Notes: Batteries, circuits

Battery
  • Cell battery - converts chemical potential energy to electrical energy
  • Batteries maintain a constant potential difference between two points
  • Batteries do NOT have charge separation, like a capacitor
Circuit
  • When charge does not flow along a conductor, its potential is the same everywhere.
  • When a conductor has a potential difference across it, charge will flow through it.
  • If charge has a path onto and off of the conductor, no charge will accumulate.
  • ↑ if potential difference is steady, current will be steady.
  • Remember, potential in the conductor gradually decreases in the direction the current flows.
  • Why? The charges lose POTENTIAL energy as they travel along the conductor.
  • NOT kinetic energy - charges at each point in the conductor have the same speed, or else they would accumulate.
Analogy
  • Imagine the mobile charges in a circuit are like water in pipes.
  • Conductors in a circuit are like the pipes.
  • The battery is like a pump, which supplies a steady amount of energy to water
  • Water flowing through a pipe will lose energy due to drag. This is like resistance in a conductor.

Worksheet: Circuit comparisons

1: compare currents
  • Thin wire → low A
  • R = ρ L / A → high R
  • I = ΔV / R → low I
2: compare ΔV
  • Battery supplies fixed ΔV → same for both.
3: compare I
  • Same for all three, because otherwise charge would accumulate.
4: compare ΔV
  • a = c < b
  • ΔV = I R, I is the same for all three, so only difference is R.

Notes: Power consumption

P
  • P can be the power supplied by a battery, or the power consumed by a device
  • The total power consumed by each device, added together, should equal the power supplied by the battery.
  • Power consumed by a resistor is not recoverable (heat).
  • Power consumed by a capacitor is recoverable.

Example: Light bulb

Solution
  • P = I ΔV = 24W
  • Bulb heats up AND shines light.