Evans SPH 3U1

Unit 5: Electricity & Magnetism

Note 3: Resistance - Ohm's Law, Series & Parallel Circuits

Reference: 16.5, 16.6

Electrical Resistance (symbol, R): The extent to which a conductor or insulator resists the flow of currents.

Human body has resistance: Wet skin has lower resistance than dry skin. (polygraph makes use of this property)

SI Unit: Ohms   (same as a volt/ampere)

It's dependent upon four factors (Table 16.4 pg. 557):

1) Length - direct variation (i.e. double length, double resistance)

2) Cross-sectional area - varies inversely (i.e. if area doubles, resistance becomes half  since charges have more space to travel)

3) Material - conductor (low resistance) vs. insulator (high resistance)

4) Temperature - resistance decreases as temperature decreases (generally).

The amount of current flow in a circuit depends on two items:

1) The potential difference of the power supply (volts)

2) The nature of the loads being used. (i.e. resistance, the degree of opposition to current flow).

Ohm's Law:

Example 1: An electric kettle is plugged into a 120V power supply It draws 8 A of current. What is the resistance of the kettle? (Ans. 15 ohms)

Review Text Examples 10, 11, 12 pages 557-558

Page 558 # 1-3

Series & Parallel Circuits:

We'll be drawing electric circuits using standard circuit symbols.

Picture of Series Circuit (current has one path to follow) and Parallel Circuit (current has several paths to follow)...........

Kirchhoff's Current Law:

At any junction in an electric circuit, the total electric current into the junction is equal to the total electric current out of the junction.

Kirchhoff's Voltage Law:

Around any complete path through an electric circuit, the sum of the increases in electric potential is equal to the sum of the decreases.

Always remember that there is not net gain or loss of electric charge in any circuit.

Discuss laws and what they mean in a circuit. Draw series and parallel circuits and talk about I, V and how they add up.

Do examples

Develop the following equations:

Resistors in Series:

Resistors in Parallel:

Review Examples 15 and 16 page 562-563

Page 563 # 1, 3, 4

Page 576 #29abc, 30ab, 31abc