Physics » Magnetism and Faraday's Law » Magnetic Field Associated With A Current

Magnetic Field Around a Straight Wire

Magnetic field around a straight wire

The direction of the magnetic field around the current carrying conductor is shown in the figure below.

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Magnetic field around a conductor when you look at theconductor from one end. (a) Current flows out of the page and themagnetic field is counter-clockwise. (b) Current flows into thepage and the magnetic field is clockwise.

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Magnetic fields around a conductor looking down on the conductor. (a) Current flows clockwise. (b) current flows counter-clockwise.

Optional Activity: Direction of a magnetic field

Using the directions given in the two figures above, 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.

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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.

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Important:

Your right hand and left hand are unique in the sense that you cannot rotate one of them to be in the same position as the other. This means that the right hand part of the rule is essential. You will always get the wrong answer if you use the wrong hand.

Optional Activity: 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.

1.

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2.

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3.

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4.

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5.

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6.

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7.

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8.

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9.

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10.

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11.

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12.

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Optional Activity: Magnetic field around a current carrying conductor

Apparatus

  1. one \(\text{9}\) \(\text{V}\) 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. Be sure to limit the current flow to \(\text{10}\) \(\text{seconds}\) at a time (Why you might ask, the wire has very little resistance on its own so the battery will go flat very quickly). 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?

 

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