19.2 Resistancepp. 700 - 707

Mr. Richter

Agenda

Review HW Chapter 19 Quiz Monday (second half) Notes:

Resistance Ohm’s Law Human Resistance Resistors and Superconductors

Objectives: We Will Be Able To… Understand the relationship between resistance,

current and voltage. Calculate using Ohm’s Law. Know what factors affect resistance. Define resistors and superconductors.

Warm-Up:

If current is like water flowing in a river, what do you think resistance is?

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

Every device that uses electrical energy adds to the total resistance to a circuit.

The more total resistance the circuit has, the less the current.

Resistance

Remember, electron flow is bumpy and indirect.

The resistance of a material largely depends on (p. 701): length cross-sectional area type of material temperature

Ohm’s Law

Ohm’s Law

The resistance (R) of a material is the ratio of the applied voltage to the current that flows through.

This ratio is known as Ohm’s Law:

Resistance is measured in Ohms [Ω]

A.K.A.:

V = IR

Ohm’s Law: Relationships

Voltage and current are directly proportional. With constant resistance

Resistance and current are inversely proportional. With constant voltage.

A.K.A.:

V = IR

Human Resistance

Human Resistance

Why will a 9-volt battery not shock you on your arm, but shock your tongue?

Human skin usually has a resistance of approximately 500,000 Ω.

A 9-volt battery creates a current of about 9/500,000 or ~1.8 x 10-5 A (0.000018 A)

Humans only feel current at about 0.01 A

Human Resistance

The resistance of wet skin can decrease to as little as 1000Ω or even 100 Ω, or about 1000 times less resistance.

Water, and especially salt water, creates a better conductor. Lots of free electrons!

This creates as much as 0.09 amps of current. This is enough for human nerves to

sense. And hurt.

Resistors and Superconductors

Resistors

Resistors can be used to control the flow of current in a circuit.

Current can also be controlled by adjusting voltage, but voltage is usually constant (like a 120-volt outlet)

A resistor is an element that provides a specified resistance.

Superconductors Superconductors have no

resistance below a critical temperature, usually near absolute zero.

Weird stuff: once current is established, it can continue even when the potential difference is removed.

This can create semi-permanent electromagnets, and super-efficient electric circuits.

Wrap-Up: Did we meet our objectives?

Understand the relationship between resistance, current and voltage.

Calculate using Ohm’s Law. Know what factors affect resistance. Define resistors and superconductors.

Homework

p707 #1-5, 7