Physics B

1
Physics B

• Chapter 17
• Sections 1 and 4

2
History of the Battery

• Luigi Galvani played with frog legs on garden
  trellis and noticed them twitch
• - said electricity was coming from frog itself

3
History of the Battery

• Alessandro Volta electricity came from the 2
  metals brought in contact and the frog was a
  meter
• - built the 1st voltaic pile (stack of small
  disks of zinc, brine-soaked cardboard, and
  silver)
• - built the 1st voltaic wet cell (glass of
  dilute acid into which a metal strip of zinc and
  copper was immersed and connected in series)

4
Battery

• Two or more cells electrically attached to one
  another
• How does it work?
• chemical energy stored in the inter-atomic
  bonds is converted into electrical PE as the
  solution and one or both of the conducting plates
  become involved in the chemical reaction

5
Voltage Sources

• Dry cells and wet cells (battery)
• - both release energy during a chemical
  reaction that is converted to electrical energy
• Generator
• - converts mechanical energy to electrical
  energy

6
Electromotive Force

• Also called emf
• Not a force
• Maximum potential difference a device can provide
• ?
• ex car battery has ? 12 V and provides 12
  J of energy per Coulomb of charge

7
Internal Resistance

• Once a voltage source is put into use it doesnt
  deliver its rated ? due to internal resistance
• In a battery internal resistance is due to
  resistance encountered by current due to
  chemicals with the battery
• In a generator the internal resistance is
  resistance of wires and components

8
Terminal Voltage

• Actual voltage delivered by a source
• Vab ? Ir where Vab terminal voltage
• ? emf
• I current
• r internal
  resistance

9
Sample Problem

• A car battery has an emf of 12 V and an internal
  resistance of 0.01 ohms. This resistance is
  relatively large because the battery is old and
  the terminals are corroded. What is the terminal
  voltage when the current drawn from the battery
  is a) 10 A and b) 100 A?

10
Sample Problem

• A battery has an emf of 12 V and an internal
  resistance of 0.15 ohms. A) What is the
  terminal voltage when the battery is connected to
  a 1.5 ohm resistor? B) What is the power
  dissipated in the 1.5 ohm resistor?

11
Sample Problem

• A 2 ohm resistor is connected across a 6 V
  battery. The voltage between the terminals of
  the battery is observed to be only 4.9 V. Find
  the internal resistance of the battery.

12
Electric Power Consumption

• Rate at which electric potential energy is used
• power energy used/time
• P U/t
• or since U qV P IV V2/R I2R
• P Watts, W J/s

13
Sample Problem

• The heating element in the rear window
  defroster of a Mazda RX-7 has a resistance of 3
  ohms. The element is connected directly across
  the cars 12 V battery. How much heat is
  produced in the element in 10 min?

14
Kilowatt - hour

• Continuous use for 1 hour at the rate of 1000 J/s
• Equal to 3.6 x 106 J

15
Sample Problem

• What would you be billed for running a 1600 W
  hairdryer for 15 min a day for 30 days if the
  utility company charges 0.08 kwh?