AP Physics 2: Magnetic Fields - Your Night-Before Guide 🧲

Hey there! Let's get you prepped for the AP Physics 2 exam with a super-focused review of magnetic fields. Remember, you've got this! We'll break it down, keep it clear, and make sure you're feeling confident.

Magnetic Fields

Properties of a Magnetic Field

• A vector field that exerts a force on moving electric charges, electric currents, and magnetic materials.
• Always produced by magnetic dipoles (or combinations), never monopoles. Think of it like a tiny bar magnet with a north and south pole.
• Represented by magnetic field lines that form closed loops. They point away from the north pole and back to the south pole of a magnet.

Caption: Magnetic field lines around a bar magnet. Notice how they loop from the north pole to the south pole.

Magnetic Behavior of Materials

• Magnetic dipoles arise from the circular or rotational motion of electric charges (electrons). Think of tiny current loops creating tiny magnets.
• Both permanent and induced magnetism result from the alignment of these magnetic dipoles.

- If you break a bar magnet, you get two smaller bar magnets, each with its own north and south pole. You can't isolate a single pole. - **Like poles repel**, and **opposite poles attract**. - The magnetic field strength from a dipole decreases with distance. - A magnetic compass (a tiny dipole) aligns itself with the external magnetic field. - How a material behaves in a magnetic field depends on its composition: - **Ferromagnetic materials** (iron, nickel, cobalt) can be permanently magnetized. Their magnetic domains (groups of aligned dipoles) can be aligned by an external field and stay aligned. - **Paramagnetic materials** (aluminum, titanium, magnesium) are weakly attracted to magnetic fields. Their dipoles align temporarily, but they don't stay aligned once the field is removed. - **Diamagnetic materials** (all materials) are weakly repelled by magnetic fields. This is due to a slight alignment of electron dipole moments opposite to the external field. -

Caption: Diagram showing the alignment of magnetic domains in ferromagnetic, paramagnetic, and diamagnetic materials.

• The Earth's magnetic field acts like a giant magnetic dipole. It's why compasses point north!

Magnetic Permeability of Materials

• Magnetic permeability is a measure of how much a material can be magnetized in response to an external magnetic field.
• Vacuum permeability (<math-inline>\mu_{0}</math-inline>) is a constant that appears in many magnetic field equations. It's the baseline for how easily a magnetic field can form in a vacuum.

- Permeability is not constant for a material and can change with temperature, orientation, and the strength of the external field. It's a complex property!

• Key Takeaway: Understand the difference between ferromagnetic, paramagnetic, and diamagnetic materials. This is a frequent topic on the AP exam. Also, remember that magnetic fields are produced by dipoles and always form closed loops.

Final Exam Focus

Okay, let's get down to the nitty-gritty. Here's what you absolutely need to nail for the exam:

• Magnetic Field Lines: Know how to draw them around bar magnets and current-carrying wires. Remember, they always form loops and point from north to south.
• Types of Magnetism: Be able to distinguish between ferromagnetism, paramagnetism, and diamagnetism. Understand what causes each and how they respond to external magnetic fields.
• Earth's Magnetic Field: Know that it acts like a giant dipole and affects compasses.
• Permeability: Understand what it measures and how it affects magnetic fields in different materials.

Practice Questions

Practice Question

Multiple Choice Questions:

1. Which of the following statements is true regarding magnetic monopoles? (A) They are commonly found in nature. (B) They are the source of all magnetic fields. (C) They have never been observed experimentally. (D) They are responsible for the Earth's magnetic field.

2. A material is placed in an external magnetic field. The material's magnetic dipoles align weakly and temporarily with the field. When the external field is removed, the dipoles return to random orientations. This behavior is characteristic of: (A) a ferromagnetic material (B) a paramagnetic material (C) a diamagnetic material (D) a superconductor

Free Response Question:

A long, straight wire carries a current I directed out of the page. A rectangular loop of wire is placed near the long wire, as shown below. The loop has sides of length a and b, and its closest side is a distance r from the long wire. The current in the loop is zero initially.

(a) Sketch the magnetic field lines produced by the long straight wire. (b) Using the right-hand rule, determine the direction of the magnetic field at the location of the loop due to the current in the long wire. (c) If the current in the loop is turned on in a clockwise direction, what is the direction of the force on the side of the loop closest to the long wire? Explain your reasoning. (d) What is the direction of the net force on the loop? Explain your reasoning.

Answer Key and Scoring:

Multiple Choice:

1. (C)
2. (B)

Free Response: (a) [1 point] Concentric circles centered on the long wire. The circles should indicate a clockwise direction. (b) [1 point] The magnetic field is directed to the right at the location of the loop. (c) [2 points] The force on the side of the loop closest to the long wire is directed toward the long wire. The magnetic field from the long wire is into the page at the location of the loop. The current in the loop is clockwise. Using right hand rule for force on a wire, the force is towards the long wire. (d) [3 points] The net force is towards the long wire. The force on the side of the loop closest to the long wire is stronger because the magnetic field is stronger closer to the wire. The forces on the other sides of the loop cancel out because they are equal and opposite. Therefore, the net force is towards the long wire.

Remember, you've got this! Take a deep breath, review these notes, and go ace that exam! 🚀