A capacitor is a passive component made of two conductive plates separated by an insulator (the dielectric). When a voltage is applied across the plates, an electric field forms in the dielectric and this field stores energy. The property that describes the amount of energy a capacitor can store is called capacitance.

Capacitance therefore describes the ability to store energy in an electric field. This is often compared with inductance, which describes the ability to store energy in a magnetic field — they both store energy but in different ways.

Memory aids:

• Capacitance = ability to store energy in an electric field
• Capacitance → stores energy in an electric field; Inductance → stores energy in a magnetic field
• Acronym: MICE — Magnetic → Inductance; Capacitance → Electric

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The basic unit of capacitance, the Farad, is named for the physicist Michael Faraday. Capacitance measures how much electric charge a component stores per volt of potential difference.

The other units listed here are:

• the Volt - basic unit of voltage
• the Ohm - basic unit of resistance
• the Henry - basic unit of inductance

Memory aids:

• "Henry was Inducted into the hall of fame, and Faraday tipped his Cap(acitance)."
• Many of us have heard of a Faraday Cage. Cages CAPture things (CAPacitance).

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An inductor is a passive electronic component made by coiling wire — often around a core, which may be magnetic (like ferrite) or non-magnetic (like air), depending on the application. When current flows through the coil, it creates a magnetic field around it — and that magnetic field stores energy. The property that describes this ability to store energy in a magnetic field is called inductance.

This is a common point of confusion because both inductance and capacitance refer to storing energy, but they store it in different kinds of fields:

• Inductance → stores energy in a magnetic field
• Capacitance → stores energy in an electric field

Memory aids:

• Inductance → stores energy in a Magnetic field
• Capacitance → stores energy in an Electric field
• Acronym: MICE — Magnetic → Inductance; Capacitance → Electric

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An inductor is a passive electrical component that stores energy in a magnetic field; its unit is the henry, which is named for Joseph Henry.

It's helpful to know what the other units listed are, since they are all real units:

• the coulomb — unit of electric charge
• the farad — unit of capacitance
• the ohm — unit of resistance

Memory aids:

• "Henry was inducted into the hall of fame, and Farad(ay) tipped his Cap(acitance)"

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Impedance is a measure of how much a circuit element resists the flow of alternating current, combining the effects of resistance, inductive reactance, and capacitive reactance. Because it quantifies opposition to current in the same way resistance does, impedance is measured in the same unit as resistance: ohms.

The important distinction is that impedance depends on frequency: inductors and capacitors change how much they oppose current as frequency changes. For example, inductors have low opposition at DC (frequency = 0) but increasing impedance at higher frequencies because they resist changes in current. Capacitors have very high opposition at DC (they block steady current) but lower impedance at higher frequencies because they pass changing voltages more easily.

Memory aids / mnemonics:

• "Impedance" and "resistance" are similar words; both use the unit ohm.
• Think: impede ≈ resist → ohms.
• Inductor: low impedance at DC, higher impedance as frequency increases.
• Capacitor: high impedance at DC, lower impedance as frequency increases.

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The correct abbreviation is kHz. The unit hertz is written "Hz" and the metric prefix for kilo (one thousand) is the lowercase "k", so the proper combined symbol is kHz.

Memory aids:

• The symbol for hertz is Hz.
• The prefix for kilo (1,000) is a lowercase k.
• Metric prefixes smaller than mega (1,000,000) are written lowercase, so kilo uses k, giving kHz.

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M is the SI prefix for mega, meaning one million (×10^6). Hz is the SI unit symbol for hertz, meaning cycles per second. The correct abbreviation combines the prefix and the unit with the proper capitalization: MHz.

Capitalization matters in SI symbols: an uppercase M means mega (10^6), while a lowercase m means milli (10^-3). Likewise, the unit symbol for hertz is properly written with an uppercase H and a lowercase z (Hz), not all uppercase or incorrect letter-case mixes.

Memory aids / mnemonics:

• Uppercase M = Mega (10^6); lowercase m = milli (10^-3)
• Hz = H (uppercase) + z (lowercase) for hertz
• MHz is common and used to denote millions of cycles per second

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The power in a DC circuit is the product of voltage and current:

\[P = E \times I\]

From this you can rearrange to solve for any one quantity:

\[E = \frac{P}{I}\]

\[I = \frac{P}{E}\]

• P = Power (watts)
• E = Electromotive force / Voltage (volts)
• I = Current (amperes)

Memory aids:

• "Pixie": P = I × E

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The electrical power delivered is the product of current and voltage. Using P = I × E, substitute the given values: P = 10 A × 13.8 V = 138 watts.

Memory aids:

• "Power (watts) equals Current (amperes) multiplied by Voltage (volts)."
• Formula: P = I × E

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Power is the rate at which electrical energy is generated or consumed. For a DC circuit, electrical power is the product of voltage and current:

P = V × I

Substitute the given values:

P = 12 V × 2.5 A = 30 W

So the circuit delivers 30 watts of power.

Memory aids:

• Formula: P = V × I (watts = volts × amperes)
• Given values: V = 12 V, I = 2.5 A
• Calculation: 12 × 2.5 = 30 W

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We use the power law:

\[P = I \times E\]

Given:

\[\begin{align} \text{(Power) } P &= 120\text{ watts}\\ \text{(Current) } I &= \text{?}\\ \text{(Voltage) } E &= 12\text{ volts}\\ \end{align}\]

Solve for current by dividing both sides by the voltage:

\[\begin{align} I &= \frac{P}{E}\\ I &= \frac{120}{12}\\ &= 10\text{ amperes} \end{align}\]

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Impedance is the measure of how much a circuit resists the flow of alternating current. The inverse of resistance is conductance (measured in siemens, historically called mhos), so the inverse of resistance is not impedance. The inverse of reactance by itself is not a commonly used single term; the inverse of impedance as a whole is admittance (also measured in siemens), whose imaginary part is called susceptance. Power-handling capability is measured in watts and is unrelated to the definition of impedance.

Impedance is measured in ohms and is represented by the symbol Z. For purely DC systems, impedance reduces to ordinary resistance. In AC circuits, impedance generally has both magnitude and phase and is represented as a complex number (real part = resistance, imaginary part = reactance).

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