Chapter 20: Magnetism Purpose: To describe magnetic field around a permanent magnet. Objectives: Describe a magnetic poles Describe magnetic field. Magnetic field lines.

Warm-up Where and how do we use magnets? What is an electro magnet and where is it used?

Describing a magnet magnet is a material or object that produces a magnetic field. A bar magnet Horse shoe magnet

Magnetic poles Magnetic poles are regions that produce magnetic forces s N North pole South pole

Like poles repel, unlike poles attract. South repels south, North Repels North, and South attracts North.

Breaking the magnet Magnetic poles cannot be isolated. When you break a magnet into two you end up with two magnets. Keep breaking further and further, you still get the same results.

Magnetic field The magnetic field (usually denoted B) is a space around a magnet where magnetic force is exerted. The SI units of magnetic field (B) is Tesla (T) Iron fillings trace out a pattern of magnetic field lines in the space around the magnet.

Magnetic field is represented by magnetic field lines where lines are closer together, the field strength is greater. The direction of magnetic field is, from North pole to south pole.

Nature of magnetic field What is in a magnet that makes it a magnet? Permanent magnets, like all other substances are composed of atoms that contain electrons. These electrons are in a continuous motion. Electrons have two types of motion; a) spinning on their positions and b) Orbiting around the nucleus. Magnetic Materials

source of all magnetism The source of all magnetism is moving electric charge (motion of electrons) Every spinning and orbiting electron is a tiny magnet that creates a magnetic field around an atom. Question; What is the source of magnetic field on a bar magnet?_____________________________

Magnetic domains Magnetic domains are large cluster of aligned atom. Each domain contains dipoles (magnetized atoms) that are aligned. Irregular shaped domains with aligned dipoles

Magnetic and non magnetic Magnetic materials have electrons spinning in the same direction. This leads to aligned atoms with a strong magnetic field around them. Examples include Iron, Nickel, and Cobalt For non magnets, the electron spins opposite to one another causing their magnetic fields to cancel each other. Therefore, no net magnetic field exists

EARTHS MAGNETIC FIELD

Electron Vs Earth

Magnetic declination Magnetic declination is the discrepancy between the orientation of a compass and true North. That means that compasses do not point to true Geographic North. Magnetic declination

Homework Using double bubble Differentiate between magnetic poles from electric charges. Textbook Pg 576 # 1 -6

Warm-up What is the source of all magnetic field?

Beam of electrons Since moving electrons create magnetism, a beam of electrons will have a magnetic field around it.

Electric currents produce Magnetic fields Straight conductors (wires) will have magnetic fields around them. The magnetic field form complete loops around the conductors.

Direction of magnetic field First Right hand rule The thumb indicate the direction of current. The four fingers indicate direction of magnetic field.

Right hand rule First Right hand rule Electric current Magnetic field

Question Consider a beam of electrons moving from west to east, what is the direction of magnetic field at point G? N E W e- G • S

Question What is the direction of a magnetic field at the center of a current carrying loop with a counter-clockwise flow of current?

answer Out of the page

Magnetic Field Produced by a Coil This is how magnetic field lines looks like in and out of an electromagnet Counter-clockwise current----North pole Clockwise current flow---South Pole.

North and south poles of an electromagnet. Second Right hand rule. Wrap the fingers around the coil in the direction of current. The thump will indicate the North pole.

question What is the direction of the magnetic field inside the coil shown on the right?

Class work Textbook Pg 576 # 7-16

Warm-up How come a magnet can attract a nail? What kind of charge, negative or positive, attracts a neutral object? What happens to the electrons within a iron nail when it gets magnetized?

Warm-up What happens when you put a small compass in a magnetic field say of a straight conductor?

Lab Lab- magnetic field

Magnetic force on moving charge What happens when a charge/current moves near a magnetic field? B N Magnetic field S A Current carrying conductor

Direction of current and magnetic field Maximum force is achieved when the current is flowing perpendicular to the magnetic field. If the current is flowing parallel to the magnetic field, there will be no interaction and therefore no force

Direction of Force on a conductor The conductor is forced to move in a certain direction. The Third right hand rule indicates the direction of the force. Concepts in Motion

What is the direction of the magnetic force on the current in each of the six cases below

Magnitude force on a current carrying wire.

Magnetic force Magnetic force formula is just the magnetic field formula rearranged as shown; When Current (I) and the length (L) of conductor is given we use the 2nd formula.

The magnetic field formula Where; B = magnetic field, V = speed of the charged particle, and q = charge of the particle, F = force of the field

Example A proton speeding at 3.0 x107 m/s experiences a magnetic field of 4.0 Teslas. What is the magnetic force pulling on the proton?

Class work Textbook page 577, # 1-7

Path of a charged particle The path of a charged particle in a uniform magnetic field is a circle. (Just as the force of gravity causes the moon to move in a circle around the earth). Force on the particle, and particle’s motion are perpendicular to each other.

Path of a charged particle Thus, a charged particle moves in a circular path with constant centripetal acceleration. Therefore;

Example; An electron travels at 2.0 x 107 m/s in a plane perpendicular to a uniform 0.01 T magnetic field. Describe the motion of the electron quantitatively.

Helical path of a charged particle If the velocity of the charged particle is not perpendicular to the magnetic field, the path of the charged particle becomes Helical.

Why Helical path The velocity component perpendicular to the field results in circular motion about the field lines. However, the velocity component parallel to the field lines results in no force. The particle continues to move in the direction of that vector.

Direction of charge Electrons (negative charges) move in the opposite direction of positive charges and current.

Magnetic field at distance r from the wire Magnetic field around a straight conductor is directly proportional to the current through the wire and inversely proportional to the radius or ( the perpendicular distance from the wire)

Magnetic field at distance r from the wire

Example An electric wire in the wall of a building carries a DC current of 25 A vertically upward. What is the magnetic field due to this current at point p 10 cm due north of the wire?

Exercise Textbook pg 577 # 8-18

Warm-up: Can a stationary electron be set into motion with a magnetic field? With an electric field? Explain.

Class work Workbook problems Pg 226; #s 1- 4 and Pg 231; #s A1- A5

Worksheet- Magnetism

Force between two parallel wires Two parallel wires carrying currents I1 and I2 separated by a distance r, exerts a force F on each other given by the formula:

Wire 1 consider the field produced by wire 1 and the force it exerts on wire 2 (call the force F 2   ). The field due to I 1   at a distance r  is given to be;

Force exerted of wire 2 This field is uniform along wire 2 and perpendicular to it, and so the force F 2  it exerts on wire 2 is given by;

We get the formula for the force F2 as; Force exerted on wire 2 Substituting for B1 from; We get the formula for the force F2 as;

Direction of force When the currents in the two wire is in the same direction, the force is attraction. When the currents are in opposite direction, the force is repulsion.

Class work Textbook pg 578 # 26-32

Putting electricity to work This force can then be used for daily work such as; sharpen pencils Cut down trees, Driving cars, In other words, we transform electrical energy to mechanical energy in order to do work.

Simple Galvanometer This is a meter that detects small electric currents. When electricity is present in the coil, each loop produces its own effect on the needle causing it to deflect.

Electric motor This is the device used to transform electrical energy to mechanical energy Concepts in Motion

Direct Current Motors Direct Current Motors

Loud speakers Loud speakers convert electric energy to mechanical energy just as Motors do. Changing current in a magnetic field creates varying forces on the conductor, and varying motions of the paper cone which is attached to the conductor.

Research Homework Explain how a motor works Explain how a speaker works