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Fundamentals of Electric Circuits Lecture 2 Basic circuit elements and concepts

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2 Open Circuits ¤Current can only exist where there is a conductive path ¤Open circuit - When there is no conductive path ¤If I = 0 - Ohm’s Law gives R = V/I = V/0 infinity ¤An open circuit has infinite resistance

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Short Circuits ¤Short circuit - the voltage is zero ¤If V = 0 - Ohm’s Law gives R = V/I = 0/I Zero ¤An short circuit has zero resistance

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Power 4 ¤ Power is the rate of doing work. ¤ Power = Work/time (dW/dt, where W is energy and t is time) ¤Power is measured in watts. ¤ One watt = one joule per second (1joule/Second)

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Power in Electrical Systems 5 ¤From V = W /Q and I = Q/t, we get P = VI (P = W/t) ¤From Ohm’s Law, we can also find that P = I 2 R and P = V 2 /R ¤Power is always in watts.

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6 Power in Electrical Systems ¤We should be able to use any of the power equations to solve for V, I, or R if P is given. ¤For example:

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Calculating Power: Example Example: What is an iPod’s power in watts if it was on for 1.30 hrs and used 210 000 J of electrical energy? Given:t = 1.30 h x 3600 t = 4700s E = 210 000 J P = E / t Power (P) is measured in watts (W) Energy (E) is measured in joules (J) Time (t) is measured in seconds (s) 7

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Required:P = ? Analysis:P = E / t Solution:P = 210 000 J / 4700 s P = 45 W Statement:Therefore, the iPod’s power is 45 watts. Calculating Power: Example (solution) 8

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Electric Power

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10 Energy ¤Energy = Power × time ¤Units are watt-seconds, watt-hours, or more commonly, kilowatt-hours. ¤For multiple loads, the total energy is the sum of the energy of the individual loads.

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Efficiency ¤Efficiency (in %) is represented by η (Greek letter eta) Ratio of power out to power input ¤Always less than or equal to 100% 11 ¤To find the total efficiency of a system Obtain product of individual efficiencies of all subsystems: Total = 1 × 2 × 3 × ∙∙∙

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SAMPLE PROBLEM 1 ¤A bulb uses 100 J of electrical energy and produces 35 J of light energy. Calculate the percent efficiency of the light bulb. Given: E out = 35 J E in = 100 J Required:percent efficiency (% efficiency) 12

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Analysis:% efficiency = E out x 100% E in Solution:% efficiency = 35 J x 100% 100J % efficiency = 0.35x100% % efficiency = 35% Statement:The efficiency of the light bulb is 35%. 13

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14 A toaster oven uses 1200 J of energy to produce 850 J of thermal energy. Calculate the percent efficiency of the toaster oven. Given: E in = 1200 J E out = 850 J Required: % efficiency = ? SAMPLE PROBLEM 2

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Analysis: % efficiency = E out x 100% E in Solution: % efficiency = 850 J x 100% 1200 J % efficiency = 70.8 % Statement: The efficiency of the toaster oven is 70.8 %. 15

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Conductors V s Insulators Conductors – material through which electric current flows easily. Insulators – materials through which electric current cannot move. ¤Conductors ¤Conductors: Metal Water ¤Insulators: Rubber Plastic Paper

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The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.

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Simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. To make the circuit, these components are connected together with metal connecting wires. cell lamp switch wires

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Circuit diagram cellswitchlampwires Scientists usually draw electric circuits using symbols;

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Circuit diagrams In circuit diagrams components are represented by the following symbols; cellbatteryswitchlamp motorammetervoltmeter buzzer resistorvariable resistor

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

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22 The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. SERIES CIRCUITS

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23 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.

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24 Measuring current This is how we draw an ammeter in a circuit. A A SERIES CIRCUIT PARALLEL CIRCUIT

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25 Measuring current SERIES CIRCUIT PARALLEL CIRCUIT current is the same at all points in the circuit. 2A current is shared between the components 2A 1A

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measuring voltage V This is how we draw a voltmeter in a circuit. SERIES CIRCUITPARALLEL CIRCUIT V 26

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Measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits. 27

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V V 6V 4A A A )a))a) 28 Measuring current & voltage

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V V 6V 4A A A A b) 29

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