Electrostatics : Charges at rest

Electric Charge  A property of matter that creates a force between objects. Can be positive or negative Can be positive or negative Like charges REPEL Like charges REPEL Opposite charges ATTRACT Opposite charges ATTRACT  What objects (particles) do we know about that carry a charge?

Electric Charge An object’s charge depends on imbalance of protons (+) and electrons ( ‐ ) More protons than electrons  positive More protons than electrons  positive More electrons than protons  negative More electrons than protons  negative Units of charge: Coulombs (C) Protons and electrons have exactly the same amount of charge: 1.6 x 10 ‐ 19 C Protons and electrons have exactly the same amount of charge: 1.6 x 10 ‐ 19 C Differ only in sign (+ or -)

Electrostatics – Elementary Charge  Since protons and electrons are the smallest whole particles, the charge on any object is a multiple of 1.6 x C  Elementary Charge (e) = 1.6 x C 1C of charge is made up of 6.25 x10 18 electrons.

Charging an Object Objects become charged if they have an imbalance of protons and electrons.  Can an object gain or lose protons? (Think: Can protons MOVE?) (Think: Can protons MOVE?)  Can an object gain or lose electrons? (Think: Can electrons MOVE?) (Think: Can electrons MOVE?) The total sum of the charge a body has is the result of a loss or gain of electrons

Electrostatics  When there is an imbalance in protons and electrons the atom will have a total sum of charge Total positive sum of the charges – more protons than electrons Total positive sum of the charges – more protons than electrons Total negative sum of the charges – more electrons than protons Total negative sum of the charges – more electrons than protons

Electrostatics  For an object to gain a total sum of charge, there must be an imbalance in the number of protons and electrons.  For the imbalance to occur, the atom can only gain or lose electrons.  If the atom lost a proton, it would change what it is.

Charging an Object  Conductors: materials that transfer and redistribute charge easily. Excess charge can move through the material. Excess charge can move through the material. Examples: ??? Examples: ???  Insulators: materials that do not transfer charge easily; retain charge within a localized region. Excess charge stays on the surface of the material. Excess charge stays on the surface of the material. Examples: ??? Examples: ???  ·Semiconductors: materials that behave as either insulators or conductors, depending on temperature Examples: ??? Examples: ???

Electrostatics and Charging  Charge can move by many methods Methods: Methods: FrictionFriction ConductionConduction InductionInduction  For charge to move, electrons must move from atoms. Positive charge does not move!

Charging by Friction  The electrons are literally rubbed off of one object and move to the other Charging a balloon Charging a balloon Hair  positiveBalloon  NegativeHair  positiveBalloon  Negative Using the fur to charge the plastic rod. Using the fur to charge the plastic rod. Fur  positiveRubber rod  NegativeFur  positiveRubber rod  Negative balloon charging balloon charging balloon charging balloon charging

Charging by Conduction  Charge is transferred from one object to another object by direct contact  Walk across the carpet in socks and touch the doorknob… ZAP! Charge is transferred and you experience a shock. Charge is transferred and you experience a shock.

Charging by Conduction: Demo  Demonstration: Touch the negatively charged magic wand to the mylar butterfly. Some electrons move from wand to butterfly. Some electrons move from wand to butterfly. Butterfly repels wand (both slightly negative). Butterfly repels wand (both slightly negative).  Static Discharge: movement of charge from one object to another by conduction using the air. Charge can JUMP! (Lightning)

Charging by Induction  A temporary charge can be induced in a neutral object by bringing a charged object close to it. The charges in the neutral object move in response to the external charge. Result: induced charge (POLARIZATION!!!) The charges in the neutral object move in response to the external charge. Result: induced charge (POLARIZATION!!!)  If a path is provided, this moved charge will escape. Result: body has a total sum of charge

Charging by Induction: Demo  Demonstration: Bring negatively charged magic wand near soda can. Electrons in soda can repel. Electrons in soda can repel. Protons in soda can are attracted and cause the can to move forward Protons in soda can are attracted and cause the can to move forward  Demonstration: Bring negatively charged balloon near wall. Electrons in wall repel. Electrons in wall repel. Protons in wall attract to balloon, and it sticks to wall Protons in wall attract to balloon, and it sticks to wall

Charging by Induction  If a charged object is brought near the spheres, the charges will POLARIZE.  The negative charges move towards the positive charges and away from other negatives.

Measuring Charge  Electroscope Movement of leaves is a “rough estimate” of amount of charge Movement of leaves is a “rough estimate” of amount of charge Other electroscopes are more precise. Other electroscopes are more precise. When a charge is present, the straw rotates.When a charge is present, the straw rotates. More rotation = more charge More rotation = more charge Can detect + and – charge, cannot differentiate Can detect + and – charge, cannot differentiate

Electric Force  Electric Force: force of attraction or repulsion between objects due to charge Depends on CHARGE and DISTANCE Depends on CHARGE and DISTANCE Increase charge  force increasesIncrease charge  force increases Increase distance  force decreasesIncrease distance  force decreases  Forces can be exerted by one charge on another from a distance through a FIELD Electric Field: region around a charged object in which other charged objects experience an electric force Electric Field: region around a charged object in which other charged objects experience an electric force

Coulomb’s Law  F = electric force in Newtons  k = constant (just a #) = 9.0x109 Nm2/C2  q1 = charge of object #1 in Coulombs (C)  q2 = charge of object #2 in Coulombs (C)  r = radius between two charges in meters F = k q 1 q 2 r 2

Coulomb’s Law calculates a force  If the calculated force is: NegativeNegative The force is attractive between particles The force is attractive between particles PositivePositive The force is repulsive between particles The force is repulsive between particles

Effect of Coulomb’s Law  If the charge of one object doubles… Force doubles (x2) Force doubles (x2)  If the charges of both objects double… Force quadruples (x4) Force quadruples (x4)  If the distance between the charges doubles… Force is quartered (divided by 4) Force is quartered (divided by 4)

EXAMPLE  Find the force exerted by one electron on another separated by a distance of 2.0 m. Draw a picture Draw a picture Table of Knowns Table of Knowns (What is the charge of each electron?) (What is a constant? k= 9x10 9 N*m 2 /C 2 ) Is the force repulsive or attractive? Is the force repulsive or attractive?

Q1Q2D (m) Force (N)Attractive or Repulsive? +2C C C+22 1 x C2 x C x C-3 x C x C-4 x C x 10 5 C7 x 10 5 C C-25 C C-67 C C2 C0.7

Electric Force  Electric Force – the force of attraction or repulsion between objects due to charge. Like charges repel. Like charges repel. Unlike charges attract. Unlike charges attract. Depends on size of charge and distance Depends on size of charge and distance Act over a distance through an electric field Act over a distance through an electric field

Coulomb’s Law  Electric force is given by Coulomb’s Law  Where :  q 1 and q 2 are the charges  r is the radius between the charges  Force decreases as r gets bigger but never will be zero – Like gravity

Electric Field  Electric Field – An electric field is the region around a charge in which the electrostatic force is felt by other charges.  Electric fields are drawn and 'mapped' in diagrams using “field lines”..

Electric Field Lines  Always extend from a positively charged object to a negatively charged object  From a positively charged object to infinity  From infinity to a negatively charged object.

Electric Field Lines  Electric field lines never cross each other.  At locations where electric field lines meet the surface of an object, the lines are perpendicular to the surface.

Electric Field Lines  The number of field lines that meet a charged particle represents the size of the charge, higher charge amounts have more field lines.

Electric Field Lines  The closer together field lines are to each other, the greater the strength of the field at that point.

Electric Field Lines  Between Particles

Electric Force  Electric field is ‘seen’ by the force on a charged body, test charge.  Electric field is depicted through Electric Field lines. Field lines point in direction of force on positive charge Field lines point in direction of force on positive charge Field lines never cross Field lines never cross Field lines travel away from positive charge Field lines travel away from positive charge Field lines travel toward negative charge Field lines travel toward negative charge

Electric Field

Electric Field Lines  When two opposite charges are brought close together,  The field lines travel from positive to negative. The lines connect the charges showing attraction The lines connect the charges showing attraction

Slide 36 Fig 15.29a, p.551

Electric Field Lines  When like charges are brought near each other,  The fields lines repel each other. Reason for the repelling force between like charges. Reason for the repelling force between like charges.

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Electric Field Equations  E = F/q test  E = kq source /r 2  The electric field only depends on the source charge  It does NOT depend on the test charge – see how the field is only created by the extra protons and electrons, NOT the puck