PHYSICS:
MAGNETISM
By Aditi, grade 7
In this chapter, the laerning objectives are:
• The behaviour of magnets
• What's inside a magnet
• The magnetic field
• The link betw een magnetism and electricity
• The electromagnet
Magnetic and non-magnetic
materials
•
Magnetic materials are materials that have magnetic
properties such as the ability to be attracted to a
magnet.
•
Non-magnetic materials cannot be magnetised. But
materials like paper and w ater can let the force of
magnetism pass through them.
The behaviour of magnets
• Magnets attract and repel other magnets. They also attract magnetic materials,
even if they are not magnetised. When a bar magnet is suspended in the air by a
thread, it w ill alw ays align itself in a north -south direction.
The strength of the magnetic force
•
The magnetic force is the strongest at the ends of the bar magnet. The ends of
the bar magnets are called poles. The pole that point tow ards the north direction
w hen the magnet is suspended in the air is called the north pole or the north seeking pole and the pole that faces the south is called the south pole or the
south-facing pole.
Repulsion and attraction
•
The north pole of a bar magnet w ill alw ays attract the south pole of a another bar
magnet w hen the tw o magnets are w ithin a certain distance from each other and
the same goes for the south pole.
•
Having said that, tw o south poles w ill repel each other, and so w ill tw o north
poles. To conclude, different poles attract, similar poles repel.
Magnetic induction
•
When a magnetic material (a material that show s magnetic properties), such as a
steel paper clip, is attracted to the poles of the magnet, It also becomes a
magnet and hence can attract other magnetic materials around it. The steel paper
clip has been made into a magnet by the process called magnetic induction.
•
But the paper clip has not become a permanent magnet. As soon as its moved
aw ay from the magnet, it w ill lose all of its magnitesm.
Inside a magnet
•
M a g ne t i c m a t e r i a l s a r e m a d e f r o m g r o u p s o f p a r t i c l e s t h a t f o r m t i n y r e g i o ns c a l l e d
d o m a i n s , a n d e a c h d o m a i n a c t s l i k e a t i n y m a g n e t . W h e n t h e d o m a i ns i n a m a t e r i a l a r e
a r r a nge d a t r a n d om , T h e m a t e r i a l w il l n o t b e a b l e t o a t t r act o t h e r m a g n e t i c m a t e r i al s, b u t i t
c a n b e a t t r a c t e d t o a m a g n e t . To a d d , m a t e r i a l w i l l n o t a l i gn i t s e l f i n t h e n o r t h - s out h
d i r e c t io n w he n s u s pe nde d i n t h e a i r.
•
T h e s e d o m a i ns c a n h ow e v e r a l i g n t h e m s e l v e s , i n w hi c h a l l t h e i r n o r t h p o l e s w i l l b e f a c i ng
o n e d i r ect i on a n d a l l t h e s o u t h p o l e s w i l l b e f a c i ng t h e o t h e r. W h e n a r r a ng ed l i ke t h i s, t h e
m a t e r i a l a s a w hol e w i l l h a v e a n o r t h a n d s o u t h p o l e . O n c e t h e m a t e r i a l i s i n t h i s c o n d i t i on,
i t h a s b e e n m a g n e t i s e d a n d h e n c e i s n ow a m a g n e t .
Magnetically hard and soft
• Materials like steel are magnetically 'hard' and w hen the domains are
arranged properly, they tend to stay that w ay.
• Materials like iron are magnetically 'soft' and domains soon rotate
again to random positions as soon as it is manually detached from the
magnet, so the material loses its magnetism.
The magnetic field
• The magnetic field is the region around the magnet, in w hich the pull of the
magnetic force acts on the magnetic materials.
• You can show the magnetic field around a magnet using some iron fillings
and a card. First, place the card over a magnet. Then sprinkle some
iron filings over the card. Since each iron filling is really small in mass, it
can be moved by the magnetic force of the magnet if you just gently tap the
card. The iron fillings w ill line up and form a pattern, w hich is know n as
the magnetic field pattern. Each line that the iron fillings form to make
the pattern represents one of the 'magnetic lines of force'.
If you place a compass
near the north pole of a
magnet, it w ill point aw ay
from it and tow ards the
south pole of the magnet.
As unlike poles attract.
This is w hy the arrow s
on the magnetic field lines
point from the magnets
north pole to the magnet's
south pole.
The Earth's magnetic field
• T h e E a r t h' s c o r e , w hi c h i s a t t h e c e n t r e o f t h e e a r t h, i s m a d e u p o f i r o n a n d n i c k e l . T h e
c o r e i s s e p a r a t e d i n t o t w o p a r t s , t h e i n n e r c o r e , w hi c h i s m a d e o f s o l i d m e t a l , a n d t h e o u t e r
c o r e , w hi c h i s m a d e o f l i q ui d m e t a l . S i n c e t h e s e t w o c o r e s s p i n a t d i f f e r e n t s p e e ds w h e n t h e
e a r t h s p i ns , t h e y c r e a t e a m a g n e t i c f i e l d a r o u nd t h e e a r t h.
• T h e e a r t h s p i n s o n i t s a x i s a n d t h e g e o g r a phi c N o r t h a n d S o u t h p o l e s a r e p o s i t i on s
p e r m a n e nt l y a t e i t he r ' e n d ' o f t h i s a x i s . Wi t h i n t h e r a n g e o f t h e g e o g r a ph i c N o r t h p o l e i s t h e
m a g n e t i c n o r t h p o l e , w hi c h c h a n ge s i t s p o s i t i o n v e r y s l i ght l y e v e r y ye a r. B u t w h a t w e k n ow a s
t h e m a g n e t i c n o r t h p o l e , a c t ua l l y i s t h e m a g n e t i c s o u t h p o l e . W h e n a m a g n e t i s s u s p e n de d, i t s
n o r t h p o l e w i l l a l w a ys f a c e t h e m a g n e t i c n o r t h p o l e . B u t w e a l l k n ow t h a t u n l i k e p o l e s a t t r a c t
a n d l i k e p o l e s r e p e l , s o i t w oul dn ’ t m a k e s e n s e i f t h e n o r t h p o l e w ou l d b e a t t r a c t e d t o t h e
m a g n e t i c n o r t h p o l e , a s t h e y a r e s u p p o s e d t o r e p e l e a c h o t h e r. T h e m a g n e t i c s o u t h p o l e , t h e
d i r e c t io n i n w hi c h t h e s o u t h p o l e o f a f r e e m a g n e t w oul d p o i n t t o , i s t h e a c t ua l m a g n e t i c n o r t h
p o l e f o r t h e s a m e r e a s o n.
The link between
magnetism and
electricity
•
When electric current passes through a wire, it produces
a magnetic field around it. If you put a compass at different
positions on the card around the w ire, you can plot the
magnetic field lines.
•
When the current flows up through the wire, the direction
of its field is anticlockwise, and w hen the current is
travelling dow nwards, the direction of its field is clockwise.
•
The lines of force in the magnetic field not only
demonstrate the direction of the field but also the strength
of it. The lines of force will be closer together when the
f o r c e i s s t r o n g e r a n d f u r t h e r a p a r t w h e n t h e f o r c e i s w e a k e r.
The link between magnetism and
electricity
•
If you tw ist a w ire into a coil and connect it to a circuit, it w ill
produce a magnetic field.
•
Now if you place a bar of steel inside the coil and sw itch the
current on, the magnetism of this set -up w ould be stronger than
the magnetism of the coil alone. The current flowing through the
coil induces magnetism in the steel, and since it is magnetically
hard, it will keeps some of its magnetism after the current is
switched off.
•
Now if the steel is replaced w ith an iron, the set-up w ould be
m a g n e t i c a l l y s t r o n g e r, b u t t h e i r o n w i l l l o s e i t s m a g n e t i s m a s s o o n
as the current is switched off as it is magnetically soft.
The electromagnet
• An e l e c t r o m a g ne t i s a m a g n e t i n w hi c h t h e m a g n e t i c f i e l d
i s p r o d uc e d b y a n e l e c t r i c c u r r e n t .
•
As s o o n a s t h e c u r r e nt i s s w i t c he d o f f , h ow e v e r, t h e
e l e c t r o m a g ne t l o s e s i t s m a g n e t i s m c o m p l e t e l y.
•
S o , a n e l e c t r om a g ne t i s a d e v i c e t h a t c a n
i n s t a n t ly b e c o m e a m a g n e t a n d t h e n l o s e i t s
m a g n e t i s m i n s t a n t ly a n d c o m p l e t e l y t o o . An e x a m pl e o f
a n e l e c t r om a gne t i s t h e l a r g e e l e c t r om a gn e t u s e d i n
s c r a p ya r d s t o m o v e s t e e l s c r a ps o r t h e s t e e l b o d i e s o f
cars.
THE CIRCUIT BELL
• W he n you pus h t he s w itch, you f or m a c l os ed c i r c uit w hich
a l l ow s c ur r e nt t o pa s s t hr ough t he c oi l. As a r e s ult, t he
e l e c trom agne t pul ls t he a r m a t ur e t o i t , w hich m a k e s t he
ha m m e r s t r i ke t he gong. Si nc e t he a r ma t ure i s pul l e d t o t he
e l e c trom agne t, t hi s c r e a t es a ga p be tw ee n t he s pr i ng a nd t he
c ont a ct s c r ew. C ons e quently, t he c i r c uit i s br ok e n a nd t he
c ur r e nt s t ops f l ow ing, w hich m a k e s t he a r m a t ur e sw ing ba c k t o
i t s i ni t ial pos i tion a s t he e l e c tr om agne t l os t a l l of i t s
m a gne t ism . N ow s i nc e t he c ont ac t s c r ew a nd t he s pr i ng t ouc h
a ga i n, i t f or m s a c om pl ete c i r cuit a nd s o t he a r m a t ur e i s pul l ed
t o t he e l e c tr om a gnet one m or e t i m e . Thi s w ill go on a nd t he
ha m m e r w ill hi t t he gong r e pe a tedly, unt i l t he s w it ch i s
r e l e ase d.
THANK YOU