AP Physics 1 : Current and Voltage

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

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Example Question #1 : Current And Voltage

Consider the following circuit:

Circuit_1

What is the current flowing through R5?

Possible Answers:

Correct answer:

Explanation:

All of the current flowing through the circuit will go through R5, since it is not in parallel with anything. To find the current flowing through the circuit, we will need to first find the total equivalen resistance of the circuit.

To do this, we first we need to condense R3 and R4. They are in series, so we can simply add them to get:

Now we can condense R2 and R34. They are in parallel, so we will use the following equation:

The equivalent circuit now looks like:

Circuit_1.1

Since everything is in parallel, we can simply add everything up:

Now that we have the total resistance of the circuit, we can use Ohm's law to find the total current:

Rearranging for current, we get:

Example Question #2 : Current And Voltage

Consider the following circuit:

Circuit_1

If 1.35A is flowing through R1, how much current is flowing through R4?

Possible Answers:

Correct answer:

Explanation:

Kirchoff's current law tells us that the same amount of current entering the junction after R1 must also leave the junction. We also know that the voltage drop across each path of the split is the same.

Consider the following circuit to help visualize things:

Circuit_1__current_at_juntion_1_

Using Ohm's law to expand the voltages, we get:

We now have two equations that we can solve simultaneously:

Since we are solve for I3, let's rearrange the first equation for I2:

Substituting this into the second equation, we get:

Rearranging to solve for I3:

We have all the values we need, so simply plug in the given values and solve:

Example Question #3 : Current And Voltage

Consider the following circuit:

Circuit_1

If 1.35 A is flowing through R1, what is the voltage drop across R2?

Possible Answers:

Correct answer:

Explanation:

Kirchoff's current law tells us that the same amount of current entering the junction after R1 must also leave the junction. We also know that the voltage drop across each path of the split is the same.

Consider the following circuit to help visualize things:

Circuit_1__current_at_juntion_1_

Using Ohm's law to expand the voltages, we get:

We now have two equations that we can solve simultaneously:

To find V2, we need to calculate I2, so let's rearrange the first equation for I3

Substituting this into the second equation, we get:

Rearranging for I2:

We have all the values we need, so simply plug in the given values and solve:

Now that we know I2, we can use Ohm's law to find the voltage drop across that resistor:

Example Question #4 : Current And Voltage

Consider the following circuit:

Circuit_1__switch_at_r2_

By how much does the current flowing through the circuit increase when the switch is closed?

Possible Answers:

There is no increase; the current decreases when the switch is closed

There is no increase; the current remains unchanged when the switch is closed

Correct answer:

Explanation:

In order to calculate the current flowing through the circuit in each scenario, we need to calculate the equivalent resistance in each scenario.

SCENARIO 1: Switch open

When the switch is open, everything is series, so we can simply add all of the resistances up to find the total equivalent resistance. Note that R2 will be excluded.

Using Ohm's Law, we can now calculate the current flowing through the circuit:

Rearranging for current:

SCENARIO 2: Switch closed

The circuit now looks like this:

Circuit_1

Since we have a few resistors in parallel, this calcluation will involve a few more steps.

First we need to condense R3 and R4. They are in series, so we can simply add them to get:

Now we can condense R2 and R34. They are in parallel, so we will use the following equation:

The equivalent circuit now looks like:

Circuit_1.1

Since everything is in series, we can simply add everything up:

Now using Ohm's law to calculate the current flowing through the circuit:

 

Now, we simply take the different of the currents in the two scenarios:

Example Question #5 : Current And Voltage

Consider the circuit:

Circuit_4

By how much does the current through the circuit decrease if R3 is removed?

Possible Answers:

Correct answer:

Explanation:

We are asked to compare two different scenarios, each involving the calculation of equivalent resistance, which will use the following formula:

Scenario 1: With R3 Present

Now using Ohm's law:

Scenario 2: Without R3

Now using Ohm's law:

 

Calculate the change in current:

 

Example Question #6 : Current And Voltage

Consider the circuit:

Circuit_4

How much current is flowing through R1?

Possible Answers:

Correct answer:

Explanation:

Although it is possible to solve this problem by calculating an equivalent resistance, calculating a total current through the circuit, and then using Kirchoff's junction rule to find the current through R1, it is much easier to simply use Kirchoff's Loop rule. What this rule says is that through any closed loop in a circuit, all voltages must add up to zero. Written as an equation:

If we consider the closed loop path consisting of only the power supply and R1, we can use Ohm's law to calculate the current:

Example Question #7 : Current And Voltage

Consider the circuit:

Circuit_4

The total current through the circuit is  and the current through R2 is . What is the value of R2?

Possible Answers:

Correct answer:

Explanation:

The total current through the circuit is unnecessary to solve this problem. We only need to know the current through the resistor, as well as Kirchoff's loop rule. The loop rule states that for any closed loop through a circuit, the voltages add up to zero.

For this problem, we will consider the loop that consists solely of the voltage source and R2. From the rule, we know that 12V is lost through the resistor. Using Ohm's law, we can write:

Rearranging for R2, we get:

Example Question #8 : Current And Voltage

Basic circuit

In this circuit above,  and . The voltage drop across  is eight volts. What is the current across the circuit?

Possible Answers:

Correct answer:

Explanation:

The voltage drop is related to the current and resistance via Ohm's law:

 

Example Question #9 : Current And Voltage

Resistor bulb series circuit

A student has created the given circuit diagram. It consists of a battery, a resistor, and a light bulb. In one minute, 1.2C of charge flows through the resistor. How much charge flows through the light bulb in one minute?

Possible Answers:

Correct answer:

Explanation:

Since the bulb and the resistor are connected in series, the current is the same in each. Electric current is just the flow of charge through the circuit, so the same amount of charge flows through each in one minute.

Example Question #10 : Current And Voltage

If a TV uses  of energy over the course of , and it has a voltage of , how many coulombs passed through it during that time?

Possible Answers:

Correct answer:

Explanation:

Because the TV uses , and it was used for , it must have used 

.

 so:

, and since the TV was used for 

 

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