1. Electric Force and Electric field 1. There are two types of electric charge (positive and negative)

2. Electric Force and Electric field 2. Static charges can be produced by the action of friction on an insulator

3. Electric force and electric field 3. Conductors contain many free electrons inside them (electrons not associated with one particular atom)

4. Electric Force and Electric field 4. Charge is conserved. The total charge of an isolated system cannot change. I’m indestructible! So am I!

5. Coulomb’s law F = kq 1 q 2 r 2 The constant k is sometimes written as k = 1/4πε o where ε o is called the permittivity of free space.

6. Calculations using Coulomb’s law The force between two charges is 20.0 N. If one charge is doubled, the other charge tripled, and the distance between them is halved, what is the resultant force between them? q1q1 q2q2 r r/2 2q 1 3q 2 F = 20N F = ? N

7. Calculations using Coulomb’s law F = kq 1 q 2 /r 2 = 20.0N x = k2q 1 3q 2 /(r/2) 2 = 6kq 1 q 2 /(r 2 /4) = 24kq 1 q 2 /r 2 x = 24F = 24 x 20.0 = 480 N q1q1 q2q2 r r/2 2q 1 3q 2 F = 20.0N x = 480 N

8. Electric field An area or region where a charge feels a force is called an electric field. The electric field strength at any point in space is defined as the force per unit charge (on a small positive test charge) at that point. E = F/q (in N.C -1 )

9. Electric field around a point charge If we have two charges q 1 and q 2 distance r apart F = kq 1 q 2 /r 2 Looking at the force on q 1 due to q 2, F = Eq 1 F = kq 1 q 2 /r 2 = Eq 1 E (field due to q 2 ) = kq 2 /r 2 q1q1 q2q2 NOT in data book

10. Electric field Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. q1q1 q2q2 Resultant field Field due to q 1 Field due to q 2

11. Electric field patterns An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. The closer the lines are together, the stronger the force felt. This is an example of a radial field

12. Field around a charged metal sphere E = 0 inside the sphere

13. Field around two point charges

15. Field between charged parallel plates Uniform field E = V/d V d “Edge effects” NOT in data book

16. Remember! The force F on a charge q in a field E is F = Eq

17. Gravitational Force and Field We already know that; 1.Masses attract each other

18. Gravitational Force and Field We will know that; 2. Mass/energy is conserved (E = mc 2 )

19. Gravitational Force and Field The force between masses was formulated (discovered?) by Isaac Newton in 1687

20. Newton’s law of universal gravitation F = Gm 1 m 2 r 2 The constant G is known as “Big G” and is equal to 6.667 x 10 -11 Nm 2 kg -2

21. Newton’s law of universal gravitation F = Gm 1 m 2 r 2 For large objects like the earth, r is the distance to the centre of mass

22. Calculations using Newton’s law What is the force of attraction between Pascal and Chris? 2 m 63kg ?70kg ?

23. Calculations using Newton’s law F = Gm 1 m 2 = 6.667 x 10 -11 x 63 x 70 = 7.3 x 10 -8 N r 2 2 2 2 m 63kg ?70kg ?

24. Force of gravity due to earth on Pascal? F = Gm 1 m 2 = 6.667 x 10 -11 x 63 x 6 x 10 24 = 615 N (= mg) r 2 (6400 x 10 3 ) 2 63kg ? R = 6400 km, m = 6 x 10 24 kg Pascal’s weight

25. Force of gravity due to earth on Pascal? F = Gm 1 m 2 = 6.667 x 10 -11 x 63 x 6 x 10 24 = 615 N (= mg) r 2 (6400 x 10 3 ) 2 In other words, for any planet; g = Gm p r p 2

26. Gravitational field An area or region where a mass feels a gravitational force is called a gravitational field. The gravitational field strength at any point in space is defined as the force per unit mass (on a small test mass) at that point. g = F/m (in N.kg -1 )

27. Gravitational field around a point mass If we have two masses m 1 and m 2 distance r apart F = Gm 1 m 2 /r 2 Looking at the force on m 1 due to m 2, F = gm 1 F = Gm 1 m 2 /r 2 = gm 1 g (field due to m 2 ) = Gm 2 /r 2 m1m1 m2m2

28. Gravitational field around a point mass If we have two masses m 1 and m 2 distance r apart F = Gm 1 m 2 /r 2 Looking at the force on m 1 due to m 2, F = gm 1 F = Gm 1 m 2 /r 2 = gm 1 g (field due to m 2 ) = Gm 2 /r 2 m1m1 m2m2 I told you, for any planet; g = Gm p r p 2 Don’t forget that for a non point mass, r is the distance to the centre of mass

29. Gravitational field Gravitational field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one mass) must use vector addition. m1m1 m2m2 Field due to m 1 Field due to m 2 Resultant Field

30. Gravitational field patterns A gravitational field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines.

31. Gravitational field patterns A gravitational field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. This is an example of a radial field The closer the lines are together, the stronger the force felt. Note, gravity is ALWAYS attractive

32. Field around a uniform spherical mass

33. Field close to the earth’s surface Uniform

34. ALL magnets have two poles NORTH seeking pole SOUTH seeking pole

35. Opposite poles attract and like poles repel

36. Magnetic materials Iron (steel), Cobalt and Nickel

37. Magnetic induction When a magnetic material is close to a magnet, it becomes a magnet itself We say it has induced magnetism N S N S magnet

38. Soft Magnetism Pure iron is a soft magnetic material It is easy to magnetise but loses its magnetism easily N S beforeafter Iron nail S N N S Not a magnet N

39. Hard Magnetism Steel is a hard magnetic material It is harder to magnetise, but keeps its magnetism (it is used to make magnets!) N S beforeafter Steel paper clip N N S It’s a magnet! N S SN

40. Magnetic field Magnets and electric currents produce magnetic fields around them. In a magnetic field, another magnet, a magnetic material or a moving charge will experience a magnetic force. www.physchem.co.za

41. Magnetic field lines The closer the field lines are, the stronger the magnetic force felt The arrows show the direction a compass needle would point at that point in the field. Note that magnetic field is a vector quantity

42. Moving charges (currents) Moving charges (electric currents) also produce a magnetic field http://www.sciencebuddies.org Conventional current – electrons flow in the opposite direction

43. Magnetic field around a straight wire Stronger field closer to wire

44. Magnetic field around a flat circular coil http://physicsed.buffalostate.edu

45. Magnetic field around a solenoid

46. The Motor Effect When a current is placed in a magnetic field it will experience a force. This is called the motor effect.

47. The Motor Effect The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Centre finger = Conventional Current First finger = Field direction Thumb = Motion

48. D.C.Motor Commutator ensures that every half rotaion the current direction reverses in the coil

49. Defining Magnetic Field B The size of the force on a wire in a field depends on the size of the field (B), the length of wire in the field (L) and the current in the wire (I)

50. Defining Magnetic Field B In other words, F α BIL, or F = kBIL

51. Defining Magnetic Field B F = kBIL We can make k = 1 by defining the Tesla as the magnetic field when the force on 1 m of wire carrying a current of 1 A is 1 N.

52. Force on a current in a field Thus the force on a length L of wire carrying a current I in a magnetic field B is given by F = BILsinθ where θ is the angle between the current and the magnetic field.

53. The force on a moving charge in a magnetic field Since a current experiences a force in a magnetic field, and a current is just made of moving charges, moving charges themselves must experience a force in a magnetic field. www.nearingzero.net

54. The force on a moving charge in a magnetic field Given that F = BILsinθ F = B(q/Δt)vΔt = Bvqsinθ v q

55. The force on a moving charge in a magnetic field The fact that this force is always at right angles to the velocity means that the charge will move in a circle (if the speed is constant) v q Note; If the force is perpendicula r to the motion, no work is done.