AP Physics 1 : Circuits

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #1 : Circuits

Consider the circuit:

 

Circuit_4

If each resistor has a value of \displaystyle 2\Omega, how much current is flowing through the circuit?

Possible Answers:

\displaystyle 6A

\displaystyle 12A

\displaystyle 24A

\displaystyle 3A

\displaystyle 48A

Correct answer:

\displaystyle 24A

Explanation:

First we need to calculate the equivalent resistance of the circuit using the following expression for condensing parallel resistors:

\displaystyle \frac{1}{R_{eq}}=\sum\frac{1}{R} = \frac{1}{2\Omega}+\frac{1}{2\Omega}+\frac{1}{2\Omega}+\frac{1}{2\Omega}

\displaystyle R_{eq}=0.5\Omega

Now we can use Ohm's law to calculate the current flowing through the circuit:

\displaystyle V = IR

\displaystyle I = \frac{V}{R}=\frac{12V}{0.5\Omega} = 24A

Example Question #1 : Ohm's Law

A light bulb requires 60 W to function properly. If it is connected to a powersupply of 120 A and functions properly, then what is the resitance of the light bulb?

Possible Answers:

\displaystyle 4m\Omega

\displaystyle 1m\Omega

\displaystyle 0.5\Omega

\displaystyle 1\Omega

\displaystyle 7m\Omega

Correct answer:

\displaystyle 4m\Omega

Explanation:

First, identify the given information:

\displaystyle P=60W

\displaystyle I=120A

Two equations are required for this problem:

1.) Ohm's law, \displaystyle V=IR

2.) Electrical power \displaystyle P=IV

Using the equation for electrical power, we can rearrange to solve for \displaystyle V:

\displaystyle P=IV

\displaystyle V=\frac{P}{I}

At this point, we can substitute in the known values and determine the voltage:

\displaystyle V=\frac{60W}{120A}=0.5V

Ohm's law can then be rearranged to solve for the resistence of the light bulb:

\displaystyle V=IR

\displaystyle R=\frac{V}{I}

The known voltage value then can be substituted into Ohm's law to determine the resistance of the light bulb:

\displaystyle R=\frac{0.5V}{120A}=0.00416\Omega\approx 4m\Omega

Example Question #1 : Ohm's Law

What is the resistance of a resistor if the current going through it is \displaystyle 2A and the voltage across it it is \displaystyle 10V?

Possible Answers:

\displaystyle 12\Omega

\displaystyle 5\Omega

\displaystyle 20\Omega

\displaystyle 0.2\Omega

Correct answer:

\displaystyle 5\Omega

Explanation:

Use Ohm's law.

\displaystyle V=IR

Plug in known values and solve for resistance.

\displaystyle R=\frac{10V}{2A}=5\Omega

Example Question #1 : Ohm's Law

What is the voltage across a resistor with a resistance of \displaystyle 10\Omega that has a current of \displaystyle 20A going through it?

Possible Answers:

\displaystyle 2V

\displaystyle 20V

\displaystyle 200V

\displaystyle 30V

Correct answer:

\displaystyle 200V

Explanation:

Use Ohm's Law.

\displaystyle V=IR

\displaystyle V=20A\cdot10\Omega =200V

Example Question #1 : Ohm's Law

What is the current through a resistor if the resistor has a resistance of \displaystyle 40\Omega and the voltage across the resistor is \displaystyle 60V?

Possible Answers:

\displaystyle 1.5A

\displaystyle 46A

\displaystyle 2A

\displaystyle 24A

Correct answer:

\displaystyle 1.5A

Explanation:

Use Ohm's law.

\displaystyle V=IR

\displaystyle I=\frac{60V}{40\Omega}=1.5A

Example Question #1 : Ohm's Law

If the current through a \displaystyle 10\Omega resistor is \displaystyle 6A, what is the voltage across the resistor?

Possible Answers:

\displaystyle 10V

\displaystyle 6V

\displaystyle 60V

\displaystyle \frac{5}{3}V

Correct answer:

\displaystyle 60V

Explanation:

UseOhm's law.

\displaystyle V=IR

Plug in known values and solve.

\displaystyle V=6A\cdot 10\Omega

\displaystyle V=60V

Example Question #2 : Circuits

Basic circuit

\displaystyle V_{in}=12V

 \displaystyle I=2 A

The voltage measured from a point between \displaystyle Z_1 and \displaystyle Z_2 to the ground is \displaystyle 8V

What is the resistance of \displaystyle Z_1 ?

 

Possible Answers:

\displaystyle 12\Omega

\displaystyle 2\Omega

\displaystyle 1\Omega

\displaystyle 3\Omega

\displaystyle 4\Omega

Correct answer:

\displaystyle 2\Omega

Explanation:

Begin by finding the total resistance in the circuit.

\displaystyle R_{tot}=\frac{V}{I}=\frac{12V}{2A}=6\Omega

Now note that the voltage identified in the problem is the same as the voltage drop across the second resistor:

\displaystyle IZ_2=V_D

\displaystyle Z_2=\frac{V_D}{I}=\frac{8V}{2A}

\displaystyle Z_2=4\Omega

Now, since \displaystyle Z_1 and \displaystyle Z_2 combine to form the total resistance:

\displaystyle Z_1=Z-Z_2=6\Omega -4\Omega=2\Omega

Example Question #1 : Ohm's Law

Basic circuit

\displaystyle V_{in}=12V

\displaystyle I=2 A, and the voltage measured from a point between \displaystyle Z_1 and \displaystyle Z_2 to the ground is \displaystyle 8V 

In the circuit above, what is the resistance of \displaystyle Z_2?

 

Possible Answers:

\displaystyle 12\Omega

\displaystyle 4\Omega

\displaystyle 1\Omega

\displaystyle 2\Omega

\displaystyle 6\Omega

Correct answer:

\displaystyle 4\Omega

Explanation:

Begin by finding the total resistance in the circuit.

\displaystyle R_{tot}=\frac{V}{I}=\frac{12V}{2A}=6\Omega

Now note that the voltage identified in the problem is the same as the voltage drop across the second resistor:

\displaystyle IZ_2=V_D

\displaystyle Z_2=\frac{V_D}{I}=\frac{8V}{2A}

\displaystyle Z_2=4\Omega

Example Question #1 : Ohm's Law

Basic circuit

\displaystyle V_{in}=12V

\displaystyle Z_1 is composted of two resistors in parallel, \displaystyle 3\Omega and \displaystyle 7\Omega

\displaystyle Z_2 is a single \displaystyle 6\Omega resistor.

In the circuit above, what is the current?

Possible Answers:

\displaystyle 2.96A

\displaystyle 0.75 A

\displaystyle 1.20A

\displaystyle 1.50 A

\displaystyle 1.48A

Correct answer:

\displaystyle 1.48A

Explanation:

To find the current, first find the total resistance of the circuit. Begin by simplifying \displaystyle Z_1, the two resistors in parallel as follows:

\displaystyle \frac{1}{Z_1}=\frac{1}{3\Omega}+\frac{1}{7\Omega}

\displaystyle \frac{1}{Z_1}=\frac{10}{21\Omega}

\displaystyle Z_1=2.1\Omega

Since \displaystyle Z_1 and \displaystyle Z_2 are in series, their combined resistance is:

\displaystyle R_{tot}=Z_1+Z_2=2.1\Omega+6\Omega=8.1\Omega

Use Ohm's law to find the current.

\displaystyle I=\frac{V}{R}

\displaystyle I=\frac{12V}{8.1\Omega}

\displaystyle I=1.48A

Example Question #1 : Circuits

A resistor with a resistance of \displaystyle 4 \Omega has a current flowing through it of 5A. What is the potential drop across the resistor?

Possible Answers:

\displaystyle \frac{2}{5}V

\displaystyle 20 V

\displaystyle \frac{5}{2}V

\displaystyle 10V

Correct answer:

\displaystyle 20 V

Explanation:

Ohm's law states that the potential drop across a resistor is equal to the product of the current flowing through the resistor and the resistance of the resistor:

\displaystyle V=IR

We were given the current, I, and the resistance, R, so we simply multiply the two together to get our final answer. 

\displaystyle V=5*4=20V

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