Chap 18 DC circuit 12 S

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D.C. Circuit

 Current and potential difference in circuits

a. draw circuit diagrams with power sources (cell or battery), switches, lamps, resistors (fixed and variable), fuses, ammeters and voltmeters, bells, light-dependent resistors, thermistors and light-emitting diodes

 Series and parallel circuits

b. state that the current at every point in a series circuit is the same and apply the principle to new situations or to solve related problems.

c. state that the sum of the potential differences in a series circuit is equal to the potential difference across the whole circuit and apply the principle to new situations or to solve related problems.

d. state that the current from the source is the sum of the currents in the separate branches of a parallel circuit and apply the principle to new situations or to solve related problems

e. state that the potential difference across the separate branches of a parallel circuit is the same and apply the principle to new situations or to solve related problems f. recall and apply the relevant relationships, including R = V/I and those for

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D.C. Circuit

Potential divider circuit

g. describe the action of a variable potential divider (potentiometer)

Thermistor and light-dependent resistor

h. describe the action of thermistors and light-dependent resistors and explain their use as input transducers in potential dividers

i. solve simple circuit problems involving thermistors and light-dependent resistors

Use of cathode-ray oscilloscope (to be covered later) (to be covered later)

j. describe the use of a cathode-ray oscilloscope (c.r.o.) to display

waveforms and to measure p.d.’s and short intervals of time (detailed circuits, structure and operation of the c.r.o. are not required)

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Recap



Charges

• Q = _{Q =}IItt

• Є = _{Є =}ww / Q / Q

• V = w / _{V = w /}QQ



Current

• V = IRV = IR



Resistance

• R = ρ l / A_{R = ρ l / A}



In series

• R = RR = R₁₁ + R + R₂₂ + …. + ….



In parallel

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Electric Circuit

 A closed path through which a charge can flow from one _{A closed path through which a charge can flow from one}

terminal of an electrical source to the other

 Components of an electric circuit_{Components of an electric circuit}

• One that provides the e.m.f._{One that provides the e.m.f.}

• One which the moving charge can do a useful job_{One which the moving charge can do a useful job}

• Conductors join them together_{Conductors join them together}

• Switches to break or complete a circuit_{Switches to break or complete a circuit}

cell +

-switch

lamp

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Series Circuits

 Only one path for current to flow_{Only one path for current to flow}

 Current through every component is _{Current through every component is}

the same

 Component with the largest _{Component with the largest}

resistance has the

highest potential difference across

it.

 Current will not flow if there is a _{Current will not flow if there is a}

break anywhere in the circuit

A₄ A₄

A₃ A₃ A₂

A₂

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Series Circuits



Potential difference

• Potential difference: _{Potential difference:} V

V₁₁ = V = V₂₂ + V + V₃₃ + V + V₄₄

• emf of circuit = total potential _{emf of circuit = total potential}

difference across all resistors

 VV₁₁ + V + V₂₂ = IR= IR₁₁ + I R + I R₂₂

=I (R

=I (R₁₁ + R + R₂₂)) emf

emf = I R= I R_T_T

 Eg.18.1, pg. 353_{Eg.18.1, pg. 353}  Try Q.1, pg. 367_{Try Q.1, pg. 367}

V₁ V₁

V₄ V₄ V₃

V₃ V₂

V₂

L₁ L₂ L₃

battery

R₁

V₁

R₂

V₂

I

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Parallel Circuits

 Current_Current

• More than one path in which charges can flow_{More than one path in which charges can flow}

• Current flow from battery split into each path_{Current flow from battery split into each path}

• Current in each path may not be equal_{Current in each path may not be equal}

• Current will still flow if one of the path is open_{Current will still flow if one of the path is open}

• II₁₁ = I = I₂₂ + I + I₃₃ + I + I₄₄ = I = I₅₅

• Current from the source is equal to the _{Current from the source is equal to the}

sum of all the current in all the

branches.

• component with smallest resistance _{component with smallest resistance}

have the largest current passing

though it. though it. A _B A₂ A₂ L₂ I₂ A₃ A₃ L₃ I₃ A₄ A₄ L₄ I₄ A₁ A₁

I₁ I₅

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Parallel circuits

 Potential difference_{Potential difference}

• all three voltmeter register same value_{all three voltmeter register same value}

• same p.d. across each component _{same p.d. across each component}

• same p.d. across separate branches _{same p.d. across separate branches}

• E.g. 18.3, pg. 358_{E.g. 18.3, pg. 358}

– _I_I₁₁_{= 6/10 = 0.6 A, I}_{= 6/10 = 0.6 A, I}₂₂_{= 6/20 = 0.3 A}_{= 6/20 = 0.3 A}

– The ratio between the current through the 2 resistors?_{The ratio between the current through the 2 resistors?} – The ratio of the 2 resistors?_{The ratio of the 2 resistors?}

– Relationship between the two ratios?_{Relationship between the two ratios?} • Q. 3, pg. 368_{Q. 3, pg. 368}

• Q. 2, pg. 368_{Q. 2, pg. 368}

V₁ V₁

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Resistors in Series

 Total resistance , _{Total resistance ,}  _{R = R}_{R = R}

1

1 + R + R22 + R + R33

 = 4 + 6 + 8_{= 4 + 6 + 8}  = 18 _{= 18}

 Current flowing through _{Current flowing through}

each of the resistors is

the same.

I

I = V / R = V / R

 = 36 / 18 _{= 36 / 18}  = 2A_{= 2A}

Given the following values, find the _{Given the following values, find the}

p.d. across each resistor.

R

R₁₁ = 4 = 4  RR₂₂ = 6 = 6 

R

R₃₃ = 8 = 8  V = 36V V = 36V

V

V₁₁ = I = I₁₁ R R₁₁= 2 x 4 = 8 V= 2 x 4 = 8 V V

V₂₂ = =II₂₂ R R₂₂ = 2 x 6 = 12 V = 2 x 6 = 12 V V

V₃₃ = I = I₃₃ R R₃₃ = 2 x 8 = 16 = 2 x 8 = 16 V

V

Check: Total p.d.

Check: Total p.d.

V

V = V= V₁₁ +V +V₂₂ + V + V₃₃ = 8 + 12 + 16

= 8 + 12 + 16

= 36V

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Challenge

 Two resistors RTwo resistors R₁₁ and R and R₂₂ are connected in parallel, form a are connected in parallel, form a

section of an electric circuit as shown in the figure. Calculate

the equivalent resistance between B and C and the current

through each resistor if p.

through each resistor if p.dd. Across BC is maintained at 6V.. Across BC is maintained at 6V.

6 V R₂ = 6  R₁ = 3 

I₂ I₁

I _B

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Challenge

 Calculate the equivalent resistance of _{Calculate the equivalent resistance of}

the above arrangement. If p.d. across

AB is maintained at 240 V, find the

p.d. across each individual resistor.

10  R₂ 20 

A

B

18 

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Extra practice

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Summary

Series circuit

Series circuit Parallel circuitParallel circuit

current

current same through every

component

 component with least resistance allows

largest current to pass through

 total current supplied from battery shared between components

voltage voltage

 component with largest resistance has the highest voltage across it

 total voltage across the battery is

distributed over each component

same across each component

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15

Potential divider circuit

g. describe the action of a variable potential divider

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Potential divider

 Variable resistor used as a rheostat _{Variable resistor used as a rheostat}

(control the current flow)

 Variable resistor used as potential divider (potentiometer)_{Variable resistor used as potential divider (potentiometer)}

(distribute the p.d. to the components)

• A potential divider is a method of using resistors to deliver only part of _{A potential divider is a method of using resistors to deliver only part of}

the battery’s voltage to the device.

• Resistance of the resistor is constant while resistance of some _{Resistance of the resistor is constant while resistance of some}

devices may not be.

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Potential divider - V



R

 Ratio of voltages across resistors is proportional to ratio of _{Ratio of voltages across resistors is proportional to ratio of}

resistances of resistors

• Greater the resistance, higher the _{Greater the resistance, higher the}

voltage across

 Calculation_Calculation

2 1 2 1 R R V V _ 300 200 100 R R

R_Total ₁ ₂

      V 2 200 100 6 100 ) R V ( R I R V total 1 1 1       V 4 200 100 6 200 ) R V ( R I R V total 2 2 2       A .02 0 I 300) ( I 6 IR V_emf _total

   V 2 (100) 0.02 IR

V₁ ₁

   V 4 (200) 0.02 IR

V₂ ₂

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18

Potential divider - using variable

resistor

 A variable resistor is used as a potential divider by _{A variable resistor is used as a potential divider by}

connecting it at X, Y and Z



When contact Y at point Z, resistance across XY is

maximum, voltage is maximum at 6V



When contact Y moves towards X, resistance across

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Potential divider vs Rheostat

Rheostat

Rheostat Potential DividerPotential Divider

Contact Y at

point Z

 _{Rheostat at max}_{Rheostat at max}

resistance

 _V_V

lamp

lamp is minimum is minimum

 _{Current minimum}_{Current minimum}

RR_XY_XY is maximum (1k is maximum (1k)) VV_xy_xy is maximum (6V) is maximum (6V) II_xz_xz unchanged (6mA) unchanged (6mA)

Contact Y at

point X

 _{Rheostat at zero}_{Rheostat at zero}

resistance

 _V_V

lamp

lamp is maximum is maximum

 Current maximum_{Current maximum}

RR_XY_XY is minimum (0 is minimum (0)) VV_xy_xy is minimum (0V) is minimum (0V) II_xz_xz unchanged (6mA) unchanged (6mA)

Y Y

Y

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Potential divider Problems

V 0 . 2 0 . 5 3300 2200 2200 ) R R V ( R I R V 2 1 in 2 2 out        450Ω R 3750 5.0 4400 3300 3300 R 0 . 5 3300 R 3300 4 . 4 ) R R V ( R I R V 1 1 1 2 1 in 2 2 out            E.G. 23.2 E.G. 23.1

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21

Thermistor and light-dependent resistor

h. describe the action of thermistors and

light-dependent resistors and explain their use as input transducers in potential dividers

i. solve simple circuit problems involving

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Transducers



Device that changes energy from one form to

another



Two types of transducers

•

Input transducer

– Changes non-electrical energy into electrical energy_{Changes non-electrical energy into electrical energy} – E.G. Solar cell, microphone, etc. _{E.G. Solar cell, microphone, etc.}

•

Output transducer

– Converts electrical energy into other forms of energy_{Converts electrical energy into other forms of energy} – E.G. Loudspeaker, LED, electrical meters, etc. _{E.G. Loudspeaker, LED, electrical meters, etc.}

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Light Dependant Resistor (LDR)

 Electrical resistance varies with amount of light falling on it_{Electrical resistance varies with amount of light falling on it}

• Low resistance when under bright light_{Low resistance when under bright light}

• High resistance when under dark conditions_{High resistance when under dark conditions}

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LDR - Application



Using LDR in photographic exposure meter



Meter measure the light intensity (brightness)



Uses a potential divider circuit

• When more light shines on LDR, its resistance drops_{When more light shines on LDR, its resistance drops}

• Voltmeter reading also drops. _{Voltmeter reading also drops.}

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LDR - Application



Using LDR in burglar alarm

system

• When room is dark. Hence _{When room is dark. Hence}

resistance of LDR is high, only small

current flowing in the circuit.

• When light from the burglar's torch _{When light from the burglar's torch}

shines on it, LDR resistance drops

and large current flows in the circuit.

• Current large enough to allow _{Current large enough to allow}

buzzer to sound.

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Thermistor



Thermal resistor



Resistance varies with temperature

• When temperature rises, resistance decreases_{When temperature rises, resistance decreases}

• When temperature drops, resistance increases_{When temperature drops, resistance increases}

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Thermistor - Application



Application in electronic thermometer

• When temperature increases, the resistance drops._{When temperature increases, the resistance drops.}

• Voltage across thermistor drops_{Voltage across thermistor drops}

• Voltmeter will register a lower value. _{Voltmeter will register a lower value.}

• Vice versa_{Vice versa}

• Resistance of thermistor does not vary linearly with _{Resistance of thermistor does not vary linearly with}

temperature.

• Calibration curve required to convert voltmeter reading to _{Calibration curve required to convert voltmeter reading to}

temperature.

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Example

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Challenge

A light dependent resistor ( LDR ) is connected in a circuit as shown below.

When no light falls on the LDR, its resistance is 3600Ω. In dim light, its resistance is 1200Ω.

a)Determine the resistance of the circuit when no light falls on the LDR. [ 2 ] b)Calculate the reading on ammeter A when no light falls on the LDR. [ 2 ]

c)Determine the change in potential difference across the LDR when the entire circuit

is moved from a dimly lit room into a closed drawer. State clearly whether there is a rise or drop in potential difference across the LDR. [ 4 ]

d)What effect would there be on the reading on voltmeter V1 when the circuit is

Link to short write-up about these transducers