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Transcript
ConcepTest 28.5 Velocity Selector 1) up (parallel to E ) What direction would a B field have to point for a beam of electrons moving to the right to go undeflected through a region where there is a uniform electric field pointing vertically upward? 2) down (antiparallel to E ) 3) into the page 4) out of the page 5) impossible to accomplish E B=? electrons v ConcepTest 28.5 Velocity Selector What direction would a B field have to point for a beam of electrons moving to the right to go undeflected through a region where there is a uniform electric field pointing vertically upward? Without a B field, the electrons feel an electric force downwards. In order to compensate, the magnetic force has to point upwards. Using the right-hand rule and the fact that the electrons are negatively charged leads to a B field pointing out of the page. 1) up (parallel to E ) 2) down (antiparallel to E ) 3) into the page 4) out of the page 5) impossible to accomplish E B=? electrons v ConcepTest 28.6b Magnetic Force on a Wire II A horizontal wire carries a current 1) left and is in a vertical magnetic field. 2) right What is the direction of the force on 3) zero the wire? 4) into the page 5) out of the page I B ConcepTest 28.6b Magnetic Force on a Wire II A horizontal wire carries a current 1) left and is in a vertical magnetic field. 2) right What is the direction of the force on 3) zero the wire? 4) into the page 5) out of the page I When the current is parallel to the magnetic field lines, the force on the wire is zero. B ConcepTest 28.7a Magnetic Force on a Loop I 1) + x A rectangular current loop is 2) + y in a uniform magnetic field. 3) zero What is the direction of the 4) - x net force on the loop? 5) - y B z y x ConcepTest 28.7a Magnetic Force on a Loop I A rectangular current loop is in a uniform magnetic field. What is the direction of the net force on the loop? 1) + x 2) + y 3) zero 4) - x 5) - y Using the right-hand rule, we find that each of the four wire segments will experience a force outwards from the center of the loop. Thus, the forces of the opposing segments cancel, so B z the net force is zero. y x