0708 electricity
DESCRIPTION
Introduction to Measurement Systems i) Introduction to measurements, Functional elements of an instrument, Performance characteristics of an instrument static & dynamic ,Errors in measurements and their statistical analysis, ii)units and dimensions: Fundamental Units, MKSA and SI Units, Derived Units, Dimensional Analysis of a Physical Quantity, Dimensional Equation.TRANSCRIPT
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ELECTRICAL CIRCUITSAll you need to be an inventor is a good
imagination and a pile of junk.-Thomas Edison
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Ohm’s LawI = V / R
Georg Simon Ohm (1787-1854)
I = Current (Amperes) (amps)
V = Voltage (Volts)
R = Resistance (ohms)
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How you should be thinking about electric circuits:Voltage: a force that pushes the current through the circuit (in this picture it would be equivalent to gravity)
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Resistance: friction that impedes flow of current through the circuit (rocks in the river)
How you should be thinking about electric circuits:
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Current: the actual “substance” that is flowing through the wires of the circuit (electrons!)
How you should be thinking about electric circuits:
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Would This Work?
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Would This Work?
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Would This Work?
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The Central Concept: Closed Circuit
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circuit diagramcircuit diagram
cell switch
lamp
wires
Scientists usually draw electric circuits using symbols;
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Simple Circuits • Series circuit
– All in a row– 1 path for electricity– 1 light goes out and
the circuit is broken
• Parallel circuit– Many paths for electricity– 1 light goes out and the
others stay on
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1
2
3
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PARALLEL CIRCUIT
• Place two bulbs in parallel. What do you notice about the brightness of the bulbs?
• Add a third light bulb in the circuit. What do you notice about the brightness of the bulbs?
• Remove the middle bulb from the circuit. What happened?
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measuring current
Electric current is measured in amps (A)
using an ammeter connected in series in
the circuit.
A
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measuring current
A A
This is how we draw an ammeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
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measuring voltageThe ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter
V
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measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUITV
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OHM’s LAW• Measure the current and voltage across each circuit.• Use Ohm’s Law to compute resistanceSeries Circuit
Voltage Current Resistance
Voltage Current Resistance
Parallel Circuit
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measuring currentSERIES CIRCUIT
PARALLEL CIRCUIT
• current is the same at all points in the circuit.
2A 2A
2A
• current is shared between the components
2A2A
1A
1A
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fill in the missing ammeter readings.
?
?
4A
4A
4A
3A?
?
1A
?
3A
1A
1A
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The circuit is no longer complete, therefore current can not flow
The voltage decreases because the current is decreased
and the resistance increases.
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The current remains the same. The total resistance drops in a parallel circuit as more bulbs are added
The current increases.
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Series and Parallel Circuits
• Series Circuits– only one end of each component is connected– e.g. Christmas tree lights
• Parallel Circuits– both ends of a component are connected– e.g. household lighting
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copy the following circuits and fill in the missing ammeter readings.
?
?
4A
4A
4A
3A?
?
1A
?
3A
1A
1A
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Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current.
measuring voltage
Unlike an ammeter, a voltmeter is connected across the components
Scientist usually use the term Potential Difference (pd) when they talk about voltage.
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V
measuring voltage
VV
V
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series circuit
1.5V
• voltage is shared between the components
1.5V
3V
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• voltage is the same in all parts of the circuit.
3V
parallel circuit
3V
3V
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measuring current & voltage
copy the following circuits on the next two slides.
complete the missing current and voltage readings.
remember the rules for current and voltage in series and parallel circuits.
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measuring current & voltage
V V
6V4A
A
A
a)a)
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measuring current & voltage
V
V
6V4A A
A
A
b)b)
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answers
3V 3V
6V
4A 4A6V
6V
6V4A 4A
2A
2A
4A
a)a) b)b)
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Voltage, Current, and Power
• One Volt is a Joule per Coulomb (J/C)• One Amp of current is one Coulomb per second
(6.24 x10^18 electrons/second). • If I have one volt (J/C) and one amp (C/s), then
multiplying gives Joules per second (J/s)– this is power: J/s = Watts
• So the formula for electrical power is just:
• More work is done per unit time the higher the voltage and/or the higher the current
P = VI: power = voltage current