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Electricity Chapter 34

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Electric Current Section 1 Lab: – What do you need to have a circuit? – What flows in a circuit? –.

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34.1 Flow of Charge Charge flows when there is a potential difference, such as from the positive terminal of a batter to the negative terminal of the battery Remember, the positive and negative sides of the battery have to do with the excess and shortage of electrons.

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Charge flows until potential is the same – If there was not difference in height, then the water would not flow out of the reservoir in the picture – Similar to heat flow- if there is no difference in temperature than there is not flow of heat

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34.2 Electric Current Electric current is the flow of charge or the amount of coulombs (charge) that flows past a point in a given amount of time Ampere (A)is the SI unit of electric current – 1A= 1C/s – Named after who? What did he do?

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I = Q/t I = symbol for current (SI unit of current = Ampere) Q = symbol for charge (SI unit for charge = Coulomb) t = symbol for time (SI unit for time = second)

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Practice Problem: 20 Coulombs of charge passes a point in 10 seconds. What is the current? – 2 A A current of 100A exists. How many coulombs of charge move past a given point in 5 seconds? – 500C

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Net charge in a conducting wire is zero What moves? – Electrons do not enter/leave a conductor, such as a wire. – Electrons transfer energy from one conductor to another.

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33.5 Electric Potential Electric potential is electrical potential energy per charge. Voltage is potential energy per coulomb available to electrons moving between terminals

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Voltage, or electrical potential difference, causes current Volt (V) is the SI unit for potential difference – 1 Volt = 1 Joule (or energy) per Coulomb (or charge) – 1V = J/C

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Electric outlets have a potential of 120V. This means there is 120 J of energy per coulomb of charge available The voltage resulting from rubbing a balloon on hair could be several thousand volts. Then why does putting your finger in an outlet be potentially deadly but static from rubbing a balloon is not?....Answers to come…

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34.3 Voltage Sources A voltage source is something that provides a potential difference – Must be a continuous potential difference for a current to flow

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Charged metal spheres are not good at maintaining a potential difference, therefore there is not a continuous current. Dry cells, wet cells, and generators are good at maintaining a potential difference, therefore will produce a continuous flow

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Section 2 Lab What is electrical resistance? How does resistance affect current?

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34.4 Electric Resistance Electric resistance is the resistance to flow Current does not stop, only ‘slowed’

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Resistance depends on: – Conductivity of the metal – Length: longer the wire the more the resistance – Diameter: thicker wires have less resistance than thin wires – Temperature: higher the temperatures, the more resistance Superconductors- zero resistance at low temperatures

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R = ρ L/A R = resistance ρ = resistivity L = length A = area

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34.5 Ohm’s Law “for a given resistance, voltage and current are proportional” V= IR V= potential difference I= current R= resistance

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Short Circuiting Something is ‘short-circuited’ when current by passes an area by following the path of least resistance

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If the red wire were not present the bulb would light When the red wire is present, the bulb does not light because the current follows the path of least resistance. It is easier for the current to flow through the wire than the filament because the filament has greater resistance.

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END OF NOTES!! Remember: your Homework: -p. 529 # p. 545 #1-9, read p chpt 35 vocabulary (quiz Monday/Tuesday)

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34.6 Ohm’s Law Ohm discovered that current in a circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit. V = IR V= voltage (Volts- V) I = current (Ampere- A) R = resistance (Ohm- Ω)

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Ohms Law Practice Problem A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it. 1. Sketch the circuit diagram 2. Calculate how many volts supply the CD player.

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?sim=Ohms_Law ?sim=Ohms_Law

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34.7 Direct and Alternating Current Direct Current (DC) – Electrons always flow in one direction – Ex: battery Alternating Current (AC) – Electrons flow in one direction and then another – Able to transmit electrical energy large distances with easy voltage setups and little heat loss – Ex: power outlets

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34.8 Converting AC to DC Diodes are devices that only allow electrons to flow in one direction – Half of each cycle will pass through – Rough DC produced, because it is off half the time – Capacitors used to smooth the effect on changes in electric currents – See p. 540 Figure 34.12

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34.9 The speed of Electronics in a Circuit Signal of on/off moves at nearly the speed of light, NOT electrons Wire acts as a guide or pipe for the electron’s energy (DC) Electrons do not make any notable progress in one direction (AC)

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34.10 The Source of Electrons in a Circuit Source of electrons is the material in the conducting circuit Power outlets are AC, so electrons move back and forth – Energy from outlet is what is transmitted, not electrons

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34.11 Electric Power P= IV P= power I= current V= potential difference Watt is the SI unit of power – 1W= 1J/s – Named after who? What did he do?

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Sometime use: – kWh for kilowatt hour – (1000W)(3600s)= 3.1 x 10 6 J

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Power Practice Problem A microwave is connected to a 120V outlet. It uses 10A of current to heat food. 1. How much power does this machine use? 2. Write this number in terms of kW.

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Series vs Parallel Circuits A series circuit is a circuit where current travels through every wire and device in one path. A parallel circuit is a circuit where current branches off at different points.

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Voltage for a series circuit and a parallel circuit does not change. Current for a series circuit and a parallel circuit changes.

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Kirchoff’s Rules Used for parallel circuits Junction Rule: the amount of charge entering a junction equals the amount of charge leaving a junction Loop Rule: the amount of voltage drops across a resistors equals the total amount of voltage from the circuit

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Resistance and Circuits – Series: as resistors are added to a circuit, total resistance increases R= R1 + R2 + R3….. – Parallel: as resistors are added to a circuit, total resistance decreases R= 1/R1 + 1/R2 + 1/R3…..

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Series Circuit and Resistance 1. Look at a round light bulb and calculate the resistance one round bulb provides. 2. Create a series circuit with the two light bulbs. 3. Calculate the total resistance for the series circuit. 4. What would happen if to the bulb brightness if another light bulb were added? 5. What would the total resistance be?

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Parallel Circuit and Resistance 1. Look at a round light bulb and calculate the resistance one round bulb provides. 2. Create a parallel circuit with the two light bulbs. 3. Calculate the total resistance for the parallel circuit. 4. What would happen if to the bulb brightness if another light bulb were added? 5. What would the total resistance be?

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