Unit 13 Electric Circuits

Electrostatics refresher: Voltage 1. ________ is the force created by a buildup of charge. And is also called a potential difference. 2. Voltage can make electrons __________. 3. Electron movement allows ____________ to flow. move current

Common Electrical Units 1. The unit of charge is the COULOMB Charge has the variable Q, the unit C 2. The unit of voltage is the VOLT Voltage has the variable is V, the unit is V 3. The unit of current is the AMP Current has the variable I, the unit is A 4. The unit of resistance is the OHM Resistance has the variable R, the unit W

Think in Pairs Electricity is two things: Voltage and Current Voltage has two polarities: + and – Two types of electrical power: AC and DC Electricity has two purposes: as an energy source (power) and as a signal Signals can be either analog or digital Alternating current and direct current Signals represent information

Electricity is…. A good power source Easy to create Easy to control Efficient to transmit Easy to measure Makes everything work – our way of life depends on it

What is a schematic? Diagram or drawing using symbols to build a circuit. There are two types of circuits. Series- one path for electrons, if one bulb goes out they all do Parallel – more than one path for electrons, if one bulb goes out the rest can still shine 6

Battery Symbol DC ( Direct Current -1 direction ) Voltage source The long side is positive The short side is negative Consists of one or more cells ( batteries ) Cells ( Batteries ) are in series Cell voltage adds to give total source voltage Dry Cell ( Battery ) – chemical paste ( Ex: A, AA, D, cell phone Nicad, Lantern 6-Volt, 9-Volt ) Wet Cell (Car Battery ) – Sulfuric acid & Distilled H2O

DC vs. AC DC ( Direct Current ) electrons travel in 1 direction EXAMPLES: Wet Cell and Dry Cell Batteries AC ( Alternating Current ) electrons travel in 2 directions ( back and forth ) EXAMPLE: AC is the type of electricity that you have in your home

AC Step-Up Transformer STEP-UP - raises the voltage of the output of the main generator to the level of the grid transmission lines. (so e-’s can travel long dist.’s to your house)

AC Step-Down Transformer STEP-DOWN – reduces the voltage so electricity can enter your home for 120V & 220V outlets ---

Fuse symbol A Fuse is 1 of the major protection elements Another protective device is called a circuit breaker Fuses & Circuit Breakers - Protects the circuit, not the person! Ground Fault Circuit Interrupter (GFCI) is a type of circuit breaker GFCI protects people Turns off the circuit, like a switch A switch is a control element, not a protection element Replaced once it is blown With the same amperage rating! Ground Fault Circuit Interrupter measures circuit current supply and circuit current return, and if there is a difference (even a small one) it means that there is some other path for current to flow, perhaps through a person to earth ground.

Circuit Breakers & Fuses Too many appliances on at the same time in a home cause the overall resistance of the circuit to be lowered. Result is a circuit carrying more electrical current than is safe it is Overloaded. Fires can result when this happens. Circuit Breakers act as a switch and open the circuit. Fuses – blow to open the circuit

Resistor Symbol One of the most common circuit elements Used to limit or restrict current Resistance amount indicated with a color code A circle or band around the resistor The physical size indicates the power rating Not the resistance value! The smaller the resistance value, the larger the amount of current A two lead device with no polarity.

FYI : Resistors information THE END   http://commons.wikimedia.org/wiki/Main_Page

Voltmeter Symbol (Must be wired in Parallel) Measures across a device Red lead to positive, black to negative Measures a voltage difference Has a high amount of internal resistance So no current flows from the circuit into the meter Does not change circuit voltage, current, or resistance Voltage range is usually a meter setting Selector switch or button Some meters have a auto range function

Ammeter Symbol Must be wired in Series Measures amperage Amp meter Must be placed in the circuit Current must flow into (through) the meter to be measured An ammeter has low (zero) internal resistance So it doesn’t reduce circuit current Leads must be changed from volts/ohms Dangerous to the meter!

An Electrical Circuit Must have a voltage source Must have a load A resistor or other device to limit current Can be a circuit component, like a light Must have a complete path from one side of the voltage source to the other Usually a wire Should have a protection device and a control device Usually a fuse or circuit breaker and a switch

A Simple Series Circuit Series – only 1 path Battery Resistor (Load) Current Flow + - The dark lines are wires Negative and positive voltages are relative to ground. When ground is placed at the most negative point, then every other voltage relative to ground will be positive. Voltage is measured across the battery or load, current flows through devices (inside wires). Ideally, this circuit should have a control device (switch) and a protection device (fuse) All of the voltage is dropped across the resistor Current flows from negative to positive

Open switch, no current flows Resistance is infinite ( electrons do not flow ) Voltage is dropped across the switch Voltage is always used up in a circuit. In other words, all of the voltage is always used. The component with the higher resistance uses more of the available voltage. When the resistance is infinite (as in this circuit with an open switch) the switch drops (uses) all of the voltage, but no current is produced because the resistance is infinite.

Closed switch – current flows Current flows from negative to positive Here the amount of current is determined by the amount of voltage and the resistance of the light bulb.

Parallel Circuit More than 1 path

OHM’s LAW – I = find this formula on your chart V R OHM’s LAW – I = find this formula on your chart An electric circuit must have a complete path from one side of a power supply (+V) to the other (-V) Voltage is caused by a build-up charge Voltage across a resistor creates current Voltage, current, and resistance follow Ohm’s Law Current is a flow of charge ( AMPS ) ampere = volts / ohms OR it could be 1 amp = 1 Coulomb / sec

The Ohm’s Law V I = V / R R = V / I V = I x R R I

Circuit Calculations R = VS = Vs = 20 V R = 250 Ω I = ? A ( Solve for I ) I = V / R 20 V / 250Ω = .08A VS equals source voltage

Circuit Calculations R = VS = Vs = 24 V I = 37 mA Convert to Amps Re-arrange formula to solve for R. R = V / I 24 V / .037 A = 648 Ω VS equals source voltage

Circuit Calculations R = VS = I = 5.9 mA R = 2.2 kΩ What is V? V = I x R = .0059 A x 2200 Ω = 13 V VS equals source voltage

A Series Circuit R1 R2 VS R3 RT = R1 + R2 + R3 VS = V1 + V2 + V3 (Resistance Adds) 1. Formula for total resistance - A series circuit has one path for current flow. The same current goes through all the circuit components. 2. Formula for total voltage across - VS = V1 + V2 + V3

A Parallel Circuit The voltage across each branch is the same The formula for Voltage: VS = V1 = V2 = V3 Hint: use the 1/x button (also shown as the x-1 button on some calculators) to solve for RT.   The formula for Resistance:

PhET Ohm’s Law Simulation What you need to keep in mind when using the simulator. How do changes in the variables affect the voltage?

Series-Parallel Circuit Work the Problem to Solve for Current 1st  Solve for Load 1 (4 Ω) and Load 2 (4 Ω) ( What formula? ) 2nd Add Load 3 (2 Ω) to answer in 1st step ( What Formula ?) 3rd  solve for current ( V = 12 Volts ) (What Formula ? )

Electrical Power Formula P = VI P = Watts ( always write this out ) V = Volts ( V ) I = Amps ( A )

Let’s Practice some Math!!