The Solenoid and Electromagnet

An electromagnetic is strengthened the more coils and more current running through a solenoid wrapped around a soft iron core.

Solenoid

The solenoid is a coiled conductive wire.  A solenoid can be used to increase a magnetic field that would be present in a single straight line wire.

Each time a wire wraps around with current flowing the same way the overall magnetic field is intensified.  If you held any wire in your hand with your thumb in the direction of current the magnetic field produced follows the curl of your hand.  The same magnetic field produced following a straight wire current.

Note: The yellow dots in this page's animations do not represent electrons.  These dots represent conventional current (I) positive flow.  This would be from the positive terminal to a negative terminal of a power source.

Counter Clockwise Solenoid Current
Counter Clockwise Solenoid Current

Using the Straight Line Current Right Hand Rule with a Solenoid

Observe how the right hand following the counterclockwise current in the solenoid.

  1. The thumb follows the direction of current
  2. The curl of the hand is in the direction of the magnetic field

The right hand is coming up through the center of the coil and curls over and down to the outside.  This is consistent with the magnetic field produced inside and out of the solenoid with counterclockwise current.

 

Counter Clockwise Current in a Solenoid

In the picture with a counter clockwise current the magnetic field on the inside the solenoid (north of a compass) is up.

In that same counter clockwise picture the magnetic field on the outside of the solenoid is down.

Counter Clockwise Solenoid Current
Counter Clockwise Solenoid Current

Clockwise Current in a Solenoid

When you change the direction of current you also flip the direction of magnetic field inside and outside the solenoid.

In the picture with a clockwise current the magnetic field on the inside of the solenoid (north of a compass) is down.

In that same clockwise picture the magnetic field on the outside of the solenoid is up.  Learn more about symbols representing direction on our right hand rule page.

Clockwise Solenoid Current

Electromagnet

An electromagnet is a solenoid wrapped around a soft iron core.  "Soft" meant that the domains will align with the magnetic field produced by the solenoid.

What makes up an electromagnet?

  1. Solenoid: Coils of Wire
  2. Soft Iron Core

Electromagnet Right Hand Rule

Note: This is different from the straight-line current right hand rule.  Your thumb will now represent the north pole of an electromagnet.  Your fingers are going to represent the direction of current.

  1. Wrap your hand around the solenoid around the soft iron core with the curl of your hand in the direction of conventional current flow.  This will be either clockwise or counterclockwise.  Follow the current from the positive terminal of the power source around the solenoid.
  2. Your thumb will now be in the direction that will be the north pole of the electromagnet.

A compass now placed on the side of your thumb will have south facing toward your thumb side.  This is because the north of the electromagnet attracts the south of the compass.

Electromagnet With Current Flowing Counterclockwise

The first electromagnet picture on this page has current flowing counterclockwise.  Observe the magnetic field lines.  The direction that field lines are drawn are the direction of the magnetic field.  That direction is the same as the north pole of a compass would face.

Electromagnet With Clockwise Current

Observe the electromagnet magnetic fields produced by clockwise current.  The negative and positive terminals of the battery were switched here switching the direction to flowing clockwise.  When this occurred the curl of the hand is wrapping around the electromagnet in the opposite direction.  Therefore this results in the poles of the electromagnet being switched.

Electromagnet With Clockwise Current

Ways to Strengthen an Electromagnet

The two ways to strengthen an electromagnet are:

  1. Increase the number of coils of the solenoid
  2. Increase the current (Increasing the voltage of the power source would result in this)

In the first animation the number of coils around the solenoid are increased.  This increases the strength of the electromagnet.  The voltage of the power source is the same so current will be the same ignoring any extra resistance from a longer wire.

In this second animation the voltage of the battery is increased.  This would increase the current through the solenoid and thus increase the electromagnets strength.

Example Problems

1. Which combination would lead to the greatest increase in strength of an electromagnet?

  • A. Increase the voltage and decrease the number of coils
  • B. Decrease the voltage and increase the number of coils
  • C. Increase the voltage and increase the number of coils
  • D. Decrease the voltage and decrease the number of coils

C. Increase the voltage and increase the number of coils

Increasing voltage increases current strengthening the electromagnet.  Increasing the number of coils in the solenoid would also increase the strength.  Both together would lead to the greatest increase in magnetic strength.

2. What would happen to an electromagnet if you switched the direction of current in the solenoid around it? 

The north and south poles of the electromagnet would flip

3. What pole would be at the bottom of this electromagnet when current is run the direction seen here?

Electromagnet Question 1

South

(Using the right hand rule for an electromagnet)

Current would flow counterclockwise.  Your thumb would be facing up so the bottom would be the south pole.

4. What poll of the electromagnet would be on the right below if a battery was attached so current flowed the way you see it below?

Electromagnet Question 2

North

(Using the right hand rule for an electromagnet)

Current would run from the positive terminal to the negative.  This would be counterclockwise and your thumb would point to the right.

5. What direction would current flow in a solenoid loop (clockwise or counterclockwise) if the magnetic field on the inside was facing towards you oriented like you see it below?

Solenoid Question 4

Counterclockwise

(Using the straight line wire right hand rule)

If you wrapped your right hand around the right side of the loop so your hand was facing downwards on the outside and fingers upwards on the inside, your thumb would be facing upwards on the right side meaning current would flow around counterclockwise.  Try to study the solenoid animation above if you are having trouble with this.

6. What direction would the magnetic field be pointing (inwards towards the screen or outwards towards you) if the current was running clockwise around the solenoid loop?

Current Carrying Magnetic Loop Question

Inwards towards the screen

(Following the straight hand current right hand rule)

If you used your right hand to hold the loop on the right side so that your thumb was facing downwards with the clockwise current, your hand would be on the outside and your fingers would be facing inwards in the center.

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