JOURNAL #15 2/22/12

Use what you learned in yesterday’s lab to predict which picture shows magnetized atoms and which shows non-magnetized atoms.

ELECTROMAGNETISMCh. 24-25

Quiz – Formulas and Vocabulary – Wednesday, 2/29Test – Ch. 24-25 – Friday, March 2

MAGNETS Magnets have been around for more

than two thousand years.

One of the earliest uses for magnets was in navigation by using compasses.

Other uses include electric motors, generators, television sets, computer hard drives, and

high speed trains.

BAR MAGNETS The north and south side represent to two poles

of the bar magnet.

Bar magnets are examples of permanent magnets.

All magnets are polarized, which mean they have two distinct and opposite ends.

Opposite poles attract while similar poles repel. Why?

MAGNETIC FIELDS The reason magnets attract and repel each

other are due to the magnetic fields around them.

Every magnet has a magnetic field around it.

Magnetic field lines are similar to electric field lines.

Field lines start at the north pole and end at the south pole.

Magnetic field strength has units of Teslas (T).

MAGNETIC DOMAINS What happens if you break a magnet in half? The two halves will still be polarized!!! A domain is a group of neighboring atoms

that have the magnetic fields of their electrons aligned in the same direction.

Magnetic materials have their domains aligned in the same direction, while non-magnetic materials have domains in random directions!

Non-magnetic

Magnetic

ELECTROMAGNETISM Electromagnetism describes the

relationship between electricity and magnetism.

We use electromagnets to generate electricity, store memory on our computers, generate pictures on a television screen, diagnose illnesses, and in just about every other aspect of our lives that depends on electricity.

Electromagnetism works on the principle that an electric current through a wire generates a magnetic field.

ELECTROMAGNETS When DC electricity is passed

through a wire, a magnetic field rotates around the wire in a specific direction.

“Right Hand Rule”-If you take your right hand and wrap it around the wire, with your thumb pointing in the direction of the electrical current (positive to negative), then your fingers are pointing in the direction of the magnetic field around the wire.

SOLENOIDS How would we model

the magnetic field in a current loop?

A solenoid is a long coil of wire consisting of many loops.

The more loops a solenoid has, the stronger the magnetic field.

MAGNETISM CONVENTIONS Current:

Current is represented by the letter I Left, right, up, and down are drawn by arrows Into the page or away from you is represented by

Out of the page or towards you is represented by

Magnetic Field: Clockwise and counter-clockwise are drawn by

arrows Into the page or away from you is represented by

Out of the page or towards you is represented by

RIGHT HAND RULE PRACTICE Use the right hand rule to draw the magnetic

field around the wire shown below

I

Current to the right

Magnetic field towards you above the wire and away from you below the wire

RIGHT HAND RULE PRACTICE Use the right hand rule to draw the magnetic

field around the wires shown below

I I

Current out of the page

Counterclockwise magnetic field

Current into the page

Clockwise magnetic field

RIGHT HAND RULE PRACTICE Use the right hand rule to draw the magnetic

field through the wire loops as shown belowI

Current clockwise

Magnetic field towards you outside the loop and away from you inside the loop

HOMEWORK Textbook p.665 #59, 60, 61, 64, 65

JOURNAL #16 2/23/12 Draw the magnetic field around the following

3 wires:

I

A) B) C)

I

I

ELECTROMAGNETIC INDUCTION When a wire moves in a magnetic field, a

force acts on the charges in the wire.

Work is done on the charges, causing the electrons to move.

Magnetic fields can induce a current in a wire!

For EM induction to occur, the wire or the magnetic field needs to be moving or changing in strength.

GALVANOMETERS A galvanometer is a device that uses

electromagnetism to measure very small amounts of current ( ~5 x 10-5 A).

The current through the wire loop creates a magnetic field.

The interaction between the loop’s magnetic field and the permanent magnetic field causes the loop to rotate.

ELECTRIC GENERATORS The electric generator was invented by

Michael Faraday and converts mechanical energy into electrical energy.

Generators utilize a wire loop called an armature that rotates in the magnetic field. As it rotates a current is induced.

Also as the loop rotates, the strength and direction of the current change, producing an alternating current.

DIFFERENCE IN AC/DC CURRENT The difference between AC and DC has to do

with the direction in which the electrons flow.

In DC, the electrons flow steadily in a single direction, or "forward."

In AC, electrons keep switching directions, sometimes going "forward" and then going "backward.”

In the US, electric utilities use a 60-Hz frequency, meaning that the current alternates direction (forward to backward and back to forward) 60 times in one second.

TRANSFORMERS The primary voltage (on the

left) induces a magnetic field in the core, which creates the secondary voltage (on the right).

What makes transformers so useful is that if you change the number of turns from one side to the other, you change the voltage in the wire on the right!

Transformers can change a high voltage to a lower one, or a low voltage to a higher one.

STEP UP TRANSFORMER Step-up transformer converts a low voltage to a

higher one. If you increase the number of turns on the right, the

voltage coming off the transformer will increase in proportion.

The right side has 4 times more turns so the voltage on the right has increased 4 times.

So the voltage has been stepped up by a factor of 4.

STEP DOWN TRANSFORMER Step-down transformer reduces voltage. If you decrease the number of turns on the right,

the voltage coming off the transformer will decrease in proportion.

The right side has 1/5 the number of turns, so the voltage is only 1/5 as large.

So the voltage has been stepped down by 5.

TRANSFORMER MATH The ratio of the number of turns is the same as the

ratio of the voltages.

An ideal transformer, which we will always assume, dissipates no power (waste heat). The power of the primary circuit is equal to the power of the secondary current.

We use this assumption about power to find the current in the secondary circuit.

p

s

p

s

NN

VV

sp PP

p p s sI V I V

FINAL THOUGHTS ON TRANSFORMERS

STEP-UP Transformers STEP-DOWN Transformers

Vp < Vs Vp > Vs

Ip > Is Ip < Is

Np < Ns Np > Ns

WHY TRANSMIT WITH AC VS. DC Advantages to using AC:

By using a step-up transformer to hike the voltage, we are able to lower the current and by lowering current, we are able to use thinner wires.

Ultimately, we are able to transfer power farther (higher voltage travels farther), for less money (cost of wire), with better efficiency (less power lost to heat caused by resistance).

TRANSFORMER EXAMPLE A transformer has 7500 turns on its primary

coil and 125 turns on its secondary coil. The voltage across the primary circuit is 7,200 V. A. What voltage is being applied across the

secondary circuit?B. If the current in the secondary circuit is 36 A,

what is the current in the primary circuit?C. Is this a step-up or a step-down transformer?

JOURNAL #17 2/27/12 The primary coil on a transformer has 100

turns and the secondary coil has 500 turns. The primary voltage is 110V and the current is 1.5 A. What type of transformer is this? What power is being used on the primary coil? What are the resulting voltage and current in the

secondary coil? What is the resulting power on the secondary

coil?

HOMEWORK Textbook p.693 #74, 75, 77, 78

PRE-LAB: ELECTRIC MOTORS Purpose of Lab:

Create a functional motor and modify it’s components to change it’s ability.

Today you will write down the function of each piece of equipment so that you don’t have to ask questions about them tomorrow.

EQUIPMENT

4 D-cell batteries provide potential difference to the circuit containing

the electromagnet

The electromagnet has a sensor on

the end that detects dark or

clear and reverses the

current alternating it’s

polarity

The commutator discs have different numbers of

alternating clear/black edges

The magnets must be arranged in the rotor in a specific pattern in order for the electromagnet to

effectively “push and pull” it continuously

The rotor is freely spinning on ball bearings, but the commutator discs must be firmly attached with the washer so that

they don’t shift

A photogate will be placed in this position so that it

can measure the frequency of color changes. That

information will be used to calculate the rotational speed of the disc/motor

DC MOTORS

WHAT IS THE FUNCTION OF THE SPLIT-RING COMMUTATOR? If you look closely, you will see that it

switches the flow of current half-way through the rotation of the armature.

By doing so, it reverses the magnetic field produced around the armature. This reversal at the exact right moment makes the armature continue to be pushed/pulled by the forces of the magnets.

The armature is a wire loop mounted on an

axle (it can be a single loop, or many repetitive loops).

JOURNAL #18 2/28/12 The ratio of turns on a transformer is 100 on

the primary to 1 on the secondary coil. The primary voltage is 12000V. What type of transformer is this?

If the resistance on the secondary coil is 100-ohms, what current is induced on the coil?

If we consider this to be an ideal transformer, what power is being used on the primary coil? How do you know?