ElectromagnetismMagnetism

Magnetic Field

Definition

Electric FieldA region of space in which a charged particle experiences an electric force.

Magnetic FieldA region of space in which a moving charged particle experiences a magnetic force.

How must it move? Discuss later.

Law of Poles

Review Law of ChargesOpposites – AttractLikes – Repel

Law of PolesOpposites – AttractLikes – Repel

Magnetic Field Lines

Rules1. Point North to South on the EXTERIOR

of a magnet.2. Point South to North on the INTERIOR of

a magnet.3. Form closed loops.4. Never cross.

Magnetic Field Lines

Examples1. Two like poles2. Two opposite poles3. Bar magnet4. Earth

B Field of a Current-Carrying Wire

1st Right Hand RuleUsing your right-hand: 1. Point your thumb in the direction of the

conventional current.2. Curl your fingers into a half-circle around

the wire, they point in the direction of the magnetic field, B

Field Notation Out of the page.

Into the page.

B Field of a Current-Carrying Wire

Examples1. Current flowing straight towards you.2. Electrons flowing away from you.3. Current flowing away from you.4. Current flowing right-to-left.5. Electrons flowing left-to-right.6. Electrons flowing straight towards you.7. Current flowing left-to-right.8. Electrons flowing right-to-left.

B Field of a Current-Carrying Loop

2nd Right Hand RuleUsing your right-hand: 1. Point your thumb in the direction of the

conventional current.2. Curl your fingers into the loop, they point

in the direction of the magnetic field, B

Field Notation Out of the page.

Into the page.

B Field of a Current-Carrying Loop

Examples1. Current flowing clockwise.2. Electrons flowing counterclockwise.3. Electrons flowing down the front edge4. Current flowing counterclockwise.5. Current flowing down the front edge.6. Current flowing up the front edge.7. Electrons flowing clockwise.8. Electrons flowing up the front edge.

Tangent Galvanometer Lab

PreLab1. Discuss set-up.2. Discuss aligning the plane of loops.3. Discuss how to wind the loops.4. Discuss how to connect batteries.5. Discuss how to measure current.6. Discuss how to measure the angle.7. Discuss B calculation.8. Discuss average calculations.

Tangent Galvanometer Lab

Post Lab1. When the current is flowing through the

loops, what are the two magnetic fields affecting the compass? Draw an overhead view of the influencing fields.

2. Calculate the average BLoop and angle for

each of the five battery readings.3. Using the averaged data, graph Bloop vs.

tan .

Tangent Galvanometer LabPost Lab4. From your graph

calculate the Earth’s magnetic field. Show your work in the space below

MagneticDeclination

Tangent Galvanometer Lab

Post Lab5. If both of the magnetic fields were the

same strength, how many degrees from due north would the compass deflect? Draw a vector diagram indicating the magnetic fields and the direction the compass would deflect.

Tangent Galvanometer Lab

Post Lab6. An important aspect of the lab was to

align your coils in the north-south direction. How would your results differ if you aligned the coils east-west direction?

Magnetic Domains

What’s required to produce a magnetic field?

MOVING CHARGES Macro and Micro Scales

DefinitionDomainA microscopic magnetic region composed of a group of atoms whose magnetic fields are aligned in a common direction.

Magnetic Force on a Charged Particle

When a charge is placed in a magnetic field, that charge experiences a magnetic force when two conditions exist:

1. The charge is moving relative to the magnetic field.

2. The charge's velocity has a component perpendicular to the direction of the magnetic field.

Magnetic Force on a Charged Particle

3rd Right Hand RuleUsing your right hand:1. Point your index fingers in the direction the

magnetic field, B.2. Point your thumb in the direction of the

charge's velocity, v, (recall conventional current).

3. The magnetic force FB is directed out of the

palm of your hand.

Magnetic Force on a Charged Particle

3rd Right Hand Rule (Optional)Using your right hand:1. Point your index finger in the direction of the

charge's velocity, v, (recall conventional current).

2. Point your middle finger in the direction of the magnetic field, B.

3. Your thumb now points in the direction of the magnetic force, FB.

Magnetic Force on a Charged Particle

3rd Right Hand Rule

Examples

Magnetic Field

UnitReview the equation for electric field.What’s required to produce a magnetic field.Discuss units of Tesla and Gauss.

Force on a Charged Particle in B.Equation

FB = qv X B = qvBsin

ExampleA proton moves straight upward (away from the ground) through a uniform magnetic field that points east to west and has a magnitude of 2.5 T. If the proton moves with a speed of 1.5 X 107 m/s through this field what force (magnitude and direction) will act on it?

Force on a Charged Particle in B.Example ContinuedDraw the subsequent path of the charge particle.

Calculate the proton’s acceleration.

Calculate the radius of its circular path.

What’s its period of revolution?

 

Force on a Current-Carrying WireEquation-Derivation

FB = BILsin

ExampleThe magnetic force on a straight 0.15 m segment of wire carrying a current of 4.5 A is 1.0 N. What is the magnitude of the component of the magnetic field that is perpendicular to the wire?  

Force on a Parallel WiresRelationshipDetermine the relationship between force and direction of current.

Parallel Currents – Attract

Anti-Parallel Currents - Repel

 

MotorsDefinitionConverts electrical energy into mechanical energy.

Let’s analyze the torque produced by the current-carrying armature.

CommutatorSplit RingMaintains unidirectional current flow.

CommutatorSlip RingMaintains bidirectional current flow.

Motor LabPreLabREAD THE DIRECTIONS CAREFULLY!

THINK BEFORE YOU SAND!

Motor LabPost LabReview sanding of armature

GeneratorsDefinitionConverts mechanical energy into electrical energy.

Let’s analyze the current produced by a rotating armature in a magnetic field.

CommutatorSplit RingMaintains unidirectional current flow.

CommutatorSlip RingMaintains bidirectional current flow.

Induced CurrentsTwo Methods1. Moving a wire in a stationary magnetic

field.2. Moving (changing) the magnetic field

about a stationary wire.

Let’s examine the current produced in a closed loop of wire in the presence of a changing magnetic field.

TransformersDefinitionA device the changes the potential difference or current.

Energy is always Conserved.Energy in equals Energy outPower in equals Power out.Equation derivation.

TransformersTypes of Transformers1. Step-Up Transformer

The secondary voltage is greater than the primary voltage.

2. Step-Down TransformerThe primary voltage is greater than the secondary voltage.

What’s the current relationship?

ExampleA transformer is used to convert 120 V to 9.0 V for use in a portable CD player. If the player needs 360 mA to operate, how much current is being supplied to the transformer? Is this a step-up or step-down transformer?