Presentation on theme: "Current And Resistance Chapter 17. Electric Current Current exists whenever charges of like signs move 17.1."— Presentation transcript:
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 ( d ). 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 ( ) –Occurs because of collisions between electrons
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
Resistors A resistor is a device which has a specific amount of resistance –Circuit diagram symbol
Examples of Resistors
Resistor Color Code
Resistivity Resistivity is a constant of proportionality which affects the resistance of a specific material. –Units: ( m) Values are in Table 17.1 (Pg. 577)
Resistance and Resistivity
Applications Involving Resistance 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 ( ) –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 o C)
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
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 10 6 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
A Bad EKG
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.
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