Chapter 4.3 Chapter 4.3 NotesNotes

Resistance in Resistance in ElectricityElectricity

Charges can easily flow through Charges can easily flow through conductorsconductors because they contain because they contain many free electrons.many free electrons.

Even conductors have Even conductors have resistanceresistance though.though.

Some conductors can be cooled to Some conductors can be cooled to very low temperatures to reduce very low temperatures to reduce the friction. These are called the friction. These are called superconductorssuperconductors..

In In insulatorsinsulators, electrons are , electrons are tightly bound and cannot tightly bound and cannot move freely and so they do move freely and so they do not allow charges to flow not allow charges to flow easily.easily.

Examples of insulators Examples of insulators include include wood, plastic, wood, plastic, glass, and rubberglass, and rubber..

Some substances are Some substances are intermediate in their ability intermediate in their ability to conduct charge and are to conduct charge and are called called semiconductorssemiconductors..

Insulators are used for Insulators are used for computer computer chips and chips and microprocessorsmicroprocessors..

In wire, free electrons move In wire, free electrons move throughout the wire, but not in throughout the wire, but not in straight straight lineslines..

The electrons continuously The electrons continuously bumpbump into into other electrons and atoms.other electrons and atoms.

Each collision causes the electron to Each collision causes the electron to change change directiondirection..

However, Electricity allows flows from However, Electricity allows flows from the negative terminal to the load (light the negative terminal to the load (light bulbs) to the bulbs) to the positivepositive terminal. terminal.

When fast moving electrons bump When fast moving electrons bump into atoms, they into atoms, they transfer energytransfer energy..

Energy is transferred from the Energy is transferred from the electron to the atom which increases electron to the atom which increases the wire’s the wire’s temperaturetemperature..

These collisions between electrons These collisions between electrons and atoms is what slows down the and atoms is what slows down the free flow of electrons and is the free flow of electrons and is the cause of electrical cause of electrical resistanceresistance..

Resistance = Resistance = voltage / voltage / currentcurrent

R = R = V / IV / I Unit for resistance is the Unit for resistance is the

ohmohm.. Symbol for resistance is Symbol for resistance is ΩΩ

• If we have a current of 2 A If we have a current of 2 A when the power supply is when the power supply is producing a voltage of 110 V, producing a voltage of 110 V, what is the resistance?what is the resistance?

R = V / IR = V / I R = 110 V / 2 AR = 110 V / 2 A R = R = 55 55 ΩΩ

Physicist George Simon Ohm Physicist George Simon Ohm discovered that the ratio of discovered that the ratio of voltage to current is constant voltage to current is constant for most for most conductorsconductors..

Ohm’s LawOhm’s Law says if its says if its resistance is constant, than resistance is constant, than change in Voltage will equal change in Voltage will equal current times resistance.current times resistance.

The amount of resistance in a The amount of resistance in a wire depends on 3 things:wire depends on 3 things:

a. Length of the wire – longer a. Length of the wire – longer increases resistanceincreases resistance

b. The radius of the wire – the b. The radius of the wire – the smaller the wire, the less smaller the wire, the less resistanceresistance

c. The material of which the c. The material of which the wire is madewire is made

ResistivityResistivity is a is a measure of the capacity measure of the capacity of a material to resist of a material to resist electric charge flow.electric charge flow.

In a In a seriesseries circuit their is circuit their is only 1 path for electricity only 1 path for electricity to flow.to flow.

3 important rules for Series circuits:3 important rules for Series circuits: Since there is only path for charges to Since there is only path for charges to

flow, the flow, the currentcurrent is the same everywhere. is the same everywhere. The current is slowed down by the first The current is slowed down by the first

light bulb and then slowed down more by light bulb and then slowed down more by a second light bulb. Therefore, the a second light bulb. Therefore, the resistance is the resistance is the sumsum of the individual of the individual resistances of each light bulb.resistances of each light bulb.

The sum of the voltage drop across each The sum of the voltage drop across each light bulb = the voltage of the light bulb = the voltage of the batterybattery..

• In a series circuit, when there is a In a series circuit, when there is a break in the circuit, the current break in the circuit, the current everywhere is everywhere is stoppedstopped..

• In a series circuit, light bulb one has a In a series circuit, light bulb one has a resistance of 90 ohms, light bulb 2 has resistance of 90 ohms, light bulb 2 has a resistance of 70 ohms, and light bulb a resistance of 70 ohms, and light bulb 3 has a resistance of 120 ohms. What 3 has a resistance of 120 ohms. What is the total resistance in the circuit?is the total resistance in the circuit?

• Total resistance in series equals sum Total resistance in series equals sum of each resistance: 90 + 70 + 120 = of each resistance: 90 + 70 + 120 = 280280

• In a In a parallelparallel circuit, the current circuit, the current from the battery flows through from the battery flows through one lamp and part of the one lamp and part of the current flows through the other current flows through the other lamp equally.lamp equally.

• If one bulb is removed, the If one bulb is removed, the current can still flow through current can still flow through the the second pathsecond path and the other and the other bulb will remain lit.bulb will remain lit.

3 rules for Parallel:3 rules for Parallel: Add the current through the bulbs to Add the current through the bulbs to

get the get the totaltotal current of the circuit. current of the circuit. To calculate resistance, use this To calculate resistance, use this

equation: equation: _1__1_ __1_1_ 1_ 1_

RRtotaltotal = R = R11 + R + R22

The voltage drop across the light The voltage drop across the light bulbs is bulbs is equalequal to the voltage of the to the voltage of the battery.battery.

• A circuit in parallel has two light A circuit in parallel has two light bulbs. Bulb 1 has a resistance of bulbs. Bulb 1 has a resistance of 90 and bulb 2 has a resistance of 90 and bulb 2 has a resistance of 70. What is the total resistance?70. What is the total resistance?

• _1__1_ __1_1_ 1_ 1_ RRtotaltotal = R = R11 + R + R22 1 / R1 / Rtotaltotal = 1/90 +1/70 = 1/90 +1/70 (Time out!)(Time out!) 1 / R1 / Rtotaltotal = .0111 +.0143 = .0111 +.0143 1 / R1 / Rtotaltotal = .0254 = .0254 RRtotaltotal = 1/.0254 = = 1/.0254 = 39.3739.37

• Sometimes, we want Sometimes, we want resistance in a circuit. A resistance in a circuit. A resistorresistor is an electrical is an electrical device that has a device that has a specific resistance we specific resistance we can add to a circuit.can add to a circuit.

How to Read ResistorsHow to Read Resistors

•First find the tolerance band, it will First find the tolerance band, it will typically be gold or silver.typically be gold or silver.•Starting from the other end, identify the Starting from the other end, identify the first band - write down the number first band - write down the number associated with that colorassociated with that color•Now 'read' the next color, here it is red Now 'read' the next color, here it is red Now read 'multiplier' band and write Now read 'multiplier' band and write down that number of zeros.down that number of zeros.

Read ResistorsRead Resistors

What is our resistor?What is our resistor?

•Blue = 6

•Red = 2

•Multiplier = 2 (Number of zeros is 2)

•6200 - Resistor