P4 Magnetism Flashcards
What happens when we pass a current through a wire?
A magnetic field is produced around the wire
What happens when you put 2 magnets near each other?
Their fields interact producing a force, attracting or repelling
what happens when we put a wire carrying current near a magnet?
The fields interact to produce a force on the wire
The motor effect
when the magnetic field surrounding the wire interacts with the fixed magnetic field between the poles of the magnet, the resulting attractive or repelling force is strong enough to move the wire
Flemmings left hand grip rule
First finger - field
Second finger - current
Thumb - movement / force
F = BIL
F = Force (N) B = Magnetic field strength / Magnetic flux density (Tesla, T) I = Current in wire (Amps, A) L = length of wire in field (m)
Permanent magnets
do not easily lose their magnetism e.g horseshoe magnet
Temporary magnets
Become magnetised in the presence of a magnetic field e.g paperclips
Electromagnets
Created by a current in a wire - the magnetism disappears when the current is turned off e.g solenoid
Common magnetic materials
Nickel, Iron, Cobalt and steel (‘kNICkerS’)
Field lines go from…
North to south pole. They never cross - closer the lines the stronger the force
Magnetic flux
- lines of a magnetic field are called magnetic flux
- How close they are together is called the magnetic field strength or magnetic flux density
- Symbol is B and is measured in Tesla (T)
Detecting magnetic field
Done with iron filings sprinkled on paper above magnet or done using a plotting compass
Bigger current (I) =
Stronger field (B) = lines closer together
Right hand grip rule
- Determines direction of field. Thumb = current, other fingers = direction of field
- can also find direction of the field from a coil or solenoid.
Plotting compass with wire
Can go through hole in middle of the page and plotting compass placed around it on a flat piece of card.
Wire - Blob
current out of the page
Wire - cross
Current into page
Motors
- Use currents and permanent magnets to create motion
- a current carrying coil is placed in the magnetic field. Flemmings left hand rule tells us that the one side experiences an upward force and the other a downward force. The coil rotates.
- to keep the motor going, a split ring commutator reverses the current direction in the coil every 180°
Electromagnet induction
Moving a wire through a magnetic field induces a pd in the wire. If the wire is connected to a circuit a current flows. It doesn’t matter what moves: the wire through the field or the magnet past a (coil of) wire
To increase the induced pd
- Move the wire faster
- Use a stronger field
- Have more wire (more turns on a coil)
Lenz’z Law
The current generated produces its own magnetic field which is in the opposite direction to the field producing it. We see this effect when a magnet is dropped down a copper pipe (it falls very slowly)
Generators
A coil is turned in a magnetic field and a pd is induced - this can be used to generate an electric current
Alternators
Use slip rings to generate an ac (alternating current)
Dynamos
Use a split ring commutator to produce a direct current
Transformers
Only work with ac’s
Look at picture equation!!
Microphones
- The compressions and rarefaction cause the diaphragm to move
- The diaphragm is connected to a coil of wire, so the coil moves
- Inside the coil is a permanent magnet, so motion of coil induces a pd in the coil
- The induced pd across the ends of the wire is the electrical signal produced
Loudspeaker
- Input wires are connected to a pd varying at the required frequency
- This pd produces a changing current in the coil
- The coil is in a magnetic field, so the coil will move in and out at the input frequency
- The coil is attached to a cone, so the cone moves in and out
- The motion of the cone causes sound waves to be produced in the air around
