Magnetism & Elctromagnistim (7) - Pt1 Flashcards
What is a magnetic field (lines)?
- it shows where other magnetic materials would feel a force
- north to south
- the closer the lines the stronger the magnetic field
- the further from the magnetic the weaker the field
- Its strongest at the poles
The force between a magnet and a magnetic field is always ………..
Attractive
Poles that are the same experience what force?
Repulsion
How do compasses do near a magnet?
- they have a small bar magnet in them
- the north of it is attracted to the south pole of the magnet it’s near
- it points in the directions of the magnetic field lines
- you can trace these to see what the magnetic field lines look like
When a compasses is not near a magnetic where is pointing?
-always point north due to the earths own magnetic field (which shows the core of the earth is magnetic)
What’s an induced magnet?
- magnetic materials that turn into magnets when they’re put into magnetic fields
- the force between induced and permeant magnets are always attractive
- when the magnetic field is taken away induced magnets lose there magnetism ans stop producing a magnetic field
What’s a permeant magnet?
-they produce they’re own magnetic field
Explain what happens magnetically when a current flows through a wire?
- a magnetic field is created around it
- use the right-hand thumb rule to know the direction of the field
- changing the direction of the current changes the direction of the magnetic field
- the strength of magnetic field changes with the current and the distance from the wire
What’s a solenoid?
-a coil of wire
How can you increase the magnetic field of a wire?
By wrapping the wire into a solenoid
Why do solenoid produce a stronger magnetic field than a wire?
- the field lines around it line up
- so this results with lots of field lines pointing in the same direction and being very close together (the closer the stronger)
- the magnetic field is strong and uniform
- it’s like the magnetic field of a bar magnet
What’s an electromagnet?and why is it done?
- a solenoid with an iron core (a magnet whose magnetic field can be switch on or off by an electric current)
- putting a black if iron in a solenoid increases the field strength. The iron core becomes induced when there’s a current flowing through
Other than in circuits what can electromagnets be used for?
- in scrap yards or in some cranes to attack and pick up things made to, magnetic materials like iron
- using an electromagnet means you can switch it on when you want to pick stuff up and off when ups want to drop then
What happens when you place a wire with a current flowing through it in a magnetic field?
- it causes the magnet and the wire to exert a force on each other
- called the motor effect that can cause the wire to move
The wire has to be at …….. to the magnetic field for it to have the greatest motor effect.
90 degrees
What increases the force that moves the wire?
- the strength of the magnetic field
- the amount of current passing through
What’s the magnetic flux density?
It’s how many field (flux) lines there are in a region which the strength of the magnetic field
When a current is at 90 degrees to the magnetic field it’s in how do you find the force acting on the wire?
Force (N) = magnetic flux density (T: Tesla) x current (A) x length (m)
F = BIL
Explain Flemings left hand rule
- shows which direction the wire moves in
- thuMb Motion (force)
- First finger Field
- seCond finger Current
What direction does the current flow?
From positive to negative
Explain how a basic dc motor works?
- forces act on both sides of the coil-wire in opposite directions
- because the coil is on a spindle and the forces move it, it rotates
- the split-ring commutator swaps the contacts every half turn to keeping it rotating in the same direction
How can you change the direction of a motor?
- by swapping the polarity if the dc supply (reversing the current)
- swapping the magnetic poles (reversing the field)
How do loudspeakers (headphones) work?
- an ac is sent to a coil of wire attached to the base of a paper cone
- the coil surrounds 1 pole of the magnet and is surrounded by the other pole, the force on the coil (causes the cone to move)
- when the current reverses makes the force acts in the opposite direction causing the cone to move in the opposite direction as well
- vibrations caused by the cone create variations in pressure that cause a sound wave
What’s the generator effect?
-the induction of a potential difference (and current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field
How is the generator effect created?
- by moving a magnet in a coil of wire
- or moving a wire (conductor) in a magnetic field
The ………… current always opposes the ………… that made it
Induced
Change
How can you change the size of the induced potential differences?
- increasing the speed of the movement-cutting more magnetic field lines in a given time
- increasing the strength of the magnetic field (so more field lines can be cut)
How do alternators generate an alternating current?
- generators rotate a coil in a magnetic field (or a magnet in a coil)
- as the coil (/magnet) spins, a Current is induced in the coil. The current changes ever half turn
- it has slip rings and brushes so the contacts don’t swap every half turn
How do Dynamos work?
- they work in the same way as alternators except
- that they have a split-ring commutator instead of slip rings
- this swaps the connection every half turn to keep the current flowing in the same direction
- dynamos generate direct currents
What do you use to see the generated pd in generators and microphones?
- an oscilloscope
- it shows how the pd generated in the coil changes over time
