Capacitance - Charge Storage in Electric Fields
Capacitance C = Q/V relates stored charge to voltage. Energy stored E = ½CV². Capacitors are essential for filtering, timing, and energy storage in circuits.
Why This Physics Calculation Matters
Why: Capacitors store energy in electric fields and are used for power supply filtering, timing circuits, and energy storage. Understanding C = Q/V and E = ½CV² is essential for circuit design.
How: C = Q/V defines capacitance. For parallel plates, C = ε₀εrA/d. Stored energy E = ½CV² = ½QV. RC time constant τ = RC governs charging/discharging.
- ●Capacitance increases with plate area and dielectric constant
- ●Capacitance decreases with plate separation
- ●Energy stored scales with V²—high voltage capacitors store more energy
- ●RC time constant: 63% charge in 1τ, 99% in 5τ
Sample Examples
Click an example to load values, or enter your own data below.
Camera Flash Capacitor
1000µF at 300V → Calculate energy stored
Power Supply Filter
470µF at 25V → Calculate charge
Timing Circuit
10µC charge at 1V → Calculate capacitance
Supercapacitor
1F storing 3.645J → Calculate voltage
Calculation Mode
Input Values
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Capacitance C is measured in Farads (F); 1 F = 1 C/V
— HyperPhysics
Parallel plate capacitance C = ε₀A/d; doubling area doubles C
— Physics Classroom
RC time constant τ = RC; capacitor reaches 63% in one τ
— HyperPhysics
Supercapacitors achieve thousands of Farads for energy storage
— NIST
📋 Key Takeaways
- • Capacitance definition: C = Q/V — capacitance measures how much charge a capacitor stores per volt applied
- • Energy storage: E = ½CV² — capacitors store energy in electric fields, with energy proportional to voltage squared
- • Parallel plate formula: C = ε₀εᵣA/d — capacitance increases with plate area and dielectric constant, decreases with separation
- • Series vs parallel: Series capacitors decrease total capacitance (1/C = 1/C₁ + 1/C₂), parallel increases (C = C₁ + C₂)
- • RC time constant: τ = RC determines charge/discharge rate — capacitor reaches 63% charge in one time constant
💡 Did You Know?
🔬 How It Works
Parallel Plate Capacitance
Where ε₀ = 8.85×10⁻¹² F/m (vacuum permittivity), εᵣ is relative permittivity, A is plate area, d is separation
Series and Parallel Combinations
Series reduces total capacitance (opposite of resistors), parallel increases it
RC Time Constant
After 5 time constants (5τ), capacitor is 99.3% charged — considered fully charged
🎯 Expert Tips
Choose capacitor type based on application — ceramics for high-frequency, electrolytics for high capacitance, film for stability
Always derate voltage by 20-50% — capacitors fail catastrophically if voltage rating is exceeded
Consider ESR (Equivalent Series Resistance) for power applications — low ESR reduces heating and improves efficiency
Watch temperature coefficients — Class 2 ceramics (X7R, X5R) lose capacitance at high temperatures and DC bias
📊 Capacitor Types Comparison
| Type | Capacitance Range | Voltage Range | Polarized | Best For |
|---|---|---|---|---|
| Ceramic (MLCC) | 0.5pF - 100µF | 6.3V - 3kV | No | High-frequency, decoupling |
| Electrolytic | 1µF - 10,000µF | 6.3V - 450V | Yes | Power supply filtering |
| Film | 100pF - 100µF | Up to several kV | No | Audio, timing circuits |
| Supercapacitor | 0.1F - 3000F | 2.5V - 2.7V per cell | Yes | Energy storage |
❓ Frequently Asked Questions
What is the difference between capacitance and charge?
Capacitance (C) is the ability to store charge per volt — it's a property of the capacitor. Charge (Q) is the actual amount of stored charge — Q = C × V. Think of capacitance as the "size" and charge as the "amount stored".
Why do capacitors in series have less total capacitance?
In series, the voltage divides across capacitors, so each capacitor sees less voltage. Since Q = CV and the same charge flows through all capacitors, the effective capacitance decreases. It's the opposite of resistors in series.
How do I choose the right capacitor for my circuit?
Consider: (1) Required capacitance value, (2) Voltage rating (use 20-50% margin), (3) Frequency response (ceramics for high-frequency), (4) Temperature stability (C0G for precision), (5) ESR for power applications.
What happens if I exceed the voltage rating?
Dielectric breakdown occurs — the insulating material fails, causing a short circuit. This can lead to fire, explosion (especially electrolytics), or component damage. Always use capacitors rated above maximum expected voltage.
Why do capacitors have polarity?
Electrolytic and tantalum capacitors use an oxide layer as dielectric, which forms only with correct polarity. Reverse polarity destroys the oxide layer, causing failure. Ceramic and film capacitors are non-polarized.
What is ESR and why does it matter?
ESR (Equivalent Series Resistance) is internal resistance causing power loss and heating. Low ESR is critical for power supply filtering and high-frequency applications. Electrolytics have higher ESR than ceramics or film.
How long does a capacitor take to charge?
Charging follows exponential curve: V(t) = V₀(1 - e^(-t/RC)). After 1 time constant (τ = RC), voltage reaches 63.2%. After 5τ, it's 99.3% charged — considered fully charged.
Can I replace an electrolytic with a ceramic capacitor?
Sometimes, for small values. Ceramics have better ESR and frequency response but are limited to ~100µF max in practical sizes. Watch for DC bias derating on Class 2 ceramics (X7R, X5R).
📊 Capacitance by the Numbers
📚 Official Sources
NIST - SI Units for Electricity
Official NIST definitions of electrical units and capacitance standards
Last verified: 2026-02-01
Electronics Tutorials - Capacitors
Practical guide to capacitor theory, applications, and circuit design
Last verified: 2026-02-01
MIT OpenCourseWare - Electromagnetism
MIT course materials on electromagnetism including capacitance and capacitors
Last verified: 2026-02-01
All About Circuits - Practical Capacitors
Engineering considerations for capacitor selection and circuit design
Last verified: 2026-02-01
⚠️ Disclaimer
This calculator is for educational and design purposes. Always verify capacitor ratings and safety requirements for your specific application. High-voltage capacitors store dangerous energy — always discharge before handling. Consult datasheets and safety guidelines when working with capacitors.