Lesson 30: Current, Magnetic Fields, and Electromagnets

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In 1820, Hans Christian Oersted discovered that when placed near a wire that was conducting electricity, the needle of a compass could be made to move in much the same way as it would when placed near a magnet. At this time, electricity and magnetism were considered different fields of physics. Oersted recognized that if both the wire and the magnet caused the same behavior in the compass, then the two fields were likely related. Oersted is responsible for uniting the two fields and discovering electromagnetism.

Oersted noticed that magnetic fields would form around wires when electricity was flowing through them, but not when the electricity was turned off. This insight led to a tremendous advancement in our understanding of both magnets and electricity. Physicists realized that it was possible to create magnetic fields without the use of magnets and that magnetic fields could be turned off and on at will by controlling the flow of electricity.

The Magnetic Field around a Conductor:

The magnetic field around a current carrying conductor is a series of concentric loops like those in the diagram below.

The black line in the diagram represents the wire along which the electricity is flowing. The red lines show the magnetic field that is produced.

The magnetic field is generated along the entire length of the wire. Although easier to visualize as a set of concentric circles, this diagram really only considers the magnetic field at one point on the wire. Like the magnetic field around a magnet, it is also three-dimensional. It is better to visualize it as a set of concentric cylinders; kind of like a wire inside of a straw, which is inside of a larger straw, which is inside of a copper pipe, which is inside of a roll of wrapping paper, and so on. And like the magnetic field around a magnet, the magnetic field around a wire also has a direction.

The Right Hand Rule for a Current Carrying Conductor:

In order to determine the direction of the magnetic field around a current carrying conductor:

1. Grab the wire with your RIGHT HAND with your thumb pointing in the direction of the current.
2. Curl your fingers around the wire.
3. Your fingers represent the magnetic field lines and they point in the direction of the magnetic field.

In the diagram to the left, the current is flowing from the right to the left. Your right thumb should also be pointing to the left. The magnetic field is shown going upwards when it is in front of the wire, away from you when it is above the wire, down when it is behind the wire, and towards you when the field is below the wire. The fingers of your right hand should do the same.

Often a small “x” is used to indicate that the electricity is flowing away from you and a dot (•) is used to indicate that the electricity is flowing towards you.

In the image to the right, your right thumb should be pointing towards you. This causes your fingers and the magnetic field to point in a counter-clockwise direction.

In the image to the right, your right thumb should be pointing away from you. This causes your fingers and the magnetic field to point in a clockwise direction.

The Magnetic Field around a Loop:

Imagine a straight conductor that has been bent into a loop. The Right Hand Rule can still be used to determine the shape and direction of the magnetic field. However, you will need to apply this rule at various points around the loop.

In the diagram to the left, the black lines show the direction of the current and the red lines show the magnetic field at various points around the loop. You should notice that at all points on the interior of the loop, the magnetic field is pointed out of the screen towards you, and at all point outside of the loop, the magnetic field is pointed into the screen away from you. Because the radius of the inside of the loop is smaller than the radius of the outside of the loop, the magnetic field lines will be more concentrated inside of the loop. This will result in a stronger magnetic field inside the loop.

The Magnetic Field around a Helix:

A helix is a series of loops, or a coil of wire. If you were to pass electricity through a helix, it would look something like the image below.

In the image to the right, the orange lines represent the wire, the black arrows indicate the direction in which the electricity is flowing, and the red lines show the direction of the magnetic field around each coil in the helix. If you use the Right Hand Rule, you will see that the magnetic field inside the helix is pointing diagonally up and to the left and that the magnetic field at all points outside of the helix is pointing diagonally down and to the right.

If you imagine that the wire in the above image is made invisible, you will get the image shown below.

This image shows only the magnetic field lines around each loop in the helix. The overall magnetic field will be the resultant sum of the individual magnetic fields.

In the diagram to the left, the overall magnetic field around the helix is shown. Hopefully you recognize this pattern as being the same as the magnetic field generated around a bar magnet.

Here we can see part of the magnetic field around a helix. Remember that the field is three dimensional. By using the right hand rule for a conductor, we can determine the north and south magnetic poles of the helix.

The Right Hand Rule for a Helix:

There is a second Right Hand Rule that can be used to quickly determine the location of the north and south pole of a helix:

1. Grab the helix with your RIGHT HAND in such a way that your fingers are the coils of wire and point in the direction of the current.
2. If you extend your right thumb it will be pointing in the direction of the north pole.

The Electromagnet:

An electromagnet is just a current carrying helix with an iron core inserted in its centre. This iron core increases the strength of the electromagnet tremendously.

Practice Questions

1. Use your knowledge of electromagnets to determine the north and the south pole.

2. Use your knowledge of electromagnets to determine the direction in which the current is flowing.

Answer

1. The current flows clockwise around the circuit. Using the Right Hand Rule for a helix, you need to grab the helix with your right hand in such a way that your fingers of your right hand go up and over the top of the helix. When you do this your right thumb will be pointing to the left. Therefore, the left side of the helix is the north pole and the right side of the helix is the south pole.
2. Using the Right Hand Rule for a helix, you need to grab the helix with your right hand in such a way that your right thumb is pointing to the left (the north pole). When you do this your fingers go up and over the top of the helix and indicate the direction of the current. Following wire you have the negative terminal on the left and the positive terminal on the right.

Applications of Electromagnets: The Electric Bell

The image above shows an electric bell. An understanding of how it works can be gained by starting at the power source and following the flow of electricity around the circuit.

1. When the switch is closed, electricity begins to flow around the circuit in a clockwise direction.
2. As the electricity passes through the helix, it creates an electromagnet with a north pole at the top of the helix and a south pole at the bottom; use the right-hand rule for a helix to figure this out.
3. The electromagnet attracts the iron armature pulling it down and ringing the bell.
4. At the same time, the contacts come apart stopping the flow of electricity. This turns off the electromagnet and the black hinge pulls the hammer back up to its original position.
5. This connects the contacts and electricity again begins to flow through the helix turning on the electromagnet which attracts the iron armature, pulling it down and ringing the bell.
6. This cycle is then repeated indefinitely until the switch is turned off.

Strength of an Electromagnet:

There are three factors that govern the strength of an electromagnet: the nature of the core, the number of loops that make up the helix, and the current running through the wire.

The Nature of the Core:

The material that is used to make the core will affect the strength of an electromagnet. Using iron alone will make the electromagnet 20 000 times stronger than no core at all. Many alloys can now be produced that will make the electromagnet several time stronger than iron.

The number of loops that make up the helix:

The strength of an electromagnet varies directly with the number of loops, or turns, wrapped around the core. If the number of loops is doubled, then the strength of the electromagnet is doubled. If you reduce the number of loops by a third, then the strength of the electromagnet is also reduced by a third.

The current running through the wire:

The strength of an electromagnet varies directly with current. If the current is doubled, then the strength of the electromagnet is doubled. If you reduce the current by a third, then the strength of the electromagnet is also reduced by a third.

Practice Questions

1. An electromagnet is strong enough to lift 200 kg of mass. If you double the number of turns of wire around the core and triple the current, how much will it be able to lift?

Answer

Given:

Current lift capacity = 200 kg

Number of turns is doubled

Current is tripled

Required:

New lift capacity:

Solution:

Doubling the number of turns means that the electromagnet will be able to double its lifting capacity so we need to multiply the old lift capacity by 2.

Tripling the current means that the electromagnet will be able to triple its lifting capacity so we need to multiply the old lift capacity by 3.

New lift capacity = (old lift capacity) x 2 x 3

New lift capacity = 200 kg x 2 x 3

New lift capacity = 1 200 kg

If the number of turns is doubled and the current is tripled, the electromagnet will be able to lift 1 200 kg.