Physics - Magnetism Flashcards
Permanent magnets
Have a North Pole and South Pole ; bar magnet/u-shaped (horseshoe) magnet
When are magnetic forces strongest?
When magnetic poles are close together and get weaker when distance between them increases
How to demonstrate a permanent magnet?
Demonstrate that it REPELS another permanent magnet (not attracts - that could be induced)
North and south-seeking
North Pole is north seeking so it is attracted towards the north geographic Pole (south magnetic Pole) and the south is south-seeking and is attracted toward the geographic South Pole of the earth
If bar magnet is pivoted at its centre
So that it is free to rotate - it will align itself in a north-south direction as shown
Why can iron fillings not be used to identify poles?
They line up with magnetic field lines but we do not know which end of the fillings is north or south ; could see magnetic field pattern but not able to distinguish between the two
Magnetic field lines
Start on north poles and end on south poles
Cannot cross each other
Point in the direction of force that would be exerted on a free North Pole
Are closer together where the field is stronger
What direction to magnetic field lines point in?
I’m the direction for the force that would be exerted if a free North Pole was brought towards it
AWAY from north (repel)
Towards south (attract)
Iron fillings vs plotting compass
Iron fillings show strength of field
Plotting compass shows direction of field
Soft magnetic materials
Easy to magnetise but also easily lose their magnetisation (iron for example)
Hard magnetic materials
Difficult to magnetise but once they are magnetised, they are difficult to demagnetise (steel)
Electromagnets use…
Soft magnetic materials as it gains and loses its magnetisation very quickly when there is a current in the coil it rapidly magnetises them core and this adds with the magnetic effect if the current ; when current is switched off the core loses its magnetisation rapidly and the electromagnetic field switches off
Can hard magnets become demagnetised?
Eventually yes - heat the magnet or hitting it can also cause demagnetisation
Needle of a magnetic compass
Hard magnet (needs to retain its magnetism)
When either Pole of a bar magnet is held close to an unmagnetised magnetic material?
ALWAYS A FORCE OF ATTRACTION ; magnetic field of the bar magnet has induced magnetism in the piece of iron ; induces the opposite pole on the nearest pile of the iron. Induced magnetism is used to make permanent magnets ; if an unmagentised sample of a hard magnetic material is placed in a strong magnetic field, magnetic poles are induced - retains magnetism
Proof to show that electric currents create magnetic field?
Demonstrate by placing a small magnetic compass close to a current carrying conductor and then switching the current on and off ; compass needle will point north when the current is off and deflect from north when the current is on (current has created a magnetic field)
Factors affecting the magnetic field created by an electric current?
Reversing the direction of the current reverses the deflection of the compass needle - direction of magnetic field depends on the direction of the current
Increasing the current increases the deflection of the compass needle - strength of magnetic field depends on the size of the eclectic current
Electric current consists of
Moving electric charges - magnetic field created by those moving charges and not the conducting material (beam of charged particles also works through a vacuum)
Iron fillings around straight wire passing through a white card
Forms concentric rings around the wire - closes together near the wire
Magnetic field pattern around a long straight current-carrying wire
Consists of concentric circles
Become farther apart at greater distances from the wire
Have a direction that can be predicted using a right hand grip
Right hand grip
Thumb - points in direction of conventional current
Fingers curl in derivation of magnetic fields lines
Solenoid
Consists of many narrow coils wound close together - magnetic field created by each of the narrow coils adds together to create a very uniform field through the centre of solenoid
Direction of solenoid magnetic field lines
Pointing away from positive end of battery towards negative end of battery
How to identify which end of solenoid is north or South Pole?
Clockwise = South Pole
Anti-clockwise = North Pole
