Magnetism

What are the ends of magnets called

Poles ( north and south)

What is the law of magnetism

When two magnets are held close together there will be an attractive or repulsive force between the magnets depending on how they are arranged

What metals are magnetic

• iron

• Cobalt

• Nickel

How do you test if a material is a magnet

• bring the material close to a magnet

• If the material can be repelled by the known magnet then the material itself is a magnet

• If it can only be attracted then it is a magnetic material

What are the types of magnets

• permanent

• Induced

What are permanent magnets

These are magnets made out of permanent magnetic materials like steel. They produce their own magnetic field and they don’t lost their magnetism

What are induced magnets

When a magnetic material is placed in a magnetic field it can temporarily be turned into a magnet. When the material is removed from the field it loses most or all of its magnetism

What is a magnetic field

This is a region around a magnet where a force acts on another magnet or on an magnetic material

What do magnetic field lines represent

• strength

• Direction of a magnetic field

Describe the strength of a magnetic field

• If the magnetic field lines are close together then the magnetic field will be strong

• If the magnetic field lines are far apart then the magnetic field will be weak

What do the magnetic field around a bar magnet look like

How do magnets look when the repel and attract

When can an attractive force be experienced

• between the opposite poles of two different permanent magnets

• A magnet and magnetic material due to induced magnetism

What is a uniform magnetic field

This is one that has the same strength and direction at all points

How do you investigate the shape and direction of a magnetic field

• Place the magnet on top of a piece of paper

• Draw a dot at one end of the magnet (near its corner)

• Place a plotting compass next to the dot, so that one end of the needle of the compass points away from the dot

• Use a pencil to draw a new dot at the other side of the compass needle

• Move the compass so that it points away from the new dot, and repeat the process above

• Keep repeating the previous process until there is a chain of dots going from one end of the magnet to the other

• Then remove the compass, and link the dots using a smooth curve – this will be the magnetic field line

What is the evidence that the earths core is magnetic

On earth with the absence of any magnets a magnetic compass will always point north this is evidence that the earth core is magnetic and created its own magnetic field

Why does the north arrows on a magnetic compass point toward the geographic North Pole

• this is because the geographic North Pole is a magnetic South Pole

• The North Pole of a magnetic compass is attracted to the earths magnetic South Pole

What is a conducting wire

This is any wire that has current flowing through it

What does the magnetic field around a current carrying wire look like

What does a circular field pattern a current carrying wire show

It indicates that the magnetic field around a current carrying wire has no poles

What is the right hand thumb rule

This can be used to work out the direction o of the magnetic field

What is the relationship the direction of current flow in a wire and the direction of the magnetic field

Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

What is the relationship between current and the strength of magnetic field

Increasing the amount of current flowing through the wire will increase the strength of the magnetic field this means field lines will become closer together

What happens when a wire is looped into a coil

The magnetic field lines circle around each part of the coil passing through the center of it

Describe the magnetic field in a solenoid

The magnetic field is strong and uniform

How do you work out the polarity of each end of a solenoid

• To work out the polarity of each end of the solenoid it needs to be viewed from the end

• If the current is travelling around in a clockwise direction then it is the south pole

• If the current is travelling around in an anticlockwise direction then it is the North Pole

  if the current changes direction then the north and south poles will be reversed

What is an electromagnet

This is a solenoid with an iron core

How can you change the strength of an electromagnet

• Increasing the current will increase the magnetic field produced around the electromagnet

• Decreasing the current will decrease the magnetic field produced around the electromagnet

Describe how an electric bell works in an electric bell

• When the button K is pressed:

  • A current passes through the electromagnet E creating a magnetic field

  • This attracted the iron armature A, causing the hammer to strike the bell B

  • The movement of the armature breaks the circuit at T

  • This stops the current, destroying the magnetic field and so the armature returns to its previous position

  • This re-establishes the circuit, and the whole process starts again

What is motor effect

This occurs when a wire with current flowing through it is placed in a magnetic field and experiences force

What is motor effect a result of

• This effect is a result of twointeracting magnetic fields

  • One is produced around the wire due to the current flowing through it

  • The second is the magnetic field into which the wire is placed, for example, between two magnets

  as a result of the interactions of the two magnetic fields the wire experiences a force

What does motor effect on a wire look like

What factors affect force on a wire in a magnetic field

• Increasing the amount of currentflowing through the wire

  • This will increase the magnetic field around the wire

• Using stronger magnets

  • This will increase the magnetic field between the poles of the magnet

• Placing the wire at 90^o to the direction of the magnetic field lines between the poles of the magnet

  • This will result in the maximum interaction between the two magnetic fields

What happens to motor effect if magnetic fields are parallel

There will be no interaction between the two magnetic fields and therefore no force is produced

What determines the direction of the force on a current carrying wire

The directions of the current and the direction of the magnetic field

What factors affect a dc motor

• The speed at which the coil rotates can be increased by:

  • Increasing the current

  • Use a stronger magnet

• The direction of rotation of coil in the d.c. motor can be changed by:

  • Reversing the direction of the current supply

  • Reversing the direction of the magnetic field by reversing the poles of the magnet

• The force supplied by the motor can be increased by:

  • Increasing the current in the coil

  • Increasing the strength of the magnetic field

  • Adding more turns to the coil

How do loudspeakers work due to motor effect

• a loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet

• An alternating current passes through the coil of the loudspeaker

  • This creates a changing magnetic field around the coil

• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing

• The magnetic field produced around the coil interacts with the field from the permanent magnet

• The interacting magnetic fields will exert a force on the coil

• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction

  • This makes the coil oscillate

• The oscillating coil causes the speaker cone to oscillate

  • This makes the air oscillate, creating sound waves

What is the generator effect

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor

What factors affect induced potential difference

• Increasing the speed will increase the rate at which the magnetic field lines are cut

  • This will increase the induced potential difference

• The number of turns on the coils in the wire:

  • Increasing the number of turns on the coils in the wire will increase the potential differenceinduced

    • This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines

• The size of the coils:

  • Increasing the area of the coils will increase the potential difference induced

    • This is because there will be more wire to cut through the magnetic field lines

• The strength of the magnetic field:

  • Increasing the strength of the magnetic field will increase thepotential difference induced

• The orientation of the poles of the magnet

What is lenz law

The direction of an induced potential difference always opposes the change that produces it

What does lenz law mean

• This means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving

Demonstrate and explain lenz law

• If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole

• Explanation

  • Due to the generator effect, a potential difference will be induced in the coil

  • The induced potential difference always opposes the change that produces it

  • The coil will apply a force to oppose the magnet being pushed into the coil

  • Therefore, the end of the coil closest to the magnet will become a north pole

  • This means it will repel the north pole of the magnet

• If a magnet is now pulled away from the coil of wire then the end of the coil closest to the magnet will become a south pole

• Explanation:

  • Due to the generator effect, a potential difference will be induced in the coil

  • The induced potential difference always opposes the change that produces it

  • The coil will apply a force to oppose the magnet being pulled away from the coil

  • Therefore, the end of the coil closest to the magnet will become a south pole

  • This means it will attract the north pole of the magnet

What are the applications of the generator effect

• generate a.c in an alternator

• Generate d.c in a dynamo

What is an alternator

This is a device which converts energy output from motion into an electrical output

What are the components of a simple alternator

permanent magnet	to provide a uniform magnetic field
rotating coil	to cut the magnetic field as it rotates and allow an induced current to flow
slip rings	to allow the alternating current to flow between the coil and the external circuit
carbon brushes	to provide a good electrical connection between the coil and the external circuit

When is maximum p.d in an alternator induced

• the position of the coil is horizontal

• The motion of the coil is perpendicular to the field

When is no pd induced in an alternator

• the position of the coil is vertical

• The motion of the cell is parallel to the field

  because no lines are cut when the coil is moving parallel to the field

What is a dynamo

This is a device which converts an electrical input into motion

What are the components of a simple dynamo

permanent magnet	to provide a uniform magnetic field
rotating coil	to provide the rotation as the current flows through it
split ring commutator	to allow the connection between the coil and the external circuit to change every half turn
carbon brushes	to provide a good electrical connection between the coil and the external circuit

What factors affect an alternator or dynamo output

• increasing the frequency of rotation of the coil

• increasing the number of turns on the coil

• increasing the strength of the magnet

• inserting a soft iron core into the coil

How does generator effect help microphones work

• Microphones convert sound wavesinto electrical current

• A moving coil microphone works using the principles of the generator effect

• When sound waves reach the microphone, the pressure variations cause the diaphragm to vibrate

• This in turn causes the coil to moveback and forth, through the magnetic field produced by the magnet

• As it does so, the coil cuts through the field lines

  • This induces a potential difference in the coil

• The potential difference will be alternating because the coil is continually changing direction due to the vibrations of the diaphragm

What is a transformer

This is a device used to change the value of an alternating potential difference using generator effect

What are the basic parts of a transformer

• a primary coil

• Secondary coil

• Iron core

How does a transformer work

• An alternating current is supplied to the primary coil

• The current is continually changing direction

  • This means it will produce a changing magnetic field around the primary coil

• The iron core is easily magnetised, so the changing magnetic field passes through it

• As a result, there is now a changing magnetic field inside the secondary coil

  • This changing field cuts through the secondary coil and induces a potential difference

• As the magnetic field is continually changing the potential difference induced will be alternating

  • The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil

• If the secondary coil is part of a complete circuit it will cause an alternating current to flow

Describe the turns on the coils of a step up Transformer

• A step-up transformer increases the potential difference of a power source

• A step-up transformer has more turnson the secondary coil than on the primary coil (N_s > N_p)

Describe the turns on the coils of a step down transformer

• A step-down transformer decreasesthe potential difference of a power source

• A step-down transformer has fewer turns on the secondary coil than on the primary coil (N_s < N_p)

What are the roles of transformers

• They are used to increase the potential difference of electricity before it is transmitted across the national grid

• They are used to lower the high voltage electricity used in power lines to the lower voltages used in houses

• They are used in adapters to lower mains voltage to the lower voltages used by many electronic devices

What are the advantages of high voltage transmission

• When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss

• To transmit the same amount of power as the input power the potential difference at which the electricity is transmitted should be increased

  • This will result in a smaller current being transmitted through the power lines

  • This is because P = IV, so if Vincreases, I must decrease to transmit the same power

• A smaller current flowing through the power lines results in less heat being produced in the wire

  • This will reduce the energy loss in the power lines

What are the similarities of step up and step down transformers

• a laminated core

• Core is made from a magnetic material

• 2 coils

• Coils are made from an electrical conductor

• Coils are insulated

• Step up transformer has more turns on secondary coil than primary

• Step down transfer has more turns on primary coil than secondary

Why is direct current induced in the coil of a dynamo when rotated

Why is a dynamo easier to turn when a lamp ( appliance) is disconnected

• after disconnecting there is no induced current

• So no magnetic field produced around coil

• To oppose the movement of the coil

Explain why alternating current is induced in a coil when the handle of alternator is turned