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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 = R0[ 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?