* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Magnetic Forces
Condensed matter physics wikipedia , lookup
Maxwell's equations wikipedia , lookup
Speed of gravity wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetic monopole wikipedia , lookup
Field (physics) wikipedia , lookup
Magnetic field wikipedia , lookup
Electromagnetism wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Superconductivity wikipedia , lookup
Magnetic Forces Magnetic Field: Magnetic Field = B=F/qv or Force SI Unit: (charge)(speed) F= 1T = 1N/Am (If v and B perpendicular) If the charge is moving as a current, I in a wire of length, L F = (If I and B perpendicular) Direction of Current Direction of Magnetic Field. (shown lines of magnetic force) If the field direction and charge direction are not perpendicular then and Note: Unlike gravitational force or electric force, magnetic force is perpendicular to the plane formed by the field and the moving charge, and is greatest when the magnetic field and current are perpendicular to each other. The force on the current carrying wire shown above is therefore into the plane of the page and is determined by using the left-hand finger rule. . 1 G = 1 x 10-4 T Another unit for magnetic field is The magnetic field at the surface of the earth is about 0.5 G or 0.5 x 10 -4 T Strong electromagnets can produce fields on the order of 2 T and superconducting magnets over 10 T Magnetic Field can also be defined as Lines moving through unit surface area perpendicular to the field B = Number of Magnetic Field Lines Area perpendicular to field = Flux Area = /A 1 Wb = 1 T m2 Flux is measured in Example A synchrotron accelerates protons up to 3 x 107 m/s and uses a steering magnet to bend them into a curved path. If the magnetic force on a proton has to be 1.9 x 10-11 N a) what strength of magnetic field is needed? b) label the direction of the field on the diagram below and c) how much stronger is this field than the earth? + F v Electromagnetic Induction If a current carrying wire moving in a magnetic field experiences a perpendicular force then a changing magnetic field must induce a current in a stationary coil of wire. So moving a magnet in and out of a wire coil must induce an alternating current in the wire. Faraday’s Law If the magnetic flux (field lines) through a given area changes over time, a voltage will be induced in the wire and a current will momentarily flow. If the number of turns of wire is increased, the voltage will increase proportionally. Potential difference = (number of turns)(change in flux) elapsed time V = - N / t Lenz’s Law The induced voltage (above) always produces a magnetic field that apposes the field that originally produced it. This is needed to conserve energy. Generators, Motors and Meters When a wire in a generator coil moves through the magnetic field, free electrons in the wire will be forced to move in a perpendicular direction. The magnitude of the force and therefore the induced voltage thus depends on the rate at which the wire moves through the field lines (changing flux). When the wire moves perpendicular to the field, it will quickly cut through the field lines and the largest induced voltage will be generated. But when the wire moves parallel to the field, the wire will not cut through any field lines and the least induced voltage will be generated. Similarly with a motor or coil meter, the rotating force on a current carrying wire will change as the coil turns in the field due to a changing rate at which the wire cuts through the field. This is why motors use multiple coil winding orientations to maximize the turning effect (torque). Coil cuts through field lines (flux) at a slow rate at the top and bottom Coil cuts through field lines (flux) at a fast rate in the middle Magnetic Forces Practice 1. In Mr. Fawcett’s TV, electron’s are shot toward the screen through a 1.0 x 10-3 T magnetic field set up in the picture tube. A) If each electron experiences a magnetic force of 2.9 x 10-15 N, at what speed is it propelled through the picture tube? B) How does this speed compare to the speed of light? 2. In the giant CERN particle accelerator in Switzerland, protons are accelerated to speeds of 2.0 x 108 m/s through a magnetic field of 3.5 T. and then collided with a fixed target. What is the magnitude of the magnetic force experienced by the protons as they are accelerated around the giant ring? 3. a) What is the force per meter on a wire carrying a 7.80 A current when perpendicular to a 0.8 T magnetic field? b) What is the angle between the wire and field is 450 ? 4. A 10 m long section of high tension power line carries a current of 20.0 A perpendicular to the earth’s magnetic field of 5.5 x 10-5 T. What is the magnetic force experienced by the power line? 5. Find the direction of the magnetic force on the charges below X + _ v v _ B v 6. Find the direction of B for each case below. F + X v + v v X F _ F 7. A square coil of side 5.0 cm contains 100 loops and is positioned perpendicular to a uniform 0.60 T magnetic field. It is quickly and uniformly pulled from the field (moving perpendicular to B) to a region where B drops abruptly to zero. It takes 0.1 s for the whole coil to reach the field free region. Find the a) change in flux through the coil and b) the induced voltage. 8. A student wears wire- rimmed glasses whose frames are shaped like two circles each with an area of 2.0 x 10-3 m2. The horizontal component of the earth’s magnetic field is 1.9 x 10-5 T. If the student turns her head 900 in 0.5 s, what is the induced voltage in the frame of one eyepiece?