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Physics 4.4

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Charge What is charge? Where do you see charge around you? Describe the atom in terms of charge?

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Coulombs and elementary charges 1 coulomb (C) is equal to 6.25 E18 elementary charges 1 elementary charge is equal to the charge of one proton (+), one electron (-), or 1.60 E-19 C

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Electrostatic forces between objects near each other What does a positively charged object do the a: Positive object repel Negative object attract Neutral object alignment

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Law of Conservation of Charge Charge cannot be created or destroyed-the total charge in the system remains constant. Ex/ think about when you rub a balloon on your hair. Your hair loses electrons and the balloon gains negative charge! When charged objects are in contact, to find individual charges, add up the total charge and divide by the number of objects. They all have the same charge.

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Coulomb’s Law EEEElectrostatic force (Fe) is directly proportional to the product of the charges (q1 x q2) and inversely proportional to the square of the distance between the charges (r²).

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??? Charge on sphere A is +5.0 E-4 C. Charge on sphere B is +3.0 E-5 C. Distance separated is 3.0 meters. Find: electrical force F e. F e = kq1xq2 / r²

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Electric Field Any place where there is an electrostatic force (attractive or repulsive) between the two charged particles, we say the there is an electric field between them. E = Fe / q Electric field strength tells how strong an electric field is. Units are N/C

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??? At point P in an electric field, the magnitude of the electrostatic force on a proton is 4.0 E-10 newton. What is the magnitude of the electric field intensity at point P? E = Fe / q

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Electric fields around point charges or spheres An electric field (electric field lines or lines of force) goes away from positive and goes toward the negative. Electric field lines never intersect (never cross) each other. Draw electric field lines around: A positive point charge A negative point charge

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??? An object with a net charge of 4.80 E-6 coulomb experiences an electrostatic force having a magnitude of 6.00 E-2 newton when placed near a negatively charged metal sphere. What is the electric field strength at this location? E = Fe / q

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Electric field lines between two parallel plates Electric field lines go away from the positive plate and go toward the negative plate. Electric field strength is the same everywhere between the plates. Where would an electron accelerate towards? A proton? A neutron? Which would accelerate faster? Why?

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Potential Differences Potential difference (or potential drop) is the work or energy required to move a charged particle towards a like charged particle divided by charge in coulombs (C). V = W / q Units = volts (V) or joules per coulomb.

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??? The energy required to move one elementary charge through a potential difference of 5.0 volts is: V = W / q

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Electric current Number of charges that reach a point in a wire in one second. I = ∆q / t Units = C / s or amperes (A).

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??? If 10. coulombs of charge are transferred through an electric circuit in 5.0 seconds, then the current in the circuit is: I = ∆q / t

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Resistance Resistance hinders the flow of charges, which means the flow of current (I). Increased temperature means bigger resistance. R = V / I is called Ohm’s Law. Units are volts per ampere or ohm (Ω).

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??? In a simple electric circuit, a 110 volt electric heater draws 2.0 amperes of current. The resistance of the heater is: R = V / I

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Resistivity Characteristic of a material at a specific temperature. Resistance is affected by temperature plus several factors which are displayed in this equation: R = ρ (resistivity) x L (length of wire) A (cross-sectional area) A (cross-sectional area)

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??? A 12.0 meter length of copper wire has a resistance of 1.50 ohms. How long must an aluminum wire with the same cross- sectional area be to have the same resistance? R = ρL / A

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Series Circuits I = I 1 = I 2 = I 3 =… V = V 1 + V 2 + V 3 +… R eq = R 1 + R 2 + R 3 +…

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Parallel Circuits I = I 1 + I 2 + I 3 +… V = V 1 = V 2 = V 3 =… 1 1 1 1 R eq = R 1 + R 2 + R 3 +…

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Ammeters and Voltmeters An ammeter is used to measure current and is connected in series with the circuit element. A voltmeter is used to measure the voltage and is connected in parallel.

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Conservation of charge in a circuit

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Power Electric power is the product of voltage and current. P = IV = I²R = V²/R

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Magnetic Field Strength Magnetic field strength is strongest where the field lines are closest.

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Electromagnetic Induction When a conductor of wire cuts across magnetic field lines, one end of the wire becomes more negative and one end more positive, producing a potential difference. This induction is greatest when the wire moves perpendicular to the magnetic field lines at a greater speed.

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