Electric Power
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AP Physics 2 › Electric Power
A motor runs from a constant $24,\text{V}$ source for $6,\text{s}$. Its current increases from $1.0,\text{A}$ to $2.0,\text{A}$. Compared to initially, the motor’s power is now
half as large
twice as large
unchanged because the voltage is fixed
greater by $6,\text{J}$ each second
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, calculated as P = VI for any electrical device. With constant voltage of 24 V, the initial power is P₁ = (24 V)(1.0 A) = 24 W, and the final power is P₂ = (24 V)(2.0 A) = 48 W. The ratio P₂/P₁ = 48 W/24 W = 2, so the power is twice as large. Choice B incorrectly assumes power remains constant with fixed voltage, ignoring that current also affects power through P = VI. When voltage is constant, power is directly proportional to current: double the current means double the power consumption.
A resistor $R$ is connected to an ideal battery. The battery voltage doubles while $R$ stays constant, so the current doubles. Compared to before, the resistor’s power dissipation becomes
twice as large because both $V$ and $I$ doubled
four times as large because $P=IV$ and both doubled
unchanged because resistance sets the power
half as large because the current spreads out
Explanation
This question tests understanding of electric power. Electric power is the rate at which electrical energy is transferred, calculated as P = IV. When both voltage V and current I double, the new power becomes P_new = (2I)(2V) = 4IV = 4P_original. This can also be verified using P = V²/R: if V doubles and R is constant, P increases by a factor of 2² = 4. Choice A incorrectly thinks doubling both quantities only doubles the power, missing that power is the product of voltage and current. When analyzing power changes, remember that P = IV means power scales with the product of changes in both V and I.
A device operates at constant voltage $V$ for $5,\text{s}$. Its current decreases from $3.0,\text{A}$ to $1.0,\text{A}$. Compared to the start, the device’s power is now
unchanged because the voltage is constant
smaller by $2.0,\text{J}$ each second
one-third as large
three times as large
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, calculated as P = VI where V is voltage and I is current. At constant voltage V, the initial power is P₁ = V(3.0 A) = 3.0V W, while the final power is P₂ = V(1.0 A) = 1.0V W. The ratio P₂/P₁ = 1.0V/3.0V = 1/3, so the power is one-third as large. Choice B incorrectly assumes power stays constant with voltage—but power depends on both voltage and current, not voltage alone. When voltage is constant, power is directly proportional to current: if current drops to one-third, power also drops to one-third.
A $4\ \Omega$ resistor has a measured potential difference of $8\ \text{V}$ across it for several seconds. Which statement about its power is correct?
It is $32\ \text{W}$ because $P=IV$ and $I=\dfrac{V}{R}=4\ \text{A}$
It is $2\ \text{W}$ because $P=\dfrac{V}{R}$
It is $16\ \text{W}$ because $P=\dfrac{V^2}{R}$
It is $64\ \text{W}$ because $P=VR$ for a resistor
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, which can be calculated using P = V²/R when voltage and resistance are known. For this 4 Ω resistor with 8 V across it, P = (8 V)²/(4 Ω) = 64/4 = 16 W. This can be verified using P = IV, where I = V/R = 8 V/4 Ω = 2 A, giving P = (2 A)(8 V) = 16 W. Choice D incorrectly uses P = VR, which has the wrong units (V·Ω = V²/A, not watts) and represents a nonsensical formula. When given voltage and resistance, use P = V²/R or first find current then use P = IV.
A resistor is connected to a variable power supply for $8\ \text{s}$. The supply is adjusted so that the current through the resistor doubles while the resistor’s resistance stays constant. Which statement about the resistor’s power dissipation is correct?
It halves, because doubling current reduces the voltage needed
It stays the same, because resistance does not change
It quadruples, because $P=I^2R$ and $R$ is constant
It doubles, because power is proportional to current
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, and when resistance is constant, we use P = I²R. If current doubles from I to 2I while resistance R stays constant, the new power becomes P_new = (2I)²R = 4I²R, which is four times the original power P = I²R. This quadrupling occurs because power depends on the square of current when resistance is fixed. Choice A incorrectly assumes a linear relationship between power and current, missing the squared dependence. When resistance is constant, remember that power scales with the square of current, not linearly.
A lamp has constant resistance $R$. Its current decreases steadily over time. Which statement about the lamp’s power dissipation over time is correct?
It stays constant because energy transfer depends only on time
It stays constant because resistance is constant
It decreases because $P=I^2R$ and $R$ is constant
It increases because decreasing current means increasing voltage
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, given by P = I²R for a resistor with constant resistance. As current I decreases steadily over time while resistance R remains constant, the power P = I²R decreases because it depends on the square of the current. Choice C incorrectly assumes constant resistance means constant power, forgetting that power depends on both current and resistance. For a fixed resistance, power varies with the square of current: halving current reduces power to one-fourth, demonstrating why decreasing current means decreasing power.
A resistor has constant voltage $V$ across it for $10\ \text{s}$. The current decreases from $2.0\ \text{A}$ to $1.0\ \text{A}$. Compared to initially, the resistor’s power at $t=10\ \text{s}$ is
four times smaller because $P=I^2R$ always dominates
unchanged because energy transfer depends only on time
half as large because $P=IV$ and $V$ is constant
twice as large because $P=IV$ and $V$ is constant
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, given by P = IV where I is current and V is voltage. When voltage V is held constant across a resistor and current decreases from 2.0 A to 1.0 A (halved), the power changes from P_initial = (2.0 A)(V) to P_final = (1.0 A)(V) = 0.5(2.0 A)(V) = 0.5P_initial. Choice D incorrectly applies P = I²R, forgetting that when V is constant and I changes, R must also change (the resistor is not fixed). The key strategy is to identify which quantities are constant and use the appropriate power formula: when V is constant, use P = IV directly.
Two identical bulbs are connected in series to an ideal battery and glow for $20\ \text{s}$. The same current flows through each bulb, and each bulb has the same resistance $R$. Compared to one bulb alone connected to the same battery, the power in each series bulb is
less because the current is smaller
zero because the voltage is shared
greater because the total resistance is greater
the same because the bulbs are identical
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, calculated as P = I²R or P = V²/R. When two identical bulbs are in series, the total resistance doubles, causing the current to halve (I_series = V/2R compared to I_single = V/R). Each bulb now has half the original current, so using P = I²R, each bulb's power becomes P_series = (I/2)²R = I²R/4, which is one-fourth the power of a single bulb. Choice C incorrectly assumes identical bulbs always have the same power, ignoring that the circuit configuration affects current. Remember: in series circuits, increased total resistance reduces current, which reduces power in each component.
A $12\ \text{V}$ battery is connected to two resistors, $R_1=6\ \Omega$ and $R_2=3\ \Omega$, in series for $10\ \text{s}$. The same current $I$ flows through both resistors, and the voltage drops satisfy $V_1=IR_1$ and $V_2=IR_2$ with $V_1+V_2=12\ \text{V}$. Which element dissipates the most power during the interval?
$R_2$ dissipates more power than $R_1$
Both resistors dissipate zero power
$R_1$ and $R_2$ dissipate equal power
$R_1$ dissipates more power than $R_2$
Explanation
This question tests understanding of electric power. Electric power is the rate at which electrical energy is transferred, calculated as P = IV, where I is current and V is voltage. In a series circuit, the same current flows through both resistors, but the voltage drops are different: V₁ = IR₁ and V₂ = IR₂. Since R₁ = 6 Ω is larger than R₂ = 3 Ω, and they share the same current, R₁ has a larger voltage drop (V₁ = 2V₂). Using P = I²R, we see that R₁ dissipates twice the power of R₂ because its resistance is twice as large. Choice D incorrectly assumes no power dissipation when current flows through resistance. Remember: in series circuits with equal current, power is proportional to resistance.
Two resistors are connected in parallel across an ideal $9\ \text{V}$ battery for $5\ \text{s}$. Each resistor has the same voltage $V=9\ \text{V}$ across it, but the currents satisfy $I_1=3I_2$. Which statement about power is correct?
Resistor 1 dissipates three times the power of resistor 2
Both resistors dissipate the same energy but different power
Resistor 2 dissipates three times the power of resistor 1
Both resistors dissipate the same power
Explanation
This question tests understanding of electric power. Electric power is the rate of electrical energy transfer, given by P = IV. In parallel circuits, all components have the same voltage across them (9 V in this case), but currents can differ. Since I₁ = 3I₂ and both resistors have the same voltage, using P = IV gives P₁ = (9 V)(3I₂) = 3(9 V)(I₂) = 3P₂. Choice C incorrectly assumes equal power when only voltage is equal, ignoring that current differs. Remember: in parallel circuits with equal voltage, power is proportional to current.