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II. Electric current 1. Definition Units: [ I ] = 1A = 1 C/s Conventional current Electron flow Example: 10 20 electrons passed through the electric conductor during 4 seconds. Find the electric current through this conductor. Example: The electric current of 0.5 A is flowing through the electric conductor. a) What electric charge is passing through the conductor during each second? b) What electric charge will pass through the conductor during 1 minute? a) b) 1

3. Resistance I V I V Nonohmic device 2. Ohm’s Law Definition:Units: [ R ] = 1Ω = 1 V/AOhm’s Law: 2

4. Resistivity L A I Definition: Temperature dependence of resistivity Example: What is the resistance of 1 m of nichrome wire of 2 mm diameter ? T 3

Example: Two cylindrical resistors, R 1 and R 2, are made of identical material. R 2 has twice the length of R 1 but half the radius of R 1. They are connected to a battery V as shown. Compare the currents flowing through R 1 and through R 2. A. I 1 < I 2 B. I 1 = I 2 C. I 1 > I 2 V I1I1 I2I2 4

R1R1 R2R2 V1V1 V2V2 V Two resistors R 1 and R 2 are in series when they are connected one after the other 5a. Resistors in series IR eq V I 5

Two resistors R 1 and R 2 are in parallel when they are connected to the same potential difference 5b. Resistors in parallel R1R1 I1I1 R2R2 I2I2 II R eq V I 6

Example: A 14-A current flows into a series combination of a 3.0-Ω and a 4.0-Ω resistors. What is the voltage drop across the 4.0-Ω resistor? A) 38 V; B) 42 V; C) 56 V ; D) 98 V. Example: A 22-A current flows into a parallel combination of 4.0 Ω, 6.0 Ω, and 12 Ω resistors. What current flows through the 12-Ω resistor? A) 18 A; B) 11 A; C) 7.3 A; D) 3.7 A. 7

Symbols for circuit elements Conductor (wire) with negligible resistance Resistors Source of emf (for example, a battery) Voltmeter (should be connected in parallel) Ammeter (should be connected in series) - + Switch Ground 8

A A G = r 6a. Ammeter (used to measure current) V A B VG = R 6b. Voltmeter (used to measure voltage or potential difference) Ammeter should be connected in series Ammeter has very small (ideally zero) internal resistance, so it does not affect the current to be measured r is small shunt resistance or shunt G is galvanometer Voltmeter should be connected in parallel Voltmeter has very large (ideally infinite) internal resistance, so that very little current is diverted through it. R is large 9

6c. Voltmeter and Ammeter Example: What connection is not correct? B C A Voltmeter should be connected in parallel Ammeter should be connected in series Voltmeter has very large (ideally infinite) internal resistance Ammeter has very small (ideally zero) internal resistance 10

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