Electrostatics Charge & Coulomb’s Law

Electrostatics Study of electrical charges that can be collected and held in one place

Examples of Static Electricity Hair brush & comb Clothes in the dryer Balloons Lightning

Charge Two kinds of charge: (+) and (-) Charged objects either gain (become negative) or lose (become positive) ELECTRONS Like charges repel Opposite charges attract Neutral objects # Protons = # Electrons Within an atom: Protons are (+) charged, Electrons are (-) charged How do objects become charged? Transfer electrons, NOT protons

Charge Neutral atom # Protons #Electrons Positive ion# Protons #ElectronsElectrons were Negative ion # Protons #ElectronsElectrons were Note: NO TRANSFER OF PROTONS!

Charge Neutral atom # Protons= #Electrons Positive ion# Protons >#ElectronsElectrons were LOST Negative ion # Protons <#ElectronsElectrons were GAINED Note: NO TRANSFER OF PROTONS!

Conductors & Insulators Conductors Outer electrons are not held as tightly – which allows charge to move easily Ex: metals, salt water, etc. Insulators Outer electrons are held tightly - charge does not move easily Ex: rubber, plastic, wood.

Separation of Charge Two neutral objects rubbed together can become charged Only electrons are transferred Total charge of both objects is still the same- Charge is conserved!!! Charge can simply be separated Thundercloud – Negative charge on bottom, positive charge on top

Charging by Friction: Two neutral objects Rub against each other One becomes positive, the other negative

Check for Understanding 1. If you comb you hair on a dry day, the comb can become positively charged. Is your hair charged or neutral? ANS: Charged! 2. Why do socks taken from the clothes dryer sometimes cling to other clothes? ANS: As the socks tumble together in the dryer they rub together and charge is exchanged.

Lightning

Electrostatic Force (F e ) and Gravitational Force (F g ) The Gravitational Force, F g, is based on mass. All objects with mass exert a gravitational force of attraction (and only attraction) on one another. The Electrostatic Force, F e, exists between two charged objects. Which is stronger? Let’s find out! ANS: The pie plates lifted off the Van de Graaf generator! This proves that the electrical force (F e ) is stronger than the gravitational force (F g ) !

Electrostatic Force (F e ) and Gravitational Force (F g ) But why is F e stronger than F g ? ANS: It has to do with the relative magnitudes of their constants. Newton’s Universal Gravitational Constant (G) = 6.67 x N*m 2 /kg 2 The Electrostatic Constant (k) = 9.0 x 10 9 N*m 2 /C 2 The electrostatic force is (or 100 Billion Billion) times stronger than the gravitational force!

Electroscopes An electroscope is a simple device used to determine charge – Sketch the diagram below. When an electroscope is touched by a charged object (like an ebonite rod), charges will move along the metal between the terminal and the leaves. If the electroscope is touched with a positively charged rod then negative charges move up to the terminal. The leaves are then (+) charged and repel one another!

If the electroscope is touched by a negatively charged rod, then the electrons in the terminal are repelled and move down to the foil leaves. The leaves become negatively charged and repel one another. You don’t even have to touch the rod to the electroscope to see this happen. Touching the rod to electroscope only transfers the charges, the electrons will start to move even (and the leaves separate) even before the rod touches the terminal!

Conduction & Induction Conduction Transfer of charge by touching two objects together Induction Charging an object without ever touching it. You just have to bring another charged object nearby.

Conduction Conduction – both objects share the same amount of charge, and the same kind too. Either both end up positive or both end up negative

Charging by Conduction: Bring a charged object near a neutral object Electrons ‘jump’ when contact is made The neutral object becomes similarly charged

Induction Bring a charged object near a neutral object The neutral object becomes oppositely charged

Why does rubbing a balloon on your head make it stick to the wall?

Calculating Electrical Force: Charge SI Unit – Coulomb (C) 1 Coulomb = 6.35 x electrons Elementary Charge Charge of ONE electron/proton Equal to 1.6 x C Electron (-), Proton (+) Lightning Bolt ~10 C

Forces of Charged Bodies Charges exert force on other charges over a distance Force is stronger as they are closer together Like charges repel, opposite charges attract

Coulomb’s Law Force on two charged particles depends on the amount of charge and distance between them F – Force between charges (N) k – electrostatic constant; 9.0 x 10 9 Nm 2 /C 2 q –charge (C) d – distance (m)

Check for Understanding The closer the charges are, the stronger the force. The larger the charges are, the stronger the force. Is force a scalar or a vector number? Electrical force is a VECTOR! Force between two particles is equal in magnitude but opposite in direction

Gravity and Electrostatics? Gravitational Force (Newton’s Law of Universal Gravitation) Electrostatic Force (Coulomb’s Law)

**Important Information** Info you will use over and over and over and over and over. I will give you the masses, but you need to know the charge of a proton and an electron! Mass (kg) Charge (Coulombs) Proton (p+)1.67 E -27 kg1.6 E -19 C Electron (e-)9.11 E -31 kg-1.6 E -19 C Neutron (n°)1.67 E -27 kg0 C

Steps to Solve Electrical Force Problems 1. CONVERT TO CORRECT UNITS! 2. Sketch the system 3. Use Coulomb’s Law to find magnitude (and direction*) of force 4. Use diagram to find direction* 5. Check units *depending on what is asks for-mutual force, or force of one charge on the other

Sketch the system Before calculating F, you MUST draw a free body diagram! Draw Force of B on A (F B on A ): Draw Force of A on B (F A on B ): +A+B +A-B F B on A F A on B F B on A F A on B

Let’s Practice!

Sample Problem Two charges are separated by 3.0 cm. Object A has a charge of 6.0 μC, while object B has a charge of 3.0 μC. What is the force on object A?

Sample Problem A negative charge of -2.0 x C and a positive charge of 8.0 x C are separated by 0.30 m. What is the force between the two charges?

Sample Problem Two protons are separated by 1 x m. What is the magnitude of the electrostatic force between them?

More Sample Problems A +4 mC charge lies 2 m to the left of a -5 mC charge. What is the magnitude and the direction of the resultant force between them?