Volts (Voltage) - Electromotive force or electrical potential difference; it pushes charge but is not the flow itself.

Ohms - Unit of electrical resistance, which opposes the flow of current.

Watts - Unit of electrical power (rate of doing work), equal to volts times amperes.

Amperes (amps) - Unit of electrical current, the rate of flow of electric charge. Current is measured in amperes.

Memory aids:

• V olts: V isn’t in Electrical Current
• O hms: O isn’t in Electrical Current
• W atts: W isn’t in Electrical Current
• A mperes: A is in Electrical Current

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Electrical power is measured in watts.

Volts — measure of electromotive force or electric potential difference (unit: volt, V). It is not a unit of power.

Watt-hours — a unit of energy (power multiplied by time). For example, one watt of power delivered for one hour equals one watt-hour (equal to 3600 joules).

Watts — the unit of power (rate of doing work or transferring energy). One watt equals one joule per second.

Amperes — the unit of electric current (rate of flow of electric charge), not power.

Memory aids:

• V in Volts is not in the word Power.
• W & H in Watt-hours are not both in Power.
• W in Watts is in the word Power.
• A in Amperes is not In Power.

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Electric current is the flow of electric charge; in most circuits this is the flow of electrons. That flow is called current and is measured in amperes (amps). Voltage is the electrical “pressure” that pushes the charges, and resistance opposes the flow.

Memory aids:

• Think of water: the flow of water = current; pressure = voltage; pipe constriction = resistance.
• Current is measured in amperes (amps).

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Frequency is the number of complete cycles an alternating current makes each second. It tells how "frequently" the current reverses direction and is measured in hertz (Hz), where 1 Hz = 1 cycle per second.

To distinguish from the other terms: pulse rate refers to pulses per second, wave number is the number of wave cycles per unit distance (inverse meters), and wavelength is the physical length of one cycle measured in meters.

Memory aids:

• Think "frequency" ~ "how frequently" something happens.
• Units: frequency = cycles per second (Hz); pulse rate = pulses per second; wave number = per meter (1/m); wavelength = meters (m).

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Voltage is the electrical term for the electromotive force that causes electron flow. It is the potential difference between two points that makes electrons move through a conductor.

Capacitance is the ability of a component (a capacitor) to store electric charge and energy; a capacitor opposes changes in voltage. Inductance is the ability of a coil or inductor to store energy in a magnetic field; an inductor opposes changes in current. Ampere-hours are a measure of electric charge or a battery's capacity (for example, a 50 ampere-hour battery can deliver 1 amp for 50 hours).

None of those other terms describe the force that pushes electrons; that role is described by voltage.

Memory aids:

• Think of voltage as electrical "pressure" pushing charge through a circuit (water pressure in a pipe).
• Capacitance = stores charge, opposes change in voltage.
• Inductance = stores energy in magnetic field, opposes change in current.
• Ampere-hours = battery capacity (current × time).

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Hertz is the standard unit for frequency in the SI system. It is defined as the number of cycles per second of a periodic event. Examples: a quartz clock ticks at 1 Hz, the US mains AC is 60 Hz, and the musical note A below middle C is 220 Hz. The unit is named after Heinrich Hertz. A graphical illustration of frequency can be found here: http://en.wikipedia.org/wiki/File:FrequencyAnimation.gif

The other (incorrect) answers are:

• the farad — unit of capacitance
• the henry — unit of inductance
• epicycles per second — not a recognized SI unit ("epicycle" is a historical astronomical concept); while "per second" indicates cycles per second, "epicycles per second" is not a standard unit of frequency

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Most metals have one or more loosely bound electrons in the outer shells of their atoms. These electrons become delocalized and form an "electron cloud" or "sea" that is free to move throughout the metallic lattice. When an electric field is applied, those free electrons can drift in a common direction, producing an electric current. The positively charged atomic cores remain fixed in the lattice and do not carry the current; it is the mobile electrons that make metals good conductors.

Memory aids:

• Picture a sea of electrons flowing between fixed metal ions.
• "Free electrons → good conduction."

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Glass is a good electrical insulator because it does not have free charge carriers (free electrons or ions) that can move easily when an electric field is applied. Metals have free electrons and conduct electricity well, so any metallic material will be a conductor.

Sea water conducts electricity because it contains dissolved salts that dissociate into ions; those ions carry charge and make the water a good electrolyte. Graphite, although not a metal, has delocalized electrons within its carbon layers which allow electrical conduction along the layers. Stainless steel is a metal and therefore a conductor.

Memory aids:

• On tests, metals are usually the safe choice for conductors.
• Non‑metallic solids like glass are typically insulators, but watch out for exceptions (graphite is a nonmetal that conducts).
• Salt water conducts because of dissolved ions.

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Alternating Current (AC) is electricity in which the current (and voltage) reverses direction periodically. The direction alternates between positive and negative values at a specific frequency measured in hertz (Hz). For example, in the United States the standard household AC reverses direction 60 times per second (60 Hz), so the voltage swings between negative and positive values around zero. "Zero" is the instantaneous point of no flow; a waveform that only went between zero and a positive value would represent switching on and off, not a reversal of direction.

Memory aids:

• Remember that AC alternates direction — positive to negative and back.
• Zero volts (or zero current) means no flow; alternating between zero and a polarity is not the same as alternating direction.
• Typical examples: US mains ~60 Hz (voltage swings between negative and positive values), many other countries use ~50 Hz.

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Power, measured in watts, is the rate at which electrical energy is used. It is equal to current times voltage (\(P = I \times E\)), sometimes written \(P = I \times V\)). That means the amount of electrical energy converted per second depends on both the voltage pushing the charge and the current (rate of charge flow).

Using a water-flow analogy: current is like the volume flow rate of water, voltage is like the pressure pushing the water, and resistance is like a constriction in the pipe that opposes flow. Mechanical power (watts) is proportional to pressure times flow rate, just as electrical power is voltage times current. Because power is a rate, energy is the integral of power over time; in simple terms, Energy = Power × Time. One watt equals one joule per second.

Resistance is the opposition to current flow and so is not itself a rate of energy usage.

Memory aids:

• \(P = I \times E\) - simple as Pie
• 1 watt = 1 joule/second
• Voltage ~ pressure, Current ~ flow, Power ~ pressure × flow

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Resistance opposes the flow of electric current regardless of the type of current. Ohm’s law (V = IR) expresses that a voltage is required to drive current through a resistance — whether the current is steady (DC) or varying (AC). At radio frequencies, the concept of resistance generalizes to impedance; the resistive (real) part of impedance still dissipates energy and opposes current flow. In short, any electrical current that must pass through a resistive element encounters opposition and energy dissipation as heat or other loss mechanisms.

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