An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive

An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive charge and the paint droplets are given a negative charge. The paint droplets experience an attractive force as soon as they are released from the sprayer. The paint droplets are originally sprayed at a distance of 30 cm from the car body, as shown below.

Which of the following changes will cause the largest increase in the attractive force on the paint droplets? A.The charge on the car body is doubled. B.The charge on the paint droplets is doubled. C. The distance between the paint droplets and the car body is halved. D. The distance between the paint droplets and the car body is doubled.

The tendency of a stationary object to resist being put into motion is known as

A. acceleration.

B. inertia.

C. weight.

D. velocity.

In a copper wire, a temperature increase is the result of which of the following?

In a copper wire, a temperature increase is the result of which of the following?

A. an increase in the size of the copper particles B. a decrease in the mass of the copper particles C. an increase in the motion of the copper

particles D. a decrease in the distance between the copper

particles

Two oppositely charged objects are separated by a small distance. The objects are then moved three times farther apart from each other.

Which of the following statements best describes what happens to the electrical force between the objects?

A. The force of attraction increases. B. The force of attraction decreases. C. The force of attraction becomes

zero. D. The force of attraction stays the

same.

Draw a circuit with a battery, a switch and a bulb and resistor in series.

Draw a circuit with a battery, a switch and a bulb and resistor in parallel.

v

A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 mA (milliAmps). Determine the new current if the voltage of the power supply was ...a. ... increased by a factor of 2 and the resistance was held constant. b. ... increased by a factor of 3 and the resistance was held constant.c. ... decreased by a factor of 2 and the resistance was held constant.d. ... held constant and the resistance was increased by a factor of 2.e. ... held constant and the resistance was increased by a factor of 4.f. ... held constant and the resistance was decreased by a factor of 2.g. ... increased by a factor of 2 and the resistance was increased by a factor of 2.h. ... increased by a factor of 3 and the resistance was decreased by a factor of 2.i. ... decreased by a factor of 2 and the resistance was increased by a factor of 2

Focus1. How is a positive ion made?2. What type of current is made with a generator?

Battery?3. What happens to the current if the voltage is

doubled?4. What happens to the current if the resistance is

doubled?5. What law is used to answer questions 3 and 4?6. What is a short circuit?7. Why would it not be a good idea to replace a

circuit breaker which is rated for 20 amps with one rated for 30amps?

Magnetic Forces

• Magnetic forces, like electrical forces, are forces that act at a distance. (Objects do not have to be in contact.)

• Like poles repel each other; opposite poles attract.

Magnetic field –

Region where a magnetic influence (force) can be felt.

Lines are drawn to represent the strength and direction of the field. Field is represented from

N to S.

FOCUS

Select your answer a or b and explain why.

What can one do to strengthen the force from a bar magnet?

What would happen if you broke a bar magnet in half?

Learning Goals

• SW describe the domain theory of magnetism.

• SW recognize the relationship between electricity and magnetism

Magnetic Poles

• North–seeking poles- point northward.

• South-seeking poles – point southward.

Important difference between electric charges and magnetic poles is that electric charges can be isolated, but magnetic poles cannot.

Domain Theory• All magnetism is due to the motion of electrons.• Electrons spin, like the earth, setting up

magnetic poles. (paramagnetism)• Electrons revolve around the nucleus, like the

earth around the sun. (diamagnetism)• Electrons are paired in orbitals. Paired

electrons spin opposite directions- creating opposite magnets. The magnetic poles therefore cancel each other. Unpaired electrons create stronger magnets. (ferromagnetism)

• Magnetic domains- clusters of aligned atoms

Magnetic Saturation – all the domains are aligned

Dropping a magnet can cause the domains to become unaligned.

Concept check 1: REVIEW

1. Must every magnet have a north and south pole? Explain.

2. How can a magnet attract a piece of iron that is not magnetized?

3. Why will a magnet not pick up a penny or a piece of wood?

Concept Check 2: Copy and Complete

1. An iron nail is strongly attracted to the north end, the south end or both ends of a magnet equally strong.

2. Magnetism is due to the motion of the electron as they __________ and ______________.

3. Several nails dangle from the north pole of a magnet. The induced pole on the top of the first nail is______. The induced pole on the bottom of the lowest most nail is _______.

Phet simulations magnetic fields

Learning Goals


Magnetism and Current

• What causes all magnetism?

• What is current?

• Will current cause magnetism?

The basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field. By using this simple principle, you can create all sorts of things, including motors, solenoids, read/write heads for hard drives, and speakers, and so on.

Put the compass on the table and, with the wire near the compass, connect the wire between the positive and negative ends of the battery for a few seconds. What you will notice is that the compass needle swings. Initially, the compass will be pointing toward the Earth's north pole (whatever direction that is for you), as shown in the figure. When you connect the wire to the battery the compass,

needle swings because the needle is itself a small magnet with a north and south end. Being small, it is sensitive to small magnetic fields. Therefore, the compass is affected by the magnetic field created in the wire by the flow of electrons.

The figure shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of the wire itself. A circular magnetic field develops around a current carrying wire, as shown by the circular lines. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire).

You can see that the field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). If you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction.

Magnetic field of a wire

Because the magnetic field around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire.

Focus (copy and complete)1. A moving electron has a ___________ field and

an ___________field around it.

2. Magnetic field lines show the ________ and the __________ of the field.

3. A magnetic field goes from _____ to ____.

4. Paramagnetism is due to _________. Diamagnetism is due to ____________.

Ferromagnetism is due to _____________.

5. The items needed to build an electromagnet include _____, ______, & _______.

Learning Goals


• SW describe how to make an electromagnet

• SW describe how to make electricity from magnetism (electromagnetic induction)

Electromagnets

The strength of an electromagnet can be increased by

•increasing the current in the wire,

•increasing the number of turns of wire, or

•by using a metal core.

Materials list:wire power supply (current)core (anything from air to metal)

Parts of an electric motor

An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion.

Electromagnet in a horseshoe magnet

You can see that this half-turn of motion is simply due to the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. .

The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field just by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet were flipped at precisely the right moment at the end of each half-turn of motion, the electric motor would spin freely.

Armature The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core.

The armature has an axle, and the commutator is attached to the axle. In the diagram you can see three different views of the same armature: front, side and end-on. In the end-on view, the winding is eliminated to make the commutator more obvious. You can see that the commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet.

Brushes and commutator The "flipping the electric field" part ofan electric motor is accomplished by two parts: the commutator and the brushes. The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator

Focus

1. What can be done to strengthen an electromagnet?

2. What are the advantages of using an electromagnet?

3. Your task is to build a model of the earth using a ball of clay and then place a magnet inside the clay to represent the earth’s magnetic field for a science fair project. How would you place the magnet inside the ball of clay (what direction?)?

3.

Generator

Mechanical energy electrical energy

Motor

Electrical energy Mechanical energy

Faraday’s Law

The induced voltage in a coil is proportional to the number of loops multiplied by the rate at which the magnetic field changes within those loops.

Lenz’s Law

The current induced in a wire is such as to oppose the force that induced it.

11/7 Focus (label, copy, and complete)

1. A moving electron has a ___________ field and an ___________field around it.

2. Magnetic field lines show the ________ and the __________ of the field.

3. A magnetic field goes from _____ to ____.

4. Paramagnetism is due to _________. Diamagnetism is due to ____________.

Ferromagnetism is due to _____________.

What is the advantage of using a horseshoe magnet over a bar magnet –assuming the two are identical as bar magnets?

What are the energy conversions for a generator?


What are the energy conversions for a motor?


• Magnetic domains- clusters of aligned atoms

Magnetic Saturation – all the domains are aligned

Dropping a magnet can cause the domains to become unaligned.

Earth

S

N

N

S

Compass

N

S

Focus

• 1. Why would someone want to use an electromagnet?

• 2. How can an electromagnet be strengthened?

• 3. What happens to a magnet if it is dropped and not handled or stored properly?

WiTricity?

• Wireless Electricity

WiTricity Corp. was founded in 2007 to

commercialize an exciting new technology for wireless

electricity invented two years earlier at the

Massachusetts Institute of Technology (MIT).

Learning Goals

• SW describe how to make electricity from magnetism (electromagnetic induction)

• SW understand Lenz’s Law and Faraday’s Law

Phet Simulations

• Review bar magnet and electromagnet

• Discovery activities pickup coil

• transformer

• Faraday’s Law and Lenz’s Law

Faraday’s LawFaraday’s Law

The induced voltage in a coil is The induced voltage in a coil is proportional to the number of loops proportional to the number of loops multiplied by the rate at which the multiplied by the rate at which the magnetic field changes within those magnetic field changes within those loops.loops.

Lenz’s Law


WiTricity?

• Wireless Electricity

SW apply understanding of magnetism to motors

In WiTricity…Why does the current have to be

AC and have to be a higher frequency?Whose law?

Faraday’s LawFaraday’s Law

The induced voltage in a coil is The induced voltage in a coil is proportional to the number of loops proportional to the number of loops

multiplied by the multiplied by the raterate at which the at which the magnetic field changes within those magnetic field changes within those loops.loops.

Lenz’s Law


Magnetism

Magnetism vs. electricityBoth due to electrons

Both force at a distanceBoth like repel unlike attractBUT magnetic poles cannot

be separated

Domain Theory

Paramagnetism- weak attraction – due to spin of electrons

Diamagnetism – weak repulsion- due to revolving electrons

Ferromagnetism – strong-due to unpaired electrons

How can a magnet attract a piece of iron that is not magnetized?

Electromagnets

The strength of an electromagnet can be increased by

•increasing the current in the wire,

•increasing the number of turns of wire, or

•by using a metal core.

Materials list:wire power supply (current)core (anything from air to metal)

Learning GoalSW understand the generator and

the motor effect

Electromagnet in a horseshoe magnet

You can see that this half-turn of motion is simply due to the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. .

Generator


Motor


Documents

An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive