Experiment 10: Ohm's Law and Resistance — Worked Examples

These examples connect circuit calculations with the physical meaning of voltage, current, resistance, resistivity, power, temperature effects, meter readings, and voltage-current graphs.

Example 1: Current through an ohmic resistor

A 12.0V12.0\,\text{V} source is connected across a 470Ω470\,\Omega resistor. Determine the current.

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Example 2: Determine resistance from measured voltage and current

A resistor has a measured voltage of 6.40V6.40\,\text{V} and a measured current of 31.8mA31.8\,\text{mA}. Determine its experimental resistance.

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Example 3: Resistance from the slope of a voltage-current graph

A best-fit line on a graph of voltage VV on the vertical axis against current II on the horizontal axis has slope 328V/A328\,\text{V/A}. Determine the resistance.

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Graph-axis reversal

If current is on the vertical axis and voltage is on the horizontal axis, the slope is conductance G=I/V=1/RG=I/V=1/R, not resistance.

Example 4: Conductance from an I-versus-V graph

An II-versus-VV graph has slope 0.00400A/V0.00400\,\text{A/V}. Determine the conductance and resistance.

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Example 5: Resistance from resistivity and geometry

A copper wire has length L=8.00mL=8.00\,\text{m}, cross-sectional area A=0.500mm2A=0.500\,\text{mm}^2, and resistivity ρ=1.68×108Ωm\rho=1.68\times10^{-8}\,\Omega\cdot\text{m}. Determine its resistance.

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Example 6: Temperature dependence of a metal resistor

A metal resistor has resistance R0=100.0ΩR_0=100.0\,\Omega at 20.0C20.0^\circ\text{C}. Its temperature coefficient is α=0.00390per C\alpha=0.00390\,\text{per }^\circ\text{C}. Estimate its resistance at 80.0C80.0^\circ\text{C}.

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Example 7: Power rating and safe voltage

A 220Ω220\,\Omega resistor is rated at 0.250W0.250\,\text{W}. Determine the maximum ideal voltage and current that correspond to this rating.

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Example 8: Electrical energy consumed during operation

A 100Ω100\,\Omega resistor operates from a 10.0V10.0\,\text{V} source for 15.0min15.0\,\text{min}. Determine the current, power, and energy converted to heat.

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Example 9: Resistor tolerance and experimental agreement

A resistor is marked 330Ω±5%330\,\Omega\pm5\%. A multimeter reads 343Ω343\,\Omega. Determine the acceptable range and decide whether the measurement is within tolerance.

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Example 10: Interpret non-ohmic behavior

A device produces a curved VV-versus-II graph whose slope becomes steeper as current increases. What does the graph imply about its resistance?

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Problem-solving checks for Ohm's law and resistance