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Science / Electronics basics / Grade 8 / se8v2

Voltage Factors: Check the Power Pattern

Use fixed-resistance comparisons to distinguish linear and squared factors.

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Review changing voltage

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For a steady ideal resistor, I = V / R and P = V x I. State that resistance is unchanged before using a voltage-scaling rule. A voltage factor k gives current factor k and power factor k x k. Convert amps to milliamps by multiplying by 1,000. Compare only the stated paper models; power alone does not establish real temperature or safety.

Worked example

At 5 V, a 500-ohm resistor carries 0.01 A = 10 mA and dissipates 0.05 W. At 10 V, that same resistor carries 20 mA and dissipates 0.2 W. Voltage and current each double; their product is four times as large. The power comparison is 0.2 / 0.05 = 4.

Worked example with 500 ohms unchanged: 5 V gives 10 mA and 0.05 W; 10 V gives 20 mA and 0.2 W. Separate current and power bars show factors of two and four.
A doubled current is not the same as doubled power when voltage changes too.
Question 1 A 400-ohm ideal resistor has 6 V across it. What current flows?
Question 2 Raise the voltage across the same 400 ohms to 12 V. What is its power?
Question 3 Compare 6 V and 18 V across that unchanged 400-ohm resistor. How does current change?
Question 4 The 400-ohm resistor dissipates 0.09 W at 6 V. How much power does it dissipate at 18 V?
Question 5 Reduce a fixed resistor from 12 V to 6 V. Which factor pair describes the new current and power?