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1. A current of 500 A flows in an electrical welding machine within a period of 0,8 s. What is

the transformed charge? Recalculate the result into kWh!

A charge of . passes through the welding machine. This equals to . kWh

2. An electric grill is connected to 230 V wall outlet. The protection allows a maximal currentof 10 A. What is the maximal allowed power, that the grill can have?

The grill may have a maximal power of . At a higher power value the protection

will ..

3. A switch is allowed to pass a maximal current of 0,4 A. The maximal connected power

should not exceed 10 W. The connected component has a resistance of 40 ?.

Is it possible to use the above mentioned switch for the consumer, which should be operated

with a voltage of 24 V?

The value of 10 W is reached at a voltage of ..

A current of . will flow through the resistance of 40?.

4. a.Explain the mechanism of propagation of a sound wave in air?

b. How is the length of a sound wave defined?

c. How is it possible to calculated the distance from an obstacle to the source of sound?

d. Explain the idea of the piezoelectric effect?

e. How do piezoelectric materials behave in elevated temperatures?

f. How is a sound wave generated?

g. What range of frequencies is employed in ultrasonic sensors?

h. What shape the area of enhanced energy of a sound wale has?

i. Which factors determine the size and cone angle of a sound wave?

j. How is it possible to determine the diameter of the cone of an ultra-sonic beam?

ANSWER

1. Q = I t

Q = 500 A 0,8 s

Q = 400 C

Q = 0,11 kWh

A charge of 400 C passes through the welding machine. This equals to 0,11 kWh.

2.

P = U I

P = 230 V 10 A

P = 2300 W

The grill may have a maximal power of 2300 W. At a higher power value the

protection will break the current flow.

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3. P = U I

U = P / I

U = 10 W / 0,4 A

U = 25 V

The value of 10 W is reached at a voltage of 25 V. Since the voltage amounts 24 Vonly, the switch may be used for this consumer.

R = U / I

I = U / R

I = 24 V / 40

I = 0,6 A

A current of 0,6 A will flow through the resistance of 40 at the voltage of24 V.

This current value is two times higher than the maximal allowed current for the

switch. Therefore, the switch can not be used in this case.

4. A. Gelombang bunyi terdiri dari molekul-molekul udara yang bergetar maju -

mundur. Tiap saat, molekul-molekul itu berdesakan di beberapa tempat, sehingga

menghasilkan wilayah tekanan tinggi, tapi di tempat lain merenggang, sehingga

menghasilkan wilayah tekanan rendah. Gelombang bertekanan tinggi dan rendah

secara bergantian bergerak di udara, menyebar dari sumber bunyi. Gelombang bunyi

ini menghantarkan bunyi ke telinga manusia,Gelombang bunyi adalah gelombang

longitudinal.

B.

C. untuk menghitung jarak dari sumber suara yaitu dengan mengkalikan antara waktu

yang dibutuhkan sumber suara untuk merambat (t) dengan cepat rambat bunyi (v) atau

s = v x t

D. Efek piezoelektrik terjadi jika medan listrik tebentuk ketika material dikenai

tekanan mekanik. Pada saat medan listrik melewati material, molekul yang

terpolarisasi akan menyesuaikan dengan medan listrik, dihasilkan dipole yang

terinduksi dengan molekul atau struktur kristal materi. Penyesuaian molekul akan

mengakibatkan material berubah dimensi. Fenomena tersebut dikenal dengan

electrostriction.

F. gelombang bunyi dihasilkan olrh benda yang bergetar,getaran ini merambat melalui

udara dan sampai ke telinga kita.