Electricity and Magnetism
After reading this section you will be able to do the following:
Discuss what happens to a compass when a wire with electrical current is near.
Describe the relationship between electricity and magnetism.
Why does the compass respond when it is near an electrical wire with current flowing through it?
We can conclude from this experiment that an electric current causes a magnetic field around it just like a magnet causes a magnetic field. When you moved the compass near a bar magnet, the needle pointed toward the magnet’s magnetic field and not toward the north. When you put the compass near the electrical wire with current flowing through it, the compass did not point north; instead, the compass needle pointed in the direction of the current’s magnetic field.
Ferromagnetic Objects in Magnetic Fields
Now we have established that a conductive wire with a current flowing through it has a magnetic field. If we put a ferromagnetic object in this magnetic field, the object will concentrate the strength of the field and cause the object to become magnetic. Once the current flow in the line stops, the magnetic field disappears and the object stops acting like a magnet. However, the magnetic field of one wire is small and does not have much strength, so it can only make temporary magnets from small objects. But, let’s say that we take a wire and coil it several times to form a long coiled piece of electrical wire, and then we turn on the current. We would have a magnetic field much bigger and stronger than we would without the coiled piece of wire, and we could magnetize even larger objects.
An iron bar placed through the center of the coiled wire would become a temporary magnet, called an electromagnet
, as long as the electric current is flowing through the wire.
When connected to a voltage source, a solenoid’s core becomes an electromagnet because of the current that flows through the wire.
Warning: Current may need to be restricted to prevent overheating the wire and to prevent damaging the battery.
You can also make an electromagnet
by passing the electric current directly through the ferromagnetic object. Electromagnets
are discussed in more detail in a following page.
If current is passed through the solenoid’s core instead of through the wires, the induced magnetic field circles around the core. This is similare to the case of current flowing through a straight wire.
Electricity and Magnetism
More on Electricity and Magnetism
After reading this section you will be able to do the following:
Explain what a galvanometer in and how it is used.
Begin to discuss how magnetism can be used to create electrical current.
In this demonstration you will use an instrument called a galvanometer. It is an instrument with a bar of iron wrapped with an electrical wire, and a magnet. It detects electrical current.
If a wire is connected to this instrument, it can detect if electricity
is flowing through the wire. If there is no current through the wire, the needle will move to the left. Similarly, if there is a current flowing through the wire, the needle will move towards the right of the scale. This demonstration should help to give you an idea of the relationship between magnetism and electricity.
Electromagnetic Induction
What you have just discovered in this experiment is that electricity
can be generated by moving a wire through a magnetic field. This process is called electromagnetic induction
. When an electrical wire cuts across magnetic lines of force, a current is produced in the wire. We know this because the current is detected by watching the needle on a galvanometer, which is an instrument that can measure electric current in wires. The same result is obtained when a magnet is moved in and out of coils of wire. It does not matter if the magnet is moved or if the coils of wire are moved. The important thing is that there is motion within the magnetic field, and that the magnetic lines of force are cut.
The discovery of electromagnetic induction
is very important in our lives because it is the principle by which electric generators can make electricity
. Through the use of magnets, a generator can convert mechanical energy to electrical energy and provide electricity
that we need for so many things. Remember that energy is the ability to do work and that mechanical energy is the energy caused by moving objects. For example, when you move your legs to peddle a bicycle, you cause energy that moves the bicycle wheels and runs the bicycle. When a rushing wave of water hits a boat and turns it over, the moving water causes the energy that moves the boat to overturn it. When there is a way to turn this moving energy into electricity
that can light a light bulb, we can get light in our home.
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