Magnetism and Electromagnetism - Y11 Flashcards
What do all magnets have?
A north pole and south pole
Where are the magnetic forces strongest?
At the poles of a magnet
What happens when you bring 2 magnets close together? Explain more
They exert a force on each other
2 like poles = repel
2 opposite poles = attract
Both non-contact forces
Whats a permanent magnet?
Produces its own magnetic field eg bar magnet
What happens when you bring a permanent magnet close to another permanent magnet?
Attract or repel each other, depending on the direction
What is an induced magnet?
Object that becomes a magnet when it is placed in a magnetic field
What is the force between permanent and induced magnets?
Always attractive
What happens when you take away the magnetic field from induced magnets?
Quickly lose their magnetism or most of it and stop producing a magnetic field
Whats a magnetic field?
Region where other magnets or magnetic materials experience a force
4 types of magnetic material, what can they be made into
Iron
Steel
Cobalt
Nickel
They can be made in a permanent or induced magnet
How can you find the direction of a magnetic field?
Compass, it points in the direction of the magnetic field its in
Why do compasses always point north?
Earth generates its own magnetic field. which shows the inside (core) of the Earth must be magnetic
What is inside every compass?
A tiny bar magnet.
What direction do the lines go on magnetic field lines?
North to south
What does it mean if the lines are closer to each other on magnetic field lines?
Stronger the magnetic field
What is created when a current flows through a wire?
Magnetic field - field made up of concentric circles perpendicular to the wire, with the wire in the centre
What happens if you change the driection of the current?
It changes the direction of the magnetic field
What is the right-hand thumb rule?
- Hold your right thumb up
- Thumb = direction of current
- Fingers = direction of magnetic field
- Flip you hand if the current is going downwards
What changes the strength of the magnetic field in a wire?
- Current
- Distance from wire
How can you increase the strength of a magnetic field that a wire produces?
Wrapping the wire into a coil called a solenoid
What pole is it if the arrowns are pointing inwards?
South
Describe the MF in a solenoid
Strong and uniform (same strength/direction at every point in that region)
Ways to increase the strength of a solenoid
- Increase the current that flows through the solenoid
- Increase no of coils
- Decrease the length of coils (no of turns is the same)
- Add an iron core inside the soledoid (become an induced magnet when solenoid is switched on, iron is a soft, magnetic material)
What is the MF around a solenoid similar shape to?
MF of a bar magnet
Uses of electromagnets
- Some cranes - to attract and pick up things made from magnetic materials in scrap yards - magnet can be switched on and off to pick and drop
- Used within other circuits to act as switches -
Why are electromagnets useful?
You can change the strength of the MF by changing size of the current + its quick to switch on and off
What is the motor effect?
- When a current-carrying wire (or any conductor) it put between magnetic poles
- Magnetic field around wire interacts with magnetic field it has been placed in
- Causes the magnet and the conductor to exert a force on each other
- Causes wire to move (upwards?)
Formula for force from motor effect. What is it only used for?
F = BIl
Force = Magnetic flux density (T) X Current (A) X length (m)
Applies to a wire which is at right angles to the MF
tesla, newtons, amperes, metres
What is the magnetic flux density?
Measure of the strength of the MF/ how many field (flux) lines there are in a region
What 3 things affect the force acting on a conductor in a MF
- Magnetic flux density
- Size of the current through the conductor
- Length of the conductor thats in the magnetic field
Flemings left hand rule
- 1st finger = direction of field
- 2nd finger = direction of the current
- Thumb - direction of mvoement
How does an electric motor work?
- A wire with a current in placed in a magnetic field
- Wire experiences a force on the right and left sides
- Using Flemings left hand rule, you see that the force on the left acts upwards, the force on the right acts downwards
- Moments on both sides = loop will rotate in clockwise direction
- Stops at 90
- Switch direction of current when loop passes 90 by adding split-ring commutator
- Commutators swaps the contacts to keep coil rotating in the same direction
How to fix an electric motor so it turns 360?
- Switch direction of current when loop passes 90 by adding split-ring commutator
2.
What is a split ring commutator?
- Split metal ring
- Connected to conducting brushes
- Brushes allow current to pass onto the ring
What is an electromagnet?
A solenoid with an iron core
How do headphones work?
- Circular south pole magnet surrounded by north pole magnet = m.field
- Paper cone over south pole wrapped around metal wires
- Power on = current in wires = m.field = electromagnetism
- 2 m.field interaction = motor effect = force
- Force causes paper cone and coil to move up and down
- Causes vibrations in the air as comrpessions and rarefractions (sound)
- If you change the frequency of the current, it wil alter the sound wave produced
When do we only see the generator effect?
If the wire passes through the magnetic field
Generator effect
When is the induced potential difference and current larger?
- If we use a stronger m.field
- if we move the wire more rapidly
- if we shape the wire into a coil (greater No of turns on coil, greater the induced pd and current)
- What happens when you have magnets with a MF between them and take a wire and move it through the m field?
- What if you stop moving it?
- What happens to the direction of the pd every time you change the direction?
- It will induce a potential diff in the wire
- The pd disappears because its the change the m field the wire experiences that creates the pd
- Direction of pd swaps each time ( plus and minus swap each time)
Generator effect
What if you joined the 2 ends to create a complete circuit between the m field?
- Induces pf would generate a current
- Electrons able to flow around the ciruit
Generator effect
What would change the direction of the current in a complete circuit if a magnet is entering the coil?
Swapping poles of the magnet (turning it around)
What do the commutators do in an alternator?
allow the current to pass out of the coil
What do split rings help with in alternators?
Prevent wires getting tangled
What causes a magnet to become demagnetised?
- Heating it
- Iron atoms vibrate, certain temp, magnetic field stops being aligned in a certain direction
- Position magnet in a solenoid and pass ac current - current causes MF to constantly change direction, domains become disorganised
What do magnetic field lines show
- strength at a particular point
- direction that a small compass would point at that point
- whether 2 magnets would repel or attract
What kind of force is between a magent and a magnetic matieral?
always attraction
What does a magnetic compass have?
small bar magnet
Does the Earth have its own magnetic field?
Yes
What does the strength of a mf depend on?
- current through the wire
- distance from wire
if you shape it into a solenoid, it increases mf
How does a microphone work?
- Sound waves cause diaphragm to vibrate
- diaphragm causes coil/wire to vibrate
- coil/wire moves through the magnetic field
- a potential difference is induced - generator effect
Simple electric motor
When there is a current in the coil, the coil rotates continously. Why?
- current creates a magnetic field, sides of the coil experience a force
- magnetic fields interact causing coil to rotate/ moments cause coil to rotate
- each half-revolution, the halves of the commutator swap from one brush to the other
- keeping forces in the same direction
Function of a microphone
to convert the pressure variations in sound waves into variations in current
Explain how a moving-coil loudspeaker produces a sound wave
- current creates a magnetic field around the coil
- interacts which permanent magnetic field
- producing a force causing cone to move - MOTOR EFFECT
- when the direction of the current reverses BECAUSE ITS ALTERNATING CURRENT, the direction of the force reverses, producing a sound wave
What effect happens in the loudspeaker
motor effect - electrical energy to mechanical energy
Function of a loudspeaker
convert electrical energy into sound energy
micrphone does the opp
What affect happens in microphones?
generator effect - sound to electric energy
How can you make the electric motor spin faster?
- Increase number of turns on coil
- Increasing current or pd
- Increasing strength of magnet
- putting poles of the magnet nearer to each other
What does a split ring commutator do in an electric motor?
swaps positive and negative connections every half turn so that direction of current swaps every half turn so forces acting on the coil will always be in the same direction - 360
In order for the motor effect to take place on a wire, what does it have to be?
at a right angle ofr FULL FORCE
Alternators vs dynamos
Both are generators of electricity
Alternators
* Slip rings and brushes
* Therefore produce AC
Dyanmos
* Have split ring commutator
* therefore direct current
How do alternators work?
- Coil of wire rotates between magnets
- this induces a MF in the coil - then a voltage and current int he coil
- slip rings and brysges mean that the contacts DONT swap every half turn
- this means they produce an AC pd and current
- As coil rotates faster, peaks of oscillations get larger and more freqeunt
How do dynamos work?
- As coil of wire spins between magnets, a MF and therefore a voltage and current induced into a coil
- Split ring commutator - contacts swap every half turn
- this means they produce a direct pd so direct current
- as coil rotates faster. peaks of oscillations get larger and more freqeunt
For both microphones and loudspeakers, the electrical signals involved are …
AC or alternating voltage
Step up vs step down
- step up comes first - increases voltage which decreases current
- step down - last decreases voltage increases current
How does a transformer work?
- Alternating pd applied to primary coil
- Creates an Alternating m field in iron core
- induces an alternating pd in the secondary coil
structure of a transformer
- primary coil
- iron core
- secondary coil
In a transformer, why cant the electricity just conduct straight across from primary to secondary
- wires insulated with plastic
- cant pass to the iron core from wire
Coils in primary vs secondary
Step up - more in secondary
step down - more in primary
for transformers to work, current in primary coil must be?
alternating
