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Electrical Resistance [Text: p.59-64] [3] Measuring Electricity 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 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 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 [] 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. 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 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] ------------------------------------------------------------------------- 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? ----------------------------------------------------------------------------------- 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? ----------------------------------------------------------------------------------- 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? -----------------------------------------------------------------------------------