Magnetic Field Patterns

A Quick Review of Magnetic Fields

• http://www.youtube.com/watch?v=uj0DFDfQajw

• When an electric current passes along a wire, a magnetic field is set up around the wire.

• For a straight wire, the magnetic field is a circle around the wire.

• We can use our right hand to determine the direction of the magnetic field.

• If a compass is placed next to a current carrying conductor, what would happen if the current were reversed?

• What would happen if you moved the compass away from the current carrying conductor?

• When a long wire is wrapped in a coil it is called a solenoid.

• Prediction: What do you think the magnetic field around a solenoid would look like?

• To break it down we could imagine what the field lines would look like for one loop.

• If we increase the number of loops next to each other we can see that the magnetic field would look like this:

• Describe the properties of the magnetic field in a solenoid:

Conclusion• The magnetic field lines around a wire are circles

centered on the wire.• The magnetic field of a solenoid is uniform inside

the solenoid and like a bar magnet on the outside.

• Increasing the current increases the strength of the magnetic fields; reversing the current reverses the magnetic field lines.

• Homework pg. 203 #1,2

The Motor Effect

• When a current passes through a wire in a magnetic field a force is exerted on the wire. This is called the motor effect.

• We can use our left hand to determine the direct of the Kinetic movement, Field, and Current (KFC).

Kinetic Movement

Field

Current

• Use the left hand rule to determine the direction of motion for the following:

• How could we increase the force?o Increase the currento Use a stronger magnet

• How could we reverse the direction of the force?o Reverse the direction of the fieldo Reverse the direction of the current

• The Force depends on the angle between the wire and the magnetic field lines. The greatest force occurs when the wire is perpendicular to the field.

• The direction of the force is always at right angles to the wire and field lines.

The Motor Effect• A current carrying conductor in a magnetic field

will experience a force (the motor effect).• In the motor effect, the force:

o Is increased if the current or the strength of the magnetic field is increased.

o Is at right angles to the direction of the magnetic field and to the wire.

o Is reversed if the direction of the current or the magnetic field is reversed.

The Electric Motor

• The motor effect can be used to create an electric motor.

• The following is a diagram of a simple motor:

Brushes (metal or graphite)

Split-ring commutator

• What direction is the current flowing in the wire?

Brushes (metal or graphite)

Split-ring commutator

• What is the direction of the magnetic field?

Brushes (metal or graphite)

Split-ring commutator

• Use the left hand rule to determine the way the wire will move. Notice that the current in the wire is flowing in two different directions between the magnets.

Brushes (metal or graphite)

Split-ring commutator

• These opposing forces will cause the wire to turn.

Brushes (metal or graphite)

Split-ring commutator

• This video demonstrates a working motor:http://www.youtube.com/watch?v=yJyVTd_O-vw&feature=related

• Why does the motor need brushes and a split-ring commutator?

Brushes (metal or graphite)

Split-ring commutator

• The split-ring commutator reverses the current round the coil every half-turn. This means the coil is pushed in the same direction every half-turn.

Brushes (metal or graphite)

Split-ring commutator

• What would happen if we increase the current?• What would happen if we reverse the current?

Brushes (metal or graphite)

Split-ring commutator

The Electric Motor• A simple electric motor has a rectangular coil of

wire that spins in a magnetic field when a current passes through a coil.

• The speed of an electric motor is:o Increased if the current is increasedo Reversed if the current is reversed

• Homework: pg. 207 #1,2

Electromagnetic Induction

• For the electric motor, when a current passes through a magnetic field a force is created.

• For a generator, when a wire moves across a magnetic field line a current is induced in the wire.

Motor Field Current Movement

Generator

Field CurrentMovement

• If a wire is moved through a magnetic field we will produce a current.

• Or, if a field is moved through a wire we will produce a current.

Motor Field Current Movement

Generator

Field CurrentMovement

Electromagnetic Induction

• When a wire passes through the lines of a magnetic field, an emf is induced in a wire.

• If the wire is part of a complete circuit, the induced emf causes a current in the circuit.

• The current is increased if the wire moves faster or a stronger magnet is used.

• The direction of an induced current opposes the change that causes it.

• Homework pg.213 #1,2

An AC Generator

AC Generator Graph

The AC Generator• The simple ac generator consists of a coil that

spins in a uniform magnetic field.• The slip rings and brush contacts enable the coil

to stay connected to the circuit.• The peak value of the induced emf is when the

sides of the coil cut directly across the magnetic field lines.

• When the sides of the coil move parallel to the field lines, the induced emf is zero.

Transformers

Not these ones…

These ones.

Remember:• When a current passes through a solenoid, a

magnetic field is created.• The more coils the solenoid has the stronger the

magnetic field.• A changing magnetic field creates an alternating

current. This creates an alternating voltage in the conductor.

• The more turns in a coil, the larger the value of the alternating voltage.

A Basic Transformer• A transformer has two coils of insulated wire, both

wound round the same iron core as shown below:

A Basic Transformer• When an alternating voltage is applied to the

primary coil and magnetic field is created in the iron core.

A Basic Transformer• This magnetic field will induce an alternating

voltage in the secondary coil.

QuestionsQ: Will the alternating voltage in the secondary coil be more or less than the primary coil?A: It will be less because there are less turns in the coil.

QuestionsQ: What would happen if both coils had the same number of turns?A: The primary and secondary voltage would be the same.

QuestionsQ: Why does the primary voltage have to be alternating?A: If it were only in one direction, the magnetic field would not change. Only a changing magnetic field induces a current.

The Transformer Equation

Where:Vp – primary voltageVs – secondary voltageNp – number of primary turnsNs – number of secondary turns

Types of Transformers• A step-up transformer has a secondary voltage

that is greater than the primary.• A step-down transformer has a secondary

voltage that is less than the primary.

Example Problem:• A transformer with 500 turns in the primary coil

and 100 turns in the secondary coil has a secondary voltage of 12V. What is the primary voltage? What type of transformer is this?

Summary• A transformer consists of a primary coil and a

secondary coil wrapped on the same iron core.• Transformers only work using alternating current.• The alternating current in the primary coil creates

an alternating magnetic field in the iron core which induces an alternating voltage in the secondary coil.

• The transformer equation is:

High-voltage transmission of electricity

Electricity is transferred using high-voltage transmission. We will examine why it is more efficient to transfer electricity using a high voltage.

Efficiency• The efficiency of a system is the percentage of

energy entering the system that is turned into useful energy.

• If I put 100J of electric energy into a lamp and 75J of light energy comes out, I would say that my lamp is 75% efficient.

• That means that 25% of the energy has gone somewhere else. Where did it go?

Efficiency• When current is traveling down a conductor why

does it lose energy?• It loses energy because the conductor has

resistance. The current in the conductor causes it to heat up, which means that electrical energy is lost to heat energy.

• How could we transfer the same amount of electrical energy, but reduce the amount of energy lost?

• If we increase the voltage we can reduce the current but transfer the same amount of energy.

Transformers• Transformers are used to step up and down

voltages, so that a lower current can be used to transfer electricity.

• Transformers are almost 100% efficient. That means that almost all the electrical power supplied to the primary coil is supplied to the secondary coil.

• Remember the equation for electrical power?• P = IV• So for a 100% efficient transformer Pp = Ps.

Efficiency Equation

WhereIp – primary currentIs – secondary currentVp – primary voltageVs – secondary voltage

IpVp = IsVs

Summary• Transformers are used to step voltages up or

down.• High voltage transmission of electricity is much

more efficient than transmission at much lower voltages.

• Homework pg. 219 #1,2

Cathode Rays

A discharge tube• Discharge tubes were invented in 1870 by William

Crookes.• He showed that by applying a high voltage across

a low pressure gas, the gas would begin to glow.

A discharge tube• Crookes also found that different gasses produce

a different colour.

A discharge tube• In a discharge tube the negatively charged plate

is called the cathode, and the positively charged plate is called the anode.

Investigating the tube• By placing a paddle wheel inside the tube,

Crookes was able to show that the that the wheel rotates due to radiation in the tube. He was able to show that the radiation began at the cathode. He named them cathode rays.

Investigating the tube• John Thomson showed that cathode rays are

deflected by an electric field. By taking careful measurements he showed that cathode rays are made of negatively charged particles. They became known as electrons.

• Cathode rays are beams of electrons.

Thermionic Emission• An electron tube is a more controlled way to

produce an electron beam.• A filament is heated so that the electrons gain

enough kinetic energy to leave. This is known as thermionic emission.

Thermionic Emission• If we have a small hole in the anode, some of the

electrons will go through the hole. This will create a narrow beam.

Deflecting Cathode Rays

• Cathode rays are negatively charged. They will be deflected by a magnetic field.

• We can use this fact to direct electron beams where we want.

Deflecting Cathode Rays

• Increasing the pd between the anode and cathode increases the speed of electrons.

• Changing the pd in the deflecting coils will change the direction of the electron beam.

Cathode Rays Summary

• Cathode rays are electrons that travel towards the anode (positive) after being emitted from the cathode (negative).

• Thermionic emission is the emission of electrons from a heated filament.

• A beam of electrons can be produced by attracting electrons from a heated filament towards a positive electrode (anode) in a vacuum tube.

• Cathode rays are deflected by electric and magnetic fields.

• Homework pg. 209 #1,2

The Cathode Ray

Oscilloscope

Using electric fields to deflect cathode rays

• Cathode rays are beams of electrons. That means they are negatively charge.

• If they pass between a positive and negative plate they will be attracted towards the positive plate.

Oscilloscopes• By using a horizontal and vertical electric field the

electrons can be deflected in any direction.• The bigger the pd between the plates, the larger

the deflection.• If the pd is reversed, the direction of deflection

will be reversed.

Oscilloscopes

We can use cathode rays to create way

paterns

What are they used for?

• They provide a way to visualize electrical voltage.• The voltage can be compared to a change in time

or with other voltages.• They can also show the time between two events,

even if those events occur very close together.

Oscilloscope Summary

• Cathode rays passing between oppositely charged plates are deflected towards the positive plate.

• The deflection is increased if the pd is increased.• In the cathode ray oscilloscope, a narrow beam of

electrons is deflected by two pairs of deflecting plates.

• Homework pg. 211 #1