Worksheet: Current Density

In this worksheet, we will practice describing current density as a vector determining the electric current through an area perpendicular to the current direction.


The current supplied to an air conditioner unit is 4.00 A. The air conditioner is wired using a 10-gauge (diameter 2.588 mm) wire. The charge density in the wire is 𝑛=8.48×10 electron/m3.

Find the magnitude of current density in the air conditioner’s wiring.

  • A7.60×10 A/m2
  • B8.35×10 A/m2
  • C8.00×10 A/m2
  • D8.21×10 A/m2
  • E7.88×10 A/m2

Find the magnitude of the drift velocity in the air conditioner’s wiring.

  • A5.17×10 m/s
  • B5.41×10 m/s
  • C5.60×10 m/s
  • D5.80×10 m/s
  • E4.82×10 m/s


An electron beam with a radius of 0.90 mm has a measured current 𝐼=30.00μA. What is the magnitude of the current density of the beam?


A high-energy proton accelerator produces a proton beam with a radius 𝑟=0.73 mm. The constant beam current 𝐼=5.0 μA. The beam’s charge density 𝑛=3.7×10 protons/m3.

What is the magnitude of the current density of the beam?

What is the magnitude of the drift velocity of the beam?

  • A2.5×10 m/s
  • B1.3×10 m/s
  • C5.0×10 m/s
  • D7.5×10 m/s
  • E6.8×10 m/s

How much time is required for 2.3×10 protons to be emitted by the accelerator?


A wire has a circular cross-section with a radius 𝑅=3.000 mm. The current density in the wire is non-uniform and is modeled as 𝐽=𝑐𝑟, where 𝑐 is a constant with units of Am/. The magnitude of 𝐽 is 5.00×10/𝑟Am. What magnitude of current is carried by the inner section of the wire from the axis to 𝑟=0.50𝑅?


A cylindrical wire with radius 𝑅=0.50mm has a current density 𝐽=𝐽𝑅𝑟, where 𝐽=16,000/Am. What is the current through the wire?


A rod has a square cross section with sides of length 2.0 cm. The current density in the rod 𝐽=𝐽𝑒kk=0.50⋅𝑒Am×, where 𝑥 is measured in meters, as shown in the diagram. Find the current through the face of the rod.


A 20 gauge wire has a cross-sectional area of 0.518 mm2. The current in the wire is modeled as 𝐼(𝑡)=2.00×(80𝜋𝑡)sin, where 𝑡 is measured in seconds. What is the current density in the wire at the instant 𝑡=10.00ms?

  • A73.4×10 A/m2
  • B15.5×10 A/m2
  • C27.8×10 A/m2
  • D81.2×10 A/m2
  • E2.27×10 A/m2


If there is one free electron per atom of zinc, find its electron number density. Use a value of 7.13 g/cm3 for the density of zinc.

  • A6.57×10 cm−3
  • B6.50×10 cm−3
  • C7.13×10 cm−3
  • D3.17×10 cm−3
  • E3.57×10 cm−3

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