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ELECTRIC FIELD Electrostatic Force ● For centuries, humans observed “strange” force – ● Force is far too strong to be gravity – ● Example: Rub amber with fur → it attracts straw Gravity between amber and straw is too weak to feel Amber and straw must have something – – – – Other than mass... Which causes a noticeable force between them This “stuff” is called electric charge Charge is measured in “Coulombs” (C) Electric Charge ● Electrostatic force can be attractive or repulsive – ● Humans named these “positive” and “negative” – – – ● There must be 2 types of charge which interact differently So the amber has positive charge... And the straw has negative charge Opposite charges attract; like charges repel This was the limit of our understanding for thousands of years – More experiments → understanding electricity in 1800's Coulomb's Law ● After much experimenting: q1 F=k d q2 ● F = Force q1 q 2 d 2 q = charge d = distance k = 9 x 109 N m2 / C2 This is an “inverse-square law” – Just like gravity! (except it can also be repulsive) Charge and Atoms ● Today we know that matter is made up of atoms – – ● Charge is always “attached” to a particle – ● Positively charged nuclei with negatively charged e– Positive charge → protons in nucleus Can never have a charge smaller than 1.6 x 10-19 C Charge can never be created or destroyed – – – It is conserved, just like energy! The universe is electrically neutral overall... ...as far as we can tell! Conductors and Insulators ● Most electrons are “bound” to the nucleus – ● Strength of this binding varies greatly – ● Some electrons are relatively easy to remove from the nucleus Conductors – – ● i.e. They orbit around the nucleus Materials with lots of “free” electrons which move easily Most metals are good conductors Insulators – Materials with few free electrons (rubber, glass, plastic) Polarization of Charge ● How do “bound electrons” respond to electric fields? – ● Shape of the electron cloud is affected Atom is said to be “polarized” – – Energy is stored inside the atom's electric field Too much polarization → electron separates from nucleus Charging Objects ● How can we transfer charge to an object? ● Conduction – – ● Put objects made of different materials in physical contact If one attracts e– better than the other → it picks them up Induction – – Bring a charged object near an uncharged one Cut uncharged object in half + – – – + + + Electric Field ● How can particles exert force without touching? – ● “Action-at-a-distance” → just like gravity Solution: The “electric field” Charge creates exerts force ● exerts force Electric Field Charge creates Electric charge affects the space around it – If another charge comes into this “field”, it feels a force Electric Potential – “Voltage” ● Charge affects space (via the electric field) – ● Need to measure the effect at a given point in space Electric potential (or “voltage”) – – Describes the overall effect of nearby charge Measured in Volts (V) + + + A – – Point A has a large positive voltage B – Point B has a large negative voltage Storing Electric Energy ● Must separate + and – charges – ● Batteries – – – ● Like stretching an imaginary rubber band Place 2 different substances in contact with acid + charges move toward one object... – charges move to the other Capacitors – – – 2 metal plates with a gap between them Place + charge on one plate and – charge on the other Good for releasing energy quickly → e.g. camera flash bulb Releasing Electric Energy – Current ● Allow + and – charges to come back together by making an electric “circuit” – ● Electric current – – ● Let the imaginary rubber band “snap” The flow of charge from one location to another Units: Coulombs / second (C/s) OR Amperes (A) Currents occur when there is a “voltage drop” – – – Difference in voltage between one location and another Often due to a battery, capacitor, or generator KE of moving charges can be used → electronic devices Resistance and Ohm's Law ● Resistor – ● Device that converts current to another form of energy Resistance – – Defined by “Ohm's Law”: Units: Measured in Ohms (Ω) Voltage drop R= Current Power in Electric Circuits ● Electric power: – – Energy used Power = time Units: Measured in Watts (W) or horsepower (hp) Depends on the electric current and the resistance 10 – 500 W 1,000 – 10,000 W About 1000 W on average