Competency 4

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Measure value of Carbon/Ceramic resistances, A.C and D.C voltages with analogue and/or digital multimeter.

4.1 Definition of current and its unit.

         Electric Current  à  Rate of flow of electrons.

 

Unit: Ampere (A)

Instrument to measure current: Ammeter

Symbol: I

4.2 State the difference between the terms e.m.f and potential difference.

12 V Battery

E.m.f (Electromotive Force) → EMF can be considered as electrical pressure that a battery exerts. It exists whether there is a circuit or not. Measured in Volts (V)

12v

Voltage or Potential difference → The voltage between two points is a short name for the electrical driving force that could determine an electric current between those points. Measured in Volts (V)

4.3 State Ohm’s law.                                                                                                                                               

        Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

Small resistance   >         Large current

        

Large resistance  >         Small current

Current (I) depends on two parameters:

1.     Voltage (V)

2.     Resistance (R)

OR

Current is directly proportional to voltage.

Ohm's Law

Resistors in series

Resistors in Parallel

4.5 Difference between A.C and D.C

In alternating current (AC, also ac) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction.

4.9 Purpose of a multimeter

Multimeter is an instrument used to measure current, voltage and electric resistance                                                                                                   

                                           e.g.; in circuits

Ø Current measured in Ampere(A)

Ø Voltage measured in Volt(V)

Ø Resistance measured in Ohm(W)

Use: To measure electrical quantities in an electrical machine, electrical appliance and a circuit.

4.10 Types of multimeter.

Multimeter

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance.

Multimeters may use analog or digital circuits—analog multimeters and digital multimeters (often abbreviated DMM or DVOM.)

Analog instruments are usually based on a microammeter whose pointer moves over a scale calibration for all the different measurements that can be made.

Digital instruments usually display digits, but may display a bar of a length proportional to the quantity measured.

                         i. Analog multimeter

•   Value indicated by a pointer.
•   Value to be read on a scale.

                     ii. Digital multimeter

                   

• Value given on a display (digital).
• No need to read on a scale.
• Value given with precision.                      

4.11 Parts of a digital multimeter

A multimeter has :

1.      a display

2.      terminals

3.       probes

4.      leads

5.      a dial(selector knob or function switch) to select various measurement ranges

 A digital multimeter has a numeric (digital) display, while an analog multimeter has a dial display

 4.12 State precautions to be observed.

Precautions

Ø  It is important that voltage and current levels do not exceed the specified maximum level. Overloading can destroy the meter, and is potentially fatal.

Ø  Always check the insulation around the probes and wires for any breaks, before using the multimeter.

Ø  Never measure voltage on a current based setting because the low resistance will act as a short, if the fuse does not blow first.

Set the function switch on the correct mode.