Section 15.3 Ohm’s Law and Resistance

8/14/2019 Section 15.3 Ohms Law and Resistance

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Manhattan Press (H.K.) Ltd. 1

Section 15.3Section 15.3

Ohms law and resistanceOhms law and resistance

Section 15.3Section 15.3

Ohms law and resistanceOhms law and resistance

Ohms lawOhms law

Definition of resistanceDefinition of resistance

Factors affecting the resistanceFactors affecting the resistance

of wireof wire

Resistor and rheostatResistor and rheostat


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opposition of current

15.3 Ohm's law and resistance (SB p. 46)

Ohms law

current flows

resistance


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Expt. 15B

Ohm's law

A. Conductors

that obey Ohm's

lawB. Conductors

that do not obey

Ohm's law


Ohms law

low voltagepower supply

ammeter

rheostat

resistancebox

voltmeter
http://movie/Expt%2015B/15B.avi


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Ohms law

relation between V& Ifor conductors

0

Voltage (V) / V

Current (I) / A


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Ohms law

Expt. 15A

Ohm's law

(data-logging)

A. Conductors

that obey Ohm's

lawB. Conductors

that do not obey

Ohm's law

data-logginginterface resistance

box
http://movie/Expt%2015A/15A.avi


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Ohm's law


Ohm's law

voltage across conductordirectly proportional

to current passing through it

(other physical conditions remain unchanged)

VI

Voltage

Current


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Forohmic conductors

Definition of resistance


CAL

Workshop 3

Resistance

and

Ohm's law

(A) Ohms law

I

R

conductorthroughCurrent

conductoracrossVoltageResistance =

I

VR= orV= IR

V

Unit: ohm ( )
http://cal/E-SimPhy_404.exe


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Definition of resistance


Voltage

Current

measure of

opposition to current

slope = resistance


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Example 1:Example 1:The voltage and current of a nichrome wire are shown

below.


Voltage / V Current / A Resistance /

2 1

4 2

6 3

8 4


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Example 1: (Cont)Example 1: (Cont)(a) Calculate the resistance in each case. Does the

resistance change as the voltage increases?

(b) Plot a graph of voltage against current.


Solut

ion

Solut

ion

By R = , the resistance is equal to 2 in each case.

Therefore, the resistance of the wire does not change as

the voltage increases.

I

V


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Example 1: (Cont)Example 1: (Cont)(b)

(c) Does the nichrome wire obey Ohms law? Explain briefly.Solut

ion


Yes. This is because V is directly proportional to I.


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Class Practice 2:Class Practice 2:A student tries to find the resistance of a semiconductor.

The data is given below.

(a) Calculate the resistance in each case.

(b) Plot a graph of voltage against current.


Voltage / V 0.1 0.2 0.3 0.4 0.5 0.6

Current / mA 0.01 0.02 0.03 0.04 0.08 0.12

Resistance /

Ans

wer

10 000 10 000 5 00010 000 10 000 6 250

Ans

wer


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Class Practice 2: (Cont)Class Practice 2: (Cont)

Voltage / V

Current / mA



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Class Practice 2: (Cont)Class Practice 2: (Cont)(c) Does the semiconductor obey Ohm's law? Explain

briefly. Ans

wer


No. This is because the voltage is not

directly proportional to the current.

Voltage / V

Current / mA


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Factors affecting the resistance of wire

resistance of wire affected by its:


dimension

temperature

material


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Change of resistance with dimension of wire


Expt. 15CFactors

affecting the

resistance of a

wire

A. Variation ofresistance of a

wire with its

length

eureka wires of different thickness

crocodile clip

multimeter

Expt. 15C

Factors

affecting theresistance of a

wire

B. Variation of

resistance of a

wire with itsthickness
http://movie/Expt%2015C/15C.avihttp://movie/Expt%2015C/15C.avi


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Change of resistance with dimension of wire


RResistance (R) /

Length () / m0

RA

1

greaterresistance for

longer/ thinnerwire


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Change of resistance with temperature


How temperature affects the resistance offilament wire?

12 V d.c. supply

light bulb

ammeter

voltmeter

switch


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not obey

Ohms law

voltage temperature resistance


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temperature

atoms vibrate more violentlyhindermotion of electrons

resistance


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Material of wire


Material Resistance of wire

Silver 0.51

Copper 0.54

Aluminium 0.84

Tungsten 1.67

Iron 3.69

Platinum 3.38

Silicon 2 038

loosely held e

small resistance

tightly bound e

high resistance


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Resistor and rheostat


resistors with fixed resistance values

thin layers of

carbon / coiled

conducting wires

fixed resistance


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rheostatadjusting resistance

sliding type

rotary type


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AB

C



potential divider

adjust voltage

across component


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To section 15.4
http://e-ch15_04.ppt/http://e-ch15_04.ppt/

Section 15.3 Ohm’s Law and Resistance

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Transcript of Section 15.3 Ohm’s Law and Resistance