scpy155_lec5_บทที่ 11_ไฟฟ้ากระแส

11 :

SCPY 155 Basic physics for medical and health science

09/09/56 http://www.sc.mahidol.ac.th/scpy/courses/scpy155_12.htm

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2

(electric current)

09/09/56 http://www.sc.mahidol.ac.th/scpy/courses/

scpy155_12.htm 3

(I) Q

A

(A) 1 A = 1 C/s

t

t

QI

- (1777-1836) 09/09/56 http://www.sc.mahidol.ac.th/scpy/courses/

scpy155_12.htm 4

(A) (V)

0.0003 3 10-20 10,000 10 120 0.1 3


5

09/09/56 http://www.sc.mahidol.ac.th/scpy/courses/scpy155_12.htm 6

1 0.2 mA 1 ( 1.6x10^-19 C)

I=Q/t N=Q/e

N=4.5x10^18


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scpy155_12.htm 8

R (V)

()

R

VI


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(L) (A)

(resistivity)

A

LR

m


10

1.59x10^-8 2.44x10^-8 2.82x10^-8 1.7x10^-8 3.5x10^-5 10^10 10^14 10^13


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2 0.5 1

r

A

LR

m 8107.1 2rA


12

3 50 cm 2 cm 500 16

A

LR


13

(direct current, DC)

(alternating current, AC)


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15

() R V=IR


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4 15 3 V

V=IR 0.2 A


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5 3 V 0.3 mA

R=10 k


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(equivalent resistance) Req = R1+R2

R1 R2 V Vab+Vbc =IR1+IR2

V = IReq


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1/Req = 1/R1+1/R2

R1 R2

V Itotal =I1+I2=V /R1+V /R2=V /Req


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(Kirchoffs rules)

I1= I2+I3 09/09/56 http://www.sc.mahidol.ac.th/scpy/courses/

scpy155_12.htm 22

6 6 V R1, R2, R3 8, 8, 4

, , ,


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1 2

3


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7 I, I1, I2


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8 R = 5


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R a b


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VQU

Q

(P) (W)


scpy155_12.htm 28

t

t

UP

t

VQ

t

U

t

QI

RIVIt

UP 2

RVP /2

40 W, 60W 100 W 220 V 0.18 A

40 W R= 40 W/(0.18 A)^2 = ??


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8 9 V 0.5mA

P = 4.5 mW


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50 Hz

(Vac) (Iac)


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(AC) (DC)

Veff (effective voltage)


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(effective current)


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2

0VVeff

2

0IIeff

effeffVIP


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