Magnetic Forces & Fields Practice Problems

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  • 0:00 Playing with Fields
  • 0:42 Magnetic Fields Near…
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Magnetism and electricity are closely related. When current flows in a wire, a magnetic field is generated. In this lesson, we will explore how magnetic fields can exert forces on parallel current-carrying wires.

Playing with Fields

Do you remember playing with bar magnets? Maybe you noticed that the magnets seemingly moved on their own if you aligned them parallel to each other. Based on which ends were aligned, you noticed the magnets moved in different directions. You were actually playing with the invisible magnetic fields generated by those pieces of metal. Something very similar happens with current-carrying wires. It is an application of Ampere's law, which deals with the magnetic field generated by an electrical charge moving through a closed loop. In this lesson, we are going to discuss current flowing through long, parallel wires.

Magnetic Fields Near Electric Wires

In previous lessons, you have learned that moving electrical charges generate magnetic fields. Electrical wires carry electric current; therefore, magnetic fields are generated around wires if they are carrying current. The strength of the magnetic field generated by a long, current-carrying wire is determined by our first equation.


Equation 1
Equation 1


  • B is the magnetic field strength in teslas (T)
  • μo is the constant 4π x 10-7 tesla-meters-per-amp (Tm/A)
  • I is the electrical current in the wire in amperes (A)
  • r is the distance from the wire in meters (m)

The magnetic field radiates out in loops around the wire. The direction of the magnetic field in relation to the direction of the current can be determined by a variation of the right-hand rule. You point your right thumb in the direction of the current, and your fingers curl around the wire. When your fingers curl this way, you can see the direction the field is flowing. If you curl your fingers in segments you can see which way the magnetic field points on all sides of the wire. In these images, we'll use a dot to represent when the magnetic field flows out of the screen and an X to represent when it goes into the screen.


Magnetic fields
RHR_wires


We can see that on the left side of the wire the magnetic field points out of the screen, and on the right side of the wire the magnetic field points into the screen. This is for positive charge-carriers. For negative charge-carriers, the magnetic field is opposite. In a negative charge-carrier situation, you could point your thumb in the opposite direction of the current and still use the curled fingers to determine the magnetic field orientation.

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