Transcript electricity - HSphysics

D.C. Circuit
Name: ________________
Class: _________________
Index: ________________
Objectives
-- 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 lightemitting diodes
--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
--state that the sum of the p.d.'s in a series circuit is equal to the p.d.
across the whole circuit and apply the principle to new situations or
to solve related problems
--state that the current from the source is the sum of the currents in
the separate branches of the parallel circuit and apply the principle
to new situations or to solve related problems
-- state that the potential differences across the separate branches of
a parallel circuit is the same and apply the principle to new
situations or to solve related problems
-- recall and apply the relevant relations, including R = V/I and those
for potential differences in series and in parallel circuits, resistors in
series and in parallel, in calculations involving a whole circuit
--describe the action of a variable potential divider (potentiometer)
--describe the action of thermistors and light-dependent-resistors
and explain their use as input transducers in potential dividers
-- solve simple circuit problems involving thermistors and lightdependent resistors
-- describe the use of a CRO to display waveforms and to measure
p.d.’s and short time intervals of time (detailed circuits, structure
and operation of the CRO are not required
-- interpret CRO displays of waveforms, p.d.’s and time intervals to
solve related problems
Electrical Circuit Symbols
Electrical Circuit symbols are used in circuit diagrams which
show how a circuit is connected together.
LED
Bell
Cells in Series and Parallel
• Cell :
In series (connected side by side)
(b)
For (b), If a cell has V = 2V, then total emf = 8V
In parallel (connected in parallel)
(b)
A cell = 2V;
emf = 2V
Examples of cell arrangement
2V 2V
2V
2V
6V
-2V
2V
2V

2V
2V
2V

2V
Resistors Arrangement
Series and Parallel Circuits
• Resistance:
In series(connected side by side)
In parallel (connected in parallel)
Resistors in series
• When resistors are connected in series,
the total resistance (effective resistance or
resultant resistance) is equal to the sum of
of the individual resistance.
Thus,
Rtotal = R1 + R2 + R3
Resistors in parallel
• When resistor connected in parallel:
The total resistance (or effective
resistance or resultant resistance) is
Two resistors connected in parallel
• If two resistors of resistance R1 and R2
are connected in parallel,
Then
Hence
Current through resistors in series
I
Current flow through R1, R2
and R3 with the same current,
I.
Current through resistors in parallel
I1
I
I2
I3
R1 = R2 = R3: I = I1 + I2 + I3
R1 > R2 > R3: I3 > I2 > I1
Voltage – Series/parallel circuit
V1
V2
E = V 1 + V2
E
E
V1 V2
E = V 1 = V2
Example
The diagram shows the magnitude and directions of
the electric currents entering and leaving junction P.
What will be the magnitude and direction of the
current in the wire PQ ?
Q
3A
5A
Solution
P
Resultant current
= (7 + 3) - (5)
= 5 A (in the direction QP)
7A
Example
A voltage of 4V is supplied to two resistors of (6 
and 2  ) connected in series. Calculate
(a) the combined resistance,
(b) the current flowing,
(c) the p.d. across the 6  resistor.
I
6
2
4V
Solution
(a) combined resistor = 6 + 2
=8
(b) since V= RI
4 = 8 I, I = 0.5 A
(c)
V6 = 6 x 0.5 = 3 V
Example
A voltage of 12 V is applied to two resistors of 3  and
6  connected in parallel. Calculate
(a) the combined resistance,
(b) the current flowing in the main circuit,
(c) the current in the 3  resistor.
3
Solution
6
(a) the combined resistance
= (3 x 6) / (3 + 6)
12 V
=2
(b) since V = RI
I = V / R = 12 / 2 = 6 A
(c) current through 3  = 12 / 3 = 4 A
Example
The battery in the circuit illustrated has an e.m.f. of
16 V and negligible internal resistance.
Calculate
(a) the combined resistance of the system.
(b) the current flowing through the 8  resistor.
16V
8
36
18
Continue on next slide
Continue ...
Solution
(a) combined resistance
= (36 x 18) / (36 + 18) + 8
= 20 
(b) since
therefore
V = RI
16 = 20 I
I = 0.8 A
hence, current through 8 resistor is 0.8A
Short Circuit
A
B
• In the fig shown, AB is a copper wire which connects
two point A and B on the circuit.
• Since copper wire has very little resistance, therefore
a large amount of current will flow through it.
• The lamp then go off. (Why ?)
• Therefore we say this circuit is now a short circuit.
Potential Divider Circuit
A voltage divider (also known as a potential divider) is a
simple linear circuit that produces an output voltage (Vout) that
is a fraction of its input voltage (Vin). Voltage division refers
to the partitioning of a voltage among the components of the
divider.
Example
Q) Calculate the output voltage from
the 4 resistor.
Solution:
Total resistance = 4.0 + 8.0 = 12
Current = 12V / 12 = 1.0A
Output voltage (4.0 ) = 1.0A x 4.0
= 4.0V
* Total potential difference for 4.0
and 8.0 resistor = 4.0V + 8.0V
= 12V
Input Transducer
Input Transducers convert a quantity to an electrical signal
(voltage) or to resistance (which can be converted to voltage).
Input transducers are also called sensors.
Variable resistor converts position (angle) to resistance
Thermistor converts temperature to resistance
LDR converts brightness (of light) to resistance
Using an input transducer (sensor) in a voltage divider
Most input transducers (sensors) vary their resistance and usually a
voltage divider is used to convert this to a varying voltage which is more
useful. The voltage signal can be fed to other parts of the circuit, such as
the input to an IC or a transistor switch.
The sensor is one of the resistances in the voltage divider. It can be at the
top (R1) or at the bottom (R2), the choice is determined by when you
want a large value for the output voltage Vo:
Put the sensor at the top (R1) if you want a large Vo when the sensor has
a small resistance. Put the sensor at the bottom (R2) if you want a large
Vo when the sensor has a large resistance.
Example
Suppose the LDR has a resistance of 0.50
k , in bright light, and 200 k in the
shade, calculate the magnitude of Vout: (a)
under the sun, (b) in the shade.
(a) Total resistance = (10 + 0.5) k = 10.5 k
I = 9.00 V / 10.5 k = 0.86 mA
Vout = 0.86 mA x 0.5 k = 0.43 V
(b) Total resistance = (10 + 200) k = 210 k
I = 9.00 V / 210 k = 0.043 mA
Vout = 0.043 mA x 200 k = 8.6 V
Cathode Ray Oscilloscope
An oscilloscope is a test instrument which
allows you to look at the 'shape' of electrical
signals by displaying a graph of voltage against
time on its screen.
The graph, usually called the trace, is drawn by
a beam of electrons striking the phosphor
coating of the screen making it emit light,
usually green or blue.
Oscilloscopes contain a vacuum tube with a
cathode (negative electrode) at one end to emit
electrons and an anode (positive electrode) to
accelerate them so they move rapidly down the
tube to the screen. The tube also contains
electrodes to deflect the electron beam up/down
and left/right.
Solution:
(a) T = 14 x 5 ms = 70 ms
(b) f = 1/T = 1 / 70 ms = 14.3 Hz
(c) Peak voltage = 4 x 20 V = 80 V
Solution:
Peak voltage = 2 x 20 V = 40 V
Solution:
Assume each small square is 1cm by 1 cm
(a) Time to complete 1 cycle = 15 ms
f = 1/T = 1 / 15 ms = 67 Hz
(b) T = 15 ms
(c) When frequency is doubled, period is halved and the length
between the 2 crests is halved.
Solution:
Assume each small square is 1cm by 1 cm
Time between X and Y = 8 x 50 ms = 400 ms
Distance = 3 x 108 m/s x 400/2 ms = 0.6 x 108 m
Reference
http://www.antonineeducation.co.uk/physics_gcse/Unit_2/Topic_6/topic_6_files/image002.gif
http://bleex.me.berkeley.edu/ME102/proj_archive/S05/18Inverted_Pendulum/images/Hardware/Potentiometer.bmp
http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Resistance_B
oxes_and_Rheostats/Greenslade170a.JPG
http://www.jestineyong.com/wp-content/uploads/2008/05/ntc-thermistor.jpg
http://www.mstracey.btinternet.co.uk/technical/Theory/ldr.jpg
http://www.hobbyprojects.com/dc_theory/potential_dividers.html
http://www.antonine-education.co.uk/New_items/DIG/Potential_Divider.gif
http://www.kpsec.freeuk.com/vdivider.htm
http://www.kpsec.freeuk.com/cro.htm