Motors Use Electromagnets

Because magnetism is a force, magnets can be used to move things. Electric motors convert the energy of an electric current into motion by taking advantage of the interaction between current and magnetism.

There are hundreds of devices that contain electric motors. Examples include power tools, electrical kitchen appliances, and the small fans in a computer. Almost anything with moving parts that uses current has an electric motor.

Motors

All motors have similar parts and work in a similar way. The main parts of an electrical motor include a voltage source, a shaft and electromagnet, and at least one additional magnet. The shaft of the motor turns other parts of the device.

Recall that an electromagnet consists of a coil of wire with current flowing through it. When current from the voltage source flows through the coil, a magnetic field is produced around the electromagnet. The poles of the magnet interact with the poles of the electromagnet, causing the motor to turn.

1)The poles of the magnet push on the like pole sof the electromagnet, causing the electromagnet to turn.

2) As the motor turns, the opposite poles pull on each other.

3) When the poles of the electromagnet line up with the opposite poles of the magnet, a part of the motor called the commutator reverses the polarity fo the electromagnet  Now, the poles push each other again the the motor continue to turn.

The illustration at the bottom of the page is simplified so that you can see all the parts. If you saw the inside of an actual motor, it might look very different. THe wires coil many times. The electromagnet is a strong motor may coil hundreds of times. The more coils, the stronger the motor.

Uses of Motors

Many machines and devices contain electric motors that may not be as obvious as the motor that turns the blades of a fan, for example. Even though the motion produced by the motor is circular, motors can move objects in any direction. For example, electric motors move power windows in a car up and down.

Motors can be very large, such as the motors that power an object as large as a subway train. They draw electric current from a third rail on the track or wires overhead that carry electric current. A car uses an electric current to start the engine. When the key is turned, a circuit is closed, producing a current from the battery to the motor. Other motors are very small, like the battery operated motors that move the hands of a wristwatch.photo (23)

Electromagnets

An electric current produces a magnetic field

Like many discoveries, the discovery that electric curent is related to magnetism was unexpected. In the 1800s, a Danish physicist named Hans Christian Oersted was teaching a physics class. Oersted used a battery and a wire to demonstrate soem properties of electricity. He noticed that as an electric charge passed through the wire, the needle of a nearby compass moved.

When he turned the current off, the needle returned to its original direction. After more experiments, Oersted confirmed that there is a relationship between magnetism and electricity. He discovered that an electric current produces a magnetic field.

Electromagnetism

The relationship between electric current and magnetism plays an important role in many modern technologies. Electromagnetism is magnetism that results from an electric current. When a charged particle such as an electron moves, it produces a magnetic field. Because an electric current generally consists of moving electrons, a current in a wire produces a magnetic field. In fact, the wire acts as a magnet. Increasing the amount of current in the wire increases the strength of the magnetic field.

A piece of iron in a strong magnetic field becomes a magnet itself. An electromagnet is a magnet made by placing a piece of iron or steel inside a coil of wire. As long as the coil carries a current, the metal acts as a magnet and increases the magnetic field of the coil. But when the current is turned off, the magnetic domains in the metal becomes random again and the magnetic field disappears.

By increasing the number of loops in the coil, you can increase the strength of the electromagnet. Electromagnets exert a much more powerful magnetic field than a coil of wire without a metal core. They can also be much stronger than the strongest permanent magnets made of metal alone. You can increase the field strength of an electromagnet by adding more coils or a stronger current. Some of the most powerful magnets in the world are huge electromagnets that are used in scientific instruments.

Thunderstorms

If you have every shuffled your shoes on a carpet, you may have felt a small shock when you touched a doorknob. Electrical charges collected on your body and then jumped to the doorknob in a spark of electricity.

In a similar way, electrical charges build up near the tops and bottoms of clouds as pellets of ice move up and down through the clouds. Suddenly, a charge sparks from one part of a cloud to another or between a cloud and the ground. The spark of electricity, called lightning, causes a bright flash of light. The air around the lightning is briefly heated to a temperature hotter than the surface of the Sun. This fast heating produces a sharp wave of air that travels away from the lightning. When the wave reaches you, you hear it as a crack of thunder. A thunderstorm is a storm with lightning and thunder.

Formation of Thunderstorm

Thunderstorms get their energy from humid air. When warm, humid air near the ground moves vertically into cooler air above, the rising air, or updraft, can build a thunderstorm quickly.

1. Rising humid air forms a cumulus cloud. The water vapor releases energy when it condenses into cloud droplets. This energy increases the air motion. The cloud continues building up into the tall cumulonimbus cloud of a thunderstorm.

2. Ice particles form in the low temperatures near the top of the cloud. As the ice particles grow large,t hey begin to fall and pull cold air down with them. This strong downdraft brings heavy rain or hail– the most severe stage of a thunderstorm.

3. The downdraft can spread out and block more warm air from moving upward into the cloud. The storm slows down and ends.

Thunderstorms can form at a cold front or within an air mass. At a cold front, air can be forced upward quickly. Within an air mass, uneven heating can produce convection and thunderstorms. In some regions, the conditions that produce thunderstorms occur almost daily during part of the year. In Florida, for example, the wet land and air warm up during a long summer day. Then, cool sea breezes blow in from both coasts of the peninsula at once. The two sea breezes together push the warm, humid air over the land upward quickly. Thunderstorms form in the rising air.

In contrast, the summer air along the coast of California is usually too dry to produce thunderstorms. The air over the land heats up, and a sea breeze forms, but there is not enough moisture in the rising warm air to form clouds and precipitation.

Effects of Thunderstorms

A thunderstorm may provide cool rain at the end of a hot, dry spell. The rain can provide water for crops and restore lakes and streams. However, thunderstorms are often dangerous.

Flash floods can be strong enough to wash away people, cars, and even houses. One thunderstorm can produce millions of liters of rain. If a thunderstorm dumps all its rain in one place, or if a series of thunderstorms dump rain onto the same area,t he water can cover the ground or make rivers overflow their banks.

Winds from a thunderstorm can be very strong. They can blow in bursts that exceed 170 miles per hour. Thunderstorm winds once knocked down a stretch of forest in Canada that was about 10 miles wide and 50 miles long. Thunderstorms can also produce sudden, dangerous bursts of air that move downward and spread out.

Hail causes nearly $1 billion in damage to property and crops in the United States every year. Hail can wipe out entire fields of a valuable crop in a few minutes. Large hailstones can damage roofs and kill livestock.

Lightning can kill or seriously injure any person it hits. It can damage power lines and other equipment. Lightning can also spark dangerous forest fires.

Hurricanes

Near the equator, warm ocean water provides the energy that can turn a low-pressure center into a violent storm. As water evaporates from the ocean, energy moves from the ocean water into the air. This energy makes warm air rise faster. Tall clouds and strong winds develop. As winds blow across the water from different directions into the low, the Coriolis effect bends their paths into a spiral. The winds blow faster and faster around the low, which becomes the center of a storm system.

A Tropical storm is a low-pressure system that starts near the equator and had winds that blow at 40 miles per hour or more. A hurricane is a tropical low-pressure system with winds blowing at speeds of 75 miles per hour or more– strong enough to uproot trees. Hurricanes are called typhoons or cyclones when they form over the Indian Ocean or the western Pacific Ocean.

Formation of Hurricanes

In the eastern united States, hurricanes most often strike between August and October. Energy from warm water is necessary for a low-pressure center to build into a tropical storm and then into a hurricane. The ocean water where these storms develop only gets warm enough or more near the end of summer.

Tropical storms and hurricanes generally move westward with the trade winds. Near land, however, they will often move north, south, or even back eastward. As long as storm stay above the water, it can grow bigger and more powerful. As soon as a hurricane moves over land or over cooler water, it loses its source of energy. The winds lose strength and the storm dies out. If a hurricane moves over land, the rough surface of the land reduces the winds every more.

At the center of a hurricane is a small area of clear weather, 10 to 30 miles in diameter, called the eye. The storm’s center is calm because air moves downward there. Just around the eye, the air moves very quickly around and upward, forming a tall ring of cumulonimbus cloud called the eye wall. This ring produces very heavy rains and tremendous winds. Farther from the center, bands of heavy clouds adn rain spiral inward toward the eye.

Effects of Hurricanes

A hurricane can pound a coast with huge waves and sweep the land with strong winds and heavy rains. The storms cause damage and dangerous conditions in several ways. The storms cause damage and dangerous conditions in several ways. Hurricane winds can lift cars, uproot tress, and tear the roofs off buildings. Hurricanes may also produce tornadoes that cause even more damage. Heavy rains from hurricanes may make rivers overflow their banks and flood nearby areas. When a hurricane moves into a coastal area, it often pushes a huge mass of ocean water known as a storm surge. In a storm surge, the sea level rises several meters, backing up rivers and flooding the shore. A storm surge can be destructive and deadly. Large waves add to the destruction. A hurricane may affect an area for a few hours or a few days, but the damage may take weeks or even months to clean up.

The National Hurricane Center helps people know when to prepare for a hurricane. The center puts out a tropical-storm or hurricane watch when a storm is likely to strike within 36 hours. People may be evacuated, or moved away for safety, from areas where they may be in danger. As the danger gets closer– 24 hours or less– the center issues as tropical storm or hurricane warning. The warning stays in effect until he danger has passed.

Buoyancy Principles

Bernoulli’s Principle

Bernoulli’s principle, named after Daniel Bernoulli, a Swiss mathematician who lived in the 1700s, describes the effects of fluid motion on pressure. In general, Bernoulli’s principle says that an increase in the speed of the motion of a fluid decreases the pressure within the fluid. The faster a fluid moves, the less pressure it exerts on surfaces or openings it flows over.

Bernoulli’s principle has many applications. One important application is used in airplanes. Airplane wings can be shaped to take advantage of Bernoulli’s principle. Certain wing shapes cause the air flowing over the top of the wing to move faster than the air flowing under the wing. Such a design improves the lifting force on a flying airplane.

Many race cars, however, have a device on the rear  of the car that has the reverse effect. The device is designed like an upside-down airplane wing. This shape increases the pressure on the top of the car. The car is pressed downward on the road, which increases friction between the tires and the road. With more friction, the car is less likely to skid as it goes around the curves at high speeds.

Pascal’s Principle

The the 1600s Blaise Pascal, a French scientist for whom the unit of measure called the pascal was named, experimented with fluids in containers. One of his key discoveries is called Pascal’s principle. Pascal’s principle states that when an outside pressure is applied at any point to a fluid in a container, that pressure is transmitted throughout the fluid with equal strength.

You can use pascal’s principle to transmit a force through a fluid. Some car jacks life cars using Pascal’s principle. These jacks contain liquids that transmit and increase the force that you apply.

Buoyancy

If you drop an ice cube in air, it falls to the floor. If you drop the ice cube into water, ti may sink a little at first, but the cube quickly rises upward until it floats. You know that gravity is pulling downward on the ice, even when it is in the water. If the ice cube is not sinking, there must be some force balancing gravity that is pushing upward on it.

The upward force on objects in a fluid is called  buoyant force, or buoyancy. Buoyancy is why ice floats in water. Because of buoyant force, objects seem lighter in water. For example, it is easier to lift a heavy rock in water than on land because the buoyant force pushes upward on the rock, reducing the net force you need to lift it.

Buoyancy

The photograph below shows a balloon that has been pushed into a beaker of water. Remember that in a fluid, pressure increases with depth. This means that there is greater pressure acting on the bottom of the balloon than on the top of it.  The pressure difference between the top and bottom of the balloon produces a net force that is pushing the balloon upward.

When you push a balloon underwater, the water level rises because the water and the balloon cannot be in the same place at the same time. The volume of the water has not changed, but some of the water has been displaced, or moved, by the balloon. The volume of the displaced water is equal to the volume of the balloon. The buoyant force on the balloon is equal to the weight of the displaced water. A deflated balloon would displace less water and would therefore have a smaller buoyant force on it.

Density and Buoyancy

Whether or not an object floats in a fluid depends on the densities of both the object and the fluid. Density is a measure of the amount of matter packed into a unit volume. The density of an object is equal to its mass divided by its volume and is commonly measured in grams per cubic centimeter.

If an object is less dense than the fluid it is in, the fluid the object displaces can weigh more than the object. A wooden ball that i pushed underwater,a s in the beaker below, rises to the top and floats. An object rising in a liquid has a buoyant force acting upon it that is greater than its own weight. If an object is floating in a liquid, the buoyant force is balancing the weight.

If the object is more dense than the fluid it is in, the object weighs more than the fluid it displaces. A glass marble placed in the beaker on the far right sinks to the bottom because glass is denser than water. The weight of the water the marble displaces is less than the weight of the marble. A sinking object has a weight that is greater than the buoyant force on it.

Measuring Mass and Weight

Different objects contain different amounts of matter. Mass  is a measure of how much matter an object contains. A metal teaspoon, for example, contains more matter than a plastic teaspoon. Therefore, a metal teaspoon has a greater mass than a plastic teaspoon. An elephant has more mass than a mouse.

Measuring Mass

When you measure mass, you compare the mass of an object with a standard amount, or unit, of mass. The standard unit of mass is a kilogram (kg). A large grapefruit has a mass of about one-half kilogram. Smaller masses are often measured in grams (g). There are 1000 grams in a kilogram. A penny has a mass between two and three grams.

How can you compare the masses of two objects? One way is to use a pan balance. If two objects balance each other on a pan balance, then they contain the same amount of matter. If a basketball balances a metal block, for example, then the basketball and the block have the same mass. Beam balances work in a similar way, but instead of comparing the masses of two objects, you compare the mass of an object with a standard mass on the beam.

Measuring Weight

When you hold an object such as a backpack full of books, you feel it pull down on your hands. This is because Earth’s gravity pulls the backpack toward the ground. Gravity is the force that pulls two masses toward each other. In this example. the two masses are Earth and the backpack. Weight is the downward pull on an object due to gravity. If the pull of the backpack is strong, you would say that the backpack weights a lot.

Weight is measured by using a scale, such as a spring scale, that tells you how hared and object is pushing or pulling. The standard scientific unit for weight is the newton (N). A common unit for weight is pound (lb).

Mass and weight are closely related, but they are not the same. Mass describes the amount of matter an object has, and weight describes how strongly gravity is pulling on that matter. On Earth, a one-kilogram object has a weight of 9.8 newtons (2.2lbs). When a person says that one kilogram is equal to 2.2 pounds, he or she is really saying that one kilogram has a weight of 2.2 pounds on Earth. On the Moon, however, gravity is one-sixth as strong as it is on Earth. On the Moon, the one-kilogram object would have a weight of 1.6 newtons (0.36lbs). The amount of matter in the object, or its mass, is the same on Earth as it is on the Moon, but the pull of the gravity is different.

Global Winds

The distance winds travel varies. Some winds die out quickly after blowing a few meters. In contrast, global winds travel thousands of kilometers in steady patterns. Global winds last for weeks.

Uneven heating between the equator and the north and south poles causes global winds. Sunlight strikes Earth’s curved surface stronger and more directly. Near the equator, concentrated sunlight heats the surface to high temperatures. Warm air rises, producing low pressure.

In regions closer to the poles, the sunlight is more spread out. Because less of the Sun’s energy reaches these regions, the air above them is cooler and denser. The sinking dense air produces high pressure that sets global winds in motion.

If Earth did not rotate, global winds would flow directly from the poles to the equator. However, Earth’s rotation changes the direction of winds and other objects moving over Earth. The influence of Earth’s rotation is called the Coriolis effect. Global winds curve as Earth turns between them. In the Northern Hemisphere, winds curve to the right in the direction of motion. Winds in the Southern Hemisphere curve to the left. The Coriolis effect is noticeable only for winds that travel long distances.

Because the Coriolis effect causes global winds to curve, they cannot flow directly from the poles to the equator. Instead global winds travel along three routes in each hemisphere. The routes, which circle the world are called global wind belts.

Air Pressure

Air molecules move constantly. As they move, they bounce off each other like rubber balls. Thy also bounce off every surface they hit. As you read this, billions of air molecules are bouncing off your body, the computer and everything else around you.

Each time an air molecule bounces off an object, it pushes, or exerts a force, on that object. When billions of air molecules bounce off a surface, the force is spread over the area of that surface. Air pressure is the force of air molecules pushing on an area. The greater the force, the higher the air pressure. Because air molecules move in all directions, air pressure pushes in all directions.

The air pressure at any area on Earth depends on the weight of the air above the area. If you hold out your hand, the force of air pushing down on your hand, the force of air pushing down on your hand is greater than the weight of a bowling ball. So why don’t you feel the air pushing down on your hand? Remember that air pushes in all directions. The pressure of air pushing down is balanced by the pressure of air pushing up from below.

Air pressure decreases as you move higher in the atmosphere. Think of a column of air directly over your body. If you stood at seas level, this column would stretch from where you stood to the top of the atmosphere. The air pressure on your body would be equal to the weight of all the air in the column. But if you stood on a mountain, the column of air would be shorter. With less air above you, the pressure is about half what it is at sea level.

Air pressure and density (the amount of mass in a given volume of substance) are related. Just as air pressure decreases with altitude, so does the density of air. Notice in the picture below that air molecules at sea level are closer together than air molecules over the mountain. Since the pressure is greater at sea level, the air molecules are pushing closer together. Therefore, the air at sea level is denser than air at high altitudes.

Magnetic Materials

Some magnets occur naturally. Lodestone is a type of mineral that is a natural magnet and formed the earliest magnets that people used. The term magnet comes from the name Magnesia, a region of Greece where lodestone was discovered. Magnets can also be made from materials that contain certain metallic elements, such as iron.

If you have ever tried picking up different types of objects with a magnet, you have seen that some materials are affected by the magnet and other materials are not. Iron, nickel, cobalt, and few other metals have properties that make them magnetic. Other materials, such as wood, cannot be made into magnets and are not affected by magnets. Whether a material is magnetic or not depends on its atoms.

Inside magnetic materials

1. In a material that is not magnetic, such as wood, the magnetic field of the atoms are weak and point in different directions. The magnetic field cancel each other out. As a results, the overall material is not magnetic and could not be made into a magnet.

2. In a material that is magnetic, such as iron,t he magnetic fields of a group of atoms align, or point in the same direction. A magnetic domain is a group of atoms whose magnetic fields are aligned. The domains of a magnetic material are not themselves aligned, so their fields cancel one another out. Magnetic materials are pulled by magnets and can be made into magnets.

3. A magnet is a material in which the magnetic domains are all aligned. The material is said to be magnetized.