Electric Energy and Power

Chapter 18
Sections 1 - 6

Energy of Electric Current

• Emf source does work on electrons
• Electrons then do work on circuit components: resistors, bulbs, motors, etc.
• One coulomb of charge moved through potential difference of one volt equals one joule of work done, energy increase also 1J
• W = qV = VIt (since q = It)
• For one electron moved through 1 volt, unit of work/energy is electron volt (eV)
• 1 eV = 1.60 x 10^-19 J

Energy and Resistance

• Work done on resistance by current appears as heat; can be desirable (oven, iron, heater) or not (motor, light, computer)
• Since resistance always present in normal circuits, some energy lost due to heat
• Joule's Law: Q = I²Rt
• Use to calculate heat produced by resistance and current over a time period

Power in Electric Circuits

• Since power is work/time, P = VI
• For a resistive element, P = I²R power dissipated in a resistance
• If current is not known, P = V²/R
• For total power in circuit, use E of emf source for V and R_T of circuit for R
• For maximum power transfer, R_L = r_source

Power Companies

• Energy sold in kilowatt-hours, a unit of energy (power x time)
• 1 kW-hr means device used 1000 watts of power for one hour
• To minimize power loss in transmission lines, high voltages and fairly low currents used

Home Electrical Circuits

• Circuits in homes are in parallel; devices are connected in parallel
• When many resistances connected in parallel, total resistance is low, current high
• Too much current through wires causes excessive heating, fire hazard
• Circuits protected from high currents by circuit breakers or fuses