From yesterday…•What are the 4 basic elements that a

circuit includes?Source, conductor, load, control

•Draw a simple circuit which includes 1 light bulb, a battery and a switch.

•What is potential difference?Difference in energy per unit of charge

between one point in the current and another- referred to as voltage

Topic 2 Review:1. Matching answers:

• Load- water wheel• Switch- valve• Electric current- flow rate• Voltage- pressure• Battery- pump• Conductor (wire)- pipe

2. Drawing a circuit with a battery, a switch, a lamp, an ammeter, and conducting wires.

4. The current passing through the electric iron would be greater because an iron requires far more energy (E in joules) than the much smaller razor. Since V = E/Q (Q = E/V) the iron would require more coulombs of charge than the razor. If I = Q/t and Q is larger than I, the current would be greater

Resisting the Movement of ChargeTopic 3Pages 279- 291

Different Types of Circuits

•Series Circuit:Only 1 current path!All moving charges travel through each

component in a circuit

•Parallel Circuit:Several current pathsThe total current is divided with charges

travelling through each branch, or part of the circuit.

House Wiring

•Most circuits around the house are parallelThe voltage across each load in a parallel

circuit is the same

Problem: Current through wires connected to the

source increases whenever another branch in the circuit is closed. Increases temperature and can cause fires!

What is resistance?•Is a property of a substance that hinders

motion of electric charge and converts electric energy into other forms of energy.

• Example: light bulb with tungsten filament

vs. light bulb with copper filament.

Electrons (-) force their way through solids, overcoming resistance caused by the attraction of positively charged atomic nuclei.

•Remember:Current is movement of electrons through

conductorsVoltage is what makes electrons moveResistance is what opposes the motion of the

electrons.

A good conductor has LOW resistanceA poor conductor has HIGH resistance

•The unit for resistance is ohm (Ω)Measured using an ohmmeter orMultimeter

Measuring Resistance•Resistance affects current flow and

potential difference in a circuit

Resistance= voltage (potential difference)

current

This is called Ohm’s Law:R= V I

Let’s try some problems!

Together:What is the resistance of an electric heater,

if a current of 12.5A runs through it when it is connected to a 120V wall outlet?

To Do:1. What is the resistance of a light bulb if a

12V battery sends a current of 2.4A through it?

2. A toaster with a resistance of 145Ω is connected to a 120V source. What current will flow through the toaster?

3. What is the potential difference across a 1500Ω resistor carrying a current of 0.075A (74mA)?

Read page 283:•Resistors:

What are resistors?What is one problem that is encountered with

resistors?What is a major use of them?What materials are resistors commonly made of?

•Variable Resistors:What changes with variable resistors?Where are variable resistors used? Give examples

•Resistors: control current or voltage to suit the specific needs of the loads in a circuit.

•Problems? Can be single-use, expensive, or hard to replace.

•Common use: circuit boards•Made of: metal oxides, metal film, resistance

wire

•Variable resistors don’t have a fixed resistanceChanges with temperature, light, voltage, etc.

•Types: rheostats, thermistors, varistorsExamples: Thermostat, dimmer-switch, surge

protector

To Do:

•Read page 291 about the cross-section of a wire.

•Resistance Calculations Worksheet •Topic 3 Review

Pg. 291, #1-5