive force keeps electrons in orbit around the nucleus.
   - **Electron-Electron Repulsion:** Electrons repel each other, which affects the distribution of electrons in an atom.

2. **Molecules and Chemical Bonds:**
   - **Ionic Bonds:** Formed between positively and negatively charged ions, these bonds are a result of the attractive forces described by Coulomb's Law.
   - **Dipole Interactions:** Polar molecules have partial positive and negative charges that interact through Coulombic attraction and repulsion.

3. **Static Electricity:**
   - **Charge Distribution:** Objects with excess electrons or holes create electric fields that exert forces on other charged objects, leading to attraction or repulsion.

4. **Electrostatic Precipitators:**
   - **Pollution Control:** Charged particles in emissions are attracted to plates with opposite charges, removing contaminants from industrial exhaust streams.

5. **Biological Systems:**
   - **Protein Folding:** Electrostatic interactions between charged amino acids influence the three-dimensional structure of proteins.
   - **DNA Structure:** The negatively charged phosphate backbone of DNA interacts with positive ions and proteins to maintain its structure.

### **Limitations of Coulomb's Law**

While Coulomb's Law is powerful, it has limitations:

- **Point Charges Assumption:** It assumes that the charges are point-like and do not account for the distribution of charge within extended objects.
- **Static Charges:** It applies to stationary charges and does not consider time-varying electric or magnetic fields.
- **Quantum Effects:** At atomic and subatomic scales, quantum mechanics provides a more accurate description of interactions.

### **Relation to Other Physical Laws**

Coulomb's Law is analogous to Newton's Law of Universal Gravitation but applies to electric charges instead of masses. Both laws describe forces that vary inversely with the square of the distance between objects, though one is attractive (gravity) for all masses, and the other can be both attractive and repulsive depending on the charges involved.

### **Summary**

Coulomb's Law is essential for understanding how charged particles interact. By quantifying the electrostatic force between charges, it explains why like charges repel and opposite charges attract. This principle forms the backbone of numerous phenomena in physics, chemistry, and engineering, influencing everything from atomic structures to electrical circuits and beyond.
Coulomb's Law describes the force between two charged particles. It states that like charges repel and opposite charges attract. The force is calculated using the formula $$ F = k_e \cdot \frac{|q_1 \cdot q_2|}{r^2} $$, where $$ k_e $$ is Coulomb's constant, $$ q_1 $$ and $$ q_2 $$ are the charges, and $$ r $$ is the distance between them.

What is Coulomb’s law, and how can it be used to describe the relationship between
attractive and repulsive forces?

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What is Coulomb’s law, and how can it be used to describe the relationship between attractive and repulsive forces?