HSC Physics Notes – Motors and Generators
9.3 - 1. Motors use the effect of forces on current-carrying conductors in magnetic fields
1. discuss the effect on the magnitude of the force on a current-carrying conductor of variations in – the strength of the magnetic field in which it is located Force is proportional to the magnetic field strength (B). i.e. ↑F = ↑B – the magnitude of the current in the conductor The force is proportional to the current (I) in the conductor. i.e. ↑F = ↑I – the length of the conductor in the external magnetic field The force is proportional to the length (l) of the conductor. i.e. ↑F = ↑l the angle between the direction of the external magnetic field and the direction of the length of the conductor The force is at a maximum when conductor is at right angles to the field. The force is equal to zero when the conductor is parallel to the field. ↑F = ↑Θ. –

F = BIlsin  Can be used to determine force in a magnetic field
2. explain qualitatively and quantitatively the force between long parallel current-carrying conductors Because we know that a wire carrying a current will produce a magnetic field, thus it will exert forces upon other fields or objects near or in the field. When two long parallel current-carrying conducting wires (known as a solenoid) are placed side by side with a finite distance between them, their magnetic fields will affect each other.

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If the current is flowing in the same direction in both conductors, the fields will attract (making a combined, larger field) If the currents of the conductors are flowing in opposite directions, the magnetic fields will repel from each other. The right-hand grip rule can also be applied to determine the direction of flow of current and thus whether the two long parallel current-carrying conducting magnetic fields exert repelling or attracting fields with relation to each other. Determining the magnitude of force between two parallel current-carrying conducting wires is given by the following equation:

d

where; F = the force acting upon the length of a conductor (N) l = length of chosen conductor (m) k = constant (derived through careful analysis + testing) = 2.0 x 10-7 N A-2 I1 and I2 = current of either conductor (amps) d = distance between conductors (m) 3. define torque as the turning moment of a force using:  = Fd Torque is the turning effect of an object when force is acting upon it. The torque of an object is greater when the distance of the force from the pivot point (where the torque occurs) is further away. Thus, as distance increasing, so does the torque of an object.

I I F = k 1 2 l d

If the force applied is perpendicular to the line joining the point of application of the force and the pivot point, the following formula can be used:

 = Fd

where; т = Torque of an object (Newton metre – Nm) F = Force (N) d = Distance from the point of application to the pivot point (m) 4. identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field A current-carrying conductor produces a magnetic field. When the current-carrying conductor passes through an external magnetic field, the magnetic field of the conductor interacts with the external magnetic field and the conductor experiences a force. This effect was discovered in 1821 and is known as the motor effect. The direction of the force on the current-carrying conductor can be determined using the right hand push rule – remembering that magnetic field lines go from north to south. 5. describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces If a current-carrying wire (loop) is present in an external magnetic field, then the current-carrying conductor will experience forces exerted upon it. Applying the right hand push rule, one can determine the direction of force on the loop in the external magnetic field. The force acting on the sides of the coil that