Physics · Circuits

Power and Energy in Circuits

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Wrap-up

What you learned

Recommended next: Open concept testCheck whether the core ideas are ready without leaving this concept.
Read next: Internal Resistance and Terminal VoltageMake the source non-ideal and track where power is wasted.

Key takeaway

Current comes from the source voltage and load resistance in the same one-loop circuit.
Power is the instantaneous energy-transfer rate, so a brighter or hotter load is taking more joules each second.
Energy is the accumulated total after that power rate acts for a chosen time.
At fixed source voltage, a larger ohmic load resistance can lower current and power rather than raising them.

Common misconception

Do not use energy and power as interchangeable words. Equal power means equal rate, but the longer-running circuit still transfers more total energy.

For one fixed source voltage and one ohmic load, increasing the resistance limits the current more strongly.

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Make the source non-idealCircuits

Internal Resistance and Terminal Voltage

Keep one non-ideal source and one load on screen so emf, internal drop, terminal voltage, current, and wasted power stay tied to the same one-loop circuit.

See how branch structure changes powerCircuits

Series and Parallel Circuits

Switch the same two loads between one loop and two branches, then track how current, voltage, brightness, and charge flow reorganize without changing the battery.

Try a mixed loadCircuits

Equivalent Resistance

Reduce one highlighted resistor group into an equivalent block, then collapse the whole mixed circuit honestly and watch how the total current and grouped behavior change together.

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Equation snapshotOhm's law for the load · Electrical powerShow 3 equations

Use Ohm's law to find the live current, use P = VI for the transfer rate right now, then let E = Pt turn that steady rate into accumulated energy.

Ohm's law for the load
$I = \frac{V}{R _{load}}$
In this one-load circuit, current is set by the source voltage and the load resistance.
Electrical power
$P = V I$
Power is the rate of electrical energy transfer right now.
Energy over time
$E = P t$
If power stays constant, energy grows linearly with time.

Worked examples

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Frozen walkthrough

Use the values shown in the current setup. The same circuit state drives the stage, readout card, overlays, and graphs, so every calculation matches what you see on screen.

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Example 1 of 2

Frozen valuesUsing frozen parameters

For the current source and load, what current flows and how much power is the load taking right now?

V

Source voltage

12 V

R_{load}

Load resistance

8 ohm

1. Use Ohm's law for the live load

I = \frac{V}{R _{load}}

2. Substitute the live voltage and resistance

I = \frac{1}{2} 8

3. Calculate the current

That gives

I = 1.5

4. Use the power relation with the same live values

P = V I = 12 \times 1.5

5. Calculate the load power

P = 18

Current and power

I = 1.5, P = 18

This is a moderate-power setup, so the load response is clear without pushing the circuit into the strongest settings.

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Starter track

Step 4 of 6

Electricity

Power and Energy in Circuits appears later in this track, so it is cleaner to start from the beginning first.

Previous step: Basic Circuits

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