Field Lines

Rules for drawing electric field lines

• At every point the direction of the field is tangent to the line

• Strength is represented by density • Lines go from positive and to negative

Rules for Gravitational Field Lines

• The force of gravity is always attractive

• The field lines will always point towards the center of a spherical mass and arrive perpendicular to the surface

Rules for Magnetic Field Lines• Field lines leave the N-

pole, enter the S-pole and continue to form a closed loop inside the magnet

• Magnetic Flux : the number of magnetic field lines passing through a particular unit area. Proportional to the magnetic field intensity

Questions from Provincial Exam:Draw appropriate lines to describe field near the

objects shown• A) • B)

Questions from Provincial

• Three points are indicated as A, B, C on the diagram. List the letters in order of increasing field strength (weakest first)

Questions from Provincial

• The diagram shows 3 magnetic poles mapped with iron filings. Identify which two poles are alike. Explain your reasoning.

ELECTRICITY and MAGNETISM

Rules for Magnetic Interactions

1. Like poles repel each other2. Unlike poles attract each other3. The force of attraction varies inversely as the

square of the distance between the poles

Magnetic Dipole: Magnets always seem to come with a N-pole

paired with an S-pole

ELECTROMAGNETISM

• Electrons produce a magnetic field and a changing magnetic field will cause electrons to move.

• Discovered by accident in 1819 by Hans Christian Oersted

USES

• Discovery marked the beginning of modern science and technology:– Radio, television, computers, tape recorders,

VCRs, CD players, lasers, electric motors and generators, etc

Magnetic Fields

• Each point of a current carrying conductor creates a magnetic field around itself

• The field lines are a set of concentric closed circles perpendicular to the direction of the current

Right-hand rule #1finding direction of magnetic field lines around a conductor

• Grasp a current carrying conductor with a your right hand, thumb lies in the direction of the conventional current (positive flow)

• The fingers encircle the conductor in the direction of the magnetic field lines caused by the current

Magnetic Fields on a Coil• Field lines are closed loops• Inside the coil, uniformly

spaced to represent the uniform nature of the field

• Outside, spread out to indicate the weakened field

• X’s indicate field lines that go into the page

• Dots indicate field lines out of the page

Magnetic Fields around a Solenoid

• Solenoid: a closely wound helix. The field from a solenoid is stronger and more uniform.

• The field lines leave one end of the solenoid, circle around and enter the other end.

Right-hand rule #2finding the N-pole of a coil of wire

• Place fingers of right hand along the wire of the coil so that your fingers point in the direction of the current in the coil

• Extend your thumb• This will indicate the

direction of the field lines as they pass through the coil, and thus the face of the coil that acts as the N-pole

INTERACTIONS OF MAGNETIC FIELDS

AMPERE

Andre Ampere

• 1820• Developed

mathematical law describing the relationship between the current in a conductor and its resulting magnetic field

Current in the same direction

• Magnetic fields between the two conductors are in opposite directions

• The magnetic fields will apply a force drawing the conductors together

Current in opposite directions

• Magnetic fields between the two conductors are in the same direction

• The magnetic fines of force will repel each other

Question from previous provincial

• Two long straight current-carrying wires are placed so that they are parallel to one another. The picture shows a cross-section of the 2 wires. Draw a representation of the field lines and draw force vectors showing the direction of the magnetic force on each wire.

Questions: Section Review

• pg 767 #1 a, b, c

The AMPERE

• The unit of measure of electric current intensity

• The amount of current in each of 2 long straight parallel conductors, one meter apart, that will cause a force of 2 x 10-7N to act on each meter of wire.

The Motor Force

F =kILB• The force exerted by a magnetic on the

magnetic field of a current carrying conductor• magnetic field that acts perpendicular to the

conductor (B )• current (I)• length of the conductor inside the field (L)• k is the proportionality constant

The Electric Motor

• Activity from resource book

Electromagnetic Induction

Electromagnetics

• Core of ferromagnetic material placed inside a solenoid increases the strength of the magnetic field inside the solenoid

• 3 things affect the strength of the electromagnet: size of current, number of turns of the coil, permeability of the core

Strong Electromagnets

• Must be super-cooled to the point where the coils become superconductors, and lose their resistance.– MRI, High speed trains, Particle accelerators

Ampere and Faraday

• 1820 Ampere shows that an electric current produces a steady magnetic field

• 1831 Faraday predicts that a steady magnetic field should produce an electric current

Lenz’s Law

• An induced electric current flows in a direction that opposes the change that produced it.

• Heinrich Friedrich Emil Lenz (1804-1865)