Electromagnetism
In previous lessons, we studied electric and magnetic fields. In this lesson, we bring together electric and magnetic fields into the study of electromagnetism. We begin with pioneering work done by the Dutch scientist Hans Christian Oersted in the 1800’s. His work led to an understanding of how an electric current through a conductor produces a magnetic field around the conductor. The principles learned in magnetic field patterns for a single straight wire are extended first into a single loop of wire, and then into many loops of wire in a device called a solenoid. We end the lesson with a discussion of electromagnets, the factors that increase or decrease the strength of electromagnets, and the various applications of electromagnets.
What did Oersted's experiment show?
Explain how to do the experiment.
Note: if you would like to try this experiment, see me for the materials!
Explain how to do the experiment.
Note: if you would like to try this experiment, see me for the materials!
Now that we know a current carrying wire has a magnetic field lets look into this a little more!
Review magnetic fields around a bar magnet
What does the magnetic field lines look like around a current carrying wire?
Explain the right hand curl rule.
Note: the current they use when doing this rule is the conventional current. Therefore "current" is the flow of positive electrons.
Review magnetic fields around a bar magnet
What does the magnetic field lines look like around a current carrying wire?
Explain the right hand curl rule.
Note: the current they use when doing this rule is the conventional current. Therefore "current" is the flow of positive electrons.
It is difficult to draw a 3 dimensional picture of the magnetic fields around magnets. So physicists have simplified matters by limiting the drawings to 2 dimensions. They use a dot to represent current traveling out of the page (picture b) and a cross to represent current travelling into the page (picture a).
What symbol do we use for current? Magnetic field?
What does the magnetic field lines look like in a loop?
Does the inside or the outside of the loop have a stronger field strength?
What is a solenoid?
What does the magnetic field of a solenoid look like?
Note: Again, if you would like to try this experiment I have the materials you will need, come and see me!
What does the magnetic field lines look like in a loop?
Does the inside or the outside of the loop have a stronger field strength?
What is a solenoid?
What does the magnetic field of a solenoid look like?
Note: Again, if you would like to try this experiment I have the materials you will need, come and see me!
We can define an electromagnet as a current carrying coiled conductor. Thus a solenoid is an example of an electromagnet. There are several factors that can affect the strength of the magnetic field inside an electromagnet.
The iron used in a solenoid core must be of a type called “soft.” This does not mean physically soft like putty. It is of a type of iron that demagnetizes quickly when the current in the solenoid is shut off and the magnetic field drops to zero. An iron core solenoid has many applications related to its ability to be a strong magnet that can be turned on and off.
- The type of material in the coil’s centre is important. If a piece of ferromagnetic material, such as iron (or steel that contains iron), is placed in the core of a solenoid, the magnetic field becomes much stronger. The domains in the iron are aligned by the magnetic field in the coil, and total magnetic field is the sum of the field due to the coil and that due to the magnetized core material. The total field can be thousands of times as strong as that of the coil alone. The ratio of the magnetic field strength with a particular core material to the magnetic field strength without it is called the permeability of the core material. For example of permeability of steel is 2000 and for iron it is 6100.
- If the size of the coil (diameter) is smaller, then the field strength is more concentrated and the magnetic field is stronger.
- If the number of coils or turns of wire is increased, then the magnetic field of the coil will also increase.
- If the amount of electrical current flowing through the coil is increased, then the magnetic field will also be stronger.
The iron used in a solenoid core must be of a type called “soft.” This does not mean physically soft like putty. It is of a type of iron that demagnetizes quickly when the current in the solenoid is shut off and the magnetic field drops to zero. An iron core solenoid has many applications related to its ability to be a strong magnet that can be turned on and off.
- Lifting electromagnets are able to lift large ferromagnetic materials.
- In a relay, the electromagnet can attract a ferromagnetic metal to a contact point, and thus acts like a switch. Such a switch can be used to turn on high-currents circuits such as the bank of lighting in the Sky Dome in Toronto.
- In an electric bell, an electromagnet oscillates on and off causing a bell to ring.
- In magnetic speakers, a current is varied in a coil of wire in a magnetic field. The coil is attached to a speaker cone which moves back and forth producing the sound.
Once you have completed the videos and the readings complete learning activity 6.3.