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Transcript
Second Semester 2014 Magnetic Fields 1 4.2.1 define magnetic field B or magnetic flux density B (and its unit the tesla) in terms of the force on a current carrying conductor; 4.2.2 state and apply Fleming’s left-hand rule to determine the direction of force on a current carrying conductor placed at right angles to a magnetic field; 4.2.3 select and use the equations BIL F= and θ sin BIL F= ; 4.2.4 select and use the equations for the motion of a charged particle: 4.2.5 analyse the motion of charged particles in both electric and magnetic fields; 4.2.6 sketch the magnetic field lines for a long straight currentcarrying wire, a long solenoid, a bar magnet and the Earth. 2 Fleming’s LHR Aims: •To know that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field. •To recall that a force is exerted on a current-carrying wire in a magnetic field, and, how this effect is applied in loudspeakers. •To predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field (Flemming’s LHR). 3 Magnetic force •When a current passes through a piece of foil a magnetic field is created around the strip. •The Permanent horseshoe magnet repels the foil upwards. 4 Reverse the current •If the direction of the electric current is reversed the magnetic field acts in the opposite direction. •The permanent horseshoe magnet attracts the foil downwards. 5 Flemming’s apparatus 6 Flemming’s apparatus •The d.c. electric current passes through one iron bar, across the moveable axle and back through the other iron bar. •The electric current passing through the axle creates a magnetic field. •The permanent magnet will either attract of repel the axle causing it to move. 7 Fleming’s Left Hand Rule 8 Get your hands ready •We are now going to look at the interaction of the current in a wire with a magnetic field. •We know that the two will either attract or repel when a current is present. •Fleming’s Left Hand Rule (LHR) will let us work out the direction of force on a wire near a magnet. •This will allow us to understand how a motor and electric speaker work. 9 LHR •Grab a pen and get ready to write on your hand. •The second finger of your left hand represents the direction of the electric current, draw a ‘C’ on this finger. •The first finger of your left hand represents the direction of the magnetic field, draw a ‘F’ on this finger. •Your left thumb represents the direction of thrust or force on the wire, draw a ‘Th’ on your thumb. •Now point these three so that they are at right angles to each other! 10 To use Fleming’s left-hand rule, hold the thumb and the first two fingers of your left hand at right angles to each other. direction of force (thumb) direction of magnetic field (first finger) direction of current (second finger) 11 We can use the ‘Left Hand Rule’ to see which way the wire moves 12 13 LHR example Which way will the wire move? 14 Direction of the Force on a Wire =? Force on a Wire Moving particles •Remember that an electric current represents a flow of positive particles from positive to negative. •If a proton was moving through a magnetic field you could use Flemming’s left hand rule to work out how its motion would be changed. •For negative particles, such as an electron, you can still use the left hand rule but the direction of force/thrust will need to be reversed. •Any particle moving along a magnetic field line will not be affected by the field. 17 How to increase the turning effect on the coil? • Increase the current • Increase a stronger magnet • Increasing the number of turns on the coil • Increasing the area of the coil in the field. Practical motors • Several sets of coils are used, each set at a different angle and with its own pair of commutator. • Each coils contain hundreds of turns of wire and are wound on a core call an armature. • The pole pieces are curved to create a radial magnetic field. Summary – Flemming’s LHR •When a proton, electron or current carrying wire are in a magnetic field they will experience a force. •Flemming’s left hand rule can be used to find the direction of the force. Thumb = Force/thrust. First finger = direction of magnetic field. Second finger = direction of electric current. •In a speaker the magnetic field from the current in the wire either attracts or repels the permanent magnets, the speaker moves in and out creating a sound wave. 22