MCAT electricity and magnetism ties together charges, fields, and circuits — and connects to the physiology of nerves and the heart. From Coulomb’s law and the electric field to Ohm’s law and the right-hand rule, this topic rewards a small set of relationships applied carefully, plus the same free-body thinking you use in mechanics.
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update Updated October 2026
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bolt The 60-second version
check_circleCoulomb’s law: force between charges falls off as 1/r²; the electric field is force per unit charge.
check_circleField lines point from positive to negative; a positive charge feels a force along them.
check_circleOhm’s law: V = IR relates voltage, current, and resistance.
check_circleSeries resistors add (R = R₁+R₂); parallel resistors combine reciprocally and lower total resistance.
check_circleCurrent is the same everywhere in series; voltage is the same across parallel branches.
check_circleMagnetic force F = qvB·sinθ is perpendicular to both velocity and field (right-hand rule).
target What you'll be able to do · aligned to AAMC content category 4C
4CApply Coulomb’s law and the electric-field concept to forces between charges.
4CRelate electric potential, potential energy, and the work to move a charge.
4CAnalyze series and parallel circuits with Ohm’s law and power relationships.
4CUse the right-hand rule to find the magnetic force on a moving charge.
How the MCAT tests electricity and magnetism
This topic is AAMC content category 4C — electrochemistry and electrical circuits and their elements. The MCAT ties it to physiology (membrane potentials, the ECG) and to lab equipment. It rewards fluency with a few core equations and clean circuit reasoning rather than heavy computation.
workspace_premium The recurring toolkit
Coulomb’s law and the field for charges; V = IR and the series/parallel rules for circuits; the right-hand rule for magnetism. Master those and most questions fall quickly.
Coulomb’s law and the electric field
Charges exert forces on each other: like charges repel, opposites attract. The electric field is the force a positive test charge would feel per unit charge — a way to describe the influence of charge at every point in space.
Field of a dipole. Electric field lines run from the positive charge to the negative charge; their density shows the field’s strength.
F = kq₁q₂ / r²Coulomb’s lawThe force falls off as the inverse square of separation. E = F/q gives the field, which also falls off as 1/r² from a point charge.
Electric potential and potential energy
Moving a charge in a field takes work, stored as electric potential energy. Electric potential (voltage) is that energy per unit charge — the driving force for current.
Potential energy: U = kq₁q₂/r for two point charges — positive for like charges, negative for opposite.
Potential (voltage): V = U/q, measured in volts; positive charges move from high to low potential.
Work: W = qΔV to move a charge across a potential difference.
Circuits: Ohm’s law and resistor networks
A circuit drives current through resistance using a voltage source. Ohm’s law links the three, and the series/parallel rules let you reduce any network.
Series vs parallel. Series resistors share the same current and add; parallel resistors share the same voltage and combine reciprocally, always giving less than the smallest resistor.
V = IROhm’s lawVoltage equals current times resistance. Power dissipated is P = IV = I²R = V²/R.
functions Worked example — two resistors in parallel
A 6 Ω and a 3 Ω resistor are connected in parallel. What is the equivalent resistance?
Use 1/R = 1/R₁ + 1/R₂ = 1/6 + 1/3.
Common denominator: 1/6 + 2/6 = 3/6 = 1/2.
So R = 2 Ω.
Answer. R = 2 Ω — less than either resistor, as parallel combinations always are.
Capacitors and power
A capacitor stores charge and energy in an electric field between two plates. Along with power, it rounds out the circuit toolkit.
Capacitance: Q = CV — a capacitor holds charge proportional to the voltage across it.
Capacitors combine oppositely to resistors: they add in parallel and combine reciprocally in series.
Power: P = IV is the rate of energy use; P = I²R gives the heat dissipated in a resistor.
Magnetism and the magnetic force
Moving charges create magnetic fields and feel forces in them. The magnetic force is always perpendicular to both the velocity and the field, so it changes direction, not speed.
Right-hand rule. For a positive charge, point your fingers along v, curl toward B, and your thumb gives the force F = qv × B — perpendicular to both.
workspace_premium A force that does no work
Because the magnetic force is perpendicular to the velocity, it does no work and cannot change a particle’s speed — it only bends its path, producing circular or helical motion.
How to study it — and the traps to avoid
E&M rewards careful bookkeeping. Reduce circuits step by step and respect the perpendicularity of magnetic forces.
Track what is constant: current in series, voltage in parallel.
Reduce resistor networks one combination at a time.
Remember both force and field fall off as 1/r² for point charges.
For magnetism, use the right-hand rule and flip the direction for negative charges.
error Highest-yield trap
Adding resistors in parallel LOWERS the total resistance below the smallest one, because you are giving current more paths. Only series resistances add directly — mixing up the two rules is the most common circuit error.
Straight from the Progress Note question bank, each with a cited explanation. No account needed.
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A noncoding variant associated with autoimmune disease lies in a DNase I–hypersensitive region bound by transcription factors in immune cells. What is the most plausible functional consequence?
Answer: B. ENCODE analyses show that many disease-linked noncoding variants map to regulatory chromatin, such as enhancers marked by TF binding and DNase hypersensitivity. Such variants can alter transcriptional regulation in cell-type–specific contexts.
At physiological pH, which amino acid has a side chain that is positively charged, making it a basic amino acid?
Answer: D. Lysine has a side chain ending in an amino group that is protonated and positively charged at physiological pH, classifying it as a basic amino acid (along with arginine and histidine). Aspartate and glutamate (A, B) have carboxylate side chains that are negatively charged (acidic), while leucine and alanine (C, E) have nonpolar, uncharged hydrocarbon side chains. Only lysine carries a positive side-chain charge at physiological pH.
In George Herbert Mead's theory of how the self develops, a child eventually learns to take the viewpoint of society as a whole — the shared norms and expectations they imagine 'people in general' hold. Mead called this internalized social viewpoint the...
Answer: D. Mead's 'generalized other' is a person's internalized sense of the overall norms, values, and expectations of society; by taking the role of the generalized other, an individual can judge their own behavior from society's broader standpoint. The looking-glass self (A) is Cooley's related but distinct idea that we form our self-image from how we think specific others see us, role conflict (B) is tension between two different statuses, anticipatory socialization (C) is rehearsing for a future role, and the presentation of self (E) is Goffman's idea about managing impressions. The internalized attitude of society as a whole is the generalized other.
What is the difference between electric field and electric potential?expand_more
The electric field is the force per unit charge at a point (a vector, in N/C), while electric potential is the potential energy per unit charge (a scalar, in volts). The field describes the force a charge would feel; the potential describes the energy landscape, and charges move from high to low potential.
How do resistors combine in series and parallel?expand_more
In series, resistors carry the same current and their resistances add: R = R₁ + R₂ + … In parallel, resistors share the same voltage and combine reciprocally: 1/R = 1/R₁ + 1/R₂ + …, which always gives a total less than the smallest individual resistor.
Why does the magnetic force do no work on a charged particle?expand_more
The magnetic force F = qv × B is always perpendicular to the velocity. Since work depends on the force component along the direction of motion, a perpendicular force does zero work and cannot change the particle’s speed — it only changes its direction, producing circular or helical motion.
What is Ohm’s law and how is power related?expand_more
Ohm’s law states V = IR: the voltage across a resistor equals the current through it times its resistance. The power dissipated can be written three equivalent ways: P = IV = I²R = V²/R, all giving the rate at which electrical energy converts to heat.
How does Coulomb’s law depend on distance?expand_more
Coulomb’s law, F = kq₁q₂/r², says the force between two charges is inversely proportional to the square of their separation. Doubling the distance quarters the force. The electric field of a point charge follows the same inverse-square dependence.