Download Resisting the Movement of Charge

Resisting the Movement of Charge
Recall:
Electric current is how fast the charges are
moving.
Voltage is the pressure that gets the charges
(electrons) moving.
Resistance (R) is the property of a substance that
hinders motion of electric charge and converts
electric charge into other forms of energy (such
as heat or sound). Resistance is slowing down
(sometimes even stopping) the charges.
The unit for resistance is the ohm, and it is
measure with an ohmmeter.
Read page 279.
A good conductor of electricity (such as copper
wire) has a low resistance, so electric charges flow
rapidly.
Poor conductors of electricity oppose the
movement of charge, and have a high resistance.
Therefore the electric current slows down.
Demonstration on page 280.
Answer questions #1-3 in “What did you find out”.
The relationship among current,
resistance, and voltage.
Pretend an electric circuit is a race track. Each
car represents a charge. Each car has a certain
amount of gasoline that provides its energy. The
gasoline that each car has would represent the
voltage. The number of cars that pass by the
starting line every second would represent the
current. As cars (charges) go up hills (resistors),
each one of them uses up gasoline (voltage). Each
car needs to get more gasoline (voltage) at the pit
stop (battery).
Ohms Law – Calculating Resistance
Read page 281-282
Ohms Law provides for the relationship between
voltage across a resistor (the energy that each
charge loses across the resistor), the current
through the resistor (the amount of charges that
are flowing through the resistor each second), and
the resistance of the resistor.
R = V/I
Where R = resistance (ohm)
V = voltage (V)
I = current (A)
Show triangle.
Question:
What is the resistance of an electric heater, if a
current of 12.5 A runs through it when it is
connected to a 120V wall outlet?
 page 282 #1-4
 BLM 4-6
Resistors (read page 283)
- it is something that resists the flow of
electric charge.
- Control current or voltage to suit the specific
needs of other electric devices in the circuit.
It slows down the current or voltage.
One type of resistor is a variable resistor. These
don’t have a fixed resistance; (ex. Rheostat).
Variable resistors are used in light dimmers, fridge
and stove temperature controls.
Types of Circuits
Series circuits
- have only one current path.
- All moving charges travel through each
component in the circuit
Parallel circuits
- have several current paths
- the total current is divided, with some of the
moving charges traveling through each branch,
or part of the circuit.
- If one path is broken there is still another
path for current to come through
- Example: your home- lamps, tv, stove,
computer, all operate from the same source of
electric energy.
Classroom analogy of parallel and series circuits
Lab on page 287.
Advantages of a series circuit
- easy to construct
- uses less wire
- allows batteries to last longer
Disadvantages of a series circuit
- existing bulbs get dimmer as more bulbs are
added
- entire circuit stops working if any one bulb
burns out
Advantages of a parallel circuit
- if one bulb burns out, the other bulbs will work
- the bulbs don’t burn less brightly when
additional bulbs are added
Disadvantages of a parallel circuit
- requires more wire
- batteries don’t last as long due to increased
current levels
BLM 4-9
House wiring
Read page 289.
Factors affecting the resistance of a wire
1) length – resistance increases with increasing
length
2)cross sectional area (thickness) – increase
area = decrease resistance (fig 4.22)
3)temperature – increase temp of wire =
increased resistance
4)material
BLM 4-10
Page 291 #1-8
Page 292 #1-17