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Transcript
Electricity Current • Electricity is a flow of electrons (negative charge) through wires and components. • The rate at which electrons flow is called current. • Current is measured in Amperes (Amps). • You can measure the current in a circuit with an ammeter. Current Equation Electric current = amount of charge amount of time Item I=q t Symbol Unit Abbreviation Current I Ampere (Amps) A Charge q Coulomb C Time t Seconds s Current Practice Problems (I = q/t) 1. What is the current if 3600 C of charge passes a point in 17 min? * What do you have to change? (HINT – Time) WTF W=> Charge = 3600 C Time = 17 minutes x (60 s/min) = 1020 seconds T => Current =? F => I = q/t Current Practice Problems (I = q/t) 1. What is the current if 3600 C of charge passes a point in 17 min? * What do you have to change? (HINT – Time) I = q/t = 3600 C/1020 s I = 3.53 Amps Current Practice Problems (I = q/t) 2. For what period of time must a current of 0.22 A, flow if 7.5 E18 electrons are passed through it? *What do you have to change? (HINT – Total Charge) WTF W=> Charge = (7.5 E18)(-1.6 E-19) = -1.2 = 1.2 C Current = .22 A T => Time =? F => I = q/t => t= q/I Current Practice Problems (I = q/t) 2. For what period of time must a current of 0.22 A, flow if 7.5 E18 electrons are passed through it? *What do you have to change? (HINT – Total Charge) t= q/I t = 1.2 C / .22 A t = 5.45 s Current Practice Problems (I = q/t) 3. What is the charge of a current that has 25 A and takes 5 seconds to round the circuit? WTF W => Time = 5 seconds => Current = 25 A T=> Charge = ? F => I = q/t => q = I t Current Practice Problems (I = q/t) 3. What is the charge of a current that has 25 A and takes 5 seconds to round the circuit? q = It q = 25 A x 5 s q = 125 C Electricity: Review & Intro to Circuits Series Circuit • A simple loop for the current to flow round • Components are connected end-to-end, one after the other. • If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. Parallel Circuits • Multiple loops for the current to flow round • Components are connected & at some point the current splits & then comes back together. These locations are called Junctions. • If one bulb ‘blows’ there is still a complete circuit so the other bulb stays alight. Junctions Direct Current (DC) • DC = Direct Current – Supported by Thomas Edison – current flows in one direction • Example: Battery Edison Alternating Current (AC) • AC = Alternating Current– Supported by Nikola Tesla – current reverses direction many times per second. This suggests that AC devices turn OFF and ON. • Example: Wall outlet Tesla Edison vs. Tesla What do you think? • Between Edison & Tesla, who do you think contributed the most to society & why? • Why do we know more about Edison than Tesla? Voltage (V) • Voltage is the difference between two charges. – also called Potential Difference & Electric Potential. • Batteries have a + charge on one side and a - charge on the other. – The difference between those 2 charges is the voltage of the battery. Voltage - Voltage use a voltmeter to measure the voltage of a battery or the voltage drop of a device. - Voltage drop is how much voltage is lost from one side of a device to the other side of the device. Current • Electricity is a flow of electrons (negative charge) through wires and components. • The rate at which electrons flow is called current. • Current is measured in Amperes (Amps). • You can measure the current in a circuit with an ammeter. Resistance (R) • Resistance (R) is the restriction of electron flow. - As current travels through a device it slows down because the electrons are attracted to the protons in the device. - Resistance is a measure of the change in current. - The unit for resistance is the OHM, W Voltage, Current & Resistance Review 𝑽 = 𝑰𝑹 Ohm’s Law “The voltage (potential difference) is directly related to the current, when the resistance is constant” Item Symbol Unit Abbreviation Voltage V Volts V Current I Amperes A Resistance R Ohm’s W Voltage Sample Problem 1. What is the voltage of a circuit with 15 amps of current and a light with 7 ohms of resistance ? W I = 15 A R=7W V = (15 A)(7 W) T F V? V = IR V = 105 V Voltage Practice Problems (V=IR) What is the voltage of a circuit that has a current of 75 A with a resistance of 10 W ? WTF W=> Current = 75 A Resistance = 10 W T => Voltage = ? F => V = IR Voltage Practice Problems (V=IR) What is the resistance of a circuit that has a current of 150 A with a voltage of 35 V? WTF W=> Current = 150 A Voltage = 35 V T => Voltage = ? F => V = IR R = V/I Voltage Practice Problems (V=IR) What is the current of a circuit that has a resistance of 50 W and a voltage of 30 V? WTF W=> Resistance = 50 W Voltage = 30 V T => Current = ? F => V = IR I = V/R Voltage Practice Problems (V=I/R) 1. V = IR = 75 A x 10 W = 750 V 2. V = IR R=V/I = 35 V / 150 A = .23 W 3. V = IR I = V/R = 30 V/50 W = .2 A 𝑷 = 𝑰𝑽 Power (P) • POWER is the rate at which work (energy) is done. • Joules/Second or Watts. • Electricity in Kilowatt hours (kW/h). Item Symbol Unit Abbreviation Power P Watts W Current I Amperes A Voltage V Volts V Power Sample Problem • Calculate the power rating of a home appliance (in kilowatts) that uses 8 amps of current when plugged into a 120-volt outlet. W I=8A V = 120 V P = (8A)(120V) T F P? P = IV P = 960 W .96 kW Power Practice Problems (P=IV) 1. What is the power of a circuit that has a current of 80 A with a voltage of 20 V? 2. What is the current of a circuit that has a a voltage of 35 V & it’s power is 770 W? 3. What is the voltage of a circuit that has a current of 50 A and 650 W of power? Power Practice Problems (P=IV) 1. P=IV P = (80 A)(20 V) P = 1600 W 2. P = IV I = P/V I = 770 W / 35 V I = 22 A 3. P = IV V = P/I V = 650 W / 50 A V = 13 V Energy (E = Pt) • Energy is the amount of Power produced in a certain time – (Watts)(seconds) or Joules Item Symbol Unit Abbreviation Energy E Joules J Power P Watts W time t Seconds s Energy Sample Problem • How much energy does a light bulb use if it uses 150 watts of power over 30 seconds? W P = 150 W t = 30 s E = (150 W)(30 s) T F E? E = Pt E = 4500 J Energy Practice Problem 1. Calculate the Energy of a 75 watt light bulb that burns for 15 seconds. 2. Find the Power of a circuit that has 1.5 x 103 Joules of energy that runs for 50 sec. 3. What is the time that is needed to produce 1000 J with the power of 250 W? Energy Practice Problem 1. E = Pt = (75 W)(15 sec) E = 1125 J 2. E = Pt P = E/t = 1.5 x 103 J / 50 s P = 30 W 2. E = Pt t = E/P t = 1000 J/250 W? t=4s