II. Electric current 1. Definition a) Conventional current Electron flow Units: [ I ] = 1A = 1 C/s Example: 1020 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)

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

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

Example: Two cylindrical resistors, R1 and R2, are made of identical material. R2 has twice the length of R1 but half the radius of R1. They are connected to a battery V as shown. Compare the currents flowing through R1 and through R2. A. I1 < I2 B. I1 = I2 C. I1 > I2 V I1 I2

5a. Resistors in series Two resistors R1 and R2 are in series when they are connected one after the other I R1 R2 I Req V1 V2 V V

5b. Resistors in parallel Two resistors R1 and R2 are in parallel when they are connected to the same potential difference I1 R1 I Req I I I2 V R2

Example: A 14-A current flows into a series combination of a 3 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.

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

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

Example: What connection is not correct? 6c. Voltmeter and Ammeter 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 Example: What connection is not correct? A C B