Current, Resistance & Power

March 11th, 2009

Current (I) The rate at which electrical charges move

through a given area Exists whenever there is a net movement of

charge through a medium

I=q/tUnit: Ampere (A) 1A= 1C/s

The current in a light bulb is 0.835 A. How long does it take for a total of 1.67 C of charge to pass a point in the wire?

Power Power = Work done / time = E / t

Unit: Watt/Wattage 1 W = 1J/s

Example: a 60 watt light bulb consumes 60 J of energy each second

**Power companies do not supply electrons!! They provide a potential difference which causes the electrons to gain energy** Remember, moving a 1 C charge (6.24 x 1018 e) across a

potential difference of 12 V gives that charge 12 J of energy

The current and power equations can be combined to form a new equation P=V * I

Determine the ...... current in a 60-watt bulb plugged into a 120-volt outlet.... current in a 120-watt bulb plugged into a 120-volt outlet.... power of a saw that draws 12 amps of current when

plugged into a 120-volt outlet. ... power of a toaster that draws 6 amps of current when

plugged into a 120-volt outlet.... current in a 1000-watt microwave when plugged into a

120-volt outlet.

Sources & Types of Current Batteries and generators supply energy to

charge carriers They maintain a potential difference across their

terminals by converting other forms of energy into electrical energy

Direct Current Charge moves in only one direction

Alternating Current Motion of charges continuously changes (moves

forwards and reverse) Because of this the electrons end up vibrating in place

Resistance The opposition to the flow of current in a

conductor

R = V/IUnit: Ohm ()

Resistance is constant over a wide range of applied potential differences

Resistance depends on length, cross-sectional area, material and temperature (see overhead)

Electrons do not move in straight line paths

Resistance in a materialmeans the electron is not ableto move as quickly

Think about it like water in a pipe:Longer wire (and pipes) means that the is a greater chance for collisions with other particles

Wider wire (and pipes) allow the particles to move more freely

Practice! The resistance of a steam iron is 19.0

. What is the current in the iron when it is connected across a potential difference of 120 V?

How to read a resistor

The top resistor

= 1 1 0 00 = 11000 The bottom resistor

= 1 0 00 = 1000

Ohm’s Law Most important law in electricity

V= IR

Can use this to determine things about a circuit As voltage increases the current will increase As resistance increases the current will decrease Current is highest with a high voltage and low

resistance

Quantity Symbol Equation Standard Unit Other UnitsPotential Difference (Voltage)

V V=PE/qV=IR

Volt (V) J/C

Current I I=q/tI= V/R

Amperes (A) C/s

Power P P= PE/t Watt (W) J/s

Resistance R R= V/I Ohm () V/A

Energy PE PE= V*q Joule (J) W*s orV*C

Summary of Terms, Equations and Units

Activity

For each set 1-4 do A and B (8 total Resisters) Use the guide to find out the resistance of

each resistor

For each power supply Find the current, and voltage for INPUT &

OUTPUT Calculate the resistance for INPUT & OUTPUT