The motor effect

1. The motor effect

2. S N S N

3. S N t F Field I S N thrust Current

5. Vectors here add positively The current is into the page N S Vectors At this side cancel

6. field N S thrust This combination of field’s is sometimes referred to as a “catapult field”

7. The force on a current carrying conductor N S

8. Predict what will happen when the current is switched on. • What will happen if the poles of the magnet are reversed? • What will happen if the current is increased?

9. Calculating the force on a conductor in a perpendicular magnetic field Flux density B L I Force L is the length of the conductor within the field. I is the current through the conductor. F = BIL

10. Exam Question At a certain point on the Earth’s surface the horizontal component of the magnetic field is 1.8 x 10-5 T. A straight piece of wire 2m long with a mass of 1.5 g lies on a horizontal woodn bench in an East-West direction. When a very large current flows in the wire momentarily it is just sufficient to cause the wire to lift off the surface of the bench. • State the direction of the current in the wire. • Calculate the current in the wire. • What other noticeable effect will this current produce?

11. Exam Question At a certain point on the Earth’s surface the horizontal component of the magnetic field is 1.8 x 10-5 T. A straight piece of wire 2m long with a mass of 1.5 g lies on a horizontal woodn bench in an East-West direction. When a very large current flows in the wire momentarily it is just sufficient to cause the wire to lift off the surface of the bench. • State the direction of the current in the wire. To the East • Calculate the current in the wire. F=BIL, so I=F/BL F= (1.5 x 10-3 x 9.81)N I= (1.5 x 10-3 x 9.81)/(1.8x10-5 x 2) I=410A 3. What other noticeable effect will this current produce? The wire melts.

12. Current into the board Field Thrust on wire N S - + 134.95

13. There is a downward force on the conducting rod. It is fixed and cannot move. By Newton’s third law there is an equal and opposite force on the magnetic “yoke” and the yoke is pulled up. This reduces the force on the balance and the reading goes down N S - + 119.48 134.95

14. Exam Question The magnitude of force in a current carrying conductor in a magnetic field is directly proportional to the magnitude of the current in the conductor. With the aid of a diagram, describe how you could demonstrate this in a school laboratory.

15. Diagram of a conductor perpendicular to a magnetic field (1) • Method of providing, varying and measuring d.c. current. (1) • Method of measuring variable force (eg top pan balance) (1) • For various values of current measure the force produced (1) • Plot F against I and straight line through origin (1) F=BIL FI Gradient = BL F/N I/A

16. Current 1 A Field 1T 1 N Defining the Tesla The tesla is defined in terms of the motor effect of a conductor in a magnetic field. One tesla is the magnetic flux density of a field in which a force of 1 newton acts on a 1 metre length of a conductor which is carrying a current of 1 ampere and is perpendicular to the field.

17. Two current carrying wires with current in the same direction have magnetic fields in the same sense. When they are brought close together they experience an attractive force This is because the field vectors cancel in the area between them and the resultant force on each wire is towards the other. We may think of a “catapult” field at both sides.

18. This is because the field vectors cancel in the area between them and the resultant force on each wire is towards the other.