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## Physics - Magnetic Fields

Now, let us talk about magnetic fields. When you say electric field, you are talking about a stationary charge and the attractive and repulsive forces it imposes on a positive charge lying in its vicinity.

Now, when you say magnetic field, you are talking about something different. You are talking about a moving positive charge and the attractive and repulsive forces it imposes on a charge moving in its vicinity.

So remember; one, an electric field arises in connection with the charge that is stationary; two, a magnetic field arises in connection with the charge that is moving. Also, a magnetic field will impose a force only on a charge that is in motion itself. Stationary charges are not affected by magnetic fields.

Electric current produces magnetic fields. You know that a moving charge creates a magnetic field. You also know that electric current is actually a way of describing a whole bunch of electrons moving through a conductor.

If the electrons are moving in one direction, we say that the current or positive charge is moving in the other. Now listen to this, whenever current moves through a conductor, it creates a magnetic field that runs in a circle around the conductor. As the magnetic field runs around the conductor, it might be directed one way or the other.

Finding the Direction

You maybe given a current and asked to find the direction of the field that produces or you maybe given a particular magnetic field and asked to find the direction of the current that produced it.

To solve these problems, we use the right-hand rule. Suppose you were shown the current and asked to tell about the direction of the magnetic field it produces. Here is what you do, take your right hand, point your thumb in the direction of the current which is the direction in which positive charge is moving, let your fingers curl naturally. The magnetic field is running in the direction that your fingers are curled from knuckle to nail.

Magnetic fields have strength. Magnetic strength is measured in a unit called the tesla (T) and the formula to find it is given in the following rule.

Rule number 37:
Field strength equals constant times current over distance from the wire. The constant use is called the permeability of free space and its value is 1.26x10-6 TM/A. Notice the field strength is directly proportional to current and inversely proportional to distance from the wire.

Another right-hand rule, you know that moving charge produces a magnetic field. You should also know that the magnetic field will also exert a force on the charge. And here is how you find the direction of that force. Take your right hand, point your thumb in the direction in which the charge is moving, open your fingers and they will be in the direction of the magnetic field.