Physics 101 Chapter 22 Electrosta4cs Electrosta4cs Electrosta4cs studies electricity at rest. Charges, forces, etc… Charge: Posi4ve + or nega4ve -­‐ Like (same) charges repel Unlike (opposite) charges aLract Normal maLer: atoms have zero net charge (electrons balance protons) Nucleus (protons and neutrons) held together by the strong nuclear force. Atom (electron and nucleus) is held together by the electric force GSCI 101, Prof. M. Nikolic 2 The Electric charge An electric charge has a magnitude and sign. It is either posi4ve (+) or nega4ve (-­‐). Electron has nega4ve electric charge Proton has posi4ve electric charge If the charges have the same sign, the forces between them are repulsive Charges of opposite sign experience aLrac4ve forces 3 Atoms and molecules are made up from electrons, protons and neutrons! Atoms are combina4ons of equal amounts of electrons and protons. The neutrons provide the glue to s4ck together the protons in the nucleus. The proton and the neutron are about 2000 4mes heavier than the electron, so the vast majority of an atom’s mass resides in the nucleus. Atoms are mostly … empty. We live in almost empty space. Example: tennis ball and a grain of sand 4 Conceptual ques4on: Nega4ve ions To make an uncharged object have a nega4ve charge we must: Add some atoms. Add some protons. Add some electrons. ✓ Add some neutrons. Prof. M. Nikolic, GSCI 101 5 Types of materials – two extreme models regarding electricity: Insulators – a material in which charges do not move freely through the interior of the sample. Examples: glass, wood, rubber, plas4cs, stone, brick, etc Conductors – material where free charges can move through the material. Examples: Ionized gases (plasmas), metals, ionic solu4ons if salts in water Semi-­‐conductors – a material intermediate between the two extreme models – GaAs, Ge, Si, are the classic examples. 6 How to Charge? Physics 150, Prof. M. Nikolic 7 How to Charge Transfer electrons from object to another 1.  Fric4on or contact 1.  Rub rod with cloth 2.  Rub clouds together 2.  Induc4on (described later) Charge is conserved. It cannot be created or destroyed. Conductors: let charges move è a lot of free electrons Insulators: do not let charges move GSCI 101, Prof. M. Nikolic 8 Conceptual ques4on: Charge transfer If something gets a posi4ve electric charge, then it follows that something else Becomes equally posi4vely charged. Becomes equally nega4vely charged. ✓ Becomes nega4vely charged, but not necessarily equally. Becomes magne4zed . Prof. M. Nikolic, GSCI 101 9 Charging by fric4on and contact Charge is transferred from one material to another through touching Balloon and Wall Charge balloon by rubbing -­‐ electrons are transferred to the balloon – balloon becomes nega4vely charged The charged balloon will then aLract neutral objects Combing your hair GSCI 101, Prof. M. Nikolic 10 Charging by Induc4on GSCI 101, Prof. M. Nikolic 11 Charging by induc4on -­‐ Thunderstorms The nega4ve charge at the boLom of the cloud induces a posi4ve charge on the buildings below. •  Opposite charges aLract è electrical discharge lightning GSCI 101, Prof. M. Nikolic 12 Polariza4on Charges in insulators can only move a short distance Charged rod The charged rod aLracts opposite charges in the wood and repels the same charges. •  Opposite charges are closer than same charges •  ALrac4ve force greater than repulsive force •  ALrac4on! GSCI 101, Prof. M. Nikolic Wooden beam 13 Conceptual ques4on: Three charges Three metal balls are suspended from thin threads. Each of the balls is then given a posi4ve charge, a nega4ve charge, or no charge. We then find that ball 1 and ball 2 repel each other and that ball 2 and ball 3 repel each other. From this we can conclude 1 and 3 carry charges of the opposite sign. 1 and 3 carry charges of the same sign. ✓ All three carry charges of the same sign. One of the objects carries no charge. Prof. M. Nikolic, GSCI 101 14 Conceptual ques4on: Three charges Three metal balls are suspended from thin threads. Each of the balls is then given a posi4ve charge, a nega4ve charge, or no charge. We then find that ball 1 and ball 2 repel each other and that ball 2 and ball 3 aBract each other. From this we can conclude 1 and 3 carry charges of the opposite sign. ✓ 1 and 3 carry charges of the same sign. All three carry charges of the same sign. One of the objects carries no charge. We need to do more experiments to determine the rela4ve signs of the charges on the different balls. Prof. M. Nikolic, GSCI 101 15 Coulomb’s law Constant Force between objects 1 and 2 Charges of objects 1 and 2 kq1q2
F= 2
d
Distance between the centers of objects 1 and 2 Charge is measured in Coulombs (C). 1 C = 6.25 × 1018 electron charges
k = 9 × 10 9 N ⋅ m 2 /C2
(G = 6.67 × 10 −11 N ⋅ m 2 /kg 2 )
GSCI 101, Prof. M. Nikolic (1 kg = 6 × 10 26 proton masses)
F posi4ve: repulsion F nega4ve: aLrac4on 16 Conceptual ques4on: Coulombs law According to Coulomb’s law, the force between a pair of charged par4cles that are brought closer together decreases. increases. ✓ increases only if the charges are of the same sign. increases only if the charges are of the opposite sign. kq1q2
F= 2
d
Prof. M. Nikolic, GSCI 101 17 Conceptual ques4on: Coulombs law When a pair of charged par4cles are brought twice as close to each other, the force between them becomes Four 4mes smaller. Two 4mes smaller. Unchanged. Four 4mes larger. ✓ Two 4mes larger. kq1q2
F= 2
d
Prof. M. Nikolic, GSCI 101 18 Conceptual ques4on: Coulombs law Two uniformly charged spheres are firmly fastened to and electrically insulated from fric4onless pucks on an air table. The charge on sphere 2 is three 4mes the charge on sphere 1. Which force diagram correctly shows the magnitude and direc4on of the electrosta4c forces? kq1q2
F= 2
d
✓ Prof. M. Nikolic, GSCI 101 19 Electric vs. Gravita4onal force Gravita4onal force Fgrav =
Gm p me
r2
Always aLrac4ve! Electric force Felec =
kq p qe
r2
Can be either aLrac4ve or repulsive! The force of gravity is 1039 4mes smaller than electric force, but since objects are mainly neutral we feel gravity force and not the electric force. GSCI 101, Prof. M. Nikolic 20 Conceptual ques4on: Coulomb’s Law vs Gravity A hydrogen atom consists of a single electron orbi4ng a single proton. The electric force between the par4cles is about 1039 4mes greater than the gravita4onal force! If we can change the distance between the two par4cles, can we find a distance at which the electric and gravita4onal forces are equal? kq1q2
Felec = 2
Yes, we must move them closer together. d
No, not at any distance. ✓ Gm1m2
F
=
grav
2
d
Yes, we must move them farther apart. Prof. M. Nikolic, GSCI 101 21 Electric field The Earth (or any mass) has a gravita4onal field. Any other mass in a gravita4onal field experiences a gravita4onal force. Fg = mg
A charged object creates an electric field. Any other charge in an electric field experiences an electric force. Electric force = charge 4mes electric field: Fe = qE
Fe
E=
q
Electric field symbol: E SI unit: [N/C] Every charged object produces an electric field around itself! GSCI 101, Prof. M. Nikolic 22 Electric field direc4on Electric field points away from posi4ve charges and towards nega4ve charges. No field lines cross. The field lines are drawn so that the thickness of lines is propor4onal to the magnitude of the electric field. GSCI 101, Prof. M. Nikolic 23 Conceptual ques4on: Electric field If it takes 10 newtons of force to hold a 0.1-­‐coulomb par4cle at rest in an electric field, the strength of the field there is 0.01 N/C. 0.1 N/C. Fe
E=
q
1 N/C. 10 N/C. ✓ E=
10N
= 100N / C
0.1C
100 N/C. Prof. M. Nikolic, GSCI 101 24 More Electric Field PaLerns hLp://phet.colorado.edu/sims/charges-­‐and-­‐fields/charges-­‐and-­‐
fields_en.html GSCI 101, Prof. M. Nikolic 25 Charge and Conductors Charges in conductors can move freely. Therefore, the electric field inside a conductor is zero. (Since the charges will keep moving around un4l the electric field is zero.) ècharges spread out on the surfaces of conductors èconductors can shield you from electric fields Charges concentrate at the points of conductors (like they’re all trying to escape the surface at those loca4ons) GSCI 101, Prof. M. Nikolic 26 Electric Shielding That means that I can place any charge inside any conductor in electrosta4c equilibrium and that charge will not be influenced by any force at all. This is the principle behind a “Faraday Cage”. GSCI 101, Prof. M. Nikolic During a thunderstorm – stay in 27 your car!!! Electric Poten4al An object in a gravita4onal field has gravita4onal poten4al energy An object in an electric field has electric poten4al energy electric poten4al energy Electric poten4al = electric charge Unit: Volt = Joule/Coulomb “Electric Poten4al” is not “Electric Poten4al Energy” •  Electric Poten4al is similar to gh •  Electric Poten4al Energy is similar to mgh GSCI 101, Prof. M. Nikolic 28 Electric Poten4al The difference between electric poten4als is called voltage. Unit: Volt = Joule/Coulomb Example: The poten4al difference between a storm cloud and the ground is 100 million V. If a charge of 2 C flashes in a bolt from the cloud to Earth, what is the change in electric poten4al energy? Poten4al energy = poten4al x charge Change in poten4al energy = voltage x charge Change in poten4al energy = 100x106 V x 2 C = 200 million J GSCI 101, Prof. M. Nikolic 29 Flashlight A #lashlight bulb uses 3 Watts (3 Joules/s) of power. A D-­‐
battery supplies a 1.5 V potential difference. What charge passes through the battery each second? •  Voltage = 1.5 V •  Electric energy (in one second) = 3 J Charge q = energy / voltage 3J
q=
=2C
1.5V
2 Coulombs of charge pass through the battery each second GSCI 101, Prof. M. Nikolic 30 Conceptual ques4on: Volts Volts are the unit of measurement for Electric charge. Electric charge energy. Electric poten4al. ✓ Electric poten4al energy. Prof. M. Nikolic, GSCI 101 31 Van de Graaf GSCI 101, Prof. M. Nikolic A motor driven balloon rubber Electric poten4al ~ 100,000 V! 1 inch spark çè 25,000 V 32