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Electromagnetism: Magnetic field associated with a current

Module by: Free High School Science Texts Project. E-mail the author

Introduction

Electromagnetism describes between charges, currents and the electric and magnetic fields which they give rise to. An electric current creates a magnetic field and a changing magnetic field will create a flow of charge. This relationship between electricity and magnetism has resulted in the invention of many devices which are useful to humans.

Magnetic field associated with a current

If you hold a compass near a wire through which current is flowing, the needle on the compass will be deflected.

Since compasses work by pointing along magnetic field lines, this means that there must be a magnetic field near the wire through which the current is flowing.

Note: Interesting Fact :

The discovery of the relationship between magnetism and electricity was, like so many other scientific discoveries, stumbled upon almost by accident. The Danish physicist Hans Christian Oersted was lecturing one day in 1820 on the possibility of electricity and magnetism being related to one another, and in the process demonstrated it conclusively by experiment in front of his whole class. By passing an electric current through a metal wire suspended above a magnetic compass, Oersted was able to produce a definite motion of the compass needle in response to the current. What began as a guess at the start of the class session was confirmed as fact at the end. Needless to say, Oersted had to revise his lecture notes for future classes. His discovery paved the way for a whole new branch of science - electromagnetism.

The magnetic field produced by an electric current is always oriented perpendicular to the direction of the current flow. When we are drawing directions of magnetic fields and currents, we use the symbols ⊙⊙ and ⊗⊗. The symbol

⊙ ⊙
(1)

represents an arrow that is coming out of the page and the symbol

⊗ ⊗
(2)

represents an arrow that is going into the page.

It is easy to remember the meanings of the symbols if you think of an arrow with a head and a tail.

Figure 1
Figure 1 (PG11C8_003.png)

When the arrow is coming out of the page, you see the point of the arrow (⊙⊙). When the arrow is going into the page, you see the tail of the arrow (⊗⊗).

The direction of the magnetic field around the current carrying conductor is shown in Figure 2.

Figure 2: Magnetic field around a conductor when you look at the conductor from one end. (a) Current flows out of the page and the magnetic field is counter clockwise. (b) Current flows into the page and the magnetic field is clockwise.
Figure 2 (PG11C8_004.png)
Figure 3: Magnetic fields around a conductor looking down on the conductor. (a) Current flows clockwise. (b) current flows counter clockwise.
Figure 3 (PG11C8_005.png)

Case Study : Direction of a magnetic field

Using the directions given in Figure 2 and Figure 3 try to find a rule that easily tells you the direction of the magnetic field.

Hint: Use your fingers. Hold the wire in your hands and try to find a link between the direction of your thumb and the direction in which your fingers curl.

Figure 4
Figure 4 (PG11C8_006.png)

There is a simple method of finding the relationship between the direction of the current flowing in a conductor and the direction of the magnetic field around the same conductor. The method is called the Right Hand Rule. Simply stated, the right hand rule says that the magnetic field lines produced by a current-carrying wire will be oriented in the same direction as the curled fingers of a person's right hand (in the "hitchhiking" position), with the thumb pointing in the direction of the current flow.

Figure 5: The Right Hand Rule.
Figure 5 (PG11C8_007.png)

Case Study : The Right Hand Rule

Use the Right Hand Rule to draw in the directions of the magnetic fields for the following conductors with the currents flowing in the directions shown by the arrows. The first problem has been completed for you.

Table 1
1.
Figure 6
Figure 6 (PG11C8_008.png)
 2.
Figure 7
Figure 7 (PG11C8_009.png)
 3.
Figure 8
Figure 8 (PG11C8_010.png)
 4.
Figure 9
Figure 9 (PG11C8_011.png)
5.
Figure 10
Figure 10 (PG11C8_012.png)
 6.
Figure 11
Figure 11 (PG11C8_013.png)
 7.
Figure 12
Figure 12 (PG11C8_014.png)
 8.
Figure 13
Figure 13 (PG11C8_015.png)
9.
Figure 14
Figure 14 (PG11C8_016.png)
 10.
Figure 15
Figure 15 (PG11C8_017.png)
 11.
Figure 16
Figure 16 (PG11C8_018.png)
 12.
Figure 17
Figure 17 (PG11C8_019.png)

Experiment : Magnetic field around a current carrying conductor

Apparatus:

  1. one 9V battery with holder
  2. two hookup wires with alligator clips
  3. compass
  4. stop watch

Method:

  1. Connect your wires to the battery leaving one end of each wire unconnected so that the circuit is not closed.
  2. One student should be in charge of limiting the current flow to 10 seconds. This is to preserve battery life as well as to prevent overheating of the wires and battery contacts.
  3. Place the compass close to the wire.
  4. Close the circuit and observe what happens to the compass.
  5. Reverse the polarity of the battery and close the circuit. Observe what happens to the compass.

Conclusions:

Use your observations to answer the following questions:

  1. Does a current flowing in a wire generate a magnetic field?
  2. Is the magnetic field present when the current is not flowing?
  3. Does the direction of the magnetic field produced by a current in a wire depend on the direction of the current flow?
  4. How does the direction of the current affect the magnetic field?

Case Study : Magnetic field around a loop of conductor

Consider two loops made from a conducting material, which carry currents (in opposite directions) and are placed in the plane of the page. By using the Right Hand Rule, draw what you think the magnetic field would look like at different points around each of the two loops. Loop 1 has the current flowing in a counter-clockwise direction, while loop 2 has the current flowing in a clockwise direction.

Figure 18
Figure 18 (PG11C8_020.png)

If you make a loop of current carrying conductor, then the direction of the magnetic field is obtained by applying the Right Hand Rule to different points in the loop.

Figure 19
Figure 19 (PG11C8_021.png)

If we now add another loop with the current in the same direction, then the magnetic field around each loop can be added together to create a stronger magnetic field. A coil of many such loops is called a solenoid. The magnetic field pattern around a solenoid is similar to the magnetic field pattern around the bar magnet that you studied in Grade 10, which had a definite north and south pole.

Figure 20: Magnetic field around a solenoid.
Figure 20 (PG11C8_022.png)

Real-world applications

Electromagnets

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 maximize 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 Grade 12. Other examples of the use of electromagnets are electric bells, relays, loudspeakers and scrapyard cranes.

Experiment : Electromagnets

Aim:

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.

Apparatus:

  1. a battery and holder
  2. a length of wire
  3. a compass
  4. a few nails

Method:

  1. If you have not done the previous experiment in this chapter do it now.
  2. 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?
  3. Repeat the experiment by changing the number and size of the coils in the wire. Observe what happens to the deflection on the compass.
  4. 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.

Conclusions:

  1. Does the number of coils affect the strength of the magnetic field?
  2. Does the iron nail increase or decrease the strength of the magnetic field?
Magnetic Fields
  1. Give evidence for the existence of a magnetic field near a current carrying wire.
  2. Describe how you would use your right hand to determine the direction of a magnetic field around a current carrying conductor.
  3. Use the Right Hand Rule to determine the direction of the magnetic field for the following situations:
    1. Figure 21
      Figure 21 (PG11C8_023.png)
    2. Figure 22
      Figure 22 (PG11C8_024.png)
  4. Use the Right Hand Rule to find the direction of the magnetic fields at each of the points labelled A - H in the following diagrams.
    Figure 23
    Figure 23 (PG11C8_025.png)

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