2. Charge, voltage, current, and power

Build the mental picture that metal wires already contain mobile electrons before a battery is connected. Distinguish slow electron drift from the much faster electrical influence that makes a whole circuit respond quickly.

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Calculate current as the rate charge passes a point: 1 ampere equals 1 coulomb per second. Use simple examples to reason about how larger current means more charge flow each second.

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Separate the standard circuit convention from the physical motion of electrons in metal. Use conventional current direction consistently while recognizing that electrons usually drift the opposite way.

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Trace a steady current through a simple one-loop circuit and see why charge does not pile up in a working device. Reason that a lamp or heater uses energy, not charge, so the current entering and leaving it matches.

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Treat voltage as energy per unit charge, measured in joules per coulomb. Calculate how much energy a given amount of charge gains from a source or gives to a device when it moves through a voltage.

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Recognize that voltage is always measured between two points, not at a point by itself. Compare common voltage labels such as 1.5 V, 9 V, and 120 V as differences in electric potential.

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Connect a battery’s voltage rating to chemical energy that separates charge inside the cell. Reason through why a higher-voltage battery gives each coulomb more energy, even before considering how much current will flow.

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Trace energy transfer around a simple battery-and-lamp circuit using voltage rises at sources and voltage drops at devices. Identify that charge circulates while electrical energy is converted into light, heat, motion, or stored chemical energy.

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Use the relationship P = VI to calculate electrical power in watts from voltage and current. Compare examples such as a small lamp, phone charger, and space heater to see how fast each transfers energy.

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Relate power to total energy with E = Pt, using joules, watt-hours, and kilowatt-hours. Estimate how much energy a device uses when it runs for a given time and connect that to household energy bills.

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Read power ratings on lamps, heaters, chargers, and appliances as limits or expected energy-transfer rates. Reason through why devices that handle more power often need larger parts, better cooling, or safer wiring.

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Interpret charger labels such as 5 V, 2 A, and 20 W as maximum output conditions. Compare older USB charging with USB Power Delivery, where a charger and device can negotiate higher voltage for faster charging.

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Use household RMS voltage as the effective voltage for power calculations in AC wiring. Estimate appliance current from wattage and voltage, then connect large currents to breaker limits and wiring safety.

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Review this chapter with practice based on your mistakes.