P7 Flashcards
Where are the magnetic forces strongest on a magnet?
At the poles
Attraction and repulsion between two magnetic poles are examples of what kind of force?
Non-contact
What is a permanent magnet?
A magnet which produces its own magnetic field
What is an induced magnet?
A material that becomes a magnet when it is placed in a magnetic field
How can you demagnetise a permanent magnet?
By applying force or heating the magnet as this disorganises the particles
What happens when an induced magnet is removed from the magnetic field?
It loses all/most of its magnetism quickly
What is the magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material
What is the force between a magnet and magnetic material always?
one of attraction
What does the strength of the magnet depend on?
The distance from the magnet
(the field is strongest at the poles)
In which direction does the magnetic field always go?
From North to South
How did explorers navigate the Earth using magnetic fields?
- In magnetic compasses there are small bar magnets
- The Earth has a magnetic field and the compass needle points in the direction of the Earth’s magnetic field
- The north of the compass points to the North Pole (Earth’s magnetic South)
- The south of the compass points to the South Pole (Earth’s magnetic North)
How can you check the direction of the magnetic field?
- right hand
- thumbs up pointing in the same direction as the current
- the way your other fingers wrap around (knuckle to tip) is the direction of the magnetic field.
What does the strength of this magnetic field rely on?
- the distance from the wire
- the current through the wire
What is a solenoid?
A coil of current carrying wire
What happens when a current flows through a wire?
A magnetic field is produced perpendicular to the wire in concentric circles around it
How does shaping a wire to form a solenoid increase the strength of the magnetic field?
The field lines around each loop of wire line up with each other and form a stronger magnetic field. The magnetic field is strongest and uniform through the middle of the solenoid
How can you further increase the strength of the magnetic field of a solenoid?
By adding an iron core
What is an electromagnet?
A solenoid with an iron core
What is the motor effect?
When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other
How must the wire be positioned to experience the full force?
At 90 degrees to the field (perpendicular)
What is Fleming’s left hand rule used for?
To determine the direction of the force
- First Finger= magnetic Field
- second finger= Current
- thumb = motion (direction of the Force)
What does the force on a conductor in a magnetic field depend on?
- The magnetic flux density measured in tesla (T)
- The current (amps)
- The length of the wire in metres
What is the equation used to calculate force?
- F= B x I x lF= force (N)B= magnetic flux density (T)I= current (A)l= length (m)
What is the basis of an electric motor?
A coil of wire carrying a current in a magnetic field tends to rotate.
How does a motor work?
- The power supply applies a potential difference across the coil
- A current flows through the coil
- A magnetic field is created around the coil
- The magnetic field interacts with the magnetic field of the permanent magnet
- This creates a force that makes the coil spin
- Every half-turn, the direction is reversed by a split-ring commutator
What is the function of a split-ring commutator?
A split-ring commutator reverses the direction of the current in the wire every half-turn
- it connects to the circuit with electrical contacts called brushes
How can you increase the speed of a motor?
- increase the current
- increase the strength of the magnetic field
- increase the number of coils
Why do loudspeakers/headphones vibrate when AC current flows through the coil?
- the current flowing through the coil creates a magnetic field
- this interacts with the permanent magnetic field and produces a resultant force
- When the direction of the current changes, the direction of the force changes to the opposite direction
- This causes vibrations in the cone which produce pressure changes in the air to produce sound
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 (and current if there’s a complete circuit) across the ends of the conductor
How does induced current work?
An induced current generates a magnetic field that opposes the original change
(either the movement of the conductor or the change in magnetic field)
Explain how alternators work
- generates AC (alternating current)
- As the coil spins, a current is induced in the coil
- every half-turn, the current changes direction
Explain how dynamos work
- generates DC (direct current)
- They use a split-ring commutator as their electrical contact
- This swaps the connection every half-turn to keep current flowing in the same direction
What do microphones use the generator effect to do?
To convert the pressure variations in sound waves into variation in current in electrical circuits
(converts sound waves into electrical signals)
How does a moving-coil microphone work?
- the sound waves cause the diaphragm to oscillate
- This causes the cone to vibrate which interacts with the permanent magnetic field (it moves through)
- As a result, a potential difference is induced
What is a transformer?
A device that changes the potential difference of a device, using the generator effect
How is the effect of an alternating current in one coil in inducing current in another used in transformers?
- The current flowing through a primary coil causes a magnetic field
- this creates an induced magnetic field in the iron
- an AC causes the induced magnetic field to also alternate
- the constantly changing magnetic field cuts through the secondary coil
- this induces an alternating potential difference in the secondary coil (due to the generator effect)
- if the secondary coil is part of a complete circuit a current will flow with some frequency of AC in the primary coil
Why are iron cores used?
Iron is easily magnetised
What does the ratio of potential differences across the primary and secondary of coils of a transformer depend on?
The number of turns on each coil
Vp/Vs = np/ns
Vp/Vs= potential difference (primary and secondary)
np/ns= number of turns on each coil (primary and secondary)
How can you manipulate the amount of potential difference induced?
by varying the number of coils wrapped around
How does potential difference and the number coils differ in step up and step down transformers?
step-up: higher potential difference in the secondary coil, more turns on the secondary coil
- increases V
step-down: higher potential difference in the primary coil, more turns on the primary coil
- decreases V
What would happen if transformers were 100% efficient?
the electrical power input and output would be equal
