# Fundamentals of Electric Circuits Lecture 2 Basic circuit elements and concepts.

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Fundamentals of Electric Circuits Lecture 2 Basic circuit elements and concepts

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

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

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)

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.

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:

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

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

Electric Power

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.

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 × ∙∙∙

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

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

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

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

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

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.

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

Circuit diagram cellswitchlampwires Scientists usually draw electric circuits using symbols;

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

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

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

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.

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

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

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

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

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

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

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