SLIDE 1
Electric Power and EMF
Electrical Power Electromotive Force (emf) Real Batteries Homework
SLIDE 2 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-
- ther, 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
☞ ✟ ☛✎✍✑✏ ✟ ✠ ✁ ✂ ✡ ✍ ✏
SLIDE 3
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?
SLIDE 4
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?
✭ ✟ ☞ ✝ ✟ ✙ ✛ ✢✎✜✮✕✗✕ ✤ ✥✦✕✬✫ ★ ✩ ✙ ✛ ✥✗✜✯✥✦✕ ✤ ✥✦✕✞✰ ✱✳✲ ★ ✩ ✟ ✢✗✢✎✜✮✕✎✴
SLIDE 5
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
SLIDE 6
Real Batteries
Real batteries have internal resistance ✶ The terminal potential of the battery in the circuit
shown below is
✁ ✂ ✟ ✂✸✷ ✹ ✂✻✺ ✟ ✵ ✹ ☛ ✶ If ☛ = 0, then ✁ ✂
=
✵
SLIDE 7
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
☛ ✟ ✵ ✏ ✽ ✶
SLIDE 8
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
☛ ✵ ✟ ☛✎✍✦✏ ✽ ☛✎✍ ✶
SLIDE 9 Example 2 A battery having an emf of 7.00 V produces a current
- f 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.
SLIDE 10 Example 2 Solution A battery having an emf of 7.00 V produces a current
- f 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.
☞ ✟ ☛✎✍✑✏ ✏ ✟ ☞ ☛ ✍ ✟ ✭ ✾ ✁ ✝ ☛ ✍ ✟ ✥✑✢✖✕✎✴ ✾❀✿ ✕✼✜✮✕❁✲ ✠❂✕✼✜✣✢✖✕✗✕✗✘ ✡ ✍ ✟ ✥✦✕✼✜✮✕✬❃ ✵ ✟ ☛✼✏ ✽ ☛ ✶ ✶ ✟ ✵ ✹ ☛✼✏ ☛ ✟ ❄ ✜✮✕✗✕ ✂ ✹ ✠❂✕✼✜✣✢✖✕✗✕✗✘ ✡ ✠❅✥✦✕✼✜✪✕✬❃ ✡ ✕✼✜❆✢✖✕✗✕✗✘ ✟ ❇❈✜✪✕✗✕✬❃
SLIDE 11
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
