Force on a current-carrying conductor
Charges confined to wires can also experience a force in a magnetic field. A
current (I) in a magnetic field (B) experiences a force (F) given by the equation F = I l ×
B or F = IlB sin θ, where l is the length of the wire, represented by a vector pointing in
the direction of the current. The direction of the force may be found by a right
‐
hand rule
similar to the one shown in Figure above. In this case, point your thumb in the direction
of the
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current - the direction of motion of positive charges. The current will
experience no force if it is parallel to the magnetic field.
Torque on a current loop
A loop of current in a magnetic field can experience a torque if it is
free to turn. Figure (a) depicts a square loop of wire in a magnetic field
directed to the right. Imagine in Figure (b) that the axis of the wire is turned
to an angle (θ) with the magnetic field and that the view is looking down on
the top of the loop. The x in a circle depicts the current traveling into the
page away from the viewer, and the dot in a circle depicts the current out of
the page toward the viewer.
Figure 3
(a)
Square current loop in a magnetic field B. (b) View from the top of the current
loop. (c) If the loop is tilted with respect to B, a torque results.
The right
‐
hand rule gives the direction of the forces. If the loop is pivoted, these
forces produce a torque, turning the loop. The magnitude of this torque is t = NI A × B,
where N is the number of turns of the loop, B is the magnetic field, I is the current, and
A is the area of the loop, represented by a vector perpendicular to the loop.
Galvanometers, ammeters, and voltmeters
The torque on a current loop in a magnetic field provides the basic principle of the
galvanometer, a sensitive current
‐
measuring device. A needle is affixed to a current coil
- a set of loops. The torque gives a certain deflection of the needle, which is dependent
upon the current, and the needle moves over a scale to allow a reading in amperes.
An ammeter is a current
‐
measuring instrument constructed from a galvanometer
movement in parallel with a resistor. Ammeters are manufactured to measure different
ranges of current. A voltmeter is constructed from a galvanometer movement in series
with a resistor. The voltmeter samples a small portion of the current, and the scale
provides a reading of potential difference – volts between two points in the circuit.
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