Electric Power and EMF Electrical Power Electromotive Force (emf) - PDF document

Electric Power and EMF � Electrical Power � Electromotive Force (emf) � Real Batteries � Homework

✏ ✂ ☛ ✁ ☞ ✟ ☛ ✁ ☞ ✂ ✟ ✟ ✠ ✁ ✂ ✡ ✍ ✟ ✂ ✁ ✠ ✄ ✟ ✡ ✁ ✂ ✟ ✄ ✂ ✁ Electric Power � The rate at which a system loses electric potential energy as a charge Q moves through a potential dif- ference is ✄✆☎ ✄✞✝ ✄✞✝ ✄✞✝ � Since power is defined as the rate at which work is done or energy is converted from one form to an- other, the electric power is defined as � The unit of power is the watt (W): 1 W = 1 V ✌ A = 1 J/s � For a resistor or some other device with resistance R, we can use Ohm’s law to express the rate of elec- tric energy dissipation due to the resistance as ☛✎✍✑✏

Example 1 An electron machine delivers 200 A at 250 kV for 1.10 ✒ s. (a) What is the power delivered during this time interval? (b) How much energy is delivered in this time interval?

✭ ✙ ✝ ☞ ✟ ✛ ✤ ★ ✤ ✩ ✟ ★✩ ✂ ✤ ✙ ✛ ✤ ★ ✟ ✂ ✁ ☛ ✟ ☞ ✩ ✟ ✟ Example 1 Solution An electron machine delivers 200 A at 250 kV for 1.10 ✒ s. (a) What is the power delivered during this time interval? ✡✚✙✛ ✠✔✓✖✕✗✕✗✘ ✓✎✜✣✢✖✕ ✥✦✕✗✧ ✢✎✜✪✕✗✕ ✥✦✕✬✫ (b) How much energy is delivered in this time interval? ✢✎✜✮✕✗✕ ✥✦✕✬✫ ✥✗✜✯✥✦✕ ✥✦✕✞✰ ✱✳✲ ✢✗✢✎✜✮✕✎✴

Electromotive Force (emf) � Any device that maintains a potential difference in a circuit is called a source of emf. � Can be thought of as a "charge pump". � The emf ✵ of a source is defined as the work done per unit charge, and thus the SI unit of emf is the volt. Figure 21.12

✵ ✹ ✂ ✁ ✶ ☛ ✹ ✵ ✟ ✟ ✂ ✁ ✶ Real Batteries � Real batteries have internal resistance � The terminal potential of the battery in the circuit shown below is ✂✸✷ ✂✻✺ � If ☛ = 0, then =

✶ ✶ ✽ ✏ ✵ ✟ ☛ ☛ ✽ ✟ ✵ Real Batteries (cont’d) � From the figure below, we see that the emf provided by the battery is equal to the voltage drops across the resistances ☛✼✏ and the current is

☛ ✵ ✟ ✽ ✶ Real Batteries (cont’d) � The rate at which energy is delivered by the battery is equal to the rate at which energy is converted to heat in the resistors ☛✎✍✦✏ ☛✎✍

Example 2 A battery having an emf of 7.00 V produces a current of 0.500 A when connected to an external resistance. If 150 J of energy are produced in 1.00 minute in the external resistor, determine the values of the internal and external resistances.

✍ ✡ ✍ ✟ ❄ ✟ ☛ ✹ ✵ ✝ ✟ ✟ ✵ ✟ ✶ ✽ ☛ ✁ ✶ ✂ ✟ ✡ ✡ ✹ ☞ ✟ ✏ ✾ ✟ ☞ ☛ ✍ ✟ ✭ ☛ Example 2 Solution A battery having an emf of 7.00 V produces a current of 0.500 A when connected to an external resistance. If 150 J of energy are produced in 1.00 minute in the external resistor, determine the values of the internal and external resistances. ☛✎✍✑✏ ✾❀✿ ✥✑✢✖✕✎✴ ✕✼✜✮✕❁✲ ✥✦✕✼✜✮✕✬❃ ✠❂✕✼✜✣✢✖✕✗✕✗✘ ☛✼✏ ☛✼✏ ✜✮✕✗✕ ✠❂✕✼✜✣✢✖✕✗✕✗✘ ✠❅✥✦✕✼✜✪✕✬❃ ❇❈✜✪✕✗✕✬❃ ✕✼✜❆✢✖✕✗✕✗✘

Homework Set 18 - Due Friday May14 � Read Sections 21.6 � Answer Questions 21.10 � Do problems 21.15, 21.16, 12.21, 21.23 & 21.24