CURRENT ELECTRICITY CURRENT q Electric Current I The

CURRENT ELECTRICITY

CURRENT q Electric Current (I)- The amount of charge that passes through an area in a given amount of time, usually electrons. Units for I are amperes (A or amps) or C/s – Why? § Current that flows in one direction is direct current (DC) § Current that alternates direction is alternating current (AC) §

POTENTIAL DIFFERENCE Another term for potential difference (V) is voltage (V) Units-volts (V) The voltage measures the electric potential difference between two points. Electric current will flow from high to low potential -the greater the difference, the greater the electron flow. Circuits: Electron Flow vs Conventional Flow

POTENTIAL DIFFERENCE SOURCES EXAMPLES Voltaic Cell- cell that converts chemical energy into electric energy. Ø Battery- several cells together to produce electric energy from chemical energy Photovoltaic- solar cell, converts light energy to electric energy. Generators, power plants

RESISTANCE – AN OPPOSITION TO THE FLOW OF CHARGE: UNITS FOR RESISTANCE = OHM’S RESISTORS - DEVICES DESIGNED TO HAVE A SPECIFIC RESISTANCE. EX. POTENTIOMETER

OHM’S LAW When charges pass through a resistance (resistor), some electrical energy is changed to other forms (usually heat). This is produced by a potential difference across the resistance (resistor) Ohm’s Law Equation: Conductors I = V/R or V = IR have little resistance to electron flow Insulators have great resistance to electron flow.

OHM’S LAW EXAMPLE While cooking dinner, Dinah’s oven uses 220 V line and draws 8 A of current when heated to its maximum temperature. What is the resistance of the oven when it is fully heated?

ELECTRIC POWER Electric Power- the product of voltage x current. Equation: P=Vx. I Power = Voltage x Current Units: Watts The unit for current is the ampere. 1 ampere = 1 Coulomb/ second Found by a French scientist Andre Ampere Manipulations & Substitutions What if you are not given Voltage? What if you are not given Current?

OHM’S LAW An alarm clock with a resistance of 60 W is plugged into a 110 V outlet. What is its power rating?

ELECTRIC ENERGY & COST High voltage transmission lines carry electrical energy over long distances with minimal loss of energy. Ex. Power Plants The kilowatt-hour is a unit of energy. Energy = Power x time (units: kilowatt-hour) Calculating Cost: Cost = Energy Used x $Price/k. Wh

A toaster with resistance 20 W draws 6. 0 A when connected to a potential difference. a) What is the power rating of this toaster? b) How much does it cost to run the toaster for 2 hours a day at $0. 10/k? What if it operates 4 days a week for 4 weeks?

POWER & COST The Garcia’s like to keep their 40 W front porch light on at night to welcome visitors. If the light is on from 6 PM to 7 AM, and the Garcia’s pay 8 cents per k. Wh, how much does it cost to run the light for this amount of time each week?

ELECTRIC CIRCUIT Electric Circuit- charges moving around a closed loop from a pump(battery) back to the pump. There are two basic types of circuits: series and parallel. There is only one path or branch for electron flow in a series circuit. There are multiple branches or paths in a parallel circuit.

SERIES CIRCUITS EXAMPLE A 4 W and a 6 W resistor are connected in series with a 12 V battery. Draw a schematic and find: • Total resistance • Total current • Voltage and current through each resistor

INTERNAL RESISTANCE EMF = a device that transforms one type of energy into electrical energy (ie battery) A battery itself has some resistance, which is called internal resistance, designated as r and can be represented is series with the terminal itself (thus can never be separated). Examples: Lights dimming in your car when you turn it on. Terminal Voltage – a measure of the actual emf (voltage) when a current flows from a battery (not the rated emf). Example: If a 12 V battery has an internal resistance of. 1 Ω and 10 A flows from the battery. The terminal voltage is 12 V – (10 A)(. 1Ω) which is equivalent to 11 V’s.

INTERNAL RESISTANCE EXAMPLE A 2Ω resistor and a 4Ω resistor are arranged in series to a 20 V emf which has an internal resistance of 1Ω. Determine the current running through the circuit and the voltage drop across each resistor to include the terminal voltage:

SHARING OF CHARGE Capacitors – A device that can store electric charge (Q) on two parallel conducting plates (Pot. Difference or V). A measure of the amount of charge (Q) stored on the conductors for a given potential difference. Equation: Capacitance (C): C = Q / V Units: Farad (1 Coulomb per Volt) For a given capacitor (C), the amount of charge (Q) acquired by each plate is found to be proportional to the potential difference (V): Q = CV Examples: Pacemakers, Computers, Camera Flashes, Human Nervous System

CAPACITANCE EXAMPLE The first capacitor was invented by Pieter van Musschenbroek in 1745 when he stored a charge in a device called a Leyden jar. If 5 x 10 -4 C of charge were stored in the jar over a potential difference of 10, 000 V, what was the capacitance of the Leyden jar? Consequently, when he touched the jar, he received a large jolt!!!

RC CIRCUITS Capacitors and resistors can work together in a circuit. Essentially it is a way to charge a capacitor while continuously supplying energy to the resistor (Remember V = Q/C). Once the capacitor is fully charged or matches the applicable emf, no further current flows. Examples: Pacemaker, Computer Screens …

CAPACITORS IN SERIES & PARALLEL Parallel: If a battery of voltage V (constant) is connected in parallel to each of the capacitors, then each capacitor will acquire a charge equivalent to the same voltage down each parallel branch: Q 1 = C 1 V, Q 2 = C 2 V, Q 3 = C 3 V … Thus Q = Q 1 + Q 2 + Q 3 and…CV = C 1 V + C 2 V + C 3 V… C = C 1 + C 2 + C 3 (Parallel Capacitance) Series: Now the voltage V (not constatnt) across the three capacitors in series must equal the sum of the voltages across each capacitor: V = V 1 + V 2 + V 3…Thus Q/C = Q/C 1 + Q/C 2 + Q/C 3… 1/C = 1/C 1 + 1/C 2 + 1/C 3 (Series Capacitance)

RC EXAMPLE (IN PARALLEL) From the Internal Resistance example, determine the charge Q on a 3μF capacitor that is in parallel with the 4Ω resistor.

ENERGY TRANSFERS IN CIRCUITS Equation: Power = Current 2 x Resistance When a capacitor is charged or discharged through a resistor, the current is high initially and falls to 0 The energy transferred is the product of power and time. Equation: Energy = I 2 Rt