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Electric Force and Electric field

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  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 = kq1q2 r2 The constant k is sometimes written as k = 1/4πεo where εo 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? q2 q1 F = 20N r 2q1 3q2 F = ? N r/2

  7. Calculations using Coulomb’s law F = kq1q2/r2 = 20.0N x = k2q13q2/(r/2)2 = 6kq1q2/(r2/4) = 24kq1q2/r2 x = 24F = 24 x 20.0 = 480 N q2 q1 F = 20.0N r 2q1 3q2 x = 480 N r/2

  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 q1 and q2 distance r apart F = kq1q2/r2 Looking at the force on q1 due to q2, F = Eq1 F = kq1q2/r2 = Eq1 E (field due to q2) = kq2/r2 q1 q2 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. Field due to q1 Field due to q2 Resultant field q1 q2

  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

  14. Field around two point charges

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

  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; • Masses attract each other

  18. Gravitational Force and Field We will know that; 2. Mass/energy is conserved (E = mc2)

  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 = Gm1m2 r2 The constant G is known as “Big G”and is equal to 6.667 x 10-11 Nm2kg-2

  21. Newton’s law of universal gravitation F = Gm1m2 r2 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 = Gm1m2= 6.667 x 10-11 x 63 x 70 = 7.3 x 10-8 N r222 2 m 63kg ? 70kg ?

  24. Force of gravity due to earth on Pascal? F = Gm1m2= 6.667 x 10-11 x 63 x 6 x 1024 = 615 N (= mg) r2(6400 x 103)2 Pascal’s weight 63kg ? R = 6400 km, m = 6 x 1024 kg

  25. Force of gravity due to earth on Pascal? F = Gm1m2= 6.667 x 10-11 x 63 x 6 x 1024 = 615 N (= mg) r2(6400 x 103)2 In other words, for any planet; g = Gmp rp2

  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 m1 and m2 distance r apart F = Gm1m2/r2 Looking at the force on m1 due to m2, F = gm1 F = Gm1m2/r2 = gm1 g (field due to m2) = Gm2/r2 m1 m2

  28. Gravitational field around a point mass I told you, for any planet; g = Gmp rp2 Don’t forget that for a non point mass, r is the distance to the centre of mass If we have two masses m1 and m2 distance r apart F = Gm1m2/r2 Looking at the force on m1 due to m2, F = gm1 F = Gm1m2/r2 = gm1 g (field due to m2) = Gm2/r2 m1 m2

  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. Field due to m2 Field due to m1 Resultant Field m1 m2

  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. The closer the lines are together, the stronger the force felt. Note, gravity is ALWAYS attractive This is an example of a radial field

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

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

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

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

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

  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. Thumb = Motion First finger = Field direction Centre finger = Conventional Current

  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