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Electrical Resistance [Text: p.59-64]
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Measuring Electricity
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An ammeter is used to measure the current flowing in a circuit.
An ammeter is connected in series with the circuit elements, the current flowing
through them is the same as that through the ammeter.
A voltmeter is used to measure the potential difference [voltage] across a
circuit element or across an EMF source.
A voltmeter must be connected across [in parallel with] the circuit element.
Current-Voltage Graphs
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The I-V characteristics [a current vs. voltage graph] for a circuit element can be
used to predict its behaviour or estimate the power it will consume.
Circuit elements can be divided into 2 groups, based on their I-V characteristics …
Devices with straight line I-V characteristics are called ohmic conductors.
Devices with non-linear I-V characteristics are called non-ohmic conductors.
Light bulbs and diodes are circuit elements that are non-ohmic conductors.
Resistors are circuit elements that are ohmic conductors.
Resistance
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Georg Ohm found that the current flowing through a wire was directly dependant
on the potential difference across it …
Ohm’s Law V = IR where V = voltage [V]; I = current [A]; R = resistance []
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Resistance [R = V/I] and is measured in ohms [] … 1  = 1 volt/ampere.
Ohm’s law [V = IR] only applies to ohmic conductors, such as wires and
resistors, whose I-V characteristics are linear.
For non-ohmic conductors, whose I-V characteristics are not linear, the ratio of
V/I is still their resistance, but they don’t obey Ohm’s Law.
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Example: What is the resistance in a coil if a 0.038 A current flows through it when
there is a 9.0 V potential difference across it?
R = V/I = 9.0/0.036  250 
Resistance and Resistivity
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The resistance of a wire is a measure of how it impedes the flow of electrons
along it and depends on its length, thickness and the type of metal.
R = L/A where L = length [m]; A = cross-sectional area [ m2]
 = resistivity (rho) of particular metal [m]
Example: What is the resistivity of nichrome if a 2.5 m long, 0.10 mm diameter
nichrome wire has a resistance of 320 ?
R = L/A …  = RA/L = 320(0.05x10-3)2/2.5  1.0x10-6 m
Problems p.65-66 Q. 1, 2, 3, 5, 7, 9 and 10
Problems: Resistance and Resistivity
----------------------------------------------------------------------1. An ammeter and voltmeter were used to measure the current in and
voltage across a circuit element as shown below. The ammeter
reading was 26.8 mA; the voltmeter read 3.40 V.
X
Y
(a) Identify the ammeter [X or Y] and the voltmeter.
(b) What is the resistance of the circuit element?
------------------------------------------------------------------------2. The I-V characteristics of two circuit elements are shown below …
Current [A]
1.00
A
B
0.50
6.0
12
18
Voltage [V]
(a) Which of the circuit elements [A or B] is an Ohmic conductor? Explain.
(b) What is the resistance of circuit element A?
------------------------------------------------------------------------3. A coiled 8.00 m length of copper wire with a diameter of 1.04 mm has a
resistance of only 0.16 .
(a) What is the resistivity of copper?
(b) What would be the resistance of the copper wire if it was:(i) only 4 m long [same diameter]
(ii) 2.08 mm diameter [same length]
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U1 Homework – Electricity No.3
Complete and submit the following homework task on A4 loose leaf, clearly showing
your name, all working and reasoning.
A coiled piece of thin nichrome wire [ resistivity = 1.0x10-6 m ] is used as a heating
element in an experiment. The wire is 2.5 m long and has a cross-sectional area of
5.0x10-6 m2. The potential difference across the wire is 9.0 volts.
1. What is the resistance of the nichrome wire?
2. What current flows in the nichrome wire?
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U1 Homework – Electricity No.3
A coiled piece of thin nichrome wire [ resistivity = 1.0x10-6 m ] is used as a heating
element in an experiment. The wire is 2.5 m long and has a cross-sectional area of
5.0x10-6 m2. The potential difference across the wire is 9.0 volts.
1. What is the resistance of the nichrome wire?
2. What current flows in the nichrome wire?
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U1 Homework – Electricity No.3
A coiled piece of thin nichrome wire [ resistivity = 1.0x10-6 m ] is used as a heating
element in an experiment. The wire is 2.5 m long and has a cross-sectional area of
5.0x10-6 m2. The potential difference across the wire is 9.0 volts.
1. What is the resistance of the nichrome wire?
2. What current flows in the nichrome wire?
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