15.2 Electric Current, Resistance and Voltagepp. 346 - 349

Mr. Richter

Agenda

Warm-Up

Check HW

Introduction to Current

Notes: Charge and Current The Source of Current (Voltage) Resistance (Conductors and Insulators)

Objectives: We Will Be Able To…

Describe the relationship between electrons and current.

Explain, at the atomic level, the difference between insulators, semi-conductors and conductors.

Identify how voltage and charge are related.

Warm-Up:

What would cause electrons to flow in a wire?

Discuss at your table and we will discuss as a class in a minute.

Current

Current

Electric current is the flow of charge.

Current is measured in amperes (A). Abbreviated: amps.

One amp of current is the equivalent of one coulomb of charge passing by in one second. Or roughly 6.24 x1018 electrons per second!

Current is a measure of how many electrons pass by a point every second.

The Direction of Current

Benjamin Franklin defined current as going from positive to negative (just like everything else).

This is before we discovered that electrons have a negative charge. Electrons actually flow away from negative to positive.

The Direction of Current

We still say current flows from positive to negative, even though it’s the electrons that really move.

It doesn’t matter if it is electrons or protons moving. Usually the direction of current does not matter.

The Source of CurrentVoltage

The Source of Current

In metals (and some other materials) electrons are shared between atoms.

Electrons can move freely from one atom to another, and they are always moving randomly.

The Source of Current (p. 347)

Because electrons move randomly, there is usually no current.

The average movements of electrons cancel each other out.

The Source of Current (p. 347)

Electrons will only flow in one direction if there is something to push them.

When a voltage source is applied across a wire, electrons drift toward the positive end, creating current.

Voltage (potential)

Voltage potential measures the energy per unit of charge.

How much energy does each electron have? How much work can each electron do?

Voltage is measured in volts (V).

One volt = 1 joule/coulomb

Video Wrap-Up!

https://www.youtube.com/watch?v=1xPjES-sHwg

Putting Voltage and Current Together

A small light bulb that draws 3 A of current is connected to a 1.5 volt battery for 10 seconds.

1. How much charge passes through the light bulb?1. 3 A = 3 coulombs of charge per second

2. 3 A * 10 seconds = 30 C of charge

2. How much energy does the light bulb use?1. 1.5 volts = 1.5 joules for every coulomb of charge

2. 1.5 V * 30 C = 45 joules

Warm-Up: Day 2

1. 60 coulombs of charge pass through a light bulb in 2.5 seconds. What is the current passing through the light bulb?

2. How much total energy does a 12 V battery supply to 6 C of charge?

Where Current Comes From

Where Current Comes From

If the wire is connected to a battery, electrons still bounce randomly, but…

Also slightly in the direction of the positive terminal of the battery.

This happens very slowly (a few mm per second) and is called drift velocity.

see p347

Where Current Comes From

If electrons move so slowly, why doesn’t it take a long time for lights to turn on?

There are already electrons in the wires. The electrons from the battery just push those along.

This happens immediately, so the light turns on right away.

Resistance

Resistance

What does resistance mean?

Resistance is the measure of how strongly an object inhibits the flow of electric current.

High resistance = low current, few electrons flow

Low resistance = high current, many electrons flow.

Resistance of Certain Materials

Just like with heat energy, some materials are better at transferring electrical charge than others.

Most materials can be classified as: conductors insulators semiconductors

Conductors

Conductors are materials that allow the flow of electrons freely.

Current flows through conductors easily.

Electrons in conductors are free to bounce around.

Examples: metals: copper, gold, silver, etc… liquids: water, salt water

Insulators

Insulators are materials that inhibit the flow of electrons.

Current does not flow through insulators. Excess charge remains on the surface.

Electrons in insulators are tightly bound and cannot move.

Examples: air, wood, glass, rubber mostly the same as thermal insulators

Semiconductors

Semiconductors are between conductors and insulators.

They transfer electrons better than insulators, but not as well as conductors.

Semiconductors can be use to control the flow of electrons. Mostly used in complex electronics like computers, cell phones and televisions.

Examples: silicon, carbon, germanium

Wrap-Up: Did we meet our objectives?

Describe the relationship between electrons and current.

Explain, at the atomic level, the difference between insulators, semi-conductors and conductors.

Identify how voltage and charge are related.

Homework

p 356-357 15.2 Reviewing Concepts #16-21