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Electricity Part 1 Electric Current • The rate of the flow of electrons (charge) – Charge Movement / Time • Symbol = I • SI unit = Ampere, A (or Coulomb/ second, C/s) – 1 ampere = 1coulomb/second Electric Current Equation • Electric current = Charge Movement / Time I = Dq /Dt • Ex. A charge of 1.67 C passes through a wire in 2 seconds. What is the current of the wire? I = Dq /Dt I = 1.67C / 2.00s I = 0.835 A Electric Resistance • Slows down electron flow in the circuit – Everything has some resistance. – Like friction for electric charges • Symbol: R • Units: W (Ohms) Resistance Factors • Electric Resistance depends on: – Length of wire • longer wires have more resistance than short wires. – Thickness of wire • thicker wires have less resistance than thin wires. – Resistivity of wire • Materials with less resistivity allow electricity to flow easier Resistivity of various materials Material Resistivity (10-8 Wm) Material Resistivity (10-8 Wm) Silver Copper Gold Aluminum 1.61 1.70 2.20 2.65 Bismuth Plutonium Graphite Germanium 106.8 141.4 1375 4.6x107 Pure Silicon Calcium 3.5 Diamond 2.7x109 3.91 1.8x1013 Sodium Tungsten Brass Uranium Mercury 4.75 5.3 7.0 30.0 98.4 Deionized water Iodine Phosphorus Quartz Alumina Sulfur 1.3x1015 1x1017 1x1021 1x1022 2x1023 Voltage • Amount of energy given to each unit of electric charge • Symbol: V • Units: V (volts) Ohm’s Law • Equation: V=IR • Voltage (V) = Current (I) * Resistance (R) • So … – If current increases, voltage increases • Directly proportional – If current increases, resistance decreases • Inversely proportional Ohm’s Law Example Problem • A potential difference of 10 V applied across a wire produces a 0.2 A current. What is the resistance? V=IR 10 V = 0.2 A * R 50 W. = R Circuits • A circuit is a closed or complete path through which electrons can flow. • An Open circuit has a break in it, electricity cannot flow. • A short circuit is just wires from the one battery terminal to the other. – Because flow is too quick, electrons back up and energy is lost as heat • Switch – can open or close circuits. – Can act as a gap or a wire Batteries • The current (electrons) flows from the negative terminal to the positive terminal in batteries. – Uses chemical energy to push electrons through Insulators vs Conductors • Conductors – conduct electricity well – Usually metals • Insulators – do not conduct electricity well – Need high energy to conduct Series Circuit • When electrons have to flow through one part to get to the next part – More components = more resistance – Increase resistance = decrease current (flow) – Less current = less bright bulbs – As voltage increases, current increases Circuit Diagram Symbols Wire Battery Light bulb Resistor Switch