 # Physics 4.4. Charge  What is charge?  Where do you see charge around you?  Describe the atom in terms of charge?

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Physics 4.4

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

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

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

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.

Coulomb’s Law EEEElectrostatic 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).

???  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

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

???  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

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

???  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

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?

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.

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

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

???  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

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).

???  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

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)

???  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

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

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 +…

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.

Conservation of charge in a circuit

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

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

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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