AP Physics 1 : Ohm's Law
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Example Questions
Example Question #21 : Ohm's Law
You connect a circuit with a resistor and a voltage source. The resistor has a resistance of 
To calculate the amount of electrons flowing through this circuit we need to first calculate the current. Use Ohm’s law:
Solve for current:
The question gives us the voltage and resistance; therefore, the current flowing through this circuit is
Recall that current is the amount of electrons flowing through a circuit per unit time.
Solving for the charge of electrons gives us
Recall that an electron contains 
This means that 
Example Question #21 : Ohm's Law
A circuit consists of a single voltage source and a single resistor. When 
Using ohm's law, the resistance is determined to be 
Example Question #21 : Ohm's Law
For a given electrical circuit, if you double the current, the resistance __________?
doubles
halves
quarters
quadruples
remains the same
halves
Recall Ohm's Law:
Since the two quantities we are interested in are on the same side of the equation, they are inversely proportional. Hence, if one increases, the other one decreases by the same ratio. Since the current is doubling, the resistance must halve for the circuit to be the same.
Example Question #24 : Ohm's Law
Consider the circuit diagram shown. In this circuit, the values of 
Which of the following expressions would give a situation in which the voltmeter in the diagram would read zero?
To answer this question, we'll need to find an expression for the value of the variable resistor that would make the voltmeter read zero.
First, it's important to realize what situation would result in a reading of zero from the voltmeter. For there to be no reading, that means that there cannot be any voltage difference between the top row of resistors and the bottom row. For this to happen, the voltage drop for the resistors on the left of the voltmeter must be equal, and the same is true for the two resistors to the right of the voltmeter. In other words, both rows of resistors will experience the same voltage decrease as current flows through, thus the difference of voltage drop in the top and bottom row will be identical.
So let's consider the top and bottom resistors on the left side first. In the top left corner, the voltage of the first resistor will be 
Now let's take a look at the other resistors on the right. The voltage of the third resistor will be 
Now that we have the two expressions shown above, we can isolate the term for the variable resistance in terms of the other three resistors to find our answer.
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