Physics - Magnetism

Question 1

Permanent magnets

Answer 1

Have a North Pole and South Pole ; bar magnet/u-shaped (horseshoe) magnet

Question 2

When are magnetic forces strongest?

Answer 2

When magnetic poles are close together and get weaker when distance between them increases

Question 3

How to demonstrate a permanent magnet?

Answer 3

Demonstrate that it REPELS another permanent magnet (not attracts - that could be induced)

Question 4

North and south-seeking

Answer 4

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

Question 5

If bar magnet is pivoted at its centre

Answer 5

So that it is free to rotate - it will align itself in a north-south direction as shown

Question 6

Why can iron fillings not be used to identify poles?

Answer 6

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

Question 7

Magnetic field lines

Answer 7

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

Question 8

What direction to magnetic field lines point in?

Answer 8

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)

Question 9

Iron fillings vs plotting compass

Answer 9

Iron fillings show strength of field
Plotting compass shows direction of field

Question 10

Soft magnetic materials

Answer 10

Easy to magnetise but also easily lose their magnetisation (iron for example)

Question 11

Hard magnetic materials

Answer 11

Difficult to magnetise but once they are magnetised, they are difficult to demagnetise (steel)

Question 12

Electromagnets use…

Answer 12

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

Question 13

Can hard magnets become demagnetised?

Answer 13

Eventually yes - heat the magnet or hitting it can also cause demagnetisation

Question 14

Needle of a magnetic compass

Answer 14

Hard magnet (needs to retain its magnetism)

Question 15

When either Pole of a bar magnet is held close to an unmagnetised magnetic material?

Answer 15

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

Question 16

Proof to show that electric currents create magnetic field?

Answer 16

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)

Question 17

Factors affecting the magnetic field created by an electric current?

Answer 17

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

Question 18

Electric current consists of

Answer 18

Moving electric charges - magnetic field created by those moving charges and not the conducting material (beam of charged particles also works through a vacuum)

Question 19

Iron fillings around straight wire passing through a white card

Answer 19

Forms concentric rings around the wire - closes together near the wire

Question 20

Magnetic field pattern around a long straight current-carrying wire

Answer 20

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

Question 21

Right hand grip

Answer 21

Thumb - points in direction of conventional current
Fingers curl in derivation of magnetic fields lines

Question 22

Solenoid

Answer 22

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

Question 23

Direction of solenoid magnetic field lines

Answer 23

Pointing away from positive end of battery towards negative end of battery

Question 24

How to identify which end of solenoid is north or South Pole?

Answer 24

Clockwise = South Pole
Anti-clockwise = North Pole

Question 25

What 3 factors does strength of magnetic field around a wire depend on?

Answer 25

Current in the wire
Distance from the wire
Medium surrounding the wire - magnetic media increases the field strength

Question 26

Effect of iron on magnetic field strength

Answer 26

Iron is ferromagnetic : each iron atom acts like a tiny bar magnet being north at one end and south at the other ; atomic magnets line up with external field to create a much stronger resultant field - iron cores are often used in electromagnetic devices such as motors, generators and transformers

Question 27

Strength of magnetic field in solenoid increased

Answer 27

Increasing number of turns in the same length of solenoid
Using a soft core inside the coil
Increasing current

Question 28

Electromagnet vs permanent magnet (action as a magnet)

Answer 28

Electromagnet - can be switched on/off
Permanent - continuous

Question 29

Strength of magnetic field created

Answer 29

Electromagnet - can vary
Permanent magnet - constant but may decay

Question 30

Polarity of electromagnet vs permanent

Answer 30

Electromagnet can be reversed by reversing current direction
Permanent magnet - constant

Question 31

Materials used for electromagnet and permanent magnet

Answer 31

Core made from soft magnetic materials so that it can magnetise and demagnetise
Magnet made from hard magnetic material - remains magnetised

Question 32

If heated above what temperature do magnets lose their magnetisation?

Answer 32

The Curie temperature

Question 33

How hard is it to magnetise hard magnetic materials?

Answer 33

Very difficult

Question 34

Electromagnets current through wire?

Answer 34

For normal conducting wires increasing the current may cause the insulation to melt - strongest electromagnets avoid this problem by using superconducting coils BUT only become like thus when cooled to extremely low temperatures using liquid helium