Basic Law

Electric Current

• Electric current is a measure of the amount of electrical charge transferred per unit time, represents the flow of electron through a conductive material

• Electric current is similar to a current of water, but electric current flows in solid metal wires so it is not visible

• Electric current can carry a lot of power

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QI

Electric Current

• An electric current exists whenever electric charge flows through a region, e.g., a simple light bulb circuit

• The magnitude of the current is measured in AMPERES (Amps/A)

• 1 ampere = 1 coulomb/second

Example

• Jika diketahui ada arus listrik sebesar 5A, berapakah banyaknya elektron yang lewat setiap detiknya? (Diketahui muatan elektron 1,602 x 10-19 Coulomb)

• Sebuah AC berdaya ½ pk (1 pk ≈ 700 Watt), berapakah arus yang dibutuhkan AC tersebut?

Current Flow

• Conventional Current assumes that current flows out of the positive side of the battery, through the circuit, and back to the negative side of the battery. This was the convention established when electricity was first discovered, but it is incorrect!

• Electron Flow is what actually happens. The electrons flow out of the negative side of the battery, through the circuit, and back to the positive side of the battery.

Electron

Flow

Conventional

Current

Engineering vs Science

• The direction that the current flows does not affect what the current is doing; thus, it doesn’t make any difference which convention is used as long as you are consistent.

• Both Conventional Current and Electron Flow are used. In general, the science disciplines use Electron Flow, whereas the engineering disciplines use Conventional Current.

• Since this is an engineering course, we will use Conventional Current

Electron

Flow

Conventional

Current

Electric Circuit

• An electric circuit is something that provides a complete path through which electricity travels

• Wires in electric circuits are similar in some ways to pipes and hoses that carry water

Electric Circuit

Voltage

• Voltage is the electrical force, or "pressure", that causes current to flow in a circuit

• Voltage is measured in volts (V)

• A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second

Q

WV

Voltage Terminology

• Drop voltage is the voltage difference between the two terminal / node in the circuit

• Node voltage is the voltage difference between one node in the circuit and the reference node / ground

Resistance

• Resistance measures how difficult it is for current to flow

• The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage.

• The more resistance the circuit has, the less current that flows

Resistance

• The SI unit of resistance is the ohm (Ω)

• One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp

Ohm's Law

• Defines the relationship between voltage, current, and resistance in an electric circuit

• Ohm's Law :

"Current in a resistor varies in direct proportion to the voltage applied to it and is inversely proportional to the resistor’s value"

• Stated mathematically :

V

I R

+ -

R

VI

Ohm's Law

V

I R

)A,amperes( R

VI

),ohms( I

VR

)V,volts( R I V

V

I R

V

I R

Example

• The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150 . When the flashlight is on, how much current will be drawn from the battery?

VT = +

-

VR

IR

Schematic Diagram

Daya & Energi

Daya & Energi

Example

• The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150 . If the battery is full and the capacity is known 400 mAh, how many hours it will turn on until the battery empty?

VT = +

-

VR

IR

Schematic Diagram

400 mAh

Example

• The flashlight shown uses a 9 volt battery and has a bulb with a resistance of 150 . If the battery is full and the capacity is known 400 mAh, how many hours it will turn on until the battery empty?

VT = +

-

VR

IR

Schematic Diagram

400 mAh

9 V

Studi Kasus

1. Diketahui sebuah mobil listrik mempunyai mesin berdaya 100 hp (1 hp = 700 Watt) dengan tegangan mesin 48 volt. Jika diinginkan mobil bisa berjalan terus menerus selama 5 jam, berapa kapasitas baterai yang diperlukan? Nyatakan dalam Ah (Ampere hour) dan Wh (Watt hour)

Studi Kasus

2. Jika kita ingin mengisi baterai mobil tsb dengan listrik di rumah (asumsi daya maksimum listrik di rumah 1300 Watt) berapa jam waktu yg diperlukan untuk mengisi baterai dari kondisi kosong sampai penuh?

Studi Kasus

ITS berhasil membuat skutik listrik yang seperti skutik bensin performanya.

Spek teknik :

daya : 7 hp ( 1 hp = 700 Watt)

tegangan motor listrik : 48 volt

batere : 48 volt, 30 Ah (Ampere jam)

Jika asumsi kecepatan motor adalah 50 km / jam, berapa jauh motor tersebut bisa berjalan sebelum berhenti karena batere habis ?

Jika diinginkan bisa menempuh 200 km, berapakah besar batere yang diperlukan

Pembahasan

• Motor 7 hp = 4900 Watt

• Batere 48 volt 30 Ah

• Kecepatan 50 km/jam

• Daya batere = 48 volt x 30 Ah = 1440 watt h

• Lama batere bertahan = 1440 wh / 4900 w = 0,29 jam

• Jarak tempuh jika kecepatan 50 km /jam adalahj 50 km/jam x 0,29 jam = 14,6 km

Pembahasan

• Motor 7 hp = 4900 Watt

• Batere 48 volt 30 Ah

• Kecepatan 50 km/jam

• Diinginkan menempuh 200 km atau 200km / 50 km/jam = 4 jam

• Sehingga batere yang dibutuhkan 4900 w x 4 jam = 48 volt x 408,3 Ah