Transcript SNC1D1 Resistance

10.10 Electrical Resistance
&
Ohm’s Law
Resistance
• The degree to which a substance opposes the
flow of electric current through it.
• All substances resist electron flow to some
extent.
Resistance
• Conductors, such as metals,
allow electrons to flow freely
through them and have low
resistance values.
• Insulators resist electron
flow greatly and have high
resistance values.
Resistance
• Resistance is measured in ohms
(Ω) using an ohmmeter.
• An ohmmeter is a device for
measuring resistance.
Resistance
• When a substance resists the flow of electrons, it
slows down the current and converts the
electrical energy into other forms of energy.
Resistance in a Circuit
• A resistor is any material that
can slow current flow.
• In a light bulb, the filament’s high
resistance to the electron’s
electrical energy causes it to heat
up and produce light.
Resistors and Potential Energy
• Resistors can be used to control electric current
or electric potential in a circuit.
• In a circuit, electrons have a higher potential energy
as they enter a resistor compared to when they leave
the resistor because they use up some energy in
passing through the resistor.
Analogy
• Imagine electrons entering a
resistor as being at the high end
of a ramp, where they have a lot
of potential energy. In this
analogy, electrons leaving the
resistor are at the bottom end of
the ramp, where their potential
energy has been converted to
another form of energy.
Resistance in a wire – water analogy
• Longer thinner pipes have more
resistance to the flow of water than
pipes with a larger diameter.
The same idea applies to electricity.
• The more resistance that you have in a circuit, the
more it will decrease current at a given voltage.
• Larger, shorter wires provide less resistance for
electrons to travel.
• Temperature and material can also affect resistance.
Ohm`s Law
• Ohm’s law established the relationship between
voltage (V) and current (I).
• Ohm`s law refers to the amount of resistance in a
circuit.
• The symbol for resistance is called the ohm (Ω) in
honour of Georg Ohm’s work in this field.
Ohm`s Law
• According to Ohm`s law, the amount of voltage (or
energy) in a circuit is equal to the current multiplied
by the resistance.
• Ohm’s law states that, as long as temperature stays
the same, V = IR
• if the resistance of a conductor remains constant,
then the current is directly proportional to the
voltage.
Ohm`s Law and Temperature
• Ohm’s law works for most circuits.
However, temperature affects resistance.
Generally, resistance is lower when a conductor is
cooler. As the temperature increases, resistance
increases.
• For example, a filament in an incandescent light
bulb often has 10 times more resistance when the
bulb is warm.
Calculating Resistance
• Resistance = voltage
current
R=V
I
… and the formula can be rearranged
Voltage
V=IxR
Current
I=V
R
V
I
R
Calculating Resistance
Quantity
Symbol
Units of
measurement
Resistance
R
Ω (ohm)
Current
I
A (Ampere)
Voltage
V
V (Volt)
Sample Problem #1
• A current of 4.0 A flows through a 40.0 Ω
resistor in a circuit. What is the voltage?
Use GRASP and sig. digits.
Sample Problem #2
• A 30 V battery generates a current through
a 15 Ω resistor. How much current does the
battery generate? Use GRASP.
Sample Problem #3
• An electric stove is connected to a 240-V
outlet. If the current flowing through the
stove is 22 A, what is the resistance of the
heating element? Use GRASP and sig figs.
Homework
• Read pg. 316 – 319
• Answer #1 – 4, 6, 8 on pg. 319