Download 17.4 and 17.5

17.4 Electricity, Magnetism, and Motion
Energy and Motion – You know that magnetic force can produce motion,
and you know that electric current in a wire produces a magnetic field.
So, a magnet can move a wire with a current, just as it would move
another magnet.
Energy Transformation – Energy can be transformed from one form to
another. When a wire with a current is placed in a magnetic field,
electrical energy is transformed into mechanical energy.
The transformation happens when the magnetic field produced by the
current causes the wire to move.
How does a Galvanometer work? You learned that a straight wire with a
current moves when it is placed in a magnetic field. But what happens
when you place a loop of wire with a current in a magnetic field?
The current in one side of the loop flows in the opposite direction than
the current in the other side of the loop. So, the sides of the loop move
in opposite directions. Once each side has moved as far up or down as it
can go, it will stop moving. As a result, the loop can rotate only a halfturn.
Inside a Galvanometer – The rotation of a wire loop in a magnetic field is
the basis of a galvanometer. A galvanometer is a device that measures
small currents. An electric current turns the pointer of a galvanometer.
In a galvanometer, an electromagnet is suspended between opposite
poles of two permanent magnets. The electromagnet’s coil is attached to
a pointer. When a current is in the electromagnet’s coil, it produces a
magnetic field. This field interacts with the permanent magnet’s field,
causing the coil and the pointer to rotate.
You can use a galvanometer to measure an unknown current, so
electricians use them. Some cars use them as fuel gauges. They are also
used in lie detectors to measure how much current a person’s skin
conducts. People who are stressed sweat more. Water conducts
electricity. Therefore, moist skin conducts more electric current.
What does an Electric Motor Do? An electric motor is a device that uses
an electric current to turn an axle. An electric motor transforms
electrical energy into mechanical energy.
A simple electric motor has four parts:
The commutator
consists of two
semicircular pieces of
metal. It conducts the
current from the
brushes to the
commutator.
Brushes conduct
current to the rest of
the commutator. They
do not move.
The armature is a loop of
wire that current flows
through (coil)
Permanent
magnets produce a
magnetic field
which causes the
armature to spin
If current only flowed in one direction, the armature could only rotate
half a turn. However, the brushes and commutator enable the current
in the armature to change direction. Current always flows from the
positive to the negative terminal of a battery. The current in the
armature is reversed each time the commutator moves to a different
brush. This causes the armature to rotate continuously.
17.5 Electricity from Magnetism
Electric motors use electrical energy to produce motion. Can motion
produce electrical energy? In 1831, scientists discovered that moving a
wire in a magnetic field can cause an electric current. This current allows
electrical energy to be supplied to homes, schools, and businesses all
over the world.
A magnet can make, or induce, current in a conductor, such as a wire, as
long as there is motion. An electric current is induced in a conductor
when the conductor moves through a magnetic field. Generating electric
current from the motion of a conductor through a magnetic field is called
electromagnetic induction.
Current that is generated in this way is called induced current.
Induction of Electric Current – Michael Faraday and Joseph Henry each
found that motion in a magnetic field will induce a current. Either the
wire can move through the magnetic field, or the magnet can move. The
direction of an induced current depends on the direction that the coil or
magnet moves. When the motion is reversed, the direction of the
current also reverses.
Alternating and Direct Current (AC/DC) – A current with charges that flow
in one direction is called direct current (DC). A battery produces direct
current when it is placed in a circuit and charges flow in one direction.
If a wire in a magnetic field changes direction repeatedly, the induced
current also keeps changing direction. A constantly reversing current is
called alternating current (AC).
Alternating current has a major advantage over direct current. An AC
voltage can be easily raised or lowered. This means that a high voltage
can be used to send electrical energy over great distances.
Then the voltage can be reduced to a safer level for everyday use. The
electric current in homes, school, and other buildings is alternating
current.
How does a Generator work? An electric generator is a device that
transforms mechanical energy into electrical energy. A generator uses
motion in a magnetic field to produce current.
The electric company uses giant generators to produce most of the
electrical energy you use each day. Huge turbines turn the armatures of
the generators. Turbines are circular devices with many blades. They
spin when water, steam, or hot air flows through them.
What does a Transformer do? The electrical energy generated by
electric companies is transmitted over long distances at very high
voltages. Transformers change the voltage in your home so you can use
the electricity.
A transformer is a device that increases or decreases voltage. A
transformer consists of two separate coils of insulated wire wrapped
around an iron core. The primary coil is connected to a circuit with a
voltage source and alternating current. The secondary coil is connected
to a separate circuit that does not contain a voltage source. The changing
current in the primary coil produces a changing magnetic field. This
changing magnetic field induces a current in the secondary coil.
The change in voltage from the primary coil to the secondary coil
depends on the number of loops in each coil.