Unit 4 Overview: Magnetic Fields

6 min readmarch 15, 2023

Riya Patel

Riya Patel


AP Physics C: E&M 💡

26 resources
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Overview

Unit 4 on Magnetic Fields is a topic in the field of physics that covers the behavior of magnets and magnetic fields, as well as the interactions between magnets and charged particles. The unit includes several key topics, including:
  1. Magnetic Fields and Magnetic Force: This topic covers the behavior of magnetic fields and how they interact with other magnets or charged particles. It also includes the concept of magnetic force and the calculation of the force on a charged particle moving through a magnetic field.
  2. Applications of Magnetic Fields: This topic covers the practical applications of magnetic fields in areas such as particle accelerators, magnetic resonance imaging (MRI), and electric motors.
  3. Electromagnetic Waves and Fields: This topic covers the relationship between magnetic fields and electric fields, and how they combine to create electromagnetic waves. It also includes the concept of electromagnetic radiation and the behavior of waves in different media.
  4. Magnetism and Matter: This topic covers the behavior of magnetic materials and how they respond to external magnetic fields. It also includes the concept of magnetization and the different types of magnetic materials.
Overall, this unit provides a comprehensive understanding of the behavior of magnetic fields and their interactions with charged particles and other magnetic materials.

4.1 Forces on Moving Charges in Magnetic Fields

When a charged particle moves through a magnetic field, it experiences a magnetic force. The magnetic force is perpendicular to both the magnetic field and the direction of motion of the charged particle. The force on a charged particle moving in a magnetic field can be described by the following equation:
F = q(v x B)
where F is the magnetic force acting on the charged particle, q is the charge of the particle, v is its velocity, and B is the magnetic field. The vector product v x B is a vector that is perpendicular to both v and B, and its magnitude is given by the product of the magnitudes of v and B multiplied by the sine of the angle between them.
The direction of the magnetic force on a charged particle is given by the right-hand rule. If you point your thumb in the direction of the velocity of the charged particle, and your fingers in the direction of the magnetic field, the direction of the magnetic force is given by the direction your palm is facing.
The magnitude of the magnetic force on a charged particle is proportional to the charge of the particle, the magnitude of its velocity, and the strength of the magnetic field. The magnetic force does not do any work on the charged particle, because it is always perpendicular to the direction of motion of the particle.
The magnetic force on a charged particle can cause it to change its direction of motion, but it cannot change the magnitude of its velocity. This means that a charged particle moving in a magnetic field will follow a curved path, called a cyclotron motion, as it moves through the magnetic field. The radius of the cyclotron motion depends on the velocity of the charged particle, the strength of the magnetic field, and the mass and charge of the particle.

4.2 Forces on Current Carrying Wires in Magnetic Fields

When an electric current flows through a wire, it produces a magnetic field around the wire. This magnetic field can interact with other magnetic fields, including the magnetic field produced by a permanent magnet or an electromagnet. This interaction can result in a force being exerted on the wire, causing it to move.
The force on a current-carrying wire in a magnetic field is given by the following equation:
F = ILBsinθ
where F is the force on the wire, I is the current flowing through the wire, L is the length of the wire, B is the magnetic field, and θ is the angle between the direction of the current and the direction of the magnetic field.
The direction of the force is given by the right-hand rule. If you point your thumb in the direction of the current in the wire, and your fingers in the direction of the magnetic field, the direction of the force is given by the direction your palm is facing.
The magnitude of the force on the wire depends on the strength of the magnetic field, the current flowing through the wire, and the length of the wire that is in the magnetic field. The force is proportional to the sine of the angle between the direction of the current and the direction of the magnetic field.
This force can be used in a variety of applications, including electric motors and generators, where a magnetic field is used to create motion in a wire or coil. It is also the principle behind many sensors and devices that measure magnetic fields, such as Hall effect sensors.

4.3 Fields of Long Current Carrying Wires

A long, straight current-carrying wire produces a magnetic field around it that is circular in shape and is known as a solenoid. The magnetic field produced by a long current-carrying wire can be calculated using Ampere's Law.
Ampere's Law states that the magnetic field around a long, straight wire is directly proportional to the current flowing through the wire and inversely proportional to the distance from the wire. The equation for the magnetic field produced by a long, straight wire is given by:
B = μ₀(I / 2πr)
where B is the magnetic field, I is the current flowing through the wire, r is the distance from the wire, and μ₀ is the permeability of free space, which is a constant with a value of approximately 4π x 10^-7 N/A^2.
The direction of the magnetic field is given by the right-hand rule. If you wrap your fingers around the wire in the direction of the current, your thumb points in the direction of the magnetic field.
The magnetic field produced by a long, straight wire is strongest close to the wire and decreases as the distance from the wire increases. The magnetic field also becomes weaker as the current through the wire decreases.
The magnetic field produced by a long current-carrying wire can be used in a variety of applications, including in electromagnets and solenoids. By wrapping a wire around a magnetic core and passing a current through the wire, the magnetic field produced can be used to create a strong magnetic field that can be used in a variety of industrial and scientific applications.

4.4 Biot–Savart Law and Ampère’s Law

The Biot-Savart Law and Ampere's Law are two important laws in electromagnetism that relate to the calculation of magnetic fields.
The Biot-Savart Law states that the magnetic field produced by a current-carrying wire at a point in space is proportional to the current in the wire and the length of the wire. The magnetic field is also proportional to a vector quantity known as the vector potential, which depends on the distance from the wire and the direction of the magnetic field. The Biot-Savart Law can be used to calculate the magnetic field at any point in space due to a current-carrying wire.
Ampere's Law relates the magnetic field around a closed loop to the current passing through the loop. It states that the line integral of the magnetic field around a closed loop is proportional to the current passing through the loop. The proportionality constant is known as the permeability of free space, μ₀. Ampere's Law can be used to calculate the magnetic field around a current-carrying wire or a group of current-carrying wires.
Both the Biot-Savart Law and Ampere's Law are important in the study of electromagnetism and can be used to calculate the magnetic fields produced by a variety of different current-carrying systems, including long straight wires, solenoids, and even more complex systems like magnetic circuits and motors.
It is important to note that the Biot-Savart Law and Ampere's Law are both based on experimental observations and are fundamental laws of electromagnetism. They are used in a variety of fields, including electrical engineering, physics, and materials science, to understand and design magnetic systems.

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