2.7 Force between current carrying conductors

1. 2.7 Force between current carrying conductors • If two current-carrying conductors are placed near to one another, then each of the conductors is in the magnetic field which the current in the other creates. • When the currents in the two long straight conductors X and Y are in the same direction, there is a force of attraction between them which tends to pull the conductors towards each other (see figure (i)).

2. I1 I2 Current – 2nd finger Current – 2nd finger Field- 1st finger Field- 1st finger Force- thumb Force- thumb 2.7 Force between current carrying conductors – same direction B– field by wire 1 using RH grib rule B– field by wire 2 using RH grib rule

3. Force Field B-field on X due to Y Field Force Current Current 2.7 Force between current carrying conductors –same direction B-field on Y due to X In figure (i), at Y, the flux density B due to the conductor X is perpendicular to Y. Thus, from Fleming’s rule, the direction of the force F on Y is towards X. By Newton’s 3rd Law, the force F on X is pointing towards Y and equal to that on Y. Hence the conductors attract each other.

4. 2.7 Force between current carrying conductors – opposite direction • When the currents flow in opposite directions, there is a repulsive force between them which tends to pull the conductors away from one another (see figure (ii)). • Fleming’s left hand rule confirms the direction of the forces.

5. Current Force Field Force Field Current 2.7 Force between current carrying conductors – opposite direction Figure ii: Forces between currents in opposite directions Figure i: Forces between currents in same directions In figure (ii), the current I’ in Y is opposite to that in figure (i). From Fleming’s left hand rule, the forces F on Y is now away from X and therefore the force is repulsive.