Resistance

Resistance is the opposition to the flow of electrical current and is measured in ohms (Ω). It arises because energy is needed to free electrons from their atoms in a conductor. When current flows through a material with resistance, some of the electrical energy is converted into heat. This is why resistors in circuits can get warm during operation.

For example, the filament in an incandescent bulb produces light because it is a poor conductor. Its resistance causes it to heat up when current flows through it.

Importantly, resistance works the same way in both direct current (DC) and alternating current (AC) circuits.

Capacitance and Capacitors

Capacitance is a property of a circuit that describes its ability to store energy in an electric field. Capacitors are components designed to create this effect, typically by placing two conductive plates close to each other with a material (called a dielectric) in between. The dielectric can be air, ceramic, or other materials.

When a voltage is applied to a capacitor, an electric field forms between the plates, storing energy. Current rushes into the capacitor and the voltage across the capacitor starts to rise. The time taken for the capacitor to charge depends on the size of the capacitor and the charging current.

Because the current is maximum when the voltage charge across the capacitor is low, the current is said to 'lead' the voltage in phase.

The capacitance value depends on the size of the plates, their distance apart, and the type of dielectric material. Capacitance is measured in farads (F), though you’ll usually encounter smaller units like microfarads (µF), nanofarads (nF), or picofarads (pF).

Capacitors are commonly used to store energy, filter signals, or block DC while allowing AC to pass. Be cautious, though: large capacitors can store dangerous amounts of energy even after a circuit is turned off. Always discharge capacitors safely using a resistor.

Inductance and Inductors

Inductance is a property related to a conductor's ability to store energy in a magnetic field. Inductors, usually coils of wire, are components that make use of this property. The amount of inductance depends on factors like the coil's size, the number of turns, and the material inside the coil (called the core).

Inductance is measured in henrys (H), and you’ll commonly see millihenrys (mH), microhenrys (µH), and nanohenrys (nH) in practical circuits.

When voltage is applied to an inductor it generates a magnetic field around the inductor which opposes the change in current through the inductor. (Remember that a changing/moving magnetic field produces a voltage in a conductor, and vice versa, and this voltage generated by the changing magnetic field opposes any change in voltage voltage placed across the inductor).

The current through the inductor slowly builds to a maximum in a time depending on the size of the inductor.

As the magnetic field across the inductor stabilises, the current through the inductor rises as the voltage across the inductor falls. The current is said to 'lag' the voltage in phase.

If the current suddenly stops, the collapsing magnetic field induces a voltage (called back-EMF) across the inductor. This effect can create very high voltages, which is why caution is needed when working with large inductors like those found in relays or motors.

Reactance: The AC Counterpart to Resistance

In AC circuits, capacitors and inductors exhibit a property called reactance, which opposes the flow of alternating current. Reactance depends on the frequency of the AC signal:

• Capacitive Reactance (XC): The opposition to changes in an electric field within a capacitor. Higher frequencies reduce capacitive reactance, making capacitors act more like a short circuit for high-frequency signals and an open circuit for low frequencies.

• Inductive Reactance (XL): The opposition to changes in a magnetic field within an inductor. Higher frequencies increase inductive reactance, making inductors act more like an open circuit for high-frequency signals and a short circuit for high frequencies.

Reactance is measured in ohms (Ω), just like resistance.

Impedance: The Combination of Resistance and All Reactance's

Impedance (Z) is the overall opposition to the flow of AC current in a circuit and combines resistance (R), capacitive reactance (XC), and inductive reactance (XL). Unlike resistance, which is a simple value, impedance is a complex number that includes both magnitude and phase angle.

For practical purposes:

• Impedance is what you see specified in devices like loudspeakers (e.g., "8 ohms impedance"), however, this is just an approximation.

• A real loudspeaker’s impedance varies with frequency because it combines resistance, inductance, and capacitance.

Impedance is crucial for audio hobbyists because it affects how components like amplifiers and speakers interact. Mismatched impedance can reduce performance or even damage equipment.

By understanding these properties, you’ll have a solid foundation for exploring more advanced audio and electronic concepts.

Thanks for reading

Phil Wait