WEL COMETO

COE RAC SECTOR

MODULE 2BASIC ELECTRICITY

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BASIC ELECTRONICS

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ELECTRO-MECHANICS

LESSON 16

REALTIONSHIP BETWEEN V,I & R IN A DC CIRCUIT

OBJECTIVES

• Define EMF & P.D. & their units.

• Define current & its units

• State the method of connecting Voltmeter

• State the method of connecting an Ameter

• Draw & Describe a simple circuit.

E.M.F.Eletro Motive force

It is now necessary to consider the reason for the motion of electrons in an electrical circuit.

In order to move free electrons in one direction & so produce a current, a source of electrical energy is required. The torch battery is a source of electrical energy.

The battery is said to have an E.M.F. which is available to drive the free electrons in the closed path in electrical circuit. The diff. in the distribution of electrons between the two terminal of the battery produce this E.M.F. Unit of E.M.F. is watt. E.M.F. of an electrical source referred to as its Voltage.

Potential DifferenceThe voltage across the mains when the load is put on is known as potential difference and its unit is volt.An electrical voltage (V) is measured in volts (V).The following expressions are used for decimal, multiple and sub multiples of 1volt1 kilovolt = 1 kv = 1000 v = 1 x 103 v1 milivolt = 1 mv = 1/1000 v = 1 x 10-3 v1 microvolt = 1 µv = 1/1000000v = 1 x 10-6 v

Electric Current• Fig. shows a simple electric

circuit. When the switch is closed, the lamp lights because the electrical current flows from the source of electrical energy (battery) electrons can move through a conductor at different rate and electric current will, therefore have different values.

• The current symbol (I) is determined by the count of electrons that pass through a cross section of a conductor in one second.

• The current of 1 amp. Means that in one second about 6.24 x 1018 electrons moves through a cross section of a conductor.

1 kilovolt = 1 kv = 1000 v = 1 x 103 v1 milivolt = 1 mv = 1/1000 v = 1 x 10-3 v1 microvolt = 1 µv = 1/1000000v = 1 x 10-6 v

State the method of connecting Voltmeter

Electrical voltage is measured with a volt meter in order to measure the voltage source. The terminal of the source must be connected to the terminal of a volt meter. Positive to the positive terminal & negative to the negative terminal as shown in figure.

State the method of connecting Ameter

In order to measure current-measuring instruments called current meter or Ameter. An ameter is used to measure current for this the current must flow through the measuring instruments. Hence Ameter is connected in terms with load

SIMPLE ELECTRIC CIRCUITA simple electrical circuit shown in figure is the complete path way of the current flow from the battery via the switch and the load back to the batteryAn electrical circuit consist of :

voltage sourceconnecting wiresloadswitch

Types of simple circuit

Open circuit

Short circuit

Closed circuit

Open circuit• In an open circuit

there is an infinitely high resistance in the circuit provided most of the time by the open switch there fore no current can flow.

Short circuit• The other important

extreme condition is the short circuit will occur,for example when two terminal of a torch battery are joined

• A short circuit may also occur if the insulation between the two cores of the cable is defective

Closed circuitWhen the switch is closed, the lamp lights because the electrical current flows from the source of electrical energy (battery)