⚡ Physics · Electricity & Magnetism

Memory tricks for circuits and fields

Ohm's law, Kirchhoff's laws, capacitors, inductors, electromagnetic induction — the core of E&M with shortcuts that stick.

⚡ Electricity & Magnetism

Memory Tricks

Proven Mnemonics & Acronyms — fast to learn, hard to forget.

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Series vs Parallel
Series: same current everywhere. Parallel: same voltage across each branch.
Series vs Parallel
The one rule that unlocks all circuit analysis
Series: current identical through all components, voltages add. Parallel: voltage identical across all branches, currents add. Christmas lights in series — one fails, all fail.
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🃏 Series vs Parallel
Series vs parallel circuits — what stays the same?
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🃏 Answer
Series: same current everywhere. Parallel: same voltage across each branch.
Series: current identical through all components, voltages add. Parallel: voltage identical across all branches, currents add. Christmas lights in series — one fails, all fail.
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Magnetic Force Direction
Right-hand rule: fingers point in current direction, curl to B-field, thumb = force
Magnetic Force Direction
Find the direction of magnetic force on a current or moving charge
Point fingers in direction of velocity (or current), curl toward B-field → thumb points in direction of magnetic force on a positive charge. Flip hand for electrons (negative charge).
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🃏 Magnetic Force Direction
Right-hand rule — how do you find the magnetic force?
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🃏 Answer
Right-hand rule: fingers point in current direction, curl to B-field, thumb = force
Point fingers in direction of velocity (or current), curl toward B-field → thumb points in direction of magnetic force on a positive charge. Flip hand for electrons (negative charge).
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AC Phase Relationships
CIVIL: in a Capacitor (C), current (I) leads voltage (V); voltage (V) leads current (I) in an inductor (L)
AC Phase Relationships
Capacitor and inductor phase relationships — one mnemonic covers both
In a Capacitor: Current leads Voltage. In an inductor (L): Voltage leads Current. CIVIL encodes both. Crucial for AC circuit analysis and power factor calculations.
C
Capacitor
I
Current leads
V
Voltage
I
In inductors
L
Voltage leads current
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🃏 AC Phase Relationships
CIVIL — what leads what in capacitors and inductors?
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🃏 Answer
CIVIL: in a Capacitor (C), current (I) leads voltage (V); voltage (V) leads current (I) in an inductor (L)
CCapacitor
ICurrent leads
VVoltage
IIn inductors
LVoltage leads current
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Electrical Power
P = IV = I²R = V²/R — electrical power in three useful forms
Electrical Power
Three equivalent expressions for electrical power — pick whichever fits
P = IV: power equals current times voltage. P = I²R: useful when you know current and resistance. P = V²/R: useful when you know voltage and resistance. Units: Watts = Joules/second.
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🃏 Electrical Power
Electrical power — the three forms of the formula?
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🃏 Answer
P = IV = I²R = V²/R — electrical power in three useful forms
P = IV: power equals current times voltage. P = I²R: useful when you know current and resistance. P = V²/R: useful when you know voltage and resistance. Units: Watts = Joules/second.
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Coulomb's Law
Coulomb's Law: F = kq₁q₂/r². Like charges repel. Opposite charges attract.
Coulomb's Law
The electric force between charges — mirrors Newton's gravity
F = kq₁q₂/r² where k = 8.99×10⁹ N·m²/C². Like charges (both + or both -): repel. Unlike charges (+ and -): attract. Inverse square law — double distance → ¼ the force. Much stronger than gravity at atomic scales.
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🃏 Coulomb's Law
Coulomb's law?
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🃏 Answer
Coulomb's Law: F = kq₁q₂/r². Like charges repel. Opposite charges attract.
F = kq₁q₂/r² where k = 8.99×10⁹ N·m²/C². Like charges (both + or both -): repel. Unlike charges (+ and -): attract. Inverse square law — double distance → ¼ the force. Much stronger than gravity at atomic scales.
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Electric Fields
Electric field: E = F/q. Field lines go from + to -. Closer lines = stronger field.
Electric Fields
The force per unit charge surrounding any charged object
Electric field E = Force/charge = F/q. Units: N/C or V/m. Field lines originate at positive charges and terminate at negative. Denser field lines = stronger field. A positive test charge would follow the field lines. Uniform field between parallel plates: E = V/d.
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🃏 Electric Fields
Electric field — formula and field-line rules?
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🃏 Answer
Electric field: E = F/q. Field lines go from + to -. Closer lines = stronger field.
Electric field E = Force/charge = F/q. Units: N/C or V/m. Field lines originate at positive charges and terminate at negative. Denser field lines = stronger field. A positive test charge would follow the field lines. Uniform field between parallel plates: E = V/d.
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Kirchhoff's Laws
Kirchhoff's Laws: junction rule (currents in = currents out). Loop rule (voltage gains = voltage drops).
Kirchhoff's Laws
Two rules for analyzing complex circuits
Junction rule (KCL): at any junction, the sum of currents entering equals the sum leaving — conservation of charge. Loop rule (KVL): around any closed loop, the sum of all voltage changes equals zero — conservation of energy. Together they let you solve any circuit.
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🃏 Kirchhoff's Laws
Kirchhoff's two laws?
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🃏 Answer
Kirchhoff's Laws: junction rule (currents in = currents out). Loop rule (voltage gains = voltage drops).
Junction rule (KCL): at any junction, the sum of currents entering equals the sum leaving — conservation of charge. Loop rule (KVL): around any closed loop, the sum of all voltage changes equals zero — conservation of energy. Together they let you solve any circuit.
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Capacitors
Capacitor stores charge: C = Q/V. Energy = ½CV². In series: 1/C total = 1/C₁ + 1/C₂.
Capacitors
How capacitors store energy in an electric field
Capacitance C = Q/V (charge stored per volt). Unit: Farads (F). Parallel plate capacitor: C = ε₀A/d. Energy stored = ½CV² = ½QV. Series capacitors: reciprocals add (like parallel resistors). Parallel capacitors: values add directly (like series resistors). Capacitors block DC, pass AC.
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🃏 Capacitors
Capacitors — capacitance, energy, and series rule?
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🃏 Answer
Capacitor stores charge: C = Q/V. Energy = ½CV². In series: 1/C total = 1/C₁ + 1/C₂.
Capacitance C = Q/V (charge stored per volt). Unit: Farads (F). Parallel plate capacitor: C = ε₀A/d. Energy stored = ½CV² = ½QV. Series capacitors: reciprocals add (like parallel resistors). Parallel capacitors: values add directly (like series resistors). Capacitors block DC, pass AC.
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Electromagnetic Induction
Magnetic flux: Φ = BAcosθ. Faraday's law: changing flux induces EMF. Lenz's law: induced current opposes change.
Electromagnetic Induction
How changing magnetic fields create electric currents
Faraday's law: EMF = -ΔΦ/Δt. More loops (N turns): EMF = -NΔΦ/Δt. Lenz's law: the induced current flows in a direction to oppose the change in flux that created it. Applications: electric generators, transformers, induction cooktops, MRI machines.
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🃏 Electromagnetic Induction
Magnetic flux, Faraday's law, Lenz's law?
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🃏 Answer
Magnetic flux: Φ = BAcosθ. Faraday's law: changing flux induces EMF. Lenz's law: induced current opposes change.
Faraday's law: EMF = -ΔΦ/Δt. More loops (N turns): EMF = -NΔΦ/Δt. Lenz's law: the induced current flows in a direction to oppose the change in flux that created it. Applications: electric generators, transformers, induction cooktops, MRI machines.
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Resistor Combinations
Resistors in series: R total = R₁ + R₂ + R₃. In parallel: 1/R total = 1/R₁ + 1/R₂ + 1/R₃.
Resistor Combinations
How to find total resistance in series and parallel circuits
Series: resistances simply add. Current is the same through all. Voltage divides proportionally. Parallel: reciprocals add. Voltage is the same across all. Current divides inversely proportionally. Total resistance always less than smallest individual resistor in parallel.
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🃏 Resistor Combinations
Resistors in series vs parallel — the formulas?
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🃏 Answer
Resistors in series: R total = R₁ + R₂ + R₃. In parallel: 1/R total = 1/R₁ + 1/R₂ + 1/R₃.
Series: resistances simply add. Current is the same through all. Voltage divides proportionally. Parallel: reciprocals add. Voltage is the same across all. Current divides inversely proportionally. Total resistance always less than smallest individual resistor in parallel.
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Transformers
Transformer: V₁/V₂ = N₁/N₂. Step-up: more secondary turns → higher voltage. Step-down: fewer turns → lower voltage.
Transformers
How transformers change voltage using electromagnetic induction
Transformer works only on AC — changing current creates changing magnetic field which induces voltage in secondary coil. Turns ratio determines voltage ratio. Power conserved (ideal): P = V₁I₁ = V₂I₂. Step up voltage → step down current. Used in power transmission: high voltage, low current = less energy lost.
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🃏 Transformers
Transformers — step-up vs step-down?
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🃏 Answer
Transformer: V₁/V₂ = N₁/N₂. Step-up: more secondary turns → higher voltage. Step-down: fewer turns → lower voltage.
Transformer works only on AC — changing current creates changing magnetic field which induces voltage in secondary coil. Turns ratio determines voltage ratio. Power conserved (ideal): P = V₁I₁ = V₂I₂. Step up voltage → step down current. Used in power transmission: high voltage, low current = less energy lost.
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Magnetic vs Electric Fields
EVEM — Electric field lines start on positive, end on negative. Magnetic field lines form closed loops — no monopoles.
Key differences between E-fields and B-fields
Electric field lines have sources and sinks; magnetic field lines are always closed loops
Electric field: created by charges, points away from positive and toward negative, field lines begin and end on charges. Magnetic field: created by moving charges or currents, field lines always form closed loops (no magnetic monopoles ever found). Gauss's law for E: flux through closed surface = enclosed charge / ε₀. Gauss's law for B: flux through any closed surface = 0 (always — because no monopoles).
E-field lines
Start on + charge, end on − charge — can be open
B-field lines
Always closed loops — no start or end point
No monopoles
Magnetic north always paired with south — never isolated
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🃏 Magnetic vs Electric Fields
Electric vs magnetic field lines — how do they differ (EVEM)?
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🃏 Answer
Electric vs magnetic field lines — EVEM
EVEM — Electric field lines start on positive, end on negative. Magnetic field lines form closed loops — no monopoles.
E-field linesStart on + charge, end on − charge — can be open
B-field linesAlways closed loops — no start or end point
No monopolesMagnetic north always paired with south — never isolated
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Maxwell's Equations Summary
GAME — Gauss (E), Ampere, Magnetic Gauss, Faraday — four laws that unify electromagnetism
The four fundamental equations of electromagnetism
Maxwell unified electricity, magnetism, and optics — light is an electromagnetic wave
Gauss's Law (E): electric flux through closed surface = Q_enclosed / ε₀. Gauss's Law (B): magnetic flux through any closed surface = 0. Faraday's Law: changing magnetic flux induces EMF — basis of generators. Ampere-Maxwell Law: currents AND changing electric fields create magnetic fields — Maxwell added the displacement current term. Together they predict electromagnetic waves traveling at c = 1/√(ε₀μ₀).
Gauss (E)
Charges create electric fields — sources and sinks
Faraday
Changing B creates E — basis of generators and inductors
Ampere-Maxwell
Currents AND changing E create B — predicts EM waves
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🃏 Maxwell's Equations Summary
GAME — Maxwell's four equations?
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🃏 Answer
GAME — Gauss (E), Ampere, Magnetic Gauss, Faraday — four laws that unify electromagnetism
Gauss (E)Charges create electric fields — sources and sinks
FaradayChanging B creates E — basis of generators and inductors
Ampere-MaxwellCurrents AND changing E create B — predicts EM waves
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Inductors and Inductance
Inductor opposes CHANGE in current — like inertia for electricity
Self-inductance — the tendency to resist changes in current flow
An inductor stores energy in its magnetic field and fights any change in current
Inductance L measured in Henries. EMF = −L(dI/dt) — the induced EMF opposes the change (Lenz's Law). Energy stored = ½LI². RL circuit time constant τ = L/R — current rises to 63% of final value in one τ. Inductors in series: L_total = L₁ + L₂. In parallel: 1/L_total = 1/L₁ + 1/L₂ (opposite of resistors). At DC steady state: inductor acts as a short circuit (wire). At high frequency AC: inductor acts as open circuit.
Opposes change
EMF = -L dI/dt — fights increases and decreases in current
Energy stored
½LI² — in the magnetic field (not electric like capacitor)
DC steady state
Acts as wire — no changing current, no induced EMF
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🃏 Inductors and Inductance
What does an inductor oppose?
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🃏 Answer
Inductor opposes CHANGE in current — like inertia for electricity
Opposes changeEMF = -L dI/dt — fights increases and decreases in current
Energy stored½LI² — in the magnetic field (not electric like capacitor)
DC steady stateActs as wire — no changing current, no induced EMF
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Dielectrics and Capacitance
Dielectric increases capacitance by factor κ — MORE charge stored at SAME voltage
How inserting an insulator between capacitor plates changes its properties
A dielectric increases capacitance, decreases electric field, and increases energy storage
Capacitance with dielectric: C = κε₀A/d where κ is the dielectric constant (always ≥ 1). κ = 1 for vacuum, ~80 for water, ~2-4 for common insulators. Effect: same voltage → more charge stored (C increases). Same charge → lower voltage (E decreases). Energy stored: U = Q²/2C = ½CV². Dielectric breakdown: if E-field exceeds dielectric strength, insulator fails — this limits maximum voltage. Polarization: dielectric molecules align with field, partially canceling the applied field → weaker net field.
κ (kappa)
Dielectric constant — multiplies capacitance, always ≥ 1
Same V
More charge stored — Q = CV, C bigger so Q bigger
Breakdown
Max E-field the dielectric can withstand before it fails
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🃏 Dielectrics and Capacitance
What does a dielectric do to a capacitor?
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🃏 Answer
Dielectric increases capacitance by factor κ — MORE charge stored at SAME voltage
κ (kappa)Dielectric constant — multiplies capacitance, always ≥ 1
Same VMore charge stored — Q = CV, C bigger so Q bigger
BreakdownMax E-field the dielectric can withstand before it fails
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