Force on current carrying conductor in magnetic field and electric motor- P10 13.03
THE PHYSICS CAFE 
CHAPTER THIRTEEN – MAGNETIC EFFECTS OF ELECTRIC CURRENT
13.03 Force on current carrying conductor in magnetic field and electric motor
If a wire carrying current is placed in
magnetic field, then
Current carrying wire will produce its
own magnetic field around it and another magnetic field is already given, so
both magnetic field will interact with each other and apply a force on each
other, so we can say when a wire carrying current is placed in magnetic field,
it experience a force
Hint: Now it become
interesting to find out the direction of this force – direction of force is
given by ‘Fleming’s left hand rule’
Fleming’s left hand rule: Arrange mutually perpendicular; first and second finger and thumb of your left hand. Now arrange
Magnitude of this force will depends on
- By
increasing strength of magnetic field
- By
increasing current in the wire
- By
increasing length of wire inside magnetic field (to increase length just
form the coil of n turns)
Hint: Magnetism in Medicine
In our body charge moves in the form of ions and due to their flow a weak ion current produced which forms weak magnetic field. This magnetic field is significant in our heart and brain which helps to obtaining their images with the help of a technology called Magnetic Resonance Imaging (MRI).
Electric Motor:
If we give electric
current and a shaft start to rotate such device is called Motor, so electric motor is a device which
converts electric energy into mechanical energy (rotating shaft has mechanical
energy)
Working principal: when a current carrying conductor is
placed in magnetic field, it experience a force
Working function of
different parts:
- A
horse-shoe (U-shaped magnet) is used to produce uniform magnetic field
- Coil
ABCD is used a current carrying conductor
- Split
rings (commutator) are used to avoid breaking of wire due to twisting of
wires (connection of external circuit to the coil)
- Carbon
brushes are used to extract the current from the coil.
- The soft iron core on which coil is wound is called Armature this increase the effectiveness of coil
Working of motor:
Current in coil ABCD, enter in Arm AB from through X and flowing to leave arm CD through Brush Y. By using Fleming’s left hand rule Force pushing arm AB downward and arm CD upwards. Thus the coil starts rotating anticlockwise. At half rotation, Q makes contact with brush with X and P with Y. now current in arm CD flow from D to C and in AB flow from D to C. Now CD arm is pushed downward and AB arm upward. Therefore the coil rotate half a turn in the same direction. Like the same direction of current in the coil changes after every half rotation and the coil thus continuous rotate.
Rotation of shaft depends
on;
- By
increasing strength of magnetic field- for this we uses electromagnet in
place of permanent magnet
- By
increasing current in the wire – but we don’t want to increase the current
because more current produces more heat. (H = i²Rt)
- By
increasing number of turns in the coil and number of coils
- By introducing soft iron core inside the coil
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