Conductance & ResistanceObjectives:3.3 Measure current intensity in simple circuits.3.4 Measure potential differences in simple circuits.3.5 Determine the conductance of a circuit element, using a graph.3.6 Distinguish between the conductance and the resistance of a circuit.

Important Background Information

By now you should have a strong understanding of conductance.

However, before we move on to circuits we must first understand the terms CURRENT INTENSITY and POTENTIAL DIFFERENCE

Symbol: IUnits: Amperes, ACurrent Intensity

Current: the amount of electrons that flow through a wire

Example: 0.7 amperes is shown by I = 0.7 A

An ammeter is the instrument used to measure current intensityA

Symbol: VUnits: Volts, V

Potential Difference(Voltage)

Potential Difference causes electrons to flow through a wire

Provided by a battery or power supply

Example: 12 volts = 12V

A voltmeter is used to measure potential difference

V

Solving for Conductance(using current intensity & potential difference)

Recall conductance is how easily current flows through a wire.

To solve use:

Remember: Units = Siemens Symbol = G

G = I V

What is the conductance of a circuit with a current intensity of 20A & a potential difference of 5V?G = I

VG = 20

5

G = 4 S

What is the conductance of a circuit with a current intensity of 5A & a potential difference of 10V?G = I

VG = 5

10

G = 0.5 S

What is the conductance of a circuit with a current intensity of 5A & a potential difference of 10V?G = I

VG = 5

10

G = 0.5 S

Sometimes the questions are tricky and ask you to solve for

conductance from a graph

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7

voltage (v)

curr

ent

(am

ps)

G = ∆I (I2– I1) ∆V (V2-V1)

Find the conductance using the following graph:

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7

voltage (v)

curr

ent (

amps

) TWO POINTS

(2, 4) and (5, 10)

V1 I1 V2 I2

G = ∆I (I2– I1) ∆V (V2-V1)

G = (10 – 4) (5-2)

G = 6 = 2 S 3

Find the conductance using

the following graph:

TWO POINTS

(4, 14) and (0, 0)

V1 I1 V2 I2

G = ∆I (I2– I1) ∆V (V2-V1)

G = (0 - 14) (0 - 4)

G = -14 = 3.5 S -4

02468

1012141618202224

0 1 2 3 4 5 6 7

Voltage (v)

Cu

rren

t (a

mp

s)

Symbol: RUnits: Ohms, Ω Resistance

Resistance is how difficult it is for current to flow

Resistance is the opposite of conductance!

A resistor is used to slow current down

Conductor – low resistance & high conductance

Resistor– high resistance & low conductance

What is the resistance of a circuit with a current intensity of 5A & a potential difference of 10V?R = V

IR = 10

5R = 2 Ω

What is the current intensity of a circuit with a resistance of 15Ω & a potential difference of 10V?R = V

I15 = 10

I15xI = 10 15 15 I = 0.67 A

What is the potential difference of a circuit with a current intensity of 5A & a resistance of 10 Ω?R = V

I10 = V

5V = 5x10

V = 50 V

Summary Table

Resistance Current Potential Difference

Conductance

DefinitionSlows current

downThe flow of electrons

Push of electrons or Energy per

charges

How easily electrons flow

Symbol R I V G

Units Ohms (Ω) Amps (A) Volts (V) Siemens (S)

Formula(if applicable)

R =V I

I =V or I = q R t

V = IR orV = E q

G = I V

How measured(if applicable) Formula

and/or graphAmmeter Voltmeter Formula and/or

graph

Conductance and Resistance of a Circuit Element

Resistors are coated with ceramic.

They have color coated bands to indicate resistance

1st & 2nd band first two digits of resistance

3rd band exponential of 10 by which the first two digits must be multiplied (multiplier)

4th band tolerance

Resistor

Example

Black 0 x1

Brown 1 x10

Red 2 x100

Orange 3 x1000

Yellow 4 x10000

Green 5 x100000

Blue 6 x1000000

Violet 7 x10000000

Gray 8 x100000000

White 9

The last band indicates the tolerance

Gold +5% Gray +10% None +20%

The resistor on the right has the following value.

23 x 104 + 10%

1st

2nd

3rd4th

Key Points

See Summary Table Finding conductance with formula

and graph (refer to examples. Very important & on exam!)

Definitions of current, potential difference, conductance and resistance

Relationship between conductance and resistance

Activities

Worksheet #9, #10

References•Student Study Guide Physical Science 416 – MEQ•Science Quest, Grenier, Daigle & Rheaune, 1998 Editions Cheneliere•Animation Factory•Google Images

•townsvilleshs.eq.edu.au/. ../Voltmeter.jpg

•electronics.howstuffworks.com/ battery1.htm

• www.8886.co.uk/ref/ ammeter_voltmeter_circuit.htm

•www.gcse.com/ohm1b.htm

•www.positiveoffset.com/ 4-7K-Ohm-resistors.htm

•www.mechatronics.me.vt.edu/ VT84Construction/r...

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