# Lesson Worksheet: Resistance and Resistivity of Conductors Physics • 9th Grade

In this worksheet, we will practice relating the dimensions of and the motion of free electrons through an object to its resistance.

**Q4: **

The wires that carry current from a power station to a substation are 7.25 km long. They are made of copper with a resistivity of Ω⋅m. The current through the wires is 450 mA. The power dissipated by the wires is required to be no more than 15 W. What is the minimum cross-sectional area required for the wires that transmit the current? Give your answer in scientific notation to one decimal place.

- A m
^{2} - B m
^{2} - C m
^{2} - D m
^{2} - E m
^{2}

**Q5: **

The diagram shows two identical circuits. Each circuit’s conducting wire has had a section of it greatly magnified to show the ions that the conducting wire is comprised of and the free electrons that move between these ions. The conducting wires are both made of the same substance.

Which of the following statements correctly states how the resistivities of the sections of conducting wire in diagram (a) and diagram (b) compare to each other?

- AThe resistivity of the section in diagram (a) is greater than the resistivity of the section in diagram (b).
- BThe resistivity of the section in diagram (b) is greater than the resistivity of the section in diagram (a).
- CThe resistivities are the same for both sections.

Which of the following statements correctly states how the cross-sectional areas of the conducting wires compare to each other?

- AThe cross-sectional area of the wire in diagram (b) is greater than the cross-sectional area of the wire in diagram (a).
- BThe cross-sectional areas of the wires are the same.
- CThe cross-sectional area of the wire in diagram (a) is greater than the cross-sectional area of the wire in diagram (b).

Which of the following statements correctly states how the average time taken for a free electron to cross from one side of the section to the other side in diagram (a) compares to the average time taken for a free electron to cross from one side of the section to the other side in diagram (b)?

- AThe time taken for the section in diagram (b) is greater.
- BThe time taken for the section in diagram (a) is greater.
- CThe time taken is the same for both sections.

Which of the following statements correctly states how the resistance of the section of conducting wire in diagram (a) compares to the resistance of the section of conducting wire in diagram (b)?

- AThe resistance of the section in diagram (b) is greater.
- BThe resistance of the section in diagram (a) is greater.
- CThe resistance is the same for both sections.

**Q7: **

A current of 1.4 A in a copper wire is carried by free electrons. The cross-sectional area of the wire is m^{2}. Find the average speed at which free electrons pass through the wire. Use a value of C for electron charge and a value of m^{−3} for the density of free electrons in copper. Give your answer in scientific notation to one decimal place.

- A m/s
- B m/s
- C m/s
- D m/s
- E m/s

**Q8: **

A current of 77 mA in a conducting wire of an unknown material is carried by free electrons. The cross-sectional area of the conductor is m^{2}. Find the density of free electrons in the material if the average speed of the free electrons along the wire is 0.18 mm/s. Use a value of C for the electron charge. Give your answer in scientific notation to one decimal place.

- A m
^{−3} - B m
^{3} - C m
^{−3} - D m
^{−3} - E m
^{−3}

**Q9: **

Diagrams (a) and (b) show sections of identical conducting objects consisting of three equally separated ions. Free electrons that have a velocity greater than a velocity move away from the nearest ion to a neighboring ion. Free electrons with velocities lower than do not move to a neighboring ion. Diagrams (c) and (d) show sections of conducting objects in which the spacing between ions is greater than it is for the objects shown in diagrams (a) and (b). The density of free electrons in the object in diagram (d) is the same as the free electron density in the objects shown in diagrams (a) and (b). The density of free electrons in the object in diagram (c) is greater than the free electron density in the objects shown in diagrams (a) and (b). The average speed at which a single typical electron moves in the objects in diagrams (c) and (d) is .

Identical-sized wires made of the materials that the objects in diagrams (a) and (d) are made of are connected across the same potential difference. Which of the following statements most correctly describes how the current through wire I, which is made of the material from diagram (a), compares to the current through wire II, which is made of the material from diagram (d)?

- AThe current in wire II is greater than the current in wire I.
- BThe current in wire I is greater than the current in wire II.
- CThe current is the same in both wires.

Identical-sized wires made of the materials that the objects in diagrams (c) and (d) are made of are connected across the same potential difference. Which of the following statements most correctly describes how the current through wire I, which is made of the material from diagram (c), compares to the current through wire II, which is made of the material from diagram (d)? Take into account the mutual repulsion of electrons.

- AThe current in wire I is greater than the current in wire II.
- BThe current is the same in both wires.
- CThe current in wire II is greater than the current in wire I.

**Q10: **

The diagram shows two very similar circuits. Each circuit’s conducting wire has had a section of it greatly magnified to show the ions that the conducting wire is comprised of and the free electrons that move between these ions.

Which of the following statements correctly states how the resistivities of the sections of the conducting wire in diagram (a) and diagram (b) compare to each other?

- AThe resistivity of the section in diagram (a) is greater.
- BThe resistivity of both sections is the same.
- CThe resistivity of the section in diagram (b) is greater.

Which of the following statements correctly states how the cross-sectional areas of the conducting wires compare to each other?

- AThe cross-sectional area of the wire in diagram (b) is greater than the cross-sectional area of the wire in diagram (a).
- BThe cross-sectional areas of the wires are the same.
- CThe cross-sectional area of the wire in diagram (a) is greater than the cross-sectional area of the wire in diagram (b).

Which of the following statements correctly states how the number of free electrons per meter of length of the conducting wire in diagram (a) compares to the number of free electrons per meter of length of the conducting wire in diagram (b)?

- AThe number of free electrons per meter of length of the conducting wire in diagram (b) is greater than the number of free electrons per meter of length of the conducting wire in diagram (a).
- BThe number of free electrons per meter of length of the conducting wire in diagram (a) is greater than the number of free electrons per meter of length of the conducting wire in diagram (b).
- CThe number of free electrons per meter of length is the same for both wires.

Which of the following statements correctly states how the average time taken for a free electron to cross from one side of the section to the other side in diagram (a) compares to the average time taken for a free electron to cross from one side of the section to the other side in diagram (b)?

- AThe average time taken for a free electron to cross from one side of the section to the other side in diagram (b) is greater than the average time taken for a free electron to cross from one side of the section to the other side in diagram (a).
- BThe average time taken for a free electron to cross from one side of the section to the other side in diagram (a) is greater than the average time taken for a free electron to cross from one side of the section to the other side in diagram (b).
- CThe average time taken for a free electron to cross from one side of the section to the other side is the same for both sections.

Which of the following statements correctly states how the resistance of the section of conducting wire in diagram (a) compares to the resistance of the section of conducting wire in diagram (b)?

- AThe resistance of the section in diagram (b) is greater.
- BThe resistance of the section in diagram (a) is greater.
- CThe resistance of both sections is the same.