2Electrostatics Static Electricity Demonstrate how you can pick up the paper pieces without touching them in any way with your body.What is occurring at the atomic level that lets you do this?Static Electricity
3ElectrostaticsStudy of electric charges that can be collected and held in one place.
5Electric ChargeThere are 2 kinds of electric charge, positive and negative. Interactions between + and – explain the attraction and repulsive forcesLike charges repel and unlike charges attract.Electric charge is not created or destroyed; it is conserved. Charging is the separation, not creation, of electric charges.
6Microscopic View of Charge The atom has positive charge in the nucleus, located in the protons. The positive charge cannot move from the atom unless there is a nuclear reactionThe atom has negative charge in the electron cloud on the outside of the atom. Electrons can move from atom to atom without that much difficulty.Ernest RutherfordJJ Thompson
7Methods of Charging Objects Charging by FrictionCharging by InductionCharging by Conduction
8Methods of Charging Objects 1. Charging by Friction• rub two different materials together• Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths.• As they pass over each other the electrons with weaker bonds are “ripped” off of that material and collect on the other material.
9How can a charged object and a neutral object attract?
10Conductors and Insulators Charges transferred to one part of an insulator remain on that part. Insulators include glass, dry wood, plastics, and dry airCharges added to a conductor quickly spread over the surface of the object. In general, examples of conductors include graphite, metals, and matter in the plasma state
11Conductors and Insulators The charges are NOT free to move aroundConductorMany of the charges free to move around
12Insulators and Conductors glassrubberoilasphaltfiberglassporcelainceramicquartz(dry) cotton(dry) paper(dry) woodplasticairdiamondpure waterSemiconductors:SiliconGermaniumcarbonConductors:silvercoppergoldaluminumironsteelbrassbronzemercurygraphitedirty waterconcrete
13How can a charged object and a neutral object attract?
14How can a charged object and a neutral object attract?
17A is either + or neutral; C is - On two occasions, the following charge interactions between balloons A, B and C are observed. In each case, it is known that balloon B is charged negatively. Based on these observations, what can you conclusively confirm about the charge on balloon A and C for each situation.A is either + or neutral; C is -A is + and C is +
18Upon entering the room, you observe two balloons suspended from the ceiling. You notice that instead of hanging straight down vertically, the balloons seems to be repelling each other. You can conclusively say ...a. both balloons have a negative charge.b. both balloons have a positive charge.c. one balloon is charge positively and the other negatively.d. both balloons are charged with the same type of charge.Explain your answer.
19Two objects are shown below. One is neutral and the other is negative Two objects are shown below. One is neutral and the other is negative. Object X will ____ object Y.a. attractb. repelc. not affectA+-
20Charge Charge is quantized comes in two forms which Ben Franklin designated positive (+) and negative (-)Charge is quantizedSmallest possible charge, designated e, is the magnitude of charge on 1 electron (-e) or 1 proton (e).e is referred to as the elementary chargee = x CThe coulomb is the SI unit of charge
21Sample ProblemA certain static discharge delivers -0.5 C of electrical charge. How many electrons are in this discharge?
22Electric Force non contact force large compared to gravity attractive or repulsive depending on chargesdepends on distancecan be analyzed using free body diagrams and Newton’s laws
23Electric ForceAn insulating rods with small conducting spheres suspended by thin wires. Coulomb charged the spheres by conduction and measured and quantified the electric force.1785French physicist, Charles Coulomb
24Distance between charges Coulombs LawCharge MAGNITUDEDistance between chargesSI units of Force: Newton (N)SI units of Charge: Coulomb (C)SI units of distance: meters (m)1 C is the charge on 6.24 x 1018 electronscharge of 1 e-, e = 1.6 x C (elementary charge)k = 9.0 x 109 Nm2/C2
25Coulomb’s Law Electrical Forces in Atoms Because most objects have almost exactly equal numbers of electrons and protons, electrical forces usually balance out.Between Earth and the moon, for example, there is no measurable electrical force.Although electrical forces balance out for astronomical and everyday objects, at the atomic level this is not always true.Often two or more atoms, when close together, share electrons.Bonding results when the attractive force between the electrons of one atom and the positive nucleus of another atom is greater than the repulsive force between the electrons of both atoms. Bonding leads to the formation of molecules.
26The diagrams show two charged objects and their separation The diagrams show two charged objects and their separation. Rank the force that the left object exerts on the right object from the STRONGEST to WEAKEST force. Explain how you made the ranking.A > B > CA)B)C)+ 2qd+ 3q+ qd+ qd
27The diagrams show two charged objects and their separation The diagrams show two charged objects and their separation. Rank the force that the left object exerts on the right object from the STRONGEST to WEAKEST force. Explain how you made the ranking.A = B = CA)B)C)- qd+ qd+ q- qd
28The diagrams show two charged objects and their separation The diagrams show two charged objects and their separation. Rank the force that the left object exerts on the right object from the STRONGEST to WEAKEST force. Explain how you made the ranking.C > A > BA)B)C)+ qd+ q2d+ q½ d
29The diagrams show two charged objects and their separation The diagrams show two charged objects and their separation. Rank the force that the left object exerts on the right object from the STRONGEST to WEAKEST force. Explain how you made the ranking.C > A > BA)B)C)+ qd+ 2q+ q2d+ 3q1/3 dWhat is
30Compare the force that the left charge exerts on the right to the force that the right exerts on the left. Explain.+ qd+ 2q- q2dWhat is+ 2q+ q1/3 dC > A > B
31Do Now: Two charged spheres 10cm apart (0 Do Now: Two charged spheres 10cm apart (0.1m) attract each other with a force of 3.0x10-6 N. What force results from each of the following changes , considered separately?Both charges are doubled and the distance remains the same.The separation is increased to 30 cmCoulombs Law
32Coulombs Lawonly valid for point charges (or uniformly charged spheres)applies to objects whose size is much smaller than the distance between themdescribes the force between 2 charges when they are at rest. The study of charges at rest is called electrostatics.Coulombs law gives force on a charge due to only one other charge. If more than one charge present, Fnet is the vector sum or SUPERPOSITION of each Coulomb force.
33Sample Problem: Sphere A with charge +6 mC is located 0 Sample Problem: Sphere A with charge +6 mC is located 0.04 m from another sphere B with charge -3mC. What is the force of sphere B on A?0.04mABFAB6mC-3mCtowards B
34Sample Problem: Electric force on electron by proton: Determine the magnitude of the electric force on the electron of a hydrogen atom exerted by the single proton that is its nucleus. Assume the electron orbits the proton at its average distance of r = 0.53 x m.+Fepqe = x C
35Sample problemTwo identical positive charges separated by 12.5 cm (0.125 m) exert a repulsive force of 1.24 N on each other. What is the magnitude of each charge?0.125mqqFF = 1.24N
36+3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.60 m. Problem: Determine the electrical force of attraction between two balloons with separate charges of+3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.60 m.+Q1-Q2d=0.60mtowards the other balloon
37SuperpositionElectrical force, like all forces, is a vector quantity.If a charge is subjected to forces from more than one other charge, all the forces must be added using vector addition.Vector addition to find the resultant vector is sometimes called superposition.
38Scalars and VectorsAll measurements are considered to be quantities. In physics, there are 2 types of quantities – SCALARS AND VECTORS.Scalar quantities have only magnitude.Vectors are quantities that have magnitude and direction.timemasstemperaturedisplacementMagnetic FieldaccelerationForcevelocityGravitational Field
39Vectors are used to describe motion and solve problems concerning motion. For this reason, it is critical that you have an understanding of how torepresent vectorsadd vectorssubtract vectorsmanipulate vector quantities.
40Vectors tip 8 units 2 units 5 units tail Magnitude represented by the length of the vector
41Vectorsy1200800002250300600 from -x3000x-600450from-yDirection represented by the direction of the arrow
42Adding Vectors - + Dx = +4 mi + (-7 mi) = -3 mi We know how to add vectors in 1-dimension.Example: If someone walks 4 mi east and then 7 mi west, their total displacement is 3 mi west.Adding vectors mathematically – In one dimension, assign direction + or – and add algebraicallywest-east+Dx = +4 mi + (-7 mi) = -3 miDirection matters (if was 7 mi west ….)1-D, direction is indicated with + and - and then the vectors are added algebraicallyAdding vectors graphically – TAIL TO TIP3 mi4 mi7 mi
43Adding VectorsWhat about if the vectors are in different directions?For example, what if I walk 5 steps north and then 4 steps east. What is my total displacement for the trip?OR what is the vector sum of the 1st displacement (5 steps north) and the 2nd (3 steps east)?4 steps eastDx = ?5 steps north
44Adding Vectors “tail to tip” Dx q To add the vectors graphically North4 steps eastTo add the vectors graphicallyDraw the first vector (5 steps north) beginning at the origin.Draw the second vector (3 steps east) with its tail at the tip of the first vector.Draw the Resultant vector (the answer) from the tail of the first vector to the tip of the last.5 steps northDxqWestEastSouth
46Determine the net electrical force on sphere A Sample Problem-Determine the net electrical force on sphere AFnet0.2m0.6mABCFABFAC2mC-3.6mC4mCFnet= (all the forces to the right) –(all the forces to the left)
47How to solve problems using Coulombs Law Make a diagram of the problemMake a force diagram of all the forces acting on the particle in question. Identify the direction of the force using the rule that opposite charges attract and like charges repel.Use Coulomb’s Law to calculate the magnitude of each of the forces acting on the particle of interest. This means ignore the + and – signs on the charges when doing the math.Find the total or net electric force on the particle of interest by adding the forces as vectors.
48Sample Problem- A charge of 6 Sample Problem- A charge of 6.00 mC is placed at each corner of a square 1.00 m on a side. Draw the forces acting on charge and determine the direction of the net force on charge 2.FnetF23F2411mF212VERY important to keep in mind that Coulombs law gives force on a charge due to only one other charge. If several or many charges are present, the net force on any one of them will be the vector sum of the forces due to each of the others.For students – find net force on sphere B43
49Sample Problem- Three point charges of magnitude +1 C, +1 C and −1 C respectively are placed on the three corners of an equilateral triangle as shown.-Which vector best represents the direction of the net force acting on the −1 C charge as a result of the forces exerted by the other two charges?Fnet++a)b)c)d)
50Sample Problem- A +6 mC and a -3 mC charge are placed 25 cm (0 Sample Problem- A +6 mC and a -3 mC charge are placed 25 cm (0.25m) apart. Where can a third charge be placed so that it experiences no net force – to the left, in the middle or to the right of the charges?0.25mAB6mC-3mC
51ConceptsTwo charged objects have a repulsive force of N. If the charge of one of the objects is doubled, then what is the new force?0.16 NTwo charged objects have a repulsive force of N. If the charge of both of the objects is doubled, then what is the new force?0.32 NTwo charged objects have a repulsive force of N. If the distance separating the objects is doubled, then what is the new force?0.02 N
53Electric FieldThe presence of a charge modifies empty space. This enables the electrical force to act on charged particles without actually touching them.We say that an “electric field is created around the charged particle.If a charged particle is placed in an electric field created by other charges, it will experience a force as a result of the field.We can calculate the electric force from the electric field.
54Electric FieldThe electric field produced by a positive charge is directed away from the chargeA positive test charge would be repelled from the positive source charge
55Electric FieldThe electric field produced by a negative charge is directed toward the chargeA positive test charge would be attracted to the negative source charge-+-
56Electric Field LinesThe arrows in a field are not vectors, they are “lines of force”.The electric field lines indicate the direction of the force on a positive test charge placed in the field.Negative charges experience a force in the opposite direction.-
57Electric Field LinesThe closer the field lines, the stronger the field. The number of field lines leaving or terminating on a charged object is proportional to the magnitude of its electric charge.
58Electric Field around Charge Electric field lines are directed away from positive charges and toward negative charges.++--
59Why use fields?Forces exist only when 2 or more particles are present.Fields exist even if no force is present.The field of one particle only can be calculated.
60Electric Field Q is the source charge that produces E The force on a charged particle placed in an electric field is easily calculatedq is a test charge in the electric field produced by QFelQ is the source charge that produces EQE depends only on the source charge, QE is a vectorSI Units are N/C
61Approximate Values of Typical Electric Fields Value (N/C)Near a charged, hard-rubber rod1 x 103In a TV picture tube1 x 105Needed to create a spark in air1 x 106At an electrons orbit in a hydrogen atom1 x 1011
63Sample ProblemA positive test charge of 5.0 x 10-6 C is in an electric field that exerts a force of 2.0 x 10-4 N on it. What is the magnitude of the electric field at the location of the test charge?+F = 2x10-4 Nq = 5.0mCin the same direction as the force (since test charge is positive)
64Sample ProblemWhat is the electric field strength at a point that is 0.30 m to the right of a small sphere with a charge of -4.0 x 10-6 C?Q = -4.0 mCE = ?d = 0.30 mThe field’s direction is to the left, into the negative charge
65Sample ProblemA negative charge of 2.0 x 10-8 C experiences a force of N to the right in an electric field. What are the fields magnitude and direction in that location?-+F = N-q = -2.0 x 10-8 CThe field’s direction is to the left, (opposite to direction of force on negative charge since neg charges move opposite the direction of the E field lines)
66+ + - Which electric field is the strongest? b a) b) c) What is the sign of the charges that produce the electric fields shown?
67Picturing the Electric Field Electric field between 2 opposite point chargesElectric field around a point chargeElectric field between charged plates
68Electric Field Line Patterns An electric dipole consists of two equal and opposite chargesThe high density of lines between the charges indicates the strong electric field in this region
69Electric Field Line Patterns Two equal but like point chargesAt a great distance from the charges, the field would be approximately that of a single charge of 2qThe bulging out of the field lines between the charges indicates the repulsion between the chargesThe low field lines between the charges indicates a weak field in this region
70Electric Field Patterns Unequal and unlike chargesNote that two lines leave the +2q charge for each line that terminates on -q
71SuperpositionWhen more than one charge contributes to the electric field, the resultant electric field is the vector sum of the electric fields produced by the various charges.Again, as with force vectors, this is referred to as superposition.
72ProblemA particle with charge -5.0 μC is placed at -2.0 m, and a particle with charge 5.0 μC is placed at +2.0m. What is the electric field at the origin?q1=-5mCq2=+5mCE12m2mq1q2E2direction
73Picturing the Electric Field The magnitude of the electric field at any point is not the field line itself, but can be determined from the field line.The direction of the electric field is always tangent to the field line at any given point.Electric field and force are vectors and vectors are never curvy.The strength of the electric field is related to the spacing between the field lines. The field is strong where the lines are close together.The electric field lines do not represent the path a test charge would follow, they represent the direction of the electric force on a particle placed in the field.How measure electric field? Test charge – place test charge at some location and if there is an electric force on it, then there is an electric field there. Tesst charge must be small enough so that doesn’t affect the other chargesDraw on board 2 opposite charges and fieldsField lines always LEAVE A POSITIVE CHARGE AND ENTER A NEGATIVE CHARGEThe never cross