How do Flashlights Work ?

Observations About Flashlights Turned on/off with a switch The more batteries, the brighter the filament The orientation of multiple batteries matter Flashlights get dimmer as batteries age Main Parts of a Flashlight 1.Battery – the ‘energy source’ 2. Wire – path for electrical charges to flow thru (current) 3. Light Bulb – the ‘load’ of your circuit 4.Switch – opens or closes the ‘circuit’ thru which current flows.

The Battery + - Battery ‘pumps’ charge from – end to + - Chemical potential energy is consumed - Produce eletrostatic potential energy Current undergoes a rise in voltage alkaline cells: 1.5 volts; lead-acid: 2.0 volts; lithium: 3.0 volts Sequence of Batteries produce larger voltage rise

The Light Bulb Contains filament which poorly conducts current (high Resistance to produce lots of heat) Consumes electrostatic potential energy and transforms it to heat/thermal energy. Across the Bulb, the current undergoes a ‘drop in voltage’ i.e. energy is lost to thermal energy. For a 2-cell alkaline flashlight, there is a 3.0 volt drop.

The Closed Loop of Current + - Switch Current I Ground 1.5 volts A B C D A : The voltage of the charges is 1.5 volts, relative to ‘Ground’ B : Voltage is still 1.5 volts (assuming perfectly conducting wires) C : Voltage is zero D: Voltage is zero Battery resupplies energy to charges when they cross the battery

Observations: Steady current requires a circuit path or loop, i.e. Charges cannot accumulate anywhere. Steady current flow requires energy replenishment Where do charges lose this energy ? - thru heat in conductors ! Circuit transports energy from battery to the load: a light bulb

Why does a battery die and the bulb dims ? In the Forward Current Flow (Discharging) battery pumps charge from – to + end battery’s chemical potential energy is consumed In the Reverse Current Flow (Charging) Circuit pushes charges from + to – battery’s chemical potential energy is replenished works for Ni-MH (nickel metal hidride batteries) Current I

What Really Flows in a Circuit ? By Convention, current points in the direction of positive flow. But….what really flows are Negatively-charged Electrons. Negative Charges flowing to the right = Positive Charges flowing to the left - + We stick to the convention but remind ourselves of the real flow of electrons.

What’s a Short Circuit ? + - Switch Current I Ground 1.5 volts A B C D What happens if the circuit bypasses the load ? A ‘short’ occurs. Energy Loss and heating occurs in the wires! Main ingredient for a fire !

The Power delivered by a Battery Power = energy delivered per unit time, units of Joules/sec or Watts Power = (Current supplied by Battery)(Voltage rise across battery) = I  V The Power consumed by a Load Power = (Current thru load)(voltage drop across load) = I  V