Topic 7 - Magnetism and Electromagnetism Flashcards
Like poles ….., unlike poles ……
Like poles repel
Unlike poles attract
What is the difference between a contact and a non-contact force?
Contact forces need to touch to act, non-contact don’t
Is magnetism a contact or non-contact force?
non-contact
What is a permanent magnet?
A permanent magnet produces its own magnetic field
What is an induced magnet?
An induced magnet becomes a magnet in a magnetic field
What are the 4 magnetic metals?
- nickel
- cobalt
- iron
- steel
(NCIS)
Where is the field of a bar magnet strongest?
at the poles
What happens to the strength of a magnetic field the further away you go from a magnet?
the strength reduces the further away you go
What is a magnetic compass made of?
a magnetic compass is made of a small bar magnet
What direction does a magnetic compass point?
geographic North but magnetic South
What direction do magnetic field lines go in?
from North to South
How can we use a compass to plot the pattern of the magnetic field lines around a magnet?
- place the compass near the magnet on a piece of paper
- mark the direction the compass needle points
- move the compass to many different positions in the magnetic field, marking the needle direction each time
- join the points to show the field lines
What causes a wire to produce a magnetic field?
a current flowing through it
What do we call a long coil of wire?
a solenoid
Describe the magnetic field inside a solenoid
strong and uniform
What kind of magnet has the same magnetic field as a solenoid?
a bar magnet
What is the main advantage of using an electromagnet rather than a permanent magnet?
electromagnets can be switched off
Name 3 ways to make an electromagnet stronger
- increase the current
- increase the number of turns on the coil
- add an iron core
Which rule do we use to draw the direction of the magnetic field around a straight wire?
the right hand rule
What does each finger stand for in Fleming’s left hand rule?
- thumb = force
- first finger = magnetic field
- second finger = current
What is the equation linked to the left hand rule?
F = BIL
force = magnetic flux density x current x length of wire
How does an electric motor work?
- a coil of wire carrying a current in a magnetic field experiences a force that makes it rotate
- a split ring commutator changes the direction every half turn
What is a magnet?
any material/object that produces a magnetic field
What is a magnetic field?
the area around a magnet where it can act on another magnet or magnetic field
How do you plot a magnetic field using iron filings?
- place a piece of paper on top of the magnet
- gently sprinkle iron filings on top of the paper
- carefully tap the paper and iron will settle on the field lines
- doesn’t show the direction of field lines (show doesn’t show which pole is which)
The force between a permanent and induced magnet is always …
attractive
What is an electromagnet?
a solenoid with an iron core
How do you change the direction of a magnetic field in an electromagnet?
change the direction of the current
Describe the right hand rule
- can determine the direction of the magnetic field around a wire
- thumb = direction of current
- fingers = direction of field lines
The motor effect occurs when:
a wire with a current flowing through it is placed in a magnetic field and experiences a force
Why does the motor effect occur?
- two magnetic fields are interacting (one around the wire, one between 2 magnets)
- this means the wire will experience a force
Name some factors that increase the size of the force (motor effect)
- increase current in wire
- use stronger magnets
- place wire at 90° to the direction of the magnetic field lines
What is the assumption we make when we use F = BIL?
that the wire is at 90° to the magnetic field lines
Describe how electric motors work
- current flows through the coil
- this creates a magnetic field around the coil
- the 2 magnetic fields interact
- so a force is exerted on the coil
- and the coil rotates
What does a split ring communtator do?
- swaps positive and negative connections every half turn
- so the direction of the current swaps every half turn
- so the forces acting on the coil will always act in the same direction
How do you increase the speed of the rotations in an electric motor?
- increase the current
- use a stronger magnet
- add more turns to the coil
Simply define the generator effect
motion is being used to create electricity
Simply define the motor effect
electricity is being used to create motion
Describe the generator effect
- coil cuts through the field lines
- a potential difference is induced
- there is a complete circuit so current flows
Name some factors affecting the size of the p.d in a generator
- speed at which the coil moves
- number of turns in coil
- strength of magnetic field
Name the 2 types of generator
alternators and dynamos
Describe the differences between alternators and dynamos
dynamos:
- have split ring commutators
- produce direct current
alternators:
- have slip rings and brushes
- produce alternating current
Describe how alternators work
- a coil of wire rotates
- the coil cuts through the field lines
- a potential difference is induced
- there is a complete circuit so current flows
- every half turn the p.d changes direction so the current changes direction
- the slip rings and brushes mean the contacts don’t swap every half turn
What are the purposes of slip rings in alternators?
- they provide a continuous connect between the coil and the meter
- they make sure the wires don’t get tangled
Describe the current graph for an alternator
a classic wave that goes positive and negative (sine graph like)
Describe how dynamos work
- coil of wire rotates
- the coil cuts through the field lines
- a potential difference is induced
- there is a complete circuit so current flows
- every half turn, the two ends swap from one brush to the other
- so the current doesn’t change direction
What is the purpose of a split ring commutator in a dynamo?
it swaps the contacts each half turn, which means the current stays in the same direction
Describe the current graph for dynamos
always positive - not a wave, looks more like mountains
What is the similarity of loudspeakers and microphones?
they both convert between sound waves and electrical signals
What effect do loudspeakers use?
the motor effect
What is a loudspeaker’s role?
loudspeakers take electrical signals and convert them into vibrations, which generate sound waves
How do loudspeakers work?
- a.c flows through the wire and creates a changing magnetic field in the coil
- the magnetic field of the coil interacts with the magnetic field of the permanent magnet
- the interacting magnetic fields exert a force on the coil
- this force will constantly change direction, making the coil oscillate and causes vibrations, creating sound waves
What effect do microphones use?
the generator effect
What is a microphone’s role?
microphones detect vibrations and convert these into electrical signals
How do microphones work?
- sound waves hit the diaphragm, causing the diaphragm to move
- this causes the coil to move back and forth through the magnetic field of the magnet
- the coil cuts through the field lines, inducing a p.d in the coil
- the p.d is alternating, because coil is constantly changing direction
A basic transformer consists of:
- a primary coil
- a secondary coil
- an iron core
How does a transformer work?
- an alternating p.d is in primary coil - this causes a current (constantly changing direction) - this causes a changing magnetic field
- this induces an alternating magnetic field in the iron core
- this induces a changing magnetic field in secondary coil - cuts through secondary coil and induces a p.d
- if part of a complete circuit, an alternating current will flow
