# What’s a Circuit?  A circuit is a closed path where positive charges flow from high to low potential. They can be manipulated on the way.

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What’s a Circuit?  A circuit is a closed path where positive charges flow from high to low potential. They can be manipulated on the way.

The Power Source  Provides the difference in potential (potential energy per unit charge). It is measured in volts (remember this)  A Cell (battery) is the easiest to see. It converts chemical energy to electrical energy.  This is also called “Electromotive force” (or emf)  Think of “Voltage” as “pressure” the charges are moving.

What is Electric Current?  An electric current is the flow of charge through wires and components.  The greater the current, the more charges are moving!  In which direction does the electrons flow?  It flows from the negative terminal to the positive terminal.  Measured in Amperes (we say Amps) + -

Conventional Current Electron flow: The direction of e - flowing from – to +. Conventional current: The motion of +q from + to – has same effect. Electric fields and potential are defined in terms of +q, so we will assume conventional current (even if electron flow may be the actual flow). + + - - +- Electron flow +-+- e-e- Conventional flow +

Resistance  When an object (like a light bulb) resists or diminishes the flow of current, it has resistance.  Resistance is measured in Ohms Symbol for Ohms: Ω

Water Analogy to EMF Low pressure PumpWater High pressure Valve Water Flow Constriction Source of EMF Resistor High potential Low potential Switch R I +- The source of emf (pump) provides the voltage (pressure) to force electrons (water) through electric resistance (narrow constriction).

Ohm’s Law  Relates the voltage (Volts), current (Amps), and resistance (Ohms) in a circuit.  V = Voltage (in Volts)  I = Current (in Amps)  R = Resistance (in Ω)

Power  Power describes the rate at which electrical energy is transferred.  It is measured in Watts (W).  P = Power (Watts)  I = Current (Amps)  V = Voltage (Volts) OR..

A light bulb has a resistance of 30Ω. What voltage would be required to run 4 Amperes of current through the bulb? A. 120 V B. 7.5 V C..13 V D. Voltron

A toaster is connected to a 120-Volt circuit and has 6 Amps of current running through it. What is the resistance of the toaster? A. 720 A. 720 Ω B. 20 B. 20 Ω C..05 C..05 Ω D. Depends on how brave he is.

If your body resistance is 100,000 Ω, how much current will you experience if you touch the terminals of a 12-Volt battery? A. 1,200,000 A B. 8,333 A C..00012 A D. Depends on how good it taste

How much power is dissipated in a toaster if it is connected to a 120- Volt circuit and uses 8 Amps? A. 960 W B. 15 W C..067 W D. Zero, it is powered by imagination

A light bulb has a power rating of 60-Watts. How much current would it pull if it has a resistance of 15 Ohms? A. 4 A B. 2 A C. 900 A D. 30 A

The End! The End!

Circuit Diagram cellswitchlampwires We draw electric circuits using specific symbols (because quite frankly most people can’t draw)….

Types of Circuits There are two basic types of electrical circuits; SERIES CIRCUITSPARALLEL CIRCUITS

The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. SERIES CIRCUITS If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. (One charge gets stuck, they all get stuck).

PARALLEL CIRCUITS The current has a choice of routes. The components are connected side by side. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.

Parts of a Circuit Electrical circuits often contain one or more resistors grouped together and attached to an energy source, such as a battery. The wires need to make a complete circle from positive to negative potential.

Circuit Diagram cellswitchlampwires We draw electric circuits using specific symbols (because quite frankly most people can’t draw)….

Types of Circuits There are two basic types of electrical circuits; SERIES CIRCUITSPARALLEL CIRCUITS

The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. SERIES CIRCUITS If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. (One charge gets stuck, they all get stuck).

PARALLEL CIRCUITS The current has a choice of routes. The components are connected side by side. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.

“Short Circuit”  Just remember that electricity is lazy, and will always take the path of least resistance.  If something (usually a wire) provides a path around a resistor, the electrons will take it!

COMPLEX CIRCUITS This is what most circuits in the “real world” are like. Is made up of both series and parallel circuits combined.

The End! The End!

Applying Ohm’s Law  It can be used to analyze a whole circuit or a single component.  For the circuit shown  What is the total current?  What is the voltage across a single resistor?  What is the voltage across two resistors?

What if the circuit isn’t so simple?  Can we find the total current through this circuit?  What information do we need?  Voltage is given  Resistance is going to be a little tougher.  We need to find the “equivalent resistance” of the whole circuit.

Equivalent Resistance  Remember that in series, we just add the resistors  R 1 + R 2 + R 3 + …  In parallel, we can use the formula at right.  Do the parallel part 1 st and then add it to the series resistor.

Now, back to the original problem:  We have a total resistance of 15Ω and a voltage of 9V

Isn’t electricity supposed to be dangerous?  Well, that depends.  You would think that 1000V would be more dangerous than 100V, right?  It all comes down to Ohm’s law  It is actually the current that kills, not the voltage.

Current kills…Voltage hurts  As current flows through muscle tissue, the muscle fibers contract.  Anything over 10mA can cause you to grab onto a wire and not be able to let go.  At around 75mA, you are unable to breathe  Between 100 – 200mA, the heart fibrillates  Actually, above 200mA, your chances are better because the heart seizes completely

What determines how much current flows?  That depends on the voltage and resistance  Your skin usually has around 100,000 – 500,000 Ω of resistance when dry.  If you get wet or sweaty, that resistance goes way down.  Whether you get hurt depends on the voltage and your skin’s resistance  This is why we could play with the batteries, but it would be dangerous to use wall outlets

What amount of current would be used if a 10Ω resistor was connected to a 240 V circuit? A. 2400 A B. 2400 I C. 24 I D. 24 A

A light bulb with a resistance of 10 Ω is hooked up to a circuit. The power used by the bulb is 160 Watts. A. A. What is the voltage difference is in the circuit? B. B. How much current is flowing through the light bulb

If a 20 ohm resistor is connected to a 30 ohm resistor in series. Which resistor will have a higher current. A. The 30 Ohm one B. The 20 Ohm one C. Both have the same D. Not enough info

If a 20 ohm resistor is connected to a 30 ohm resistor in series. Which resistor will have a higher voltage. A. The 30 Ohm one B. The 20 Ohm one C. Both have the same D. Not enough info

If a 20 ohm resistor is connected to a 30 ohm resistor in parallel. Which resistor will have a higher current. A. The 30 Ohm one B. The 20 Ohm one C. Both have the same D. Not enough info

AC/DC Direct vs. Alternating Direct vs. Alternating A circuit containing a battery is a DC circuit. A circuit containing a battery is a DC circuit. In a DC circuit the current always flows in the same direction. In a DC circuit the current always flows in the same direction. The electricity that you get from the power company is AC. The electricity that you get from the power company is AC. In an AC circuit the current reverses direction periodically. In an AC circuit the current reverses direction periodically. Duracell + The voltage reverses polarity 60 times a second. The current through the bulb also reverses direction 60 times a second. AC/DC

Effects of Resistance  Resistors oppose or “resist” the current flowing through them.  Light bulbs do as well. The filament inside the bulb glows hot from resisting current.  The longer the resistor or light bulb filament, the more resistance. The more stuff the current has to push through  The thicker the wire the less resistance. It has more room to pass through the material.  Even wire has a resistance, but it is usually very small

If a 20 ohm light bulb is connected to a 30 ohm bulb in parallel. Which light bulb will be the brightest. A. The 30 Ohm one B. The 20 Ohm one C. Both have the same D. Not enough info

If a 20 ohm light bulb is connected to a 30 ohm bulb in series. Which light bulb will be the brightest. A. The 30 Ohm one B. The 20 Ohm one C. Both have the same D. Not enough info

Electrical Stored Energy  Electrical energy can be stored in a device called a capacitor.  Capacitors are found in nearly all electronic circuits.  The simplest capacitor is a pair of conducting plates separated by a small distance, but not touching each other. When the plates are connected to a power source, charge is transferred from one place to the other. This occurs as the positive battery terminal pulls electrons from the plate connected to it. The capacitor plates then have equal and opposite charges.

Capacitors  The charging process is complete when the voltage between the plates equals the voltage between the battery terminals.  The greater the battery voltage and the larger and closer the plates, the greater the charge that is stored.  The net charge on the capacitor is still zero.

Discharge Capacitors  A charged capacitor is discharged when the plates make contact.  Capacitors can still shock you even when the device is turned off (ex: TVs).

Capacitance Capacitance is the measure of the ability of a device to store charge for a given voltage. Capacitance is the measure of the ability of a device to store charge for a given voltage. q = C V C = capacitance, in Farads q = magnitude of charge on each plate V = voltage difference SI unit for capacitance is the farad, F SI unit for capacitance is the farad, F 1 F = 1 C/V 1 F = 1 C/V

Remember  Because some charges are very small, we will use smaller units.  You will need to remember how to convert. 10 -6 Coulomb = 1 microCoulomb (μC) 10 -6 Farad = 1 microFarad (μF)

Series Circuits  Only one path for electricity to flow! If connected in series, the capacitors charged in consecutive fashion. If connected in series, the capacitors charged in consecutive fashion. Components in a series circuit share the same charge Components in a series circuit share the same charge q total = q 1 = q 2 … q total = q 1 = q 2 … In a series circuit, adding more capacitors causes the overall charged stored within the circuit to decrease. In a series circuit, adding more capacitors causes the overall charged stored within the circuit to decrease. Total Voltage is equal to the sum of the individual voltage drops V total = V 1 + V 2 +…. Total Voltage is equal to the sum of the individual voltage drops V total = V 1 + V 2 +…. To find total capacitance in a series circuit, you have to so something similar to a resistors in parallel because total capacitance decreases To find total capacitance in a series circuit, you have to so something similar to a resistors in parallel because total capacitance decreases

Parallel Circuits  More than one path for electricity to flow!  All capacitors in a parallel circuit are charged all at the same time  Components in a parallel circuit share the same voltage V total = V 1 = V 2 … V total = V 1 = V 2 …  Total charge stored in a parallel circuit is equal to the sum of the individual branch charges q total = q 1 + q 2 +…. q total = q 1 + q 2 +….  For parallel circuits, as the number of capacitors increases, the overall charge stored also increases. To find total capacitance in a parallel circuit, add the individual capacitance To find total capacitance in a parallel circuit, add the individual capacitance C total = C 1 + C 2 + C 3 + …

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