Physics 7C SS1 Lecture 6: Electricity Analogies with Gravity: Electric Force, Electric Field, Electric Potential Energy, Electric Potential, As you wait for class to start, ponder what the following statement means…Gravity is bigger on Earth than the moon.

2 New Model(s): Electric Field and Forces Field, Forces, Potential Energy, and Potential

3 True or False Gravity is more on Earth than the moon.

4 True or False Gravity is more for a feather on Earth than for an elephant on the moon.

5 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today

6 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today True/False: Temperature is a field

7 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today True/False: Temperature is a field

8 Temperature Field What do places with the same color have in common?

9 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today True/False: Height (as measured relative to Roessler Hall entrance) is a field.

10 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today True/False: Height (as measured relative to Roessler Hall entrance) is a field.

11 What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework due today True/False: The gravitational field we defined for Earth is a vector field.

12 Applying Field to Electrical Phenomena A charge Q generates an Electric Field E Q Charge q, placed in an electric field E Q, experiences a force F elec Q on q =qE Q. How might we draw the electric field?

13 Applying Field to Electrical Phenomena A charge Q generates an Electric Field E Q Charge q, placed in an electric field E Q, experiences a force F elec Q on q =qE Q. For + test charge, force points in the same direction as field. For - test charge, force points in opposite direction of field

14 Field vs. Force How many objects are required to create a electrical field? At least… a)0 b)1 c)2 d)3

15 Field vs. Force How many objects are required to create a electrical field? At least… a)0 b)1 c)2 d)3 Field depends only on source charge! Field does not change depending on what type of charge is placed in it.

16 Field vs. Force How many objects are required to create a electrical force? At least… a)0 b)1 c)2 d)3

17 Field vs. Force How many objects are required to create a electrical force? At least… a)0 b)1 c)2 d)3 Force depends on both source charge and test charge--as before, you need two objects to have a force! Force direction and magnitude both can change depending on what type of charge is placed in the field.

18 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! Which way is the electric field at the marked spot? - 

19 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! ab cd 1) Which vector might be the electric field from the top charge? - 

20 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! ab cd 2) Which vector might be the electric field from the bottom charge? - 

21 Superposition of Fields Which direction does the net Electric Field point?  - E+E+ E-E- c E tot = 0 d Neither a nor b (but not 0) b a

22 Checking Understanding: If I put a charge at the marked location, which way will the force be?  - d b c a e Insufficient Information

23 Checking Understanding: If I put a charge at the marked location, which way will the force be?  - d b c a e Insufficient Information But…if I tell you the type of charge, you can tell me the direction of the force.

24 Models of Electric Phenomena: Electric Field and Forces Each source charge Q generates an Electric Field E Q The net Electric Field is the sum of all the source fields Charge q, placed in an electric field E tot, experiences a force F elec on q =qE tot. For + test charge, force points in the same direction as field. For - test charge, force points in opposite direction of field Field, Forces, Potential Energy, and Potential

25 Reviewing what you’ve previously studied… Gravitational Potential Energy 1 2 3

26 Relationship between Potential Energy and Force Potential Energy r - 0

27 Relationship between Potential Energy and Force Potential Energy r - 0 Negative means decrease of PE with decreasing r

28 Relationship between Potential Energy and Force Potential Energy r - 0 More slope closer to earth means F is greater there F = -  PE /  r, the - slope Force increases with greater slope

29 Reviewing what you’ve previously studied… Relationship between Potential Energy and Force Magnitude of Force = slope of PE vs. r graph.

30 Quiz 3: Reflection, Refraction, Lenses You should bring a straight edge You may bring and index card (3’x5’) You will not be allowed a calculator

31 Ray Model in Review Hitting a boundary can lead to reflection or refraction, often both. We can calculate the angles for any outgoing ray- always with respect to normal Lenses are constructed carefully to bend light in particular ways to create images We can locate images with the thin lens equation or a ray tracing, and describe them in many ways

32 Light passes from water (n=1.3) into an unknown substance water ? At larger angles, will there be total internal reflection? a)Yes, past the critical angle b)No, not possible c)Not enough information

33 Light passes from water (n=1.3) into an unknown substance water ? At larger angles, will there be total internal reflection? a)Yes, past the critical angle b)No, not possible c)Not enough information

34 You have a 2-lens system. You know only what the information shown below: ? f=-25cm Image created by first lens 25cm 100cm T/F: There is enough information to locate the image produced by the second lens.

35 You have a 2-lens system. You know only what the information shown below: ? f=-25cm Image created by first lens 25cm 100cm T/F: There is enough information to determine what type of lens the first lens must be.

36 You have a 2-lens system. You know only what the information shown below: ? f=-25cm Image created by first lens 25cm 100cm T/F: There is enough information to determine what type of lens the first lens must be. Note: Only true because I told you in class that the light originated from the left, so the original object was left of the lens.

37 Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction. ? f=-25cm Image created by first lens 25cm 100cm How will the blue arrow change: Bigger or smaller? Upright or inverted? Closer or further?

38 Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction. ? f=-25cm Image created by first lens 25cm 100cm How will the blue arrow change: Bigger or smaller? Upright or inverted? Closer or further? Stumped? 1) Try to locate the original object, making a guess about the focal length

39 Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction. ? f=-25cm Image created by first lens 25cm 100cm How will the blue arrow change: Bigger or smaller? Upright or inverted? Closer or further? Stumped? 2) Next: how will focal length change if index of refraction increases? 3) Draw new image.

40 Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction. ? f=-25cm Image created by first lens 25cm 100cm How will the blue arrow change: Bigger or smaller? Upright or inverted? Closer or further?