# MCAT Physical : Resistivity

## Example Questions

### Example Question #1 : Resistivity

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

Decreasing the cross-sectional area of the cord

Increasing the resistivity

Increasing the length of the cord

Increasing the cross-sectional area of the cord

Increasing the cross-sectional area of the cord

Explanation:

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 #3 : 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  ?

960m

10cm

2.6cm

38m

9.6m

960m

Explanation:

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 #4 : 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.

Explanation:

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: