### All MCAT Physical Resources

## Example Questions

### Example Question #1 : Resistivity

Which of the following factors would decrease the resistance through an electrical cord?

**Possible Answers:**

Decreasing the cross-sectional area of the cord

Increasing the cross-sectional area of the cord

Increasing the length of the cord

Increasing the resistivity

**Correct answer:**

Increasing the cross-sectional area of the cord

The equation for resistance is given by .

From this equation, we can see the best way to decrease resistance is by increasing the cross-sectional area, , of the cord. Increasing the length, , of the cord or the resistivity, , will *increase* the resistance.

### Example Question #1 : Resistivity

An electrician wishes to cut a copper wire that has no more than of resistance. The wire has a radius of 0.725mm. Approximately what length of wire has a resistance equal to the maximum ?

**Possible Answers:**

9.6m

960m

10cm

38m

2.6cm

**Correct answer:**

960m

To relate resistance R, resistivity , area A, and length L we use the equation.

Rearranging to isolate the quantity we wish to solve for, L, gives the equation . We must first solve for A using the radius, 0.725mm.

Plugging in our numbers gives the answer, 960m.

### Example Question #2 : Resistivity

What is the current in a circuit with a resistor followed by a resistor that are both in parallel with a resistor? The voltage supplied to the circuit is 5V.

**Possible Answers:**

**Correct answer:**

Resistors in serries add according to the formula:

Resistors in parallel add according to the formula:

We can find the total equivalent resistance:

Now we can use Ohm's law to find the current:

Solve: