Lecture Eleven Physics 100 Fall 2012

Energy and Power

Applying V=iR to circuits1. Calculate the total resistance in the circuit

using: Resistors in Series

R = R1 + R2

Resistors in Parallel

2. Then apply V=iR where V is the voltage of the battery.

2. Then apply

V=iR

1 1 1

1 2R R R

1 1 1

1 2R R R

V=iR

We can apply this relationship to individual resistors OR To the circuit as a whole.

• Once you know the total current in the circuit, you can use what you know about how current divides in a circuit to figure out the current through each resistor.

• Once you know that, you can use V=iR to calculate the voltage across each resistor.

Bulbs Don’t Use Up Current

• Why do batteries go “dead”?

• Chemical Energy stored in the battery gets “used up”.

• Energy is not electrons in motion (current). It is not the strength of the battery’s push.

Energy

• Energy is not matter. You can not touch it.• Energy is the quantity necessary to make

something happen….to do work, make a sound, generate light, heat something up.

• Energy is only indirectly observable. Energy is detected by observing its effect….see a motion, hear a sound, see light, feel heat and so on.

Energy

• Symbol is E• Unit is the Joule• Abbreviation for the unit Joule is J

• So, E= 6 J

Energy

• Energy is conserved.• It can be transferred from one object to another

and from one “form” to another….for example from chemical energy in a battery to light.

• But, the total amount of energy you start with is the amount you end with…Any “missing” energy just went to another object and/or changed form.

Bulbs Don’t Use Up Current

• Why do batteries go “dead”?

• Chemical Energy stored in the battery gets “used up”….transformed into heat and light.

• Energy is not matter. It is not electrons in motion (current). It is not the strength of the battery’s push (voltage).

• We will be wrapping up electricity and circuits and then will study energy in more depth.

Power

Conceptually, power is energy used or provided per unit time.

Therefore, it is the (time) rate of energy transfer into or out of a system.

The standard unit then is J/s which is called a WATT (W)

Amount of energy transfered

how long it took for the transferPower P

Power defined for electric circuit components

Amount of energy transfered

how long it took for the transferPower P General Expression:

Always true

iVP Specific Equation for electric circuit elements

Power defined for electric circuit components

P iV

Power issues associated with electric circuits components are critical for understanding things like: Energy loss in power transmission, Heating in circuit elements, Electric bills.

Power Bill or Energy Bill?

Amount of energy transfered

how long it took for the transferPower P

( ) (how long you use it) Amount of energy transferedPower x

Watt x hour or a kilowatt-hour is a unit of energy which can be converted into a Joule.