6.2 Ohm’s Law p. 230 Resistance George Simon Ohm (1787 – 1854) found that for metal conductors at a given temperature, the current was directly proportional to the voltage across the conductor. I α V V I = constant This constant is called the resistance, R, and is measures in ohms (1 Ω = 1 V/A) V I = R V R = I V = IR or

6.2 Ohm’s Law Resistors p. 231 Every conductors offers some resistance to current flow. A resistor is a device specifically created to control the current in a circuit. Two main types of resistors: In a resistor electrical energy is transferred to thermal energy. There are some materials when cooled to super low temperatures that offer no resistance. These materials are called superconductors and are important parts of the magnets in controlling the beam of protons entering the large HADRON collider at Cern, Switzerland.

6.2 Ohm’s Law Resistor Code p. 232 Resistors either are labelled with their value or use a colour code to identify the resistance value. Colour Band Meaning 1 st First digit value 2 nd Second digit value 3 rd Power-of-ten multiplier 4 th Manufacturer’s tolerance Entire Resistor Code: For example: 33 x % Ω Ohm’s Law does not apply to all conductors. For example light bulbs, conducting solutions and gas-discharge tubes are examples of non-ohmic resistors.

6.2 Ohm’s Law Joule’s Law p James Prescott Joule (1818 – 1889) found out that amount of energy released in a unit of time (power) by a resistor is proportional to the square of the current. P α I 2 P = constant x I 2 The constant is the same as the resistance in the circuit. P = R x I 2 = I 2 R By bringing in Ohm’s law: V = IR P = I 2 R = VI = V2V2 R or

6.2 Ohm’s Law EMF, Terminal Voltage, and Internal Resistance: p. 235 When little current is flowing through a cell the EMF and the terminal voltage of the cell are nearly equal. However, when significant current is flowing through the circuit and the cell the terminal voltage will be less than the EMF. V ab < Ɛ V ab = Ɛ - V r When a cell is connected to an external resistor the current drawn by the complete circuit flows through the cell and there would be some loss of potential energy per coulomb or voltage by the internal resistance, r, of the cell. V r = Ir and V ab = Ɛ - Ir For a cell in use: V ab = Ɛ + Ir For a cell being charged:

6.2 Ohm’s Law Key Questions In this section, you should understand how to solve the following key questions. Page 231 – Practice Problems #2 Page 233 – Practice Problems #3 Page 234 – Quick Check #2 Page 239 – 240 – Review 6.2 #2,4,7,8,13 & 16