chapter 1 fundamentals in electrical measurements

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Chapter 1

Fundamentals in electrical measurements

22Prepared by Sandy Tay

Chapter 1 – Fundamentals in electrical measurements

Objectives1. State the factors affecting the resistance of a

resistor and perform resistance calculation. 2. Explain basic electrical units of measurements.3. Compute electrical parameters using Ohm’s

law.



Electrical Fundamentals

Watch the video from:

http://www.youtube.com/watch?v=dDIy-tLy4Ts

Torture flies (electric circuit)





Voltage (V) source of potential energy also called electromotive force (or EMF) energy can be obtained from:

• a battery

• power supply

• solar cell

• generator Units: Volts (V).



Simple Electric Circuit



Current If voltage is applied across conductive or semi

conductive material, one end becomes +ve, the other end is –ve.

Results in movement electrons

Symbol : I,

Unit: ampere (A)



Resistance

Opposition that restricts the flow of current.

Symbol : R

Unit : Ohm (Ω)



Schematic Symbols

Voltage Source

Resistor

Conductor



Ohm’s Law

States that the ratio of potential difference (V) between any two points on a load to the current (I) flowing between them is constant provided the temperature of the load does not change.

I

VR

R is the resistance of the load between the two points considered



Example 1

A heater with a resistance of 8 is connected across the 120 V power line. Find the current flowing through the heater.

I = V/R I = 120V/8

=15A

10



Example 2

How many amperes of currents are flowing in the circuit?

I100V

25



Example 3

A current of 5A passes through a circuit having a resistance of 10. Find the voltage across the circuit.



Example 4

An electric circuit is supplied by 12V battery. Calculate the resistance when the current draws out from the battery is 3 amperes.



Power in An Electrical Circuit

When current flows through resistance, there is a loss of energy resulting in heat.

There is always a certain amount of power in an electrical circuit.

I

VR



Power Calculations

P = I2 R

= IV

= V2

R

Unit: Watts (W)



Power Example 1

Calculate the power in the circuitI

105V



Power Example 2

Calculate the power in the circuit

R

2A

10V



Power Example 3

Calculate the power in the circuit2A

50V



Power Example 4

Calculate the supply voltage, current and resistanceI

1.5W50V



FACTORS AFFECTING THE RESISTANCE OF RESISTORS

At constant temperature, the resistance of a conductor is determined by 3 factors

a) length of conductor

b) Cross-sectional area ( A ) of the conductor

c) material used.

AR

is known as its specific resistance or resistivity

Resistance,



Example 1

Calculate the length of a 1.5 mm diameter copper wire, having a resistance of 0.3 ohm and that the resistivity of copper is 0.017 µΩ-m.



Example 2

Calculate the resistance of 100m length of copper wire having a cross-sectional area of 0.1 mm2 if its resistivity is 50 x 10-8 ohm-cm. (Answer: 5 )

chapter 1 fundamentals in electrical measurements

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