AP Physics 2 : Circuit Power

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

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Example Question #77 : Circuits

Consider the circuit:

Circuit_1

What is the rate of power consumption in the circuit if every resistor has a resistance of ?

Possible Answers:

Correct answer:

Explanation:

To calculate the power consumption of the circuit, we need to first reduce it to an equivalent circuit with a single resistor. Since each resistor has the same resistance, this solution will keep resistance calculations as multiples of  until the circuit is fully reduced.

Start with the two branches in parallel. We can condense R3 and R4, then solve for the total resistance of R2, R3, and R4.

The equivalent circuit now has three resistors in series (R1, Req, and R5), so we can simply add them all up:

Plug in the value for R:

Now we can use the equation for power:

Substituting in Ohm's law for current, we get:

Example Question #78 : Circuits

You have 4 resistors, , , , and , set up like this:

4resistorcircuit

Their resistance are as follows:

If the battery has 8V, what is the total power dissipated through the resistors?

Possible Answers:

Correct answer:

Explanation:

The equation for power is

In order to get the power, we need the current. To find the current, we need to get the total resistance, and use Ohm's Law ().

To find the total resistance, remember the equations for adding resistors is this:

Resistors  and  are in series, resistors  and  are in parallel, and resistors  and  are in series.

Now, we can find the current.

Finally, we can find the power.

Therefore, the power is 16W (watts).

Example Question #79 : Circuits

Physics2set1q6

Calculate the power consumed across resistor .

Possible Answers:

Correct answer:

Explanation:

Physics2set1q6

To calculate power, we need two of the following three quantities: voltage, current, and resistance.

In this case, since we are lacking the voltage, let's try to find the current.

We can use Kirchoff's junction law to calculate current .

The current coming into the junction = the current coming out of the junction.

Let's take a look at the central junction to the right of resistor .

Now that we know  and , we can calculate power across the resistor.

Example Question #80 : Circuits

Physics2set1q4aPhysics2set1q4b

Elements A-D represent light bulbs.

Which of the following is true about these two circuits? Assume voltage sources have the same value and all the light bulbs are all identical.

Possible Answers:

Bulbs A and B have different brightnesses.

Bulb A will be as bright as bulb C, but bulbs B and D will have a different brightness.

Bulbs A and B will be brighter than bulbs C and D.

All bulbs will have the same brightness.

Bulbs C and D have different brightnesses.

Correct answer:

Bulbs A and B will be brighter than bulbs C and D.

Explanation:

Since bulbs A and B are in parallel, they will have the same voltage, and since the bulbs are identical in resistance, they will have the same current running through them and will be just as bright. 

Let's say the voltage source as a value of  and each bulb has a resistance of .

The current going through bulbs A and B is .

However, the current going through bulbs C and D is .

The current going through bulbs C and D is half as much as the other two, so their brightness will be less.

So, bulbs A and B will be brigher than bulbs C and D.

Example Question #1 : Circuit Power

Photo 4 1

If the circuit above is connected to a  battery, what is the total power dissipated by the circuit?

Possible Answers:

Correct answer:

Explanation:

The equation for power dissipated in a circuit is 

The three resistors are in parallel with each other, so the total resistance is 

Use Ohm's law to find current.

Finally, solve for power.

Example Question #2 : Circuit Power

Combined circuit 

In the circuit above, find the power being dissipaited by .

Possible Answers:

None of these

Correct answer:

Explanation:

First, find the total resistance of the circuit.

 and  are in parallel, so we find their equivalent resistance by using the following formula:

Next, add the series resistors together.

Use Ohm's law to find the current in the system.

Because it is not in parallel, the total current in the circuit is equal to the current in .

The equation for power is as follows:

 

Example Question #3 : Circuit Power

Combined circuit

In the circuit above, find the power being dissipated by .

Possible Answers:

None of these

Correct answer:

Explanation:

First, find the total resistance of the circuit.

 and  are in parallel, so we find their equivalent resistance by using the following formula:

Next, add the series resistors together.

Use Ohm's law to find the current in the system.

The current through  and  needs to add up to the total current, since they are in parallel.

Also, the voltage drop across them need to be equal, since they are in parallel.

 Set up a system of equations.

 

Solve. 

The equation for power is as follows:

Example Question #4 : Circuit Power

Combined circuit

In the circuit above, find the power being dissipated by .

Possible Answers:

None of these

Correct answer:

Explanation:

First, find the total resistance of the circuit.

 and  are in parallel, so we find their equivalent resistance by using the following formula:

Next, add the series resistors together.

Use Ohm's law to find the current in the system.

The current through  and  needs to add up to the total current, since they are in parallel.

Also, the voltage drop across them need to be equal, since they are in parallel.

Set up a system of equations.

Solve.

The equation for power is as follows:

 

 

Example Question #5 : Circuit Power

Three parallel resistors

What can be said about the power being dissipated by  and ?

Possible Answers:

None of these

will dissipate more power than

 will dissipate more power than

It is impossible to tell

Their values will be equal

Correct answer:

Their values will be equal

Explanation:

 and are in series with each other, therefore, they will have the same current values. They also have the same resistance values. Thus, they will have the same power dissipated, as .

Example Question #6 : Circuit Power

3 sets of parallel resistors

Calculate the power being dissipated by

Possible Answers:

None of these

Correct answer:

Explanation:

The first step is to find the total resistance of the circuit.

In order to find the total resistance of the circuit, it is required to combine all of the parallel resistors first, then add them together as resistors in series.

Combine  with with with .

 

Then, combining  with  and :

Ohms is used law to determine the total current of the circuit

Combing all voltage sources for the total voltage.

 

Plugging in given values,

 

 

We know that the voltage drop across parallel resistors must be the same, so:

Using ohms law:

 

It is also true that:

Using Subsitution:

Solving for :

 Plugging in values:

 

Using the definition of electric power, where is current and is the resistance of the component in question.

 

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