Resistance, Resistivity, and Ohm's Law - AP Physics 2
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What does the variable $V$ represent in Ohm's Law?
What does the variable $V$ represent in Ohm's Law?
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Voltage. The electric potential difference across a component.
Voltage. The electric potential difference across a component.
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What unit is used to measure resistance?
What unit is used to measure resistance?
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Ohms ($\text{Ω}$). Named after Georg Ohm, measures opposition to current.
Ohms ($\text{Ω}$). Named after Georg Ohm, measures opposition to current.
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Calculate the current if $V = 12\text{V}$ and $R = 4\text{Ω}$.
Calculate the current if $V = 12\text{V}$ and $R = 4\text{Ω}$.
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$I = 3\text{A}$. Using $I = \frac{V}{R} = \frac{12}{4} = 3\text{A}$.
$I = 3\text{A}$. Using $I = \frac{V}{R} = \frac{12}{4} = 3\text{A}$.
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State the formula for resistivity.
State the formula for resistivity.
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$\rho = R \frac{A}{L}$. Rearranged from $R = \rho \frac{L}{A}$.
$\rho = R \frac{A}{L}$. Rearranged from $R = \rho \frac{L}{A}$.
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What does an increase in cross-sectional area do to resistance?
What does an increase in cross-sectional area do to resistance?
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Decreases resistance. Larger area provides more parallel paths for current.
Decreases resistance. Larger area provides more parallel paths for current.
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What does a negative temperature coefficient of resistance mean?
What does a negative temperature coefficient of resistance mean?
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Resistance decreases with temperature. Negative coefficient means resistance drops as temperature rises.
Resistance decreases with temperature. Negative coefficient means resistance drops as temperature rises.
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What is the relationship between voltage and current if resistance is constant?
What is the relationship between voltage and current if resistance is constant?
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Directly proportional. From Ohm's Law, $V = IR$ with constant $R$.
Directly proportional. From Ohm's Law, $V = IR$ with constant $R$.
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How does increasing length of a wire affect resistance?
How does increasing length of a wire affect resistance?
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Increases resistance. Longer path means more resistance to current flow.
Increases resistance. Longer path means more resistance to current flow.
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What is the reciprocal of resistance called?
What is the reciprocal of resistance called?
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Conductance. Conductance $G = \frac{1}{R}$, measures ease of current flow.
Conductance. Conductance $G = \frac{1}{R}$, measures ease of current flow.
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What is the effect of resistance on current?
What is the effect of resistance on current?
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Resistance reduces current. Higher resistance opposes and limits current flow.
Resistance reduces current. Higher resistance opposes and limits current flow.
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What does the symbol $\text{Ω}$ represent?
What does the symbol $\text{Ω}$ represent?
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Ohm, unit of resistance. Greek letter omega represents the ohm unit.
Ohm, unit of resistance. Greek letter omega represents the ohm unit.
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Calculate the current if $V = 12\text{V}$ and $R = 4\text{Ω}$.
Calculate the current if $V = 12\text{V}$ and $R = 4\text{Ω}$.
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$I = 3\text{A}$. Using $I = \frac{V}{R} = \frac{12}{4} = 3\text{A}$.
$I = 3\text{A}$. Using $I = \frac{V}{R} = \frac{12}{4} = 3\text{A}$.
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What happens to current if voltage is doubled while resistance is constant?
What happens to current if voltage is doubled while resistance is constant?
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Current doubles. From Ohm's Law, current is proportional to voltage.
Current doubles. From Ohm's Law, current is proportional to voltage.
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What is Ohm's Law formula?
What is Ohm's Law formula?
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$V = IR$. Relates voltage, current, and resistance linearly.
$V = IR$. Relates voltage, current, and resistance linearly.
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What unit is used to measure resistance?
What unit is used to measure resistance?
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Ohms ($\text{Ω}$). Named after Georg Ohm, measures opposition to current.
Ohms ($\text{Ω}$). Named after Georg Ohm, measures opposition to current.
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Calculate the resistance if $V = 10\text{V}$ and $I = 2\text{A}$.
Calculate the resistance if $V = 10\text{V}$ and $I = 2\text{A}$.
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$R = 5\text{Ω}$. Using $R = \frac{V}{I} = \frac{10}{2} = 5\text{Ω}$.
$R = 5\text{Ω}$. Using $R = \frac{V}{I} = \frac{10}{2} = 5\text{Ω}$.
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How is resistivity different from resistance?
How is resistivity different from resistance?
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Resistivity is material property; resistance depends on dimensions. Resistivity is intrinsic; resistance includes geometry.
Resistivity is material property; resistance depends on dimensions. Resistivity is intrinsic; resistance includes geometry.
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Calculate the power if $I = 5\text{A}$ and $V = 12\text{V}$.
Calculate the power if $I = 5\text{A}$ and $V = 12\text{V}$.
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$P = 60\text{W}$. Using $P = IV = 5 \times 12 = 60\text{W}$.
$P = 60\text{W}$. Using $P = IV = 5 \times 12 = 60\text{W}$.
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Find the resistivity if $R = 8\text{Ω}$, $L = 4\text{m}$, and $A = 2\text{m}^2$.
Find the resistivity if $R = 8\text{Ω}$, $L = 4\text{m}$, and $A = 2\text{m}^2$.
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$\rho = 4\text{Ωm}$. Using $\rho = R \frac{A}{L} = 8 \times \frac{2}{4} = 4\text{Ωm}$.
$\rho = 4\text{Ωm}$. Using $\rho = R \frac{A}{L} = 8 \times \frac{2}{4} = 4\text{Ωm}$.
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State the formula for power in terms of voltage and resistance.
State the formula for power in terms of voltage and resistance.
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$P = \frac{V^2}{R}$. Alternative power formula using voltage and resistance.
$P = \frac{V^2}{R}$. Alternative power formula using voltage and resistance.
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How does resistance vary with temperature for a semiconductor?
How does resistance vary with temperature for a semiconductor?
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Decreases with temperature. Semiconductors have negative temperature coefficient.
Decreases with temperature. Semiconductors have negative temperature coefficient.
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What is the effect of doubling the cross-sectional area on resistance?
What is the effect of doubling the cross-sectional area on resistance?
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Resistance is halved. Larger area provides more paths for current flow.
Resistance is halved. Larger area provides more paths for current flow.
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What is the relationship between resistance and resistivity?
What is the relationship between resistance and resistivity?
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$R = \rho \frac{L}{A}$. Resistance equals resistivity times length over area.
$R = \rho \frac{L}{A}$. Resistance equals resistivity times length over area.
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How does resistivity change with temperature for metals?
How does resistivity change with temperature for metals?
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Increases with temperature. Thermal energy increases atomic vibrations in metals.
Increases with temperature. Thermal energy increases atomic vibrations in metals.
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What is the unit of conductance?
What is the unit of conductance?
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Siemens ($\text{S}$). Named after Ernst Werner von Siemens.
Siemens ($\text{S}$). Named after Ernst Werner von Siemens.
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What does $I$ represent in Ohm's Law?
What does $I$ represent in Ohm's Law?
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Current. The flow of electric charge through a conductor.
Current. The flow of electric charge through a conductor.
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Calculate resistance using resistivity $\rho = 1.68 \times 10^{-8}\text{Ωm}$, length $L = 2\text{m}$, and area $A = 1\text{m}^2$.
Calculate resistance using resistivity $\rho = 1.68 \times 10^{-8}\text{Ωm}$, length $L = 2\text{m}$, and area $A = 1\text{m}^2$.
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$R = 3.36 \times 10^{-8}\text{Ω}$. Using $R = \rho \frac{L}{A} = 1.68 \times 10^{-8} \times \frac{2}{1}$.
$R = 3.36 \times 10^{-8}\text{Ω}$. Using $R = \rho \frac{L}{A} = 1.68 \times 10^{-8} \times \frac{2}{1}$.
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What is the formula for power in terms of current and voltage?
What is the formula for power in terms of current and voltage?
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$P = IV$. Power equals current times voltage.
$P = IV$. Power equals current times voltage.
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Identify the unit for electric current.
Identify the unit for electric current.
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Ampere ($\text{A}$). Named after André-Marie Ampère.
Ampere ($\text{A}$). Named after André-Marie Ampère.
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What happens to resistance if temperature increases?
What happens to resistance if temperature increases?
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Resistance increases. Higher temperature increases atomic vibrations, impeding current.
Resistance increases. Higher temperature increases atomic vibrations, impeding current.
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