AP Physics 1 : Circuits

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #23 : Circuit Power

A capacitor with a capacitance of \(\displaystyle 10 F\) stores \(\displaystyle 500 J\) of energy. What is the voltage across this capacitor?

Possible Answers:

\(\displaystyle 100 V\)

\(\displaystyle 20 V\)

\(\displaystyle 10 V\)

\(\displaystyle 50 V\)

\(\displaystyle 500 V\)

Correct answer:

\(\displaystyle 10 V\)

Explanation:

The correct answer is \(\displaystyle 10 V\).

We can obtain this answer with the following equation regarding capacitors:

\(\displaystyle U= \frac{1}{2}CV^{2}\)

\(\displaystyle U\) is the energy stored on the capacitor, \(\displaystyle C\) is the capacitance, and \(\displaystyle V\) is the voltage.

\(\displaystyle (500J)=\frac{1}{2}(10F)(V^{2})\)

Solving for \(\displaystyle V\) we obtain:

\(\displaystyle V= 10 V\)

Example Question #31 : Circuit Power

What is the power of a circuit with a resistance of \(\displaystyle 12\Omega\) and \(\displaystyle 4A\) current?

Possible Answers:

\(\displaystyle 406W\)

\(\displaystyle 12W\)

\(\displaystyle 192W\)

\(\displaystyle 92W\)

Correct answer:

\(\displaystyle 192W\)

Explanation:

We calculate the power in a circuit using the following equation

\(\displaystyle P=I^2R\)

Given \(\displaystyle R=12\Omega\) and \(\displaystyle I=4A\)

\(\displaystyle P=(4)^2(12)=192W\)

Example Question #32 : Circuit Power

What is the power of a circuit with a resistance of \(\displaystyle 6\Omega\) and \(\displaystyle 15A\) current?

Possible Answers:

\(\displaystyle 4050W\)

\(\displaystyle 560W\)

\(\displaystyle 1350W\)

\(\displaystyle 192W\)

Correct answer:

\(\displaystyle 1350W\)

Explanation:

We calculate the power in a circuit using the following equation

\(\displaystyle P=I^2R\)

Given \(\displaystyle I=15A\) and \(\displaystyle R=6\Omega\)

\(\displaystyle P=(15)^2(6)=1350W\)

Example Question #261 : Electricity And Waves

What is the power of a circuit with a voltage of \(\displaystyle 6V\) and \(\displaystyle 15A\) current?

Possible Answers:

\(\displaystyle 90W\)

\(\displaystyle 75W\)

\(\displaystyle 45W\)

\(\displaystyle 192W\)

Correct answer:

\(\displaystyle 90W\)

Explanation:

We calculate the power in a circuit using the following equation

\(\displaystyle P=IV\)

Given \(\displaystyle I=15A\) and \(\displaystyle V=6V\)

\(\displaystyle P=(15)(6)=90W\)

Example Question #262 : Electricity And Waves

What is the power of a circuit with a voltage of \(\displaystyle 9V\) and \(\displaystyle 12A\) current?

Possible Answers:

\(\displaystyle 92W\)

\(\displaystyle 192W\)

\(\displaystyle 108W\)

\(\displaystyle 88W\)

Correct answer:

\(\displaystyle 108W\)

Explanation:

We calculate the power in a circuit using the following equation

\(\displaystyle P=IV\)

Given \(\displaystyle I=12A\) and \(\displaystyle V=9V\)

\(\displaystyle P=(12)(9)=108W\)

Example Question #263 : Electricity And Waves

What is the power of a circuit with a voltage of \(\displaystyle 9V\) and resistance of \(\displaystyle 12\Omega\)?

Possible Answers:

\(\displaystyle 1.92W\)

\(\displaystyle 4.67W\)

\(\displaystyle 8.54W\)

\(\displaystyle 6.75W\)

Correct answer:

\(\displaystyle 6.75W\)

Explanation:

We calculate the power in a circuit using the following equation

\(\displaystyle P=\frac{V^2}{R}\)

Given \(\displaystyle V=9V\) and \(\displaystyle R=12\Omega\)

\(\displaystyle P=\frac{9^2}{12}=6.75W\)

Example Question #264 : Electricity And Waves

What is the power of a circuit with a voltage of \(\displaystyle 150V\) and resistance of \(\displaystyle 1000\Omega\)?

Possible Answers:

\(\displaystyle 22.5W\)

\(\displaystyle 42.6W\)

\(\displaystyle 32W\)

\(\displaystyle 19.2W\)

Correct answer:

\(\displaystyle 22.5W\)

Explanation:

What is the power of a circuit with a voltage of \(\displaystyle 150V\) and resistance of \(\displaystyle 1000\Omega\)?

We calculate the power in a circuit using the following equation

\(\displaystyle P=\frac{V^2}{R}\)

Given \(\displaystyle V=150V\) and \(\displaystyle R=1000\Omega\)

\(\displaystyle P=\frac{(150)^2}{(1000)}=22.5W\)

Example Question #191 : Electricity

Series curcuit

What is the power in the given circuit if \(\displaystyle R_1=10\Omega\)\(\displaystyle R_2=15\Omega\) and \(\displaystyle V=25V\)?

Possible Answers:

\(\displaystyle 10W\)

\(\displaystyle 5W\)

\(\displaystyle 45W\)

\(\displaystyle 25W\)

Correct answer:

\(\displaystyle 25W\)

Explanation:

To find the current we must first find the equivalent resistance. For resistors in series, the equivalent resistance is 

\(\displaystyle R_{eq}=R_1+R_2+...\)

For this problem

\(\displaystyle R_{eq}=R_1+R_2=10+15=25\Omega\)

Now we use Ohm's law, \(\displaystyle V=IR_{eq}\), to find the current, \(\displaystyle I\)

\(\displaystyle V=IR_{eq}\Rightarrow\)

\(\displaystyle I=\frac{V}{R_{eq}}=\frac{25}{25}=1A\)

Now that we have found the current in the resistors we use the following equation to find the power in the circuit:

\(\displaystyle P=IV\)

\(\displaystyle P=IV=1*25=25W\)

Example Question #266 : Electricity And Waves

Parallel circuit

What is the power in the below circuit if \(\displaystyle R_1=10\Omega\)\(\displaystyle R_2=15\Omega\) and \(\displaystyle V=25V\)?

Possible Answers:

\(\displaystyle 192.067W\)

\(\displaystyle 19.384W\)

\(\displaystyle 104.167W\)

\(\displaystyle 91.32W\)

Correct answer:

\(\displaystyle 104.167W\)

Explanation:

To find the current we must first find the equivalent resistance. For resistors in parallel, the equivalent resistance is 

\(\displaystyle \frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+...\)

For this problem

\(\displaystyle \frac{1}{R_{eq}}=\frac{1}{10}+\frac{1}{15}=\frac{3+2}{30}=\frac{5}{30}\)

\(\displaystyle \frac{1}{R_{eq}}=\frac{5}{30}\)

\(\displaystyle R_{eq}=\frac{30}{5}=6\Omega\)

Now we use Ohm's law, \(\displaystyle V=IR_{eq}\), to find the current, \(\displaystyle I\)

\(\displaystyle V=IR_{eq}\Rightarrow\)

\(\displaystyle I=\frac{V}{R_{eq}}=\frac{25}{6}\approx 4.167A\)

Now that we have found the current in the resistors we use the equation

\(\displaystyle P=IV\)

to find the power in the circuit.

\(\displaystyle P=IV=4.167*25\approx 104.167W\)

Example Question #267 : Electricity And Waves

Combo circuit

What is the power in the given circuit if \(\displaystyle R_1=10\Omega\)\(\displaystyle R_2=15\Omega\)\(\displaystyle R_3=5\Omega\) and \(\displaystyle V=25V\)?

Possible Answers:

\(\displaystyle 192W\)

\(\displaystyle 60W\)

\(\displaystyle 300W\)

\(\displaystyle 412W\)

Correct answer:

\(\displaystyle 300W\)

Explanation:

To find the current we must first find the equivalent resistance. This problem is a combination of resistors in series and parallel. First we find the equivalent resistance of resistors in series.

For resistors in series, the equivalent resistance is:

\(\displaystyle R_{eq}=R_1+R_2+...\)

For this problem:

\(\displaystyle R_{eq1}=R_1+R_2=10+15=25\)

The resistors in series can be treated as one \(\displaystyle 25\Omega\) resistor. Now we address the resistors in parallel. For resistors in parallel, the equivalent resistance is 

\(\displaystyle \frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+...\)

For this problem

\(\displaystyle \frac{1}{R_{eq}}=\frac{1}{R_{eq1}}+\frac{1}{R_3}=\frac{1}{25}+\frac{1}{5}=\frac{1+5}{25}\)

\(\displaystyle \frac{1}{R_{eq}}=\frac{6}{25}\)

\(\displaystyle R_{eq}=\frac{25}{6} \approx 4.167\Omega\)

Now we use Ohm's law, \(\displaystyle V=IR_{eq}\), to find the current, \(\displaystyle I\)

\(\displaystyle V=IR_{eq}\Rightarrow\)

\(\displaystyle I=\frac{V}{R_{eq}}=\frac{25}{4.167}=6A\) 

Now that we have found the current in the resistors we use the equation

\(\displaystyle P=IV\)

to find the power in the circuit.

\(\displaystyle P=IV=6*25=300W\)

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