**Evans SPH 3U1**

**Physics Grade
11**

**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**