Transcript Current Electricity

Current Electricity
SNC
Grade 9
A Simple
Circuit
Connecting
wires
Power source
Load/Resistance
Circuit Models
Circuit diagrams
Introducing Some terms
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Current:
Charge:
Coulomb:
Ampere:
Time:
represented by the symbol (I)
represented by the symbol (Q)
represented by the symbol (C)
represented by the symbol (A)
represented by the symbol (s)
Measuring Current
• Current is the flow of electrons
• It is measured with an ammeter
• Current (I) is measured in amps (A)
Current in Household Appliances
Appliance
Current (A)
Radio
0.4
100 W lamp
0.8
Colour TV
1.7
Toaster
8.8
Microwave oven
11.7
Electric kettle
12.5
Electric stove
40
Electric Shock
Electricity can Kill
Use it safely
Caution:
Electrical Hazard
Charge
• Charge is measured in coulombs.
• 1 coulomb = 6.25 X 1018 electrons
• Current = charge moving past a point
time
I=Q
t
Example: GRASS Method
If 240C of charge pass a point in a conductor in 5.0 mins, What is the current
through that point in the conductor?
Given: Q = 240C, t= 5.0 mins
Required: Current (I) in Amperes
Analysis: Use the formula I = Q
t
Convert time from minutes to seconds = 5.0 x 60 = 300s
Solution: I = 240 C
300 s
= 0.80A
Paraphrase: When 240C of charge pass a point in a conductor every 5.0
minutes, the current in the conductor is 0.80A
Batteries
• Are portable power
sources
• Can be wet cells or
dry cells
• Come in a variety of
shapes & sizes
• Have different
voltages
How a battery works
Changing energy from one form to
another
• A battery changes chemical energy into
electrical potential energy.
• This energy is used to move electrons
through a circuit.
• A battery works like a pump
The battery “pumps” the electrons around the
circuit.
Measuring the “pump”
• The pressure that the battery pumps the
electrons around the circuit is called Voltage.
(volts) and is measured with a voltmeter
connected in parallel to the load or the battery.
Types of Circuits
• Series circuit. – the electricity has only ONE
path.
• Parallel Circuit – the electricity has a choice of
paths.
Electricity is like grade 9 science
students. It always takes the
easiest path to get to the goal
SERIES CIRCUIT CALCULATIONS
If, for example, two or more lamps
(resistances R1 and R2, etc.) are
connected in a circuit as follows, there is
only one route that the current can take.
This type of connection is called a series
connection. The value of current I is
always the same at any point in a series
circuit.
The combined resistance RO in this
circuit is equal to the sum of individual
resistance R1 and R2. In other words:
The total resistance (RO) is equal to
the sum of all resistances (R1 + R2 +
R3 + .......)
Symbol
Component
Ammeter
Unit of
measurement
Amperes
Cell
Volts
Resistor
Ohms Ω
Switch
Voltmeter
X
Volts
Connector
Circuit Symbols
X
V
Make the following
• A series circuit with 3 bulbs, a switch and
power source.
• Draw the circuit diagram
• What do you notice about the brightness
of the bulbs?
• Unscrew one bulb what happens?
AND..
• A parallel circuit with 3 bulbs, 3 switches –
one for each bulb, and power source.
• What do you notice about the brightness
of the bulbs?
• Unscrew one bulb, what happens?
Electricity terms and formulae
Term
Symbol
Unit
Formula
Definition
Flow of electrons
past a given point in
a given time
Current
I
Ampere
(amps)
I=Q
t
Potential
Difference
V
Volts
V=
Resistance
Ω
Ohms
R=V
I
Power
W
Watts
P=E
t
Charge
Q
Coulombs
Q =I X t
Efficiency
%
efficiency=
Ohm’s Law
• Resistance = Potential Difference (V)
Current (A)
• R= V
I
• symbol for resistance is Ω
• If the potential difference of 1 volt causes
a current of 1amp to flow then the
resistance is 1Ω.
• Ohm's Law deals with the relationship
between voltage and current in an ideal
conductor. This relationship states:
• The potential difference (voltage)
across an ideal conductor is
proportional to the current through it.
Resistance can
be determined
by using:
Slope = Rise
Run
This graph shows a linear relationship
between voltage and current.
This resistor would be said to follow Ohm’s
law.
You practice..
• Use the data provided to create a graph of
voltage vs. Current, with current plotted on
the x-axis.
Potential Difference (V)
(V)
Electric Current (I)
(1mA = 1 X 10-3 A)
0.0
0.00
0.50
0.720
1.0
1.25
1.5
1.67
3.0
2.50
4.5
3.0
6.0
3.33
• Describe the shape of the graph
• Is the resistor shown on the graph an
ohmic or a non-ohmic resistor? Explain.