# Aim: How can we explain electric current and resistance? Do Now: Two charged plates are connected by a wire conductor. What will happen? + -

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Aim: How can we explain electric current and resistance? Do Now: Two charged plates are connected by a wire conductor. What will happen? + -

Electron Flow Current flows from negative to positive + -

Electric Current The “flow of charge” – the amount of charge that passes a single point in a period of time

Units = Ampere André Marie Ampère 1775 - 1836

A current of 2.0 A is flowing in a circuit. How much charge passes a given point in the circuit in 30 seconds? I = Δq t 2.0 A = Δq 30 s (2.0 A)(30 s) = Δq 60 C = Δq

5 electrons pass a given point in 1 x 10 -18 s. What is the current?

Resistance A measure of how easy or difficult it is for current to flow

Ohm’s Law Units = ohm (represented as Ω) Georg Simon Ohm 1787-1854

A potential of 120 V draws 15 A of current. What is the resistance? A transistor radio of 0.2 A of current is operated by a 9 V battery. What is the resistance?

When solving for other expressions, here’s the electrician’s trick: V IR R = V =IR I =

75 V flows through a 15 Ω resistor. What is the current? A 60 Ω resistor allows 0.4 A of current to flow. What is the voltage?

What is the potential difference across a 2.0 Ω resistor that draws 2 C of charge per second?

Graph of V vs. I Current (amps) Voltage (volts) Slope is RESISTANCE

Resistance Increases when: Length of conductor increases/decreases Cross sectional area of conductor increases/decreases Temperature of metal conductor increases Temperature of non-metal conductor decreases Area length

Resistance of Conducting Wire R = Resistance (Ω) ρ = Resistivity (Ωm) L = Length (m) A = Cross Sectional Area (m 2 )

Resistivity

Calculate the resistance of a 2.0-m long Aluminum wire with a cross sectional area of 1.5 x 10 -3 m 2. R = ρL A R = (2.82 x 10 -8 Ωm)(2.0 m) (1.5 x 10 -3 m 2 ) R = 3.8 x 10 -5 Ω

Calculate the resistance of a 5.0-m long Nichrome wire with a radius of 7.5 x 10 -6 m. A = πr 2 A = π(7.5 x 10 -6 m) 2 A = 1.77 x 10 -10 m 2

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