Download Chapter 7: Magnetism and Its Uses

Chapter 7:
Magnetism and Its Uses
• Section 1: Magnetism
• Section 2: Electricity and Magnetism
• Section 3: Producing Electric Current
Section 1: Magnetism
Magnetism – the properties and interaction of magnets
• Magnetic field – the
region around a
magnet that exerts the
magnetic force
 The magnetic field
weakens with
distance
 The magnetic field
can be represented
by curved lines called magnetic field lines
• Magnetic poles – regions on a magnet where the
magnetic force is strongest
 All magnets have a north and south pole
 Magnetic field lines start at the north pole and end on
the south pole
 Two magnets will either attract or repel each other
• Like poles will repel, unlike poles will attract
S
N
N
S
N
S
S
N
Section 1: Magnetism
• Earth’s magnetic field and compasses
 A compass is a device consisting of a tiny bar magnet that
is free to rotate
 When a compass is placed near a magnet, the needle
will align with the field lines of the magnet
 The Earth acts like a huge bar magnet, so a compass
needle will align with the Earth’s magnetic field line
and the need le will point to Earth’s magnetic north
pole
• Magnetic materials – not all metals are attracted to a
magnet
 Magnetic metals include iron, cobalt, and nickel
 All electrons have magnetic properties. In the atoms of
most elements these properties cancel out
 The magnetic properties in the atoms of iron, cobalt,
and nickel do not cancel, so each atom of these metals
behave like little magnets
Section 1: Magnetism
 Magnetic domains – groups of atoms with aligned
magnetic poles
 Placing a magnetic metal near a permanent magnet
will cause the atoms in the metal to align, and the
metal will become a temporary magnet
 Because of the random motion of the atoms in the
metal this alignment is soon lost and the metal will
no longer act as a magnet
 In permanent magnets the magnetic field inside the
magnet is several thousand times stronger than the
magnetic field outside the magnet so the magnetic
domains do not get bumped out of alignment
Section 2: Electricity and Magnetism
Electric Current and Magnetism
• Moving charges, like an electric current, produce magnetic
fields
Magnetic field lines
Electron flow
Electron flow
Magnetic field lines
• The field forms a circular pattern around a currentcarrying wire
• The direction of the field depends on the direction of the of
the current
• The strength of the magnetic field depends on the amount of
current
Electromagnetism – the interaction between electric charges
and magnets
• Electromagnetic force – the attractive or repulsive force
between electric charges and magnets
Section 2: Electricity and Magnetism
Electromagnets
• An electromagnet is a temporary
magnet made by placing a piece
or iron inside a current-carrying
coil of wire
• Electromagnets are temporary
magnets because the magnetic field is
present only when the current is flowing
Electric Motors
• An electric motor is a device that converts electrical energy
into mechanical energy
 Contains an electromagnet that is free to rotate between
the poles of a permanent, fixed magnet
 Converts alternating current to rotary motion
Section 3: Producing Electric Current
Electromagnetic induction – the production of an electric
current by moving a loop of wire through a magnetic field or
moving a magnet through a wire loop
• Generator – a device that
produces electric current by
rotating a coil of wire in a
magnetic field
 Power plants use huge
generators to produce electric
current
• Turbine – a large wheel that
rotates when pushed by water,
wind, or steam
Direct and Alternating Currents
• Direct current – the electric current flows in only one
direction
 Ex.: the current produced by a battery
• Alternating current – reverses the flow of current in a regular
way
 In North America, generators produce alternating current
at a frequency of 60 Hz (cycles per second)
 The electric current produced changes direction twice
during each cycle, so a 60 Hz alternating current changes
direction 120 each second
Section 3: Producing Electric Current
Transformers
• Transformer – a device that increases or decreases the
voltage of an alternating current
 Made of two coils of wire, called the primary and
secondary coils, wrapped around the same iron core
 As the alternating current passes through the primary coil
the core becomes an electromagnet
 Because the current is changing direction many times
each second, the magnetic field of the iron core changes
direction.
 The changing magnetic field induces an alternating
current in the secondary coil
• Step-up transformer – if the secondary coil has more turns of
wire than the primary, then the transformer increases
voltage
• Step-down transformer – if the secondary coil has fewer
turns of wire than the primary, then the transformer
decreases voltage
 Before the electric current enters your house it must
travel through a step-down transformer
Section 3: Producing Electric Current
Example 1: What is the output voltage of a transformer if the
input voltage is 240 V and the primary coil has 20 wraps and
the secondary coil has 30 wraps?
Solution
V1 = 240 V
N1 = 20
N2 = 30
V2 = ?
V1
V2
=
, cross-multiply
N1
N2
V1N2 = V2N1
V1N2
V2 N1
=
N1
N1
V 1N 2
N1
240V(30)
V2 =
20
V2 = 360V
V2 =
Example 2: What is the output voltage of a transformer in the
input voltage is 1,200 V and the primary coil has 40 wraps and
the secondary coil has 4 wraps?
Solution
V1 -1,200V
N1 = 40
N2 = 4
V2 = ?
V1
V2
=
, cross-multiply
N1
N2
V1N2 = V2N1
V1N2
V2 N1
=
N1
N1
V 1N 2
N1
1,200V(4)
V2 =
40
V2 = 120V
V2 =