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Chapter 1 Fundamentals in electrical measurements

Chapter 1 Fundamentals in electrical measurements. Objectives. State the factors affecting the resistance of a resistor and perform resistance calculation. Explain basic electrical units of measurements. Compute electrical parameters using Ohm’s law. Electrical Fundamentals.

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Chapter 1 Fundamentals in electrical measurements

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  1. Chapter 1 Fundamentals in electrical measurements

  2. Objectives • State the factors affecting the resistance of a resistor and perform resistance calculation. • Explain basic electrical units of measurements. • Compute electrical parameters using Ohm’s law.

  3. Electrical Fundamentals Watch the video from: • http://www.youtube.com/watch?v=dDIy-tLy4Ts Torture flies (electric circuit)

  4. 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).

  5. Simple Electric Circuit

  6. 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)

  7. Resistance • Opposition that restricts the flow of current. • Symbol : R • Unit : Ohm (Ω)

  8. Schematic Symbols Voltage Source Resistor Conductor

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

  10. 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

  11. 25 I 100V Example 2 • How many amperes of currents are flowing in the circuit?

  12. Example 3 • A current of 5A passes through a circuit having a resistance of 10. Find the voltage across the circuit.

  13. Example 4 An electric circuit is supplied by 12V battery. Calculate the resistance when the current draws out from the battery is 3 amperes.

  14. I V R 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.

  15. Power Calculations • P = I2 R • = IV • = V2 R • Unit: Watts (W)

  16. I 10 5V Power Example 1 • Calculate the power in the circuit

  17. 2A 10V R Power Example 2 • Calculate the power in the circuit

  18. 2A 50 V Power Example 3 • Calculate the power in the circuit

  19. I 1.5W 50V Power Example 4 • Calculate the supply voltage, current and resistance

  20. FACTORS AFFECTING THE RESISTANCE OF RESISTORS • At constant temperature, the resistance of a conductor is determined by 3 factors • length of conductor • Cross-sectional area ( A ) of the conductor • material used. Resistance, is known as its specific resistance or resistivity

  21. 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.

  22. 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  )

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