How are the first 3 different from the last 3?

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1 Unit 3: Fields 1

2 What is meant by the term: Fields (in Physics)? What makes the following move? A) A puck on ice B) A car C) A snowmobile on ice D) A skier or snowboarder going downhill E) A coin dropped from your hand F) A compass needle How are the first 3 different from the last 3? These are examples of contact forces These are forces acting at a distance In Physics we use the field concept to explain how a force can act over a distance. 2

3 Consider all examples where a force is acting at a distance. These are called field forces Field theory explains why you do not have to be in contact with an object in order to feel the effects of a force. The field concept explains the phenomenon how a force can act over a distance.

4 Fields in Physics Field: a region of influence in space surrounding a body that is the source of the field and through which its influence is felt For example, consider a street light at the end of a long, dark street on a dark night. The light from the lamp extends outwards from the lamp in ALL directions, becoming dimmer the further away you go. A field of light is formed around the lamp. The lamp is the source of the field.

5 Overview In this complete unit you will study the following topics: Universal Law of Gravitation [4h] Electrostatics [5h] **MIDTERM CUTOFF** Circuits [10h] Electromagnetism [6h] Magnetic Induction [4h] Generators/Transformers [6h] Comparison of Fields [1h] 5

6 Introduction to Fields We will learn about three fields in this unit: gravitational, electric, and magnetic fields.

7 Gravitational Field A planet affects an object many kilometres away without touching it. The object experiences a force due to the gravitational field surrounding the planet. In fact, there is a gravitational field around every object that has mass.

8 Gravitational Field The gravitational field acting around an object can be shown using a map of the field called a gravitational field diagram. In the map, both the direction and strength of the gravitational field is shown. Each vector in the map shows the direction a mass would move if released in that vicinity.

9 Gravitational Field Diagram The lines are denser near the surface, and sparser further away from the planet. The density of the lines indicate the strength of the field If you move into a region of less density, you enter a lesser gravitational field thus becoming lighter! You still have the same mass though!

10 Note: In a field map 1. The field lines (vectors) never cross each other. 2. The lines meet the surface of the planet at right angles (normal to the surface) 3. The map (and field) is 3-dimensional

11 Mathematically Speaking Gravitational field strength (g) Force of gravity on object g mass of object On the surface of the earth g N kg 9.81 m s

12 Question: If the Gravitational field strength on the moon is 1/6 earth s gravitational field, what is the moon s g? Calculate your weight on the moon.

13 Electric Fields How can we tell if we are in the vicinity of an electric field?

14 Electric Fields An electric field is the region of space surrounding a charged object in which another charged object experiences a force of attraction or repulsion.

15 15

16 Drawing electric field: The direction of any field, whether it be a gravitational field, electric field, or magnetic field, can be determined by testing the field. What was used to test gravitational fields? A test mass.

17 What is used to test electric fields? A small test charge. By convention electric fields are drawn based on the direction that a small POSITIVE test charge will move when placed in that field

18 The arrows drawn representing the electric field show the electrostatic force that acts on the positive test charge based on the charged object(s) that are generating the field.

19 Draw the electric field around the following point charges. A) B)

20 Notes on electric fields 1. The direction of the lines of force is the direction in which a small positive test charge will move when placed in that field. Lines of force BEGIN on a positively charged object Lines of force END on a negatively charged object

21 2.The lines of force meet the charged object at right angles. 3.The lines of force never touch or cross. 4.The density of the lines of force indicates the strength of the field. The electric field strength is greatest near the charged object and diminishes as the distance from the charged object increases.

22 5. Electric fields are 3-dimensional

23 1. Draw the electric fields around the following charged objects - +

24 2. - -

25 3. + +

27 The electric field bulges out on the ends because there is not an equal amount of charge on either side of the test charges at the ends

28 Extra Fun Ones

29 Extra Fun. A)

30 B) + - +

31 B) + - +

33 Magnetic Fields How can we tell if we are in the vicinity of a magnetic field?

34 What is used to determine the direction of a magnetic field? A compass

35 Magnetism Origin of the term magnetism The Ancient Greeks discovered a mysterious mineral which attracted iron, and would point North if it were allowed to rotate freely. This mineral was found near a place called Magnesia, in Greece, hence the term magnetism. This mineral later became known as lodestone because it always pointed towards the leading, or "lode" star. Today this star is called the North Star, or Polaris. 35

37 Definition: North and South Poles The North end of a compass or any magnet is the NORTH SEEKING end, and is called the North Pole. The South end of a compass or any magnet is the SOUTH SEEKING end, and is called the South Pole. 37

38 Properties of Magnets Like Poles REPEL Different Poles ATTRACT 38

39 Magnetic Field Around every magnet there is a magnetic field of force (magnetic field). A magnetic field is the region of space surrounding a magnetic pole in which another magnetic pole experiences a force of attraction or repulsion (defined by the effect on a N pole). It acts at a distance - similar to gravitational field or electrical field. The direction of the field lines is given in the way that a compass would point. The lines point away from the North end of the magnet and towards the South end. When drawing magnetic fields, remember that the field lines never cross and are perpendicular to the surface. 39

40 Draw magnetic fields around the following: N S 40

41 N N S 41

42 S S N 42

43 N S N 43

44 What kind of magnetic pole is by the north pole of earth? ( Up by Santa s House!) 44

46 Earth's Magnetic Field The earth's magnetic field enables people to navigate using a compass. It is believed that certain animals have a magnetic sense of direction which enables them to navigate. (biomagnetism) It protects us from harmful cosmic radiation by deflecting them into outer space. (Science 1206) Provides us with entertainment. It produces the Aurora Borealis and Aurora Australis. Northern Lights and the Southern Lights. 46

47 What causes the earth's magnetic field? It is believed that it results from the continuous motion of magnetic material (mainly molten iron and nickel) in the liquid outer core of the earth. 47

48 Magnetic field reversals Reversals have been documented as far back as 330 million years. During that time more than 400 reversals have taken place, one roughly every 700,000 years on average. However, the time between reversals is not constant, varying from less than 100,000 years, to tens of millions of years. 48

49 1. Sample questions

50 2. Which object shows the electric field around a negative charge?

51 3. Which object shows the magnetic field around Earth?

52 4. Draw the magnetic field around this horseshoe magnet N S

Conceptual Physics. Chapter 24: MAGNETISM

Conceptual Physics. Chapter 24: MAGNETISM Conceptual Physics Chapter 24: MAGNETISM Magnetism The term magnetism comes from the name Magnesia, a coastal district of ancient Thessaly, Greece. Unusual stones, called lodestones, were found by the