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ZingPath: Electromagnetic Force and Induction

Magnetic Field of a Current-Carrying Infinity Wire

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Magnetic Field of a Current-Carrying Infinity Wire

Physics

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Students learn the factors that affect the magnitude and direction of the magnetic field of a current-carrying infinite wire.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Define magnetic field.
  • Describe the behavior of the magnetic field around a straight current-carrying infinite wire.
  • Determine the direction of the magnetic field around a straight current-carrying infinite wire.
  • Explain the right-hand rule.

Everything You'll Have Covered

Some naturallyoccurring objects are capable of exerting a force on magnetic objects. The area over which this force acts is called the magnetic field. Magnetic field lines describe the shape and direction of the force. For example, Earth's magnetic poles produce a field described by arching longitudinal lines connecting the North and South Poles. The field lines around a bar magnet can be seen using iron filings.

The first indication that the phenomenon of magnetism was related to electricity came from ěrsted'sdiscoveryin 1820 thatacompass needledeflected (moved from pointing toward the Earth's magnetic North Pole) when a current passed through a nearby wire. Further experiments confirmed that the magnetic field around a long, straight current-carrying wire is cylindrical in shape and counterclockwise in direction (when viewed from the end of the wire from which current is emitted).

The right-hand rule helps to ascertain the direction of the field. Think of the right hand wrapped around a wire, with the thumb extended in the direction of the current. The fingers then curl in the direction of the magnetic field. Magnetic field lines are represented as concentric circles around the wire coming into, or out of, the plane with arrows indicating the direction of the field. A wire with a current coming out of the plane is represented by a circle with a dot in the center, and a current heading into the plane is represented by a circle with a cross.Since the magnetic field has a direction, it is a vector quantity.By switching the direction of the current, the direction of the magnetic field changes from clockwise to counterclockwise, or vice-versa. The magnitude of the magnetic field decreases as distance from the wire increases. The magnitude of the magnetic field also varies proportionally with the magnitude of the current. The relationship between the magnetic field B, distance from the wire r, and current I, is given by the equationwhereB = magnetic field, I = current, r = distance, andk is a constant.The magnetic field is measured in tesla (T), distance is measured in meters, and current is measured in amperes(A).

The magnetic field produced by current-carrying wires is an essential aspect in many technologies, such as simple doorbell mechanisms and electric motors. In a simple doorbell circuit, there is an electromagnet made up of a length of coiled wire. When someone presses the doorbell, it closes the circuit. This magnetizes the electromagnet inside the buzzer, causing it to pull up on the contact arm. When the contact arm rises, it breaks the doorbell circuit, which shuts off the electromagnet. The arm then falls back down, closing the circuit again. The connection between electricity and magnetism is also applied in rechargeable toothbrushes and other gadgets. The aurora borealis (northern lights) occur due to aninteraction between solar winds and the Earth's magnetic field.

Tutorial Details

Approximate Time 30 Minutes
Pre-requisite Concepts Students understand the concepts and behaviors of electric currents and electric charges, and can describe the properties of a force (such as a magnetic force) as a vector quantity.
Course Physics
Type of Tutorial Concept Development
Key Vocabulary amperes, compass needle deflection, currents