# Current And Resistance

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Current And Resistance
Chapter 17

Electric Current Current exists whenever charges of like signs move
17.1

Current is the rate of flow of charge

The unit for current is the Ampere (A).
1 Ampere = 1 Coulomb/Second

Conventional (positive) current flow is used.
Charge carriers may be positive or negative 75

Drift Speed The speed of the charge carriers is an average speed called the drift speed (nd). n is the number of charge carriers per unit volume 17.2

Current Produces Heat Internal collisions between charge carriers produce heat. 17.4A

Current Flow Analogies
Water flow in a pipe Marbles in a long pipe Links in a bicycle chain 20-21

Resistance Resistance (R) is opposition to the flow of current
Measured in units called ohms Symbol (W) Occurs because of collisions between electrons

Ohm’s Law

Current And Voltage Measurements
Multimeters can measure voltage,current and resistance in an electrical circuit.

It is very important to choose the correct range and to connect the multimeter leads properly.

Ohmic and Non-Ohmic Resistance
Ohm’s Law Ohmic resistance is constant over a wide range of voltages. Non ohmic resistance varies with voltage Semiconductor diodes One way valves for electricity 17.5, 226, 175

Ohmic Resistance

Non-Ohmic Resistance

Resistors A resistor is a device which has a specific amount of resistance Circuit diagram symbol

Examples of Resistors

Resistor Color Code

Resistivity Resistivity (r) is a constant of proportionality which affects the resistance of a specific material. Units: (W.m) Values are in Table 17.1 (Pg. 577)

Resistance and Resistivity

Applications Involving Resistance
The carbon microphone

The carbon microphone Sound waves cause a change in resistance.
Frequency response is very poor. (20 Hz to 4,000Hz) The normal human hearing range is between 20 Hz and 20,000 Hz.

Temperature Variation Of Resistance
Usually, the resistivity of a metal increases with an increase in temperature.

Temperature Variation Of Resistance
Temperature coefficient of resistivity (a) Values are in Table 17.1

Superconductors The resistance of some metals and compounds falls to virtually zero below the critical temperature. (Tc) Values are in Table 17.2 (Pg. 584)

Currents in a superconductor, could flow forever without an applied voltage.

The Meissner Effect

The goal is to produce high temperature superconductors
Mercury-oxide-based The highest temperature so far is 150 K (-123 oC)

What is the ultimate goal?
Superconductivity at room temperature!

Applications of Superconductors
Microwave devices Superconducting power lines Mag-Lev trains Motors

Cells and Batteries A dry cell transforms chemical energy into electrical energy Some of this energy becomes thermal energy 178, 179

Resistive Circuits A simple resistive circuit
Assume no loss of energy in the wires. Joule heating occurs in the resistor. 17.11

Electrical Power Electrical power is measured in Watts (W)

Electrical Energy Electrical energy is measured in kilowatt hours (kWh) Consumers Energy bill? 1 kWh = 3.60 x 106 J

A Kilowatt-Hour Meter

Electrical Activity In The Heart
Muscle action is initiated by electrical activity Voltage pulses cause the heart to beat These electrical waves travel through the body Monitored by electrocardiographs EKG (electrocardiogram) 177

If the atrial and ventricular contractions become irregular, we have fibrillation.
Defibrillators can be used to restore the normal heart rhythm 17.13

Cardiac Pacemakers Cardiac pacemakers may be implanted under the skin and attached with a wire to the right ventricle. A lithium battery is used to charge a capacitor. It discharges 60 times per minute to stimulate the heart beat. If the heart is beating normally, the capacitor is not allowed to charge.

Pacemaker Location

An Actual Pacemaker

Cardioverter Defibrillators
Implanted Cardioverter Defibrillators (ICDs) can monitor, record, process information, and send corrective signals to the heart. The lithium batteries last 4 – 5 years. Table 17.3 (Defibrillator Properties)

Electrical Activity In The Brain
The brain produces electrical waves Monitored by electroencephalographs EEG

An EEG

Questions 1, 3 - 5, 7, 8, Pg. 587