12.5 Ampère’s Law

Ampère's law connects electric currents to the magnetic fields they create. It's a key principle in electromagnetism, showing that moving charges generate magnetic fields proportional to the current's strength.

For straight wires, the magnetic field strength decreases as you move away. The right-hand rule helps visualize field direction: point your thumb along the current, and your curled fingers show the field's circulation.

Ampère's Law and Magnetic Fields

Ampère's law and magnetic fields

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• Ampère's law establishes a fundamental relationship between electric currents and the magnetic fields they generate
• States the magnetic field around a closed loop is directly proportional to the total electric current passing through the area enclosed by the loop
• Mathematically expressed as $\oint \vec{[B](https://www.fiveableKeyTerm:B)} \cdot d\vec{l} = \mu_0 I_{enc}$
  • $\oint \vec{B} \cdot d\vec{l}$ represents the line integral of the magnetic field $\vec{B}$ along the closed loop
  • $I_{enc}$ denotes the total current enclosed by the loop
  • $\mu_0$ is the permeability of free space, a constant equal to $4\pi \times 10^{-7} \text{ [T](https://www.fiveableKeyTerm:T)} \cdot \text{[m/A](https://www.fiveableKeyTerm:m/A)}$
• Demonstrates that electric currents (moving charges) serve as sources of magnetic fields
• The strength of the generated magnetic field is directly proportional to the magnitude of the electric current
• Ampère's law is a key principle in electromagnetism, alongside Gauss's law, Faraday's law, and the Ampère-Maxwell law
• Ampère's law is particularly useful in magnetostatics, where currents and magnetic fields are time-independent

Magnetic fields around straight wires

• For an infinitely long straight wire carrying a steady current $[I](https://www.fiveableKeyTerm:I)$, the magnetic field at a distance $[r](https://www.fiveableKeyTerm:r)$ from the wire can be calculated using Ampère's law
• The magnetic field magnitude is given by $B = \frac{\mu_0 I}{2\pi r}$
  • $B$ represents the magnitude of the magnetic field
  • $I$ is the current flowing through the wire
  • $r$ is the perpendicular distance from the center of the wire to the point of interest
• To derive this formula using Ampère's law:
  1. Choose a circular loop of radius $r$ centered on the wire as the path of integration
  2. Due to the symmetry of the situation, the magnetic field is constant in magnitude along the loop
  3. Simplify the line integral: $\oint \vec{B} \cdot d\vec{l} = B(2\pi r)$, where $2\pi r$ is the circumference of the loop
  4. Apply Ampère's law: $B(2\pi r) = \mu_0 I$, equating the line integral to the enclosed current
  5. Solve for $B$ to obtain the final expression: $B = \frac{\mu_0 I}{2\pi r}$
• The magnetic field around a straight wire decreases inversely with increasing distance from the wire (falls off as $1/r$)
• This result is useful for calculating magnetic fields in situations involving long, straight current-carrying wires (power lines, electrical cables)

Right-hand rule for current direction

• The right-hand rule is a convention used to determine the direction of the magnetic field circulation in relation to the current direction when applying Ampère's law
• To use the right-hand rule:
  1. Point your right thumb in the direction of the current flow
  2. Your fingers will naturally curl in the direction of the magnetic field circulation
• When applying Ampère's law, the direction of the line integral (circulation) must be consistent with the right-hand rule
  • If the current is pointing out of the loop (towards you), the circulation is counterclockwise
  • If the current is pointing into the loop (away from you), the circulation is clockwise
• The right-hand rule ensures that the relationship between the current direction and the magnetic field circulation is consistent with Ampère's law
• This convention is crucial for correctly determining the direction of the magnetic field when solving problems involving Ampère's law (solenoids, toroidal coils)

Ampère's Law and Maxwell's Equations

• Ampère's law is one of Maxwell's equations, which collectively describe the behavior of electric and magnetic fields
• The curl of the magnetic field is related to the current density and the time-varying electric field
• Ampère's law in its original form applies to steady currents, but Maxwell's equations extend it to include time-varying fields
• Electromagnetic induction, as described by Faraday's law, is closely related to Ampère's law in Maxwell's equations