Учебное пособие Казань-2016 Сайфуллина М. Н., Хабирова Н. М. English for Physicists
Force on a current-carrying conductor
жүктеу/скачать 1,1 Mb. Pdf көрінісі
|
Posobie dlya fizikov Habirova Sajfullina ispravv
- Бұл бет үшін навигация:
- Torque on a current loop
- Galvanometers, ammeters, and voltmeters
| 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 69 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. 70 |