AC Generators The output of an ac generator –ac generator symbol –Formula for instantaneous voltage ( v) 21.1/21.2
Resistors In An ac Circuit Current and voltage are in step with each other. (in phase)
The average value of the current over one cycle is zero. Power formula:
rms Current rms current (root mean square) –It is the direct current that would dissipate the same amount of energy in a resistor as an ac current of that value.
Rms Voltage Rms voltage –120 V rms in our homes
Ohm’s Law In An ac Circuit Ohm’s Law is valid for resistors in ac circuits.
Review Capacitors in a dc circuit –Charging As the voltage across the capacitor increases, the current decreases. –RC time constant 21.4
Capacitors In An ac Circuit In a capacitive ac circuit, the current always leads the voltage by 90 o. 21.5
Capacitive Reactance Capacitive reactance (ohms) –Similar to resistance in a dc circuit Decreases with frequency –Opposes a change in ac voltage –Formulas:
Inductive Reactance Inductive reactance (ohms) –Similar to resistance in a dc circuit Increases with frequency –Opposes a change in ac current –Formulas: 21.6/21.7
In an inductive ac circuit, the voltage always leads the current by 90 o. ELI the ICE man 21.7
ELI the ICE man Three important relationships: –The instantaneous voltage across the resistor is in phase with the instantaneous current. –The instantaneous voltage across the inductor leads the instantaneous current by 90 o. –The instantaneous voltage across the capacitor lags the instantaneous current by 90 o.
The RLC Series Circuit Impedance (Z) –The total resistance (ohms) in an ac circuit –Formula: 221
Phasors –Rotating vectors which are used to represent the voltage across each component. Separated by 90 o –Phase angle 222
Transformer Voltage A transformer steps up or steps down ac voltages Primary and secondary coils 218
Transformer Power The power input to the primary equals the power output at the secondary.
Transformer Efficiency Efficiencies are between 90% and 99%.
The Transformer Equation N p /N s is the turns ratio
The Transformer Applications: AC/DC electronic equipment Power distribution Automobile ignition systems 219, 220
Maxwell’s Theory Electric field lines originate on positive charges and terminate on negative charges. Magnetic field lines always form closed loops. A varying magnetic field induces an emf and hence an electric field. Magnetic fields are generated by moving charges (or currents). 21.18
Maxwell’s Hypothesis A changing electric field should produce a magnetic field. This could not be proven experimentally because the electric fields were too weak to detect at the time.
Maxwell’s Prediction Maxwell came up with the concept of electromagnetic waves and predicted that they would travel at the speed of light. 21.20
Maxwell’s Conclusion He concluded that light waves are electromagnetic in nature.
Hertz’s Discovery Hertz was the first to generate and detect electromagnetic waves in the laboratory. 21.17
EM Resonance Hertz achieved resonance and transferred electromagnetic energy between two coils which were several meters apart. Resonant frequency (f o ) formula
Properties EM Radiation Hertz showed that electromagnetic radiation exhibited wave properties. Reflection Refraction Diffraction Interference Polarization 231, 232
The Speed of EM Waves Hertz showed that electromagnetic radiation traveled at the speed of light.
Hertz’s Contribution Hertz’s experiments provided the first proof of Maxwell’s theories.
Production Of EM Waves By An Antenna Energy stored in an LC circuit is constantly transferred between the electric and magnetic fields. Electromagnetic waves are generated if this happens rapidly. Antennas are used to produce electromagnetic waves. 177, 178