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Electric Circuit Charges in Motion OCHS Physics Ms. Henry

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Current Amount of charge per time that passes through an area perpendicular to the flow: 1 ampere = 1 coulomb/second A coulomb is a unit of electric charge: 6.241x10 18 electrons I (A) = ∆q (C) ∆t (s)

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Resistance We know that current is proportional to voltage for conductors. Resistance is the proportionality constant within the circuit (what causes a variation). The relationship is called: Ohm’s Law 1 Ohm (Ω)=1Volt/1 Ampere V(Voltage)=I(Current)R (Resistance) (Volt, V) = (Amp, A) (Ohm, Ω)

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Resistivity Resistance depends upon a conductor’s length ( l ), it’s cross-sectional area (A), and it’s resistivity (ρ). Resistance to current happens when the flow of moving charges is hindered by the material of the wire. Thus, Unit: ohm-meter. R = l ρ A Note: A resistor is specifically designed to resist current in a circuit. For example: a light bulb or heating element.

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Voltage Provided by the battery between it’s terminals. A constant potential difference comes from the battery - for example, 6 Volts. When current passes through the light bulb, charges move from a higher potential to a lower, with a difference of 6 volts. Energy is then being converted into light or heat. Often represented by: ε

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The electric field set up in the wire causes the current to flow; which happens when the circuit is complete. Current flows toward the positive charges; from the negative end of the battery to the positive.

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Electrical Power & Energy Power (Watts): rate of energy usage. 1 watt = 1 Joule/second = 1 ampere-volt ε ε + + P = IV = V 2 = I 2 R R

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Direct Current (DC) Circuit Direct current means current flowing in only one direction. Series Current is the same at any point in the circuit. (I = I 1 =I 2 ) The potential difference supplied by the battery equals the potential drop over R 1 and the potential drop over R 2. Thus, Parallel Current branches off at the intersection point; part of the current goes one way part of the current goes the other. The potential drop of current is the same regardless of which path is taken; thus, the voltage difference is the same over either resistor. (V batt =V 1 =V 2 ) Currents sum to the total current: V=V 1 +V 2 Ohm’s Law V=IR 1 +IR 2 V=I(R 1 +R 2 ) Equivalent Resistor R eq = R 1 +R 2 I=I 1 +I 2 I= V + V I=V( 1 + 1) 1 = 1 + 1 R 1 R 2 R 1 R 2 R eq R 1 R 2

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Kirchhoff’s Rules Junction Rule: The sum of the currents entering the junction must equal the sum of the currents leaving the junction; referring to the conservation of charge. Conservation of Energy: the charge moving around any loop must gain as much energy from batteries as it loses when going through resistors. When applying the rules: Use consistent sign conventions. (i.e. counterclockwise or clockwise) If you choose an incorrect direction initially the solution will have a negative current.

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