resistance and ohm’s law

Resistance and Ohm’s Law

When electric charges flow they experience opposition or resistance which reduces the amount of energy they have

Greater resistance -> greater amount of energy each charge has to give up

Filament – high resistance

Therefore, lot’s of energy taken from each electron and turned into light

Wire – low resistanceTherefore, little energy lost – turned mainly into heat

The amount of energy (voltage) required to push electrons (current) through a conductor

Electric Resistance

German Physicist Georg Ohm determined that for a given conductor, the ratio of voltage to current (V/I) is constant

We call this constant resistance (Measured in Ohms - )

= IVConstantResistance

Relates voltage, current and resistance

Note: E stands for electric potential (commonly known as voltage)

Ohm’s Law

IVR

Electrical Insultator ◦Prevents the transfer of electric charges

◦Ex: Air, glass, rubber, paperElectrical conductor◦Low resistance which allows electric charges to flow easily

◦Ex: copper

A single cell is set-up in a circuit with a switch and a resistor. For the resistor, a voltmeter is set-up and it measures 1.3V and an ammeter is set-up and it measures 3.5A.

a) Draw the circuit with the correct set-up of a voltmeter and ammeter

b) Calculate the resistance of the resistor

Example 1:

If a resistor has a resistance of 1000 and the current is 2.0A. What will be the voltage drop across the resistor?

Example 2:

Pg 330 # 1aPg 332 # 1, 3-5Pg 331 # 5ab,7,8

Work

Circuit Analysis

Series◦Only one path for current to flow

Types of Circuits

Parallel◦More than one path for current to take

Types of Circuits

We have four tools we can use to analyze circuits:◦Kirchoff’s Current Law (KCL)◦Kirchoff’s Voltage Law (KVL)◦Ohm’s Law◦Equivalent Resistance (Series and Parallel)

Analysis of Circuits

At any junction (Point) in the circuit, the current going in equals the current going out

At a point, Iin = Iout

Note: In any series circuit, every point has the same current

Kirchoff’s Current Law (KCL)

Examples

In any complete path in a circuit, the sum of voltage rises (sources) is equal to the sum of voltage drops (loads)

For a path,Vrises = Vdrops

Kirchoff’s Voltage Law (KVL)

Examples

Pg 337 # 1-3Pg 343 # 1, 2, 4

Work

If you have many resistors (or loads) in a circuit, finding the equivalent resistance allows you to replace multiple resistors with a single resistor

This allows easier analysis of circuits

Equivalent Resistance

Equivalent Resistance in Series

ns RRRRR ...321

Equivalent Resistance in Parallel

nP RRRRR1...1111

321

The easiest way to put this formula into a calculator is to use your inverse button (x-1)

Pg 339 # 4-6Pg 340 # 7

Work