RESISTANCE & WATER MODELText11.1: 441-445

Agenda

1. Review2. Resistance

What is it? Resistors

3. Water Model4. Lab Consolidation

Learning Goal

By the end of this class, we should be able to: Define and identify the correct units and

measurement tool of resistance Describe resistance and current in terms of

the water model

Minds On How do we control the flow of

electricity?

Minds On

How does power get to your house?

Minds On How can we use that power in our

house?

Minds On How can my electronics use/control the

electricity?

Resistance Is the degree to which

a substance opposes the flow of electric current through it

All substances allow electron flow to through them to some extent So nothing is entirely

resistant to current

Resistance

Conductors, such as metals, allow electrons to flow through it fairly easily and therefore have limited resistance Insulators are vice-

versa More resistance the

lower the conductivity

Resistance

Resistance is measured with an ohmmeter Units are in ohms (Ω) Multimeters can also

be used which have a combination of all 3 meters (potential difference, current or resistance)

Resistance Resistance results in

the slowing of electrons and transfers the electrical energy into other forms of energy Like heat and/or light i.e. heaters are

resistors for a purpose, filaments in a light bulb are resistors

Other Factors Effecting Resistance

Resistors: Type 1

Wire wound: Has a wire mask of heat-

resistant metal wrapped in an insulating core

The longer/ thinner the wire the more the resistance

Usually 0.1 Ω -200k Ω 200k Ω = thinnest

Resistors: Type 2 Carbon-composition:

Made of carbon mixed with other materials Carbon mixture is moulded

into a cylinder with a wire at each end

By varying the size and composition of the cylinder the resistances can vary from 10 Ω to 20 M200k Ω

Cheaper but less precise

Resistors

Resistors can be used to control current (or the potential difference) in a circuit Always remember

that they can create heat and can burn so you should be cautious when using them

Water Model Think of these new

concepts in terms of pipes filled with water Not all pipes transport water

equally well. The longer and thinner the pipe is, the more resistant it is to water flowing through it The easiest way for water to

flow through is a wide short tube

Water Model

Think of the circuit as a pipe Amount of water

following through a specific portion on the tube per second is like current

Resistance is the size of the tube

Lets Test Some More! Mini-Lab – The

Effect of Resistance on an Electric Circuit