Applications of current in a magnetic field (motors, meters and loudspeakers)
The simple d.c. motor is based on the principle outlined in Sample Problem 20B as is the moving-coil galvanometer, see Fig 20.14 and Fig 20.13 respectively.The moving-coil loudspeaker (Fig 20.15) is also based on the principle that a current-carrying conductor in a magnetic field experiences a force.1
Pointer N S
Uniform scale Spring
Moving coil Fixed soft-iron core
Split ring commutator
Carbon brushes
Magnet N S
Rotation produced
Coil Pivot
Fig 20.13: Moving-coil galvanometer Magnet
Fig 20.14: d.c. motor
Vibration
Coil attached to cone and free to move
Cone
Varying alternating current from amplifier
Fig 20.15: Moving-coil loudspeaker (a) Positively charged particle Force (b) Negatively charged particle Force
Fig 20.16: Force on a mov- ing charge in a magnetic field
V Path Force Path V
Flux density B into page
Force on a Charged Particle Moving
through a Magnetic Field Current is the flow of electric charge. Even when only single charge moves, it is still a current, and as such it experiences a force in a magnetic field. Fleming’s left-hand rule can still be used to find the direction of the force on the particle. Recall that the direction of (conventional) current is from positive to negative.Therefore, the direction of the velocity of a positive particle is the same as the directions of the current, while it is in the opposite direction for a negative particle. Fig 20.16 shows the directions of the forces acting on (a) a positive particle and (b) a negative particle as each one enters a uniform magnetic field.The size of the force on the particle is equal to the product of the size of its charge, its velocity and the flux density of the magnetic field.
1The simple d.c. motor, the moving-coil galvanometer and the moving-coil loudspeaker are all dealt with in greater detail in Chapter 24 for those who choose the Further Electricity option.
