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DiscussionQuestions

Today’s lecture

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Self-attempt Questions

DC Circuits Websitehttp://dccircuits.wordpress.com/

Simplifying circuits takes time to master

Four Stages of Life:

1.) You believe in Santa2.) You don’t believe in Santa3.) You are Santa4.) You look like Santa

Only a kid would think that these are true

World is more complex than you previously thought

DC Circuits: Full of traps

You think that you got it right, but actually, you were tricked

• Current flow as water flow• Current takes the path of least resistance

All the current will flow here“short circuit”

No current flows here

Visit website to find out why

D1

Key Lessons Working and Calculations

AssumptionVoltmeter : Infinite Resistance

EquationeqRRR

111

21

Solve for equivalent resistance:

eqAC RR

111

ACeq RR Voltmeter doesn’t affect resistance of circuit much

8

D1

Key Lessons Working and Calculations

AssumptionLight Bulb : Finite Resistance

EquationeqRRR

111

21

Solve for equivalent resistance:

eqR

1

10

1

10

1

5eqREquivalent

Resistance is lower than expected

Larger Resistor gets a larger share of emf

D2

Key Lessons Working and Calculations

Diode allows current to flow in one direction only

For Diode, V = RI does not work. Instead, we read the graph.

For ammeter, V = RI = 100 x 0.008 = 0.8VCurrent through diode = 6A (read from graph)

Total Current = 6mA + 8mA=14mA

I

V

D2

Key Lessons Working and Calculations

Why does current not flow like this? Energy will not be conserved.

Show Excel Simulation

D3

Parallel circuit?

Is this series?

Visit website to find out why

Follow the wire

Split into Three

Three combine into one

D3

Split into Two

Combine back

5 Ω

0.80 Ω

Component C

D3

Key Lessons Working and Calculations

Parallel Circuit: 5 Ω resistor and component C have same potential difference across them

For 5 Ω resistor, V = RI = 0.85 x 5.0 = 4.25V

For Component C, p.d = 4.25V (parallel)Current = 1.3 A (read graph)

5 Ω

0.80 Ω

C

D3

Key Lessons Working and Calculations

Around a closed loop, sum of p.d. is equal to the sum of the emf

For 0.80 Ω resistor, V = RI = 0.8 x 2.15 = 1.72V

Sum of emf = sum of p.d.= 1.72 + 4.25 = 5.97V

1.3A

0.85 A 2.15A4.25V 1.72V

5 Ω

0.80 Ω

C

D3

Key Lessons Working and Calculations

Around a closed loop, sum of p.d. is equal to the sum of the emf

For 0.80 Ω resistor, V = RI = 0.8 x 2.15 = 1.72V

Sum of emf = sum of p.d.= 1.72 + 4.25 = 5.97V

4.25V 1.72V

D4

Key Lessons Working and Calculations

Misconception

Practice Question

Resistance =

I

V

NOT

I

V

V

I

ResistanceIs NOT

Gradient

NIL

D1

Key Lessons Working and Calculations

Larger resistor gets a proportionally larger share of battery emf (closed loop)

Potentiometer

O

E

XS

R

1197

32

005.0

R

R

R

total

R

total

R

R

R

R

R

R

V

V

Battery emf (2V)

0.005V1.995V

wireR

3Ω??

Revision Time!

A B

E

PD

“No Deflection” Concept

A B

E

PD

A B

E

PD

A B

E

PD

A B

E

PD

A B

E

PD

No deflectionNo currentPD = E

P

P is called balance pointAP is called balance length

D1

Key Lessons Working and Calculations

“No Deflection” Concept

Potentiometer

O

E

XS

R

0.005V

60% of lengthp.d. is _____

E = 0.003V

D1

Key Lessons Working and Calculations

“No Deflection” Concept

Potentiometer

O

E

XS

R

0.005V

60% of lengthp.d. is _____

E = 0.003V

D2

Key Lessons Working and Calculations

1. Know how to simplify circuits

2. Larger Resistor Proportionally larger share of emf

3. Potentiometer

Potentiometer

0.005V

60% of lengthp.d. is _____

E = 0.003V

80

1.50 V

20

2.00 V

A C

J

B

S1 S2

a ii)

E

S

Galvanometer No deflectionNo Current Flow in lower circuitPg from lecture notes Resistance does not matter

Does the resistance matter?

E

XS

Galvanometer No deflectionNo Current Flow in lower circuitRefer to Page 16 of lecture notes

Any Current Flow?

Does internal resistance matter?

Why?

No No No current means no power dissipated due to internal

resistance. I2R = 0

Yes Yes There is power loss! I2R > 0. Part of the energy provided by the battery will be lost due to

internal resistance.

V = RIIf I is zero, V = 0

Does the resistance matter?

E

XS

1Ω2Ω3Ω5Ω10Ω1337Ω

No current no energy dissipated p.d is zero resistor is useless

Answer to b(ii) : No change in answer. Lower circuit resistance does not matter

D2

Key Lessons Working and Calculations

Know how to simplify circuits

Potentiometer

2V

Need to balance 1.5V

80

1.50 V

20

2.00 V

A C

J

B

S1 S2

Length is 0.75m (75% of 1m)

Bottom Circuit

Top Circuit

1.5V

1.5VOpposes

Net emf is zero No current flow

Bottom Circuit

Top Circuit

1.5V

1.2VOpposes

Net emf is not zero current still flows

D2

Key Lessons Working and Calculations

Use DC Circuit Construction Kit to understand this problem fully!

Potentiometer

80

1.50 V

20

2.00 V

A C

J

B

S1 S280% 20%

Blue Circuit(Top) is opposing 80% of red circuit80% of 1.5V = 1.2V

2V

Need to balance 1.2V

There is still current flow. Resistance cannot be neglected

Balance length: 60cmIf battery has internal resistance, BL drops

Summary of part i and ii

 Top

CircuitBottom Circuit

Current flow in lower circuit Resistance

ii 1.5V 1.5V noDoesn’t matter

i 1.2V 1.5V yes matters

No current no power no heat loss resistor does nothing