Resistance

Definition

• Opposition to the flow of electrons • When electrons pass through materials with high resistance, they collide with its molecules more often and in doing so transfer some of their energy into heat • Examples: heating coils light bulb filaments fuses – metal that will heat up and melt and break if too much current passes through it

Relationships

• Resistance is measured by comparing the potential difference with the current • Plot of V vs I, V = slope(I) , slope = V/I

Ohm’s Law

• For any given conductor, the V/I ratio is constant • Ohms law – the potential difference between any two points in a conductor varies directly with the current between them • The constant slope is call the resistance ( R) R = V/I • Units V/amp = 1 ohm (Ω) • Some non-metals are conductors that don’t obey ohm’s law (called non-ohmic)

Examples

• Find the resistance of an electric light if there is a current of .80A and the potential drops 120V as it goes through the bulb.

• What is the potential difference across a toaster which has a resistance of 13.7 Ω when the current is 8.75A?

• What is the current through a heater with a resistance of 38 Ω and a potential difference of 240V across it?

• A small flashlight bulb draws 300mA from a 1.5V battery. A) what is the bulb’s resistance? B) if the voltage drops to 1.2V, how would the current change?

Resistors

• Devices used to control the amount of current in a circuit • Symbol of resistor on diagrams • Two types Wire round resistors – coil of thin wire Composition resistors – made of some material that offers resistance (semi-conductor) • Resistors are rated by color code banding

Resistivity

• Thicker wires have more area for electrons to pass through – less resistance • Longer wires have more obstacle for electrons – higher resistance • Resistance ( R) of material directly proportional to L and inversely proportional to A R = ρ (L/A) • Resistivity – ρ – constant that depends on the type of material

Example

• Suppose you want to connect your stereo to some speakers. A) If each wire must by 20m long, what diameter of copper wire should you use to keep the R less than .10 Ω? B) If the current to each speaker is 4.0A, what is the voltage across each wire?

Temperature and Resistance

• Resistivity of a material depends on the temperature • Faster moving atoms interfere with electrons more – higher temp, higher R R = R 0 [ 1 + α  T ] α – temperature coefficient (pg. 508)

Example

• Variation of the resistance of metals with temperature can be used as a way of measuring temperature. A piece of platinum at 20°C is found to have a resistance of 164.2 Ω. When placed into a solution its resistance is measured at 187.4 Ω. What is the solution’s temperature?