Voltage and Current Whenever positive and negative charges are separated, Voltage is the energy per unit charge created by the separation. Where v = the voltage in volts w = the energy in joules q = the charge in coulombs ECE 201 Circuit Theory I

Voltage and Current (continued) The rate of charge flow is electric current Where i = the current in amperes q = the charge in coulombs t = the time in seconds ECE 201 Circuit Theory I

Power and Energy Power is the time rate of expending or absorbing energy. Where p = the power in watts w = the energy in joules t = the time in seconds 1 Watt = 1 Joule/second ECE 201 Circuit Theory I

Power Associated with the flow of charge Using the definitions of Voltage and Current Where p = the power in Watts v = the voltage in volts i = the current in amperes ECE 201 Circuit Theory I

The Basic Ideal Circuit Element The ideal basic circuit element has only two terminals is described mathematically in terms of current and/or voltage Cannot be subdivided into other elements ECE 201 Circuit Theory I

Passive Sign Convention If the power is positive (p>0), power is being delivered to the circuit inside the box. If the power is negative (p<0), power is being extracted from the circuit inside the box. ECE 201 Circuit Theory I

Circuit Theory What is an electric circuit? A mathematical model that approximates the behavior of an actual electrical system. Example: Automobile Headlights Here the headlights are OFF and draw no current from the battery because the switch is open. ECE 201 Circuit Theory I

Automobile Headlights (continued) Turn the headlights ON by closing the switch. The headlights are ON. They draw a current of 4.138 Amperes from the battery. ECE 201 Circuit Theory I

Jump Starting Example The current is measured to be 40 Amperes. Which car has the dead battery? ECE 201 Circuit Theory I

The current is flowing into the + terminal of the battery in car A. p = vi = (12V)(40A) = 480W Since the power is positive, the battery in car A is absorbing power. Car A has the dead battery. ECE 201 Circuit Theory I

Additional Calculation If this connection is maintained for 1.5 minutes, how much energy is transferred to the dead battery? ECE 201 Circuit Theory I