P4 Flashcards
What is a force?
A push/pull
How does the distance affect force?
All three get smaller as you move further away from the mass/magnet/charge
Describe magnet
An object with a magnetic field
Exert a force on a magnetic material
How do field lines reflect the magnetic strength?
The more denser the field lines are , the stronger the magnetic field
What are permanent magnets?
Always has a magnetic field
Attract/repel other permanent magnetic and also non magnets
How do permanent magnets exert a force on non-magnet?
When brought close , non magnets have magnetism induced in them.
What direction do magnetic field lines go in?
From North Pole to South Pole
Describe the 🌎’s magnetic field
The geographic North Pole is the magnetic South Pole.
Because the magnetic north Pole points to the geographic south Pole
What happens when two magnetic fields interact?
The lines will best around
Creating a high density of field lines between the two magnets, however weak in other places (weak attraction)
What happens when current flows in a wire?
A magnetic field is generated around it
What is a magnetic field?
A region of space within which a magnet experiences a magnetic force of attraction or repulsion.
Describe the look of magnetic fields exist around wires?
Concentric rings
The space between the lines increases with increasing distance
How does a stronger current affect the magnetic field?
Stronger magnetic field
the space between the lines are smaller
What is the corkscrew rule?
RIGHT HAND
Thumb = direction of current
Fingers= direction of magnetic field
What is a solenoid?
A device that increases the strength of the magnetic field around a current carrying wire
How do you make a solenoid?
A wire wrapped many times around a metal that can be magnetised
*not permanent magnets
What is an electromagnet?
A magnet which runs on electricity, the field can be switched on and off
What effects the strength of a solenoid?
🧲coils in the wire
🧲size of current
🧲material of the core inside the wire
How would you show there is a field around a solenoid?
Use iron fillings
What is the relationship between the distance and the strength of a wire?
inversely proportional
The strength is the square of the distance
Eg: double distance , strength decreases by a factor of 4
What is magnetic flux density?
The number of lines of force passing though a unit of area of material
Measured in Tesla’s
Newton per amp meter
What does a circle with an x mean ?
The arrow is going into the page
What does a circle with a point mean?
The arrow is coming towards me
How do you calculate the force on the conductor?
F = BIL
F= force (N)
B= magnetic flux density (T)
I= current (A)
L= length of conductor (m)
What does Flemings left hand rule state ?
The direction of force exerted is always perpendicular to both the direction of the magnetic field and the direction of the current in the wire.
How do you use Fleming’s left hand rule?
Thumb =force
First finger= field
Second finger =current
FFC
What does an electric motor do?
Uses the motor effect to transfer electric energy to kinetic energy
What does an electric motor consist of?
A loop of wire , with a current in a magnetic field
Why must the current be reversed every half turn?
If the current was not reversed , the direction would be (force).
Causing it to spin anti clockwise and then clockwise,
Shaking and flopping -not turning
What does a split ring commutator do?
Reverse the direction of the current every half turn.
Allowing the coil to rotate in the same direction
Why does increasing the current increase the force ?
( motor effect )
1.Magnetic field lines exist around a wire with a current
2. The larger the current the greater magnetic fie,s strength in the interaction between the wire and permanent magnet
Why would some motors use electromagnets rather than permanent?
Electromagnets produce stronger fields
Describe the motor effect :
🔘a looped wire carrying current is placed within a magnetic field
🔘therefore the wire experiences a force, causing it to turn
🔘to make it spin the same direction a split ring commutator is required to reverse the current every half turn
🔘this allows the loop to spin in one direction
🔘as it spins electrical energy is transferred to kinetic energy
What is an electrostatic field in motion?
A magnetic field
How do you draw a magnetic field around current carying wires?
Use the right hand corkscrew rule.
The direction the thumb points in is the North Pole
How do you draw a magnetic field around a solenoid?
Use the right hand corkscrew rule, but align your fingers with the coils (thumb points to the North Pole).
Draw as a bar magnet
*but the strength of the field lines is now greatest in between the coils
Why does a current carrying wire have a magnetic field?
The electrons within the wire have an electrostatic field, as they move the electro static field is in motion creating a magnetic field
What is a magnetic field?
An electrostatic field in motion
What is the generator effect (electromagnetic induction)?
When a conductor cuts the magnetic field lines of a magnet, a potential difference is induced across the ends of the conductor
Describe the generator effect:
🟣wire with no current , within a magnetic field is experiencing a force
🟣this force is perpendicular to the direction of magnetic field
🟣as the wire cuts field lines, electrons are accelerated inducing a p.d
🟣this creates a current
What happens when the motor and generator effect take place together?
The force and current produced by the motor effect counteracts the generator effect.(law of conservation of energy)
How do you increase the size of the generated p.d?
F= increase the velocity/accelerate the wire
B= increase the magnetic field strength (bring closer/new magnets
L= increase the coils on the wire
What happens when you reverse the direction of the wire in the generator effect?
The direction of p.d is reversed
(Negative p.d)
How does a stronger magnetic field produce a larger current in the generator effect?
The stronger the magnetic field, the more dense the field lines are , the rate at which magnetic field lines are cut is faster, larger current
State Lenz’s Law
The direction of the induced p.d is such that it opposes the change producing it
How can you use the generator effect to slow down a falling lift (no cable) ?
🔴a lift with permanent magnets, inside a metal tunnel
🔴as the loft falls , magnets move past the metal surfaces of the tunnel
🔴the lifts (metal surfaces), will cut the magnetic field lines , inducing a p.d and current within
🔴the current carrying surface will interact with the magnetic field of the tunnel giving rise to a force in the opposing direction (motor effect)
🔴this force will counteract the weight force of the falling lift , causing it to fall slower🔴
What does a voltage time graph look like for a generator?
A sin graph.
*the highest peaks are for when the field lines were cut and the closest angle (90°)
Describe the alternator
*generator affect
Produced a.c (magnetic field lines as cut from above and below
Uses a slip ring(doesn’t change direction of the current)
Describe a dynamo
*generator effect
Produces d.c
Split ring (reverses the current every half turn)
How do you increase the induced p.d of an alternator?
🟣rotate coil faster (more field lines cut per second)
🟣stronger magnetic field (field lines closer together, so more cut)
🟣more coils
* doubling frequency
Compare dynamos and alternators
What are transformers?
Devices that use a.c in the primary circuit to induce an a.c in the secondary current
How do transformers work?
How do transformers work?
Describe the difference between step up and step down transformers
What is the transformer equation?
How do microphones work?
🟢generator effect
🟢sound waves hit diaphragm, causing it to move forwards and backwards
🟢so the coil attached also moves backwards and forwards
🟢the coil moves inside a magnetic field , so cuts field lines perpendicularly
🟢a p.d is now induced within coil, producing an electrical signal fed to a speaker
How do loudspeakers work?
🟢a current carrying wire, enters a magnetic field . MOTOR EFFECT. This causes the wire to experience a force
🟢as the coil moves forwards and backwards, the diaphragm moves forwards and backwards
🟢therefore creating sound waves , at the same frequency diaphragm vibrates
Describe the circular magnet
In loudspeakers,
The charge
Draw the magnetic field around a solenoid
Describe electromagnet induction:
A chaos in the magnetic field around a conductor can give rise to an induced potential difference across it’s ends , which could drive a current , generating a magnetic field that would pose the original change
What is happening in the photo
Moving magnet = changing magnetic field
Induces a p.d in coils
Magnetic field of coil(wires) is produced due to current
The North Pole will be generated on left hand side to oppose the original magnetic field (Lenz’ law)
Repelling
What is happening in the photo
What are some ways to produce a changing magnetic field ?
How do transformers work?
- Alternating p.d across the primary coil produces an alternating current in the primary coil
- An alternating current produces in the primary coil produces a magnetic field in the iron core that is always changing
- This induced a changing p.d in the secondary coil
How do dynamic microphones work?
Sound waves are pressure waves.
Areas of high pressure (compression ) push the diagram in
Areas of low pressure (rarefactions ) pull diagram in.
Coil moves in and out in magnetic field, inducing p.d across ends of wire
