1 ELECTRICAL TECHNOLOGY EET 103/4 Define and explain the meaning of current, voltage, resistance, power, conductor, and insulator

2 Ohm’s Law  Every conversion of energy from one form to another can be related to this equation.  In electric circuits the effect we are trying to establish is the flow of charge, or current. The potential difference, or voltage between two points is the cause (“pressure”), and resistance is the opposition encountered.

3  Simple analogy: Water in a hose  Consider the pressure valve as the applied voltage and the size of the hose as the source of resistance.  Electrons in a copper wire are analogous to water in a hose. Ohm’s Law

4  The absence of pressure in the hose, or voltage across the wire will result in a system without motion or reaction.  A small diameter hose will limit the rate at which water will flow, just as a small diameter copper wire limits the flow of electrons Ohm’s Law

5  Developed in 1827 by Georg Simon Ohm  For a fixed resistance, the greater the voltage (or pressure) across a resistor, the more the current.  The more the resistance for the same voltage, the less the current.  Current is proportional to the applied voltage and inversely proportional to the resistance.

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7 Ohm’s Law Where: I = current (amperes, A) E = voltage (volts, V) R = resistance (ohms,  ) E is the source voltage and V is voltage drop across R

8 Ohm’s Law For any resistor, in any network, the direction of current through a resistor will define the polarity of the voltage drop across the resistor

9 Ohm’s Law

10 Ohm’s Law

11 Example 4.3. Calculate I :

12 Example 4.4. Calculate the voltage that must be applied across the soldering iron to establish a current of 1.5 A through the iron it its internal resistance is 80 

13 Power  Power is an indication of how much work (the conversion of energy from one form to another) can be done in a specific amount of time; that is, a rate of doing work. P in watt (W), W in joule (J), t in second (s)

14  Power can be delivered or absorbed as defined by the polarity of the voltage and the direction of the current.  The power delivered or absorbed in terms of voltage and current can be found as follows; Power

15 Power Or;

16 Example 4.6 Find the power delivered to the dc motor in the following figure; Solution Power

17 Example 4.9 Determine the current a 5 k  resistor when the power dissipated by the element is 20 mW Solution Power

18 Energy  Energy ( W ) lost or gained by any system is determined by:  Since power is measured in watts (or joules per second) and time in seconds, the unit of energy is the wattsecond (Ws) or joule (J)

19 Energy The watt-second is too small a quantity for most practical purposes, so the watt-hour (Wh) and kilowatt-hour (kWh) are defined as follows:

20 The killowatt-hour meter is an instrument used for measuring the energy supplied to a residential or commercial user of electricity. Energy

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22 Example 4.11 How much energy (in kWh) is required to light a 60-W bulb continuously for 1 year (365 days)? Solution:

23 Example 4.14 What is the total cost of using all of the following at RM0.25 per kWh? A 1200 W toaster for 30 min Six 50 W bulbs for 4 h A 400 W washing machine for 45 min A 4800 W electric clothes dryer for 20 min.

24 Solution:

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26 Efficiency

27 Efficiency  Efficiency (  ) of a system is determined by the following equation: Where:  = efficiency (decimal number) P o = power output P i = power input

28  The basic components of a generating (voltage) system are depicted below, each component has an associated efficiency, resulting in a loss of power through each stage. Efficiency

29 Efficiency Overall efficiency;