Current Electricity (Chapter 12.1, 13.3)
Electric Charge:
• Since one electron creates a very ______ negative charge,
it was necessary to think of a bunch of electrons to work
with the charges we use in life.
• The common unit of charge is known as the ___________
• Unit Symbol __
• Quantity Symbol ___
• 1 C = ______________ electrons
• 1 e = 1.60 x 10-19 C
Electric Current:
• The movement or flow of electric charge from one place to
another. It can be measured much like the flow of water
can be measured. It is the ________ of charges.
• Unit Name ___________ or _______
• Unit Symbol __
• Quantity Symbol is ___ in equations
• 1 A = 1 C/s
• Example: electrons flowing through a wire
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Sample Problems (Use the GRASS method)
1. If 500C of charge passes a point in a conductor in 2.5 min,
what is the current through that point in the conductor?
2.
a. How much charge passes through a starting motor, if
it takes 7.1 s to start a car and there is a current of
230 A during that time?
b. BONUS: Approximately how many electrons passed
through the motor in that time? (
).
Use scientific notation.
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Electric Circuits: (Chapter 12.2)
• The ___________ _______ in which electric current will
flow.
• Example: current flowing from the power source, through a
light bulb, and back to the power source
Components of a Simple Circuit:
1. Source of electrical energy (ie: a battery)
2. Electrical load :name given to anything that converts
electrical energy to another form of energy (ie:
toaster converts electrical energy to heat)
3. Electric circuit control device (ie: a switch, a timer)
4. Connectors: wires that connect the components of a
circuit together
Schematic Diagram of a simple circuit:
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Current Electricity Circuit Symbols: (13.1)
Circuit: controlled path in which electrons flow
Conductor (ie: copper wire)
Cell
Battery (connected cells)
Open Switch (electrons cannot flow)
Closed Switch (electrons can flow)
Light bulb (electrons flow through – light is
produced)
Motor (electrons flow through – rotary motion
occurs)
Voltmeter (measures how much energy the
flowing electrons have – measured in volts)
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Ammeter (measures how fast the electrons are
moving)
Fuse (made of thinner wire than the rest of the
circuit – melts if the circuit gets too hot)
Resistor (resists the flow of electrons through
the wire)
Conducting wire
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Electric Potential (13.5)
Electric Potential:
• the amount of energy stored in an electron
• SI Unit: Volt
• Symbol: V
(Read the falling Water Analogy on Page 302-303)
Voltmeter: Used to measure the electric potential
• A voltmeter is connected into the circuit, by connecting
the positive terminal of the power source to the positive
terminal on the voltmeter, and the negative terminal of the
power source to the negative terminal of the voltmeter.
Diagram of a circuit with a voltmeter:
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Electrochemical Cells: (p.512)
• Cells in which a chemical reaction generates electricity.
• Divided into primary and secondary cells
Primary Cell:
• Disposable energy source.
• As electrons flow through the cell, the chemical reaction
consumes the materials in the cell.
• Two kinds: primary wet and primary dry cells
Primary Wet Cell:
• Also called a voltaic cell
• 2 pieces of metal, usually zinc and copper, are placed in a
liquid
• The metals act as the electrodes, and the liquid is the
electrolyte.
• The zinc acts as the negative electrode, by reacting with
the electrolyte and accumulating a negative charge
• The copper acts as the positive electrode, by accumulating
a positive charge.
• When the circuit is closed, the electrons will flow from the
zinc plate through the circuit and back to the copper plate.
Primary Dry Cell:
• This acts in the same way as a wet cell, but the electrolyte
is a moist paste instead of a liquid
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Secondary Cell:
• These cells can be recharged: 2 chemical processes are
involved: one to discharge the cell, and the other to
recharge it.
Diagram of A Voltaic Cell:
Some possible Electrolytes: dill pickle
Lemon
Orange
Potato
Clove of garlic
Salt water
Vinegar
Red wine
Etc.
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Batteries in Series and Parallel (p.552)
Hypothesis:
We can measure the voltage of a single cell, two batteries in
series, and two batteries in parallel.
Materials:
Voltmeter
Two 1.5 V cells
Switch
4 connecting wires
Procedure:
1. Wire the apparatus as shown below. Make sure you leave
the switch open
2. Estimate the voltage of the cell. Record this hypothesis in
your observation table. Call the teacher over to verify your
connections.
3. Close the switch and record the voltage reading
4. Wire two batteries in series with each other and with the
switch and voltmeter. --- Wiring them in series means to
connect them from negative to positive. Leave the switch
open
5. Hypothesize as to the voltage, and record the value in your
table.
6. Test your hypothesis by throwing the switch
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7. Connect the batteries in parallel. A parallel connection
involves wiring from positive to positive, and negative to
negative. Wire in the open switch and voltmeter. Call your
teacher over to confirm your circuit.
8. Hypothesize as to the voltage and write it down.
9. Test your hypothesis by throwing the switch.
Observations:
Hypothesis (Volts)
True Value (Volts)
1 cell
2 cells in series
2 cells in parallel
Conclusions:
1. What is the difference of connecting batteries in series
and in parallel?
-when connecting 2 cells in series, you double the voltage.
-when connecting 2 cells in parallel, the voltage does not
change.
2. If three 1.2 V cells are connected in series, what would be
the output voltage?
-3.6 V (3 x 1.2V)
3. If three 1.2 V cells are connected in parallel, what would be
the output voltage?
-1.2 V
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4. What do you think are the advantages and disadvantages of
connecting cells in series?
- more voltage can be acquired
- if one goes out, the whole circuit is broken
5. What do you think are the advantages and disadvantages
of connecting cells in parallel?
-can multiply the voltage
- if one goes out, the circuit still flows
- can run many items at one time.
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Current, Resistance, and Ohm’s Law (13.9)
Current:
• A measure of how many electrons pass through the load per
second
• Measured in amperes (amps)
• SI symbol: A
• 1 ampere = 6.24 x 1018 electrons
• to measure current in a circuit, an ammeter must be
connected in series with the load, source, and switch
Sample household currents:
Appliance
Radio
100 watt lamp
Colour TV
Toaster
Microwave
Kettle
Stove element
Calculator
Water heater
Car starter
Current (A)
0.4
0.9
4.1
13.6
11.7
12.5
6.8
0.002
27.3
500.0
How dangerous is Current Electricity?
• 0.001 A is the perception level
• At 0.002, your muscles will tingle
• 0.005 A is the maximum safe level
• At 0.02 A, muscles convulse, human suffocates
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Electric Resistance: (13.7)
• The ability to impede (hinder, slow down) the flow of
electrons
• All substances will resist electron flow
• Good conductors have low resistance
• Good insulators have high resistance
• The symbol for resistance is R
• The unit is the ohm
Ohm’s Law: (13.9)
• States that the potential difference (voltage drop) across
a resistor is proportional to the current flowing through it.
• This can be demonstrated using the following equation:
V=IxR
(potential = current x resistance)
Examples of household resistances:
Load
Resistance (ohms)
Flashlight bulb
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Light bulb (60 W)
240
Toaster oven
8.6
Water heater
12.8
Resistor:
• Device (load) put into circuit to decrease the current.
Resistors come in many shapes and sizes. They convert
electrical energy into heat.
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Problem Solving using Ohm’s Law: (13.9)
1. List the information given
2. Write the equation
3. Simplify or manipulate the formula
4. Substitute in the values
5. Solve the equation
6. State your answer using the correct unit
Example: What is the voltage drop across the grill of a toaster
oven if it has a resistance of 9.0 ohms and 12 A of current is
passed through it?
Given:
V=?
I = 12 A
R = 9 ohms
V=IxR
V = 12 A x 9.0 ohms
V = 108 V
Therefore, the voltage drop is 108 V
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Series and Parallel Circuits (13.10)
Series Circuit:
• One path from the source and back
• If one device burns out, you have an open circuit, and
everything shuts off.
• The more loads you add in series, the lower the current
becomes
• The potential difference remains the same, but it is
divided up among all parts of the circuit.
• The resistance is added together
• The current splits in proportion to the loads
• Examples: Christmas tree lights
• Diagram:
Parallel Circuit:
• More than one path from the source and back
• If one device goes out, the rest are unaffected
• You can add more loads in parallel without affecting the
operation of the others.
• Resistances are not added together because the electrons
can go down separate paths.
• Current is the sum of the currents from all paths
• Examples: the plugs in your home
• Diagram:
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