CHAPTER 24 - MAGNETISM AP Physics 2 – Mrs. Lorfing

Magnetism Does the compass needle rotate? A.) Yes, clockwise B.) Yes, counterclockwise C.) No, not at all.

Magnetism A bar magnet is sawed in half. What are the results?

 A compass needle placed at the dot will point which way?

Key Facts about Magnetism 1. Magnetic poles and electric charges share similar behavior but they are not the same. 2. Magnetism is a field force. 3. Magnets have two poles – north and south. 4. Poles of a magnet can be identified by using a compass. (Opposite poles attract) 5. Only certain materials (magnetic materials) are attracted to magnets.

Magnetic Field Much like an electric field surrounds the area around a charge, the magnetic field surrounds the area around a magnet.

Magnetic Fields and Compasses Remember how an electric field can cause a dipole to rotate? (it exerts a torque) – same thing applies to a magnetic field and a compass.

‘Seeing’ the magnetic field It’s possible to use iron fillings to visualize the magnetic field around a bar magnet. The field of a magnet points away from the north pole and toward the south pole.

Magnetic Field Vectors and Lines We draw the magnetic field around a bar magnet very similar to how we draw e-field lines.

Magnetic Field of the Earth A compass needle always points north – but if opposite poles attract each other, it is pointing toward the Earth’s magnetic south pole. (the north geographic pole)

Animals and the Magnetic Field Magnetotactic bacteria have small bits of iron in their bodies that they use to align to the magnetic field of the earth. This helps them to determine depth of water and to travel deeper if they need to.

Other animals.. Bats Salmon Sea Turtles

Electricity and Magnetism Magnetic fields exist around a magnet (of course), but they can also be created from electric current. This can be proved by placing a compass close to a current carrying wire – it will deflect. The shape of the magnetic field around the wire depends on the shape of the wire. We will deal with 3 different shapes of wire – straight, loop, and solenoid.

Magnets vs. Currents Earlier we mentioned that field lines around a bar magnet start and end at the poles – but the field lines created by currents have no beginning or end.  If we extend our field lines through a bar magnet, we would see the same continuous representation.

Straight wire To find the direction of the magnetic field around a current carrying wire, we will use our RHR – most specifically, the RHR for fields. 1. Point your R thumb in the direction of the current. 2. Wrap your fingers around the wire 3. Fingers curl in the direction of the magnetic field.

Symbols for Directions Magnetism requires us to think more in 3D – although it’s tough to draw that in 2D, so we use the following symbols to represent our z-axis.

Check! A long, straight wire extends into and out of the screen. The current in the wire is A.Into the screen. B.Out of the screen. C.There is no current in the wire. D.Not enough info to tell the direction.

Check! Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P?

Magnetic Field of a Loop Because a loop is a straight wire bent into a circle, the same RHR can apply. Again, point your thumb in the direction of the current and curl your fingers through the center of the loop.

Solenoid A solenoid is a long coil of wire with the same current passing through each loop in the coil. A solenoid can generate a uniform magnetic field. The field within a solenoid is strong, parallel to it’s axis – and relatively weak outside the solenoid.