Transcript Slide 1
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 C + B A Current I 1.5 volts - D Ground 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 Current I + - 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 C + B A Current I 1.5 volts - D Ground 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) = IV The Power consumed by a Load Power = (Current thru load)(voltage drop across load) = IV