Download Sc9 - D 2.2 (teacher notes)

Learning outcomes:
1 I can calculate resistance using Ohm’s law.
2.2 Modeling and Measuring Electricity

The flow of a Rocky Mountain River is used
as a model to demonstrate electricity.

Energy source: energy must come from
somewhere and so must water.
 Energy comes from a battery or
other, and water comes from
glaciers.

Resistance and current:
 Resistance in electricity is when the electrons encounter friction.
 In the river resistance is when the water smashes into rocks in
rapids.

Current:


The more electrons used, the more powerful the current in a
conductor.
The more water in the river, the more powerful the current in the
river.

Unless there is a change in elevation, water doesn’t
flow…it sits in a pool.
 It will only flow if it gains potential
energy. (going down a hill)

In electricity, electrons will not flow if there is no
“pump” for the electrons.
 It will only flow if there is an electric
potential. (voltage)

We can use water in pipes, to demonstrate resistance and current.

The size of the pipe determines the volume of water allowed through it.


Thin and long pipe = lots of resistance, restricted flow.
Large and short pipe = no resistance, lots of flow.
o The amount of resistance, in a circuit, determines the size of the
current.
Need a computer - Activity #1: http://phet.colorado.edu/en/simulation/ohms-law
Activity #2 : http://phet.colorado.edu/en/simulation/resistance-in-a-wire
Ohms law proves a Mathematical link between VOLTAGE (V), CURRENT (I), and
RESISTANCE (R).

The unit of resistance was named after Georg Simon Ohm, the Ohm.
Graph analysis
1. What is the subject?
2. What conclusion can be made with this graph?
3. What are the controlled variables? Manipulated? Responding?
Ohm's Law states that as long as temperature stays the same:
the resistance of a conductor stays constant, and
the current is directly proportional to the voltage applied
Using Ohms law:
Quantity
Symbol
Ohm’s Law
Unit
Voltage
Current
Resistance
V
I
R
Volts (V)
Amperes (A)
Ohms (Ω)
Calculated with
Ohm’s law
V=I *R
I=V/R
R=V/I
Measured with
Voltmeter
Ammeter
ohmeter
Solving a Problem…
An electric stove is connected to a 240 – V outlet. If the current flowing
through the stove is 20 A, what is the resistance of the heating element?
Steps to solving the Problem
Information and Solution
1. Identify know quantities.
Current (I) = 20 A, voltage (V) = 240 V
2. Identify the unknown quantity.
Resistance (R)
3. Use the correct formula.
R=V/I
4. Solve the problem.
R=V/I
R = 12 Ω
=
240V / 20 A
Practice Questions…
1. A 30-V battery creates a current through a 15 Ω resistor. How much current is
created?
2. A motor has an internal resistance of 40 Ω. The motor is in a circuit with a current
of 4.0 A. What is the voltage?
3. A current of 625 mA runs through a bulb that is connected to 120 V. What is the
resistance of the bulb?
4. A bulb of 15 Ω resistance is in a circuit powered by a 3 V battery. What is the
current in this circuit?
5. A bulb of 40 Ω resistance is in a circuit powered by a 3 V battery. What is the
current in this circuit?
6. An electric heater draws 10 A from a 120 – V source. What is the heaters
resistance?
7. A current of 1.5 A flows through a 30 Ω resistor that is connected across a
battery. What is the battery’s voltage?