Atlas MCAT Course

Equations & How to Use Them → Equations: Charge

Equations: Charge

Fields, circuits, capacitors, and electrochemistry.

01The idea

Charges move, fields exist, or energy is transferred electrically. Electrons move; cells generate voltage; ion gradients create potentials.

THREAD · Gradients and flow

02Build the model

AElectric charge: the property that creates electrical interaction

Use when: counting charge; comparing magnitudes of charge; discrete charges are involved.

Passage cues: “Elementary charge”, “Number of electrons”, “Ionization”.

Don’t use it for: continuous charge distributions; field or force questions directly.

BCoulomb’s law: how charges exert forces on each other

Use when: two (or more) point charges interact; you care about force magnitude or direction; distance changes matter.

Passage cues: “Attraction / repulsion”, “Separated by distance r”, “Point charges”.

Don’t use it for: continuous charge distributions; circuit problems.

CElectric field: force per unit charge

Use when: a charge experiences force due to surroundings; you want a location-based description; test charge language appears.

Passage cues: “Field strength”, “Test charge”, “Region of space”.

Don’t use it for: energy-per-charge questions (use electric potential).

DElectric potential (voltage): energy per unit charge

Use when: energy changes are discussed; charges move due to potential difference; batteries or membranes appear.

Passage cues: “Potential difference”, “Energy gained/lost”, “Voltage source”.

Don’t use it for: force-based questions; pure current calculations.

EElectric field and voltage: how voltage creates force

Use when: uniform fields; parallel plates; distance between electrodes matters.

Passage cues: “Uniform field”, “Parallel plates”, “Separated by distance d”.

Don’t use it for: non-uniform fields; point charges.

FOhm’s law: what sets current

Use when: steady-state circuits; comparing current, resistance, or voltage; resistors are present.

Passage cues: “Battery”, “Resistor”, “Steady current”.

Don’t use it for: capacitors charging/discharging; non-ohmic materials.

GElectric power: how fast electrical energy is transferred

Use when: energy per time matters; heat dissipation; comparing device output.

Passage cues: “Power”, “Energy per second”, “Heat generated”.

Don’t use it for: static fields; pure charge calculations.

HCapacitance and capacitor energy: ability to store charge and energy

Use when: charge storage is discussed; capacitors appear; voltage changes without current flow; energy storage is asked.

Passage cues: “Capacitor”, “Dielectric”, “Stored charge”, “Charged capacitor”.

Don’t use it for: steady current flow; resistor-only circuits.

IRC time constant: how fast capacitors charge and discharge

Use when: charging/discharging curves; time-dependent behavior; exponential changes.

Passage cues: “Time constant”, “Exponential”, “Approaches equilibrium”.

Don’t use it for: pure DC steady-state; no capacitor present.

JSeries and parallel circuits: how components share current and voltage

Use when: multiple resistors present; circuit architecture matters.

Passage cues: “Same current” (series), “Same voltage” (parallel).

KCell potential and free energy: electrical work available from redox

Use when: galvanic/voltaic cell spontaneity; relating voltage to free energy.

LNernst equation: how concentration gradients change voltage

Use when: non-standard conditions (cell, physiology, electrochem); ion gradients / membrane potentials.

Passage cues: “Not standard conditions”, “Ion gradient”, “Membrane potential / equilibrium potential”.

03See the connections

The lesson names these links itself. Read the map left to right.

04Reason with it

Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.

05MCAT lens

06Common traps

07Dependency check

08Remember

09Forward hook