# Electric Charge and Fields

Computer Science I · Physics · https://tryals.app/learn/computer-science-i/electric-charge-and-fields

## Electric Charge & Coulomb's Law

All electronics rest on **electric charge**, coming in **positive** and **negative** signs. The fundamental rule: **like charges repel, opposite charges attract**. Charge is **conserved** (never created or destroyed) and **quantized** (multiples of $e$).

The force between two point charges is given by **Coulomb's law**:

$$F = k\frac{q_1 q_2}{r^2}$$

| Symbol | Meaning |
| --- | --- |
| $F$ | Electric force |
| $k$ | Coulomb constant |
| $q_1, q_2$ | Point charges |
| $r$ | Distance between charges |

**Common pitfall:** Coulomb's force falls off as $1/r^2$. Doubling the distance cuts the force to a **quarter**, not a half. Never confuse like repulsion with attraction.

## Fields & Voltage

Rather than charges reaching across empty space, physics uses the **electric field** $E$: a charge creates a field, and another charge placed there feels a force $F = qE$. The field is force per unit charge, pointing away from positive and toward negative.

**Voltage** (electric potential) is potential energy per unit charge, the electrical height driving charges to move. A charge moves from high to low potential like a ball rolling downhill.

| Concept | Definition |
| --- | --- |
| **Electric Field** | Force per unit charge in space |
| **Voltage** | Potential energy per unit charge |

**Common pitfall:** Do not confuse force ($F$), field ($E$), and voltage. Voltage difference is what pushes current through a circuit.

## Practice questions

3 of this lesson's 10 practice questions, with answers. The full set is in the app.

### 1. A data centre spends nearly as much energy on cooling as on computing. Which physical fact makes that unavoidable?

A. Essentially all energy delivered to a chip ends up as heat
B. Cooling systems are less efficient than the computers they cool
C. Processors are inefficient and waste most of their energy as heat
D. Heat is generated by the cooling equipment itself

**Answer:** A. Essentially all energy delivered to a chip ends up as heat

**Why:** A chip performs no mechanical work and accumulates no energy, so virtually everything in comes out as heat, computing is electric heating that happens to calculate. This is why data centres are sited near cold water or cheap power, and why waste heat is increasingly piped into district heating.

Page: https://tryals.app/practice/computer-science-i/electric-charge-and-fields/a-data-centre-spends-nearly-as-much-energy-on-cooling-as-on

### 2. Select every statement that is TRUE of the electric force compared with gravity.

A. Both are proportional to the product of the two sources
B. Both fall off as the inverse square of the distance
C. Gravity can be repulsive between ordinary masses
D. The electric force can be attractive or repulsive
E. The electric force is vastly stronger than gravity for typical particles

**Answer:** A. Both are proportional to the product of the two sources; B. Both fall off as the inverse square of the distance; D. The electric force can be attractive or repulsive; E. The electric force is vastly stronger than gravity for typical particles

**Why:** The two laws share their mathematical shape entirely — inverse square, proportional to the product of the sources — but differ in strength and in sign. Ordinary mass only ever attracts, whereas charge comes in two signs, and that difference is what makes electronics possible.

Page: https://tryals.app/practice/computer-science-i/electric-charge-and-fields/select-every-statement-that-is-true-of-the-electric-force-compared

### 3. Coulomb's law and Newton's gravitation share the inverse-square form, yet gravity dominates at astronomical scales while electromagnetism dominates everywhere else. Why?

A. Because electrostatic forces require a medium and space is empty.
B. Because gravity acts on all matter while charge exists only in metals.
C. Because gravity falls off more slowly with distance than the electrostatic force.
D. Because charge comes in two signs that cancel, and mass in one.

**Answer:** D. Because charge comes in two signs that cancel, and mass in one.

**Why:** Between a proton and an electron the electrostatic force exceeds the gravitational by about 10³⁹, the disparity is not close. What suppresses it in bulk is cancellation, since ordinary matter holds almost exactly equal positive and negative charge, where mass has one sign and always adds. Both laws are inverse-square, so neither falls off faster; the asymmetry is entirely about cancellation.

Page: https://tryals.app/practice/computer-science-i/electric-charge-and-fields/coulombs-law-and-newtons-gravitation-share-the-inverse-square-form