An electromagnet is a piece of wire intended to generate a magnetic field with the passage of electric current through it. Though all current-carrying conductors produce magnetic fields, an electromagnet is usually constructed in such a way as to maximise the strength of the magnetic field it produces for a special purpose. Electromagnets are commonly used in research, industry, medical, and consumer products. An example of a commonly used electromagnet is in security doors, e.g. on shop doors which open automatically.
As an electrically-controllable magnet, electromagnets form part of a wide variety of “electromechanical” devices: machines that produce a mechanical force or motion through electrical power. Perhaps the most obvious example of such a machine is the electric motor which will be described in detail in a previous lesson. Other examples of the use of electromagnets are electric bells, relays, loudspeakers and scrapyard cranes.
Optional Video: Magnetic Field
Optional Experiment: Electromagnets
A magnetic field is created when an electric current flows through a wire. A single wire does not produce a strong magnetic field, but a wire coiled around an iron core does. We will investigate this behaviour.
a battery and holder
a length of wire
a few nails
If you have not done the previous experiment in this section do it now.
Bend the wire into a series of coils before attaching it to the battery. Observe what happens to the deflection of the needle on the compass. Has the deflection of the compass grown stronger?
Repeat the experiment by changing the number and size of the coils in the wire. Observe what happens to the deflection on the compass.
Coil the wire around an iron nail and then attach the coil to the battery. Observe what happens to the deflection of the compass needle.
Does the number of coils affect the strength of the magnetic field?
Does the iron nail increase or decrease the strength of the magnetic field?
Optional Case Study: Overhead power lines and the environment
Power lines are a common sight all across our country. These lines bring power from the power stations to our homes and offices. But these power lines can have negative impacts on the environment. One hazard that they pose is to birds which fly into them. Conservationist Jessica Shaw has spent the last few years looking at this threat. In fact, power lines pose the primary threat to the blue crane, South Africa’s national bird, in the Karoo.
“We are lucky in South Africa to have a wide range of bird species, including many large birds like cranes, storks and bustards. Unfortunately, there are also a lot of power lines, which can impact on birds in two ways. They can be electrocuted when they perch on some types of pylons, and can also be killed by colliding with the line if they fly into it, either from the impact with the line or from hitting the ground afterwards. These collisions often happen to large birds, which are too heavy to avoid a power line if they only see it at the last minute. Other reasons that birds might collide include bad weather, flying in flocks and the lack of experience of younger birds.
Over the past few years we have been researching the serious impact that power line collisions have on Blue Cranes and Ludwig’s Bustards. These are two of our endemic species, which means they are only found in southern Africa. They are both big birds that have long lifespans and breed slowly, so the populations might not recover from high mortality rates. We have walked and driven under power lines across the Overberg and the Karoo to count dead birds. The data show that thousands of these birds are killed by collisions every year, and Ludwig’s Bustard is now listed as an Endangered species because of this high level of unnatural mortality. We are also looking for ways to reduce this problem, and have been working with Eskom to test different line marking devices. When markers are hung on power lines, birds might be able to see the power line from further away, which will give them enough time to avoid a collision.”
Impact of fields
The fact that a field is created around the power lines means that they can potentially have an impact at a distance. This has been studied and continues to be a topic of significant debate. At the time of writing, the World Health Organisation guidelines for human exposure to electric and magnetic fields indicate that there is no clear link between exposure to the magnetic and electric fields that the general public encounters from power lines, because these are extremely low frequency fields.
Power line noise can interfere with radio communications and broadcasting. Essentially, the power lines or associated hardware improperly generate unwanted radio signals that override or compete with desired radio signals. Power line noise can impact the quality of radio and television reception. Disruption of radio communications, such as amateur radio, can also occur. Loss of critical communications, such as police, fire, military and other similar users of the radio spectrum, can result in even more serious consequences.
Discuss the above information.
Discuss other ways that power lines affect the environment.
When lightning strikes a ship or an aeroplane, it can damage or otherwise change its magnetic compass. There have been recorded instances of a lightning strike changing the polarity of the compass so the needle points south instead of north.