天津一汽夏利汽车股份有限公司.ppt

ism
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东莞叉车培训/
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ic fields can be caused in three different ways
1. A moving electrical charge such as a wire with current flowing in it
2. By electrons or protons which act like little bar s
3. By some ic atoms such as iron (ism)
All of the appearances of ism can be attributed to the movement of electrical charges or the ic moment from elementary particles as a result of their spin.

In order to describe the phenomena of ism, we need the concept of a ic field. ic fields can be caused by
ic material such as a permanent
electric current such as current flowing through an inductor
3. the change in time of an electric field.
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Inductor
ic field caused by current flowing through some coils.
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Lorentz Force
The ic field has an effect on the force of moving charges. This is called the Lorentz force. The Lorentz force is the force on a point charge due to ic fields. It takes effect perpendicular to the field lines of the ic field and perpendicular to the direction of the movement of the charge.
where
F is the force (in newtons)
E is the electric field (in volts per metre)
B is the ic field (in teslas)
q is the electric charge of the particle (in coulombs)
v is the instantaneous velocity of the particle (in metres per second)
Biot-Savart Law
Consider a current I flowing through a wire. Break the wire into
little pieces of length ds. The ic field due to this little piece of current is then found experimentally to be
or
This is the law of Biot-Savart. r is the distance from the current element I ds to the field point P where we wish to find the ic field B. is a unit vector pointing along r. is a constant of nature, the permeability of free space.
The relationship between ds, er, and dB is shown in the figure. To determine the direction of use your right hand and point your fingers along ds and curl them toward er. Your right thumb will point along dB.
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To find the total ic field due to a conductor, we add up the
contributions from each current e