Magnetism and electromagnetism

Magnets

• A magnet is any material or object that produces a magnetic field

  

• Field lines go from north to south

• The denser the field lines, the stronger the magnetic field 

  • The closer to the ends of the magnet, the stronger

• Compasses can be used to find magnetic fields

  • They interact with it’s filed and point to the south pole

• Like poles of a magnet repel, and opposite poles attract

  • South attracts north, north repels north

Permanent and induced magnets

• A magnet is any object that produces a magnetic field

• A magnetic material is an object that can be influenced by a magnetic field, and has the potential to become a magnet

  • Nickel, cobalt, iron and steel are all magnetic materials

• Permanent magnets can produce their own magnetic fields all the time

• Induced magnets only have temporary magnetic fields

  • They are created when interacting with a permanent magnet, and they always attract 

• Some magnetic materials lose their magnetic fields slowly

  • Steel

  • Are magnetically hard

• Some magnetic materials lose their magnetic fields quickly

  • Nickel and iron

  • Are magnetically soft

Electromagnetism

• Electric currents produce their own magnetic fields

• A current carrying wire has a magnetic field in concentric fields around it

• The right hand rule shows the direction of the magnetic field relative to the direction of the current

• In a circular coil of wire, there would be a concentric magnetic field around the wire, and a field directly through the centre as well

• A solenoid is a coil of current carrying wire 

  • A field generated acts as a bar magnet, and passes through the centre of the coil

• Solenoids with a current carrying wire make a magnet - electromagnets

• Electromagnets only last as long as current flows 

  • This means they can turn on or off

  • The direction of the magnetic field can change, when the current switches direction

    • The north and south would switch

• A solenoid is weak, but its strength can be increased by:

  • Decrease length (denser coil)

  • Increase the current

  • Increase the number of coils

  • Add an iron core, which acts as an induced magnet and is magnetically soft so doesn’t stay magnetic after

Motor effect

• A current carrying wire in the presence of a magnetic field will experience a force

• The two magnetic fields interact, and a force pushes the wire

  • The full force possible only acts at a 90 degree angle between the two fields

  • There is no force created if the fields go the same direction

• To calculate the direction of the force, we need the magnetic field direction and the current’s direction

  • We use Fleming’s left hand rule

• The strength of the force, when the magnetic fields are at a right angle, is equal to the magnetic flux density x current x length of wire

  • F = B(T) x I(A) x L(m)

    • T equals tesla’s, the unit for magnetic flux density

Electric motors

• The coil of wire will rotate, turning clockwise in the diagram

• Split-ring commutators swaps the positive and negative connections every half turn, so the direction of current swaps, and the force always acts in the same direction

  • This allows the motor to turn in a consistent direction, and not alternate direction every half turn

• Increasing the speed of rotations can be done by:

  • Increasing the current, and therefore force

  • Increasing the magnetic flux density

  • Adding more turns to the coil 

The generator effect

• The generator effect is also called electric magnetic induction

• If you move a wire through a magnetic field, a potential difference is induced

• When the wire stops moving, the potential difference disappears, as there is no change

• The direction of the potential difference swaps each time we change the direction of the wire

• If a circuit was created, the potential difference would generate a current

  • It also works if the magnets are moved up or down

• Changing the size of the induced potential difference (and therefore current):

  • The strength of the magnets

  • Moving the wire or magnets quicker

  • Shaping the wire into a coil (a solenoid) - a larger potential difference

• The movement of a magnetic field generates a current 

  • Changing the direction of the movement, changes the direction of the current

  • Can also be done by flipping the magnet

Alternators and dynamos

• The generator effect is the idea that we can generate an electrical current by moving a wire relative to a magnetic field

  • This is used in generator - either alternators or dynamos

• Dynamos

  • They are used to create a direct potential difference, and current

  • The split ring commutator and brushes allows a constant connection that swaps the direction of the connection every half turn

  • As the coil turns, a magnetic field and hence a voltage and current are induced in the coil

  • The split ring commutators mean the contact swaps every half turn

• Alternators

  • They are used to create alternating current - the faster the coil rotates, the larger the peaks of oscillations, and higher frequency

  • The slip rings and brushes keep a constant connection without changing the direction of current 

  • The coil rotates relative to the magnetic field, which induces a magnetic field in the coil

  • The magnetic field then induces a voltage and current in the coil

  • The slip rings and brushes mean the contacts are always there and don’t swap every half turn

Loudspeakers and microphones

• Loudspeakers and microphones both convert between sound waves and electrical signals

• Loudspeaker convert from electrical signals to sound waves (vibrations), via the motor effect

  • The motor effect generates small movement - vibrations - which produce sound

• The coil of wire and cone are permanently attached, but can move back and forth along the magnet

• An alternating current flows through the wire and create a magnetic field in the coil

  • This magnetic field interacts with the magnetic field of the permanent magnet

  • The interacting fields exert a force on the coil (the motor effect)

  • This causes the speaker cone to move back and forth

    • These vibrations move so fast that they cause pressure variations in the air - sound waves

• Microphones convert from sound waves to electrical signals via the generator effect

  • The generator effect converts small vibrations (sound waves) into electrical signals

• The sound waves move the diaphragm, which is connected to the coil that generates a potential difference, and therefore current, when moving through a magnetic field

• Sound waves hit the diaphragm

  • This causes the diaphragm and coil of wire to move

  • As the wire moves through the magnetic field of the permanent magnet, it will generate a current (generator effect)

• The frequency and amplitude of the sound waves will determine how much the diaphragm will vibrate and determine the frequency and amplitude if the current (A.C)

• Both microphones and loudspeaker use an alternating current

Transformers

• Most electricity in the UK is generated in power stations and transported across the country through transformers

• Step up transformers increase the voltage to 40000v, which decreases the current and minimises energy loss

• Step down transformers decrease the voltage to 230volts and 50 hertz, which is safe for domestic use

• An alternating potential difference is applied across the primary coil

  • This causes a current to flow through the wire, which generates a magnetic field around the coil

  • The direction of the potential difference, current and magnetic field alternate constantly

  • This induces an alternating magnetic field in the iron core

    • A soft magnetic material, so becomes induced only while the magnetic field is present

  • The iron core’s magnetic field induces a potential difference in the secondary coil, and if it is part of a complete circuit, current will flow

• Current is transferred across from the primary coil to the secondary coil

  • This allows us to change the size of the potential difference between the coils by changing the number of coils on either side

• Step up transformers have less coils in the primary coil, so there is a lower potential difference in the primary coil

  • In the secondary coil, there are more coils, which increases the potential difference in the output

• Step down transformers have more coils in the primary coil, so there is a higher potential difference in the primary coil

• In the secondary coil, there are less coils, so a lower potential difference is produced

• V_p/V_s = n_p/n_s

  • Potential difference in the primary coil/ potential difference in the secondary coil = no. of turns in the primary coil/ no. of turns in the secondary coil

• V_pI_p=V_sI_s

  • The potential difference in the primary coil x current in the primary coil = potential difference x current in the secondary coil

  • Is a constant

DONE!!!

QUESTIONS

What is a magnet?

Field lines go from…to…

The denser the field lines, the…

How do compasses help to find magnetic fields?

Like poles… and opposite…

A magnetic material is… 

The magnetic materials are…

Permanent magnets are…

Induced magnets are…

They are created when… and they always attract/repel?

Magnetically hard materials lose magnetic fields… 

For example, …

Magnetically soft materials lose magnetic fields…

For example,... and…

Electric currents produce… 

A current carrying wire has a magnetic field in…

The right hand rule shows…

In a circular coil of wire, there is a… magnetic field and a field …

A solenoid is…

The field generated acts as a… and passes through… 

Solenoids with a current carrying wire makes a…

This is an …

Electromagnetics last as long as…

They can…

The direction of the magnetic field depends on the… 

Solenoids are …

Their strength can be increased by … (4)

A current carrying wire in a magnetic field experiences…

This is because the magnetic fields… 

The full force only happens at…

If in the same direction…

Fleming’s left hand rule is used to…

The strength of the force (at right angles)=... x…x… 

F=

Magnetic flux density is measured in…

In motors, a current carrying wire is placed… which causes a… 

Split-ring commutators… every…

This causes the current to switch every… so the force…

The motor therefore turns in a … direction

The speed of rotation can be increased by… (3)

If you move a wire through a magnetic field,...

When the wire stops moving, the… 

The direction of potential difference changes when… 

If a circuit was created, potential diff would cause…

We can increase the potential difference created by… (3)

The movement of a magnetic field generates a…

Changing the direction of the movement causes…

This can also be done by…

The generator effect is the idea that…

The types of generators are… or …

Dynamos are used to create a… current

Split ring commutators and brushes allow… that ensures direction is…

As the coil turns, a… is induced in the coil so therefore a… and… are induced

Alternators are used to create… current

The faster the coil rotates, the oscillations become… with … frequency

Slip rings and brushes are used to ensure there is… so current is…

The coil rotates relative to the magnetic field, which causes… in the coil

This then induces a … and …

As rotation is constantly in … direction, the coils direction is… every…

This creates a … 

Loudspeakers convert from … to… via the…

The motor effect generates… which produce sound

A current carrying wire and cone are permanently attached and can move back and forth along the…

As an … current flows, a … is generated around the wire

These interacting fields cause a…

The cone then …, which creates sound waves in the air through vibrations

Microphones convert from… to… via the…

The generator effect converts movement into…

The sound waves move the…, which is connected to the coil

This movement generates a… and therefore… when moving through the…

The frequency and amplitude of the sound waves determines … 

Both microphones and loudspeakers use… current

Electricity in the UK is generated in… and transported across the country

Step up transformers increases… to… which decreases… 

This minimises…

Step down transformers decrease… to… 

The frequency is… 

This is used as…

Transformers have… core and a … and… coil

An alternating potential difference is applied across the primary coil

This causes… which generates a… around the coil

The directions of the P.D, current and MF are…

This induces a… in the…

The … core is a … magnetic material so is induced… 

The core’s magnetic field induces a… in the secondary coil which will flow if part of a complete circuit

Step up transformers have … coils in the primary coil, so there is a lower potential difference in the… coil

Step down transformers have… coils in the primary coil, so there is a lower potential difference in the… coil

The voltage x current is a constant on both sides of the core