Science 8.2 seventh grade :)

Strength of the magnetic field can be increased by

adding more turns to the wire coil increasing the current passing through the wire

Electric Current and Magnetism

-Hans Christian Oersted -Danish Physics teacher -In 1820 he found that electricity and magnetism are related --He had a compass near a piece of wire when he was demonstrating the operation of electric circuits and realized that as the current flowed through the wire the compass needle was turned or deflected. If the current was reversed, the compass needle was deflected in the opposite direction. When the current stopped the needle returned to its original position.

Electromagnets

An electromagnet is a temporary magnet made by placing a piece of iron inside a current-carrying coil of wire. When a current flows through a circular loop of wire, magnetic-field lines form all around the wire. If more loops of wire are added to make a coil, the magnetic field lines formed around each loop will overlap and add together. This results in a stronger magnetic field If an iron core is inserted into the coil, the magnetic field inside the coil causes the iron core to become magnetized. When a magnetized iron core is combined with a coil the magnetic field comes mostly from the iron core

Useful

Can be controlled by changing the electric current flowing through the wire coil When current flows through the electromagnet and it moves toward or away from another magnet, it converts electrical energy into mechanical energy to do work. Electromagnet do work in various devices such as stereo speakers and electric motors. They are good for lifting large metal objects.

Electric Motors

Electric motor: a device that changes electrical energy into mechanical energy Example: fan

Switching Poles

Electric motors are like galvanometers because the coil turns so that its north and south poles are aligned along the magnetic-field lines of the permanent magnet. Once the coil is aligned, there is no longer a force that will keep the coil rotating. The direction of the coil's magnetic field can be flipped by reversing the direction of the electric current in the coil. Then, the like poles of the coil and magnet will be next to each other. Once the field is flipped the coil will be repelled and will rotate further which causes the coil to rotate until it is aligned along the field lines of the permanent magnet. Then the current is reversed again and the coil is kept rotating.

Controlling Electric Motors

Electric motors can be more useful if their rotation speed can be controlled.

Oersted's Hypothesis

From his observation he developed a hypothesis that the electric current must produce a magnetic field around the wire, and the direction of the field changes with the direction of the current

Galvanometers

Galvanometers: devices that use an electromagnet to measure electric current Example: gauges in the dashboard of a car

It behaves like other magnets

Has a north pole and south pole If places in a magnetic field, an electromagnet will align itself along the magnetic field lines, like a compass needle will. Will attract magnetic materials and be attracted or repelled by other magnets

Using Galvanometers

If the coil is connected to a small spring, then the coil can act as a galvanometer. If the current through the coil is small, only a weak magnetic force is produced in the electromagnet. This makes the magnetic force between the electromagnet and the permanent magnet weak. The coil can rotate only a small amount against the resistance of the spring and the needle moves by only a small amount. When a large current flows in the coil, the magnetic force between the electromagnet and the permanent magnet is stronger. The coil rotates further, and the needle moves further along the scale. To be used as a gauge, a galvanometer must be calibrated by sending a known current through the coil and seeing how much the needle is deflected.

continued

In some motors a switch called a commutator reverses the current in the coil. Other motors don't need a commutator because they use household alternating current, which reverses direction 120 times a second.

how?

One way their rotation speed can be controlled is to vary the amount of current flowing through the coil. Because the coil is an electromagnet, its magnetic field becomes stronger if more current flows through the coil causing the magnetic force between the coil and the permanent magnet to increase. This results in the coil turning faster.

How does musical information stored on a CD become sound you can hear?

The electromagnet changes electrical energy to mechanical energy that vibrated parts of the speaker to produce sound.

Using Electric Motors

The first electric motor to be widely used was developed in 1873 The motor used direct current The first motor to use alternating current was invented in 1888 Since that time many additional developments have made electric motors smaller, more powerful, and more efficient.

Moving Charges and Magnetic Fields

The magnetic field around a current-carrying wire forms a circular pattern about the wire. The direction of the field depends on the direction of the current. The strength of the magnetic field depends on the amount of current flowing in the wire. When no current flows in a wire, the magnetic field disappears.

Temporary magnets

The magnetic field is only present when current is flowing in the wire coil