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Electricity did not become an integral
Part of our daily lives until
Scientists learned to control the
Movement of electric charge.
This is known as current
Electric currents are responsible
For many things; computers,
Cars, and every move you make.
Current is the rate at which
Electric charges move through
A given area.
Conventional current is defined
In terms of positive movement.
Electricity is the movement of
Electrons, the negative charge,
Moving in a direction. This direction
Is in the positive direction.
Solutions that have dissolved
Ions can be charge carriers,
These are called electrolytes.
Drift velocity is the net velocity
of a charge carrier moving in
An electric field.
Drift speeds are relatively small.
The speed of an electron in a
Copper wire is only about
0.000246 m/s! The electric
Field, on the other hand, is
Almost the speed of light.
Batteries and generators
Supply energy to charge carriers.
Both batteries and generators
Contain a potential difference
(volts) across their terminals.
Batteries use chemical energy
And generators use mechanical
Energy.
Current can be direct or alternating.
In Direct Current (DC) the charge
Moves only one way through the
Wire. (like my electric truck)
In Alternating Current (AC), the
Charges are constantly changing,
Creating no real movement of
Electrons.
Our house current in the US is
60 Hz.
Q
I=
t
I = electric current
Q = charge through a given area
t = time
The SI unit for current is the
Ampere (A).
The SI unit for charge is the
Coulomb (C).
So the Ampere is 1 C per second.
The amount of charge that passes
Through the filament of a
Certain light bulb in 2 s is
1.67 C. Determine the current in
The light bulb.
I = 0.835 A
There are insulators and
Conductors, but there are also
Not so good conductors.
The impedance of the motion of
Charge through a conductor
Is the conductor’s resistance.
Resistance is the ratio of the
Potential difference across
A conductor to the current
It carries.
V
R=
I
SI unit for resistance is the Ohm,
And is represented by Ω.
Ohm’s law states that the
Resistance is constant over a
Wide range of applied
Potential differences.
It is usually shown by…
V = IR
Ohm’s Law: V = I x R
Used to determine V, I, & R at all parts of
a circuit.
It means:
• A 1V circuit with a 1Ω Resistance will
produce a current of 1A.
• Adding more resistors to a circuit will
decrease the current produced.
• Increasing the voltage will increase the
current produced.
Resistance depends on length,
Cross-sectional area, material,
And temperature.
Resistors can be used to control
The amount of current
In a conductor.
The plate on an iron states that
The current in the iron is 6.4 A
When the iron is connected across
A potential difference of 120V.
What is the resistance of the iron?
R = 19 Ω
Electric power is the rate of
Conversion of electrical energy.
P = IV
The SI unit is the Watt.
Power = the rate of doing work
(Work/time)
Electrical Power = Current x Voltage
P=IxV
Units: Watts
An electric space heater is
Connected to a 120 V outlet. The
Heater dissipates 3.5 kW of power
In the form of heat. Calculate
The resistance of the heater.
R = 4.1 Ω
An electric circuit is a
Continuous path through which
Electric charges can flow.
There are two types of circuit
Connections…
Parallel & Series
A parallel circuit is one with
Several current paths, whose
Total current equals the sum of the
Currents in its branches.
A series circuit is one in which
Current passes through one
Device and then another.
Electric companies measure
Energy consumption in
Kilowatt-hours.
1 kW h = 3.6 X 106 J
Electrical energy is transferred at
High potential differences to
Minimize loss. (up to 1000000 V)
When the wires are strung, they are
Very tight and straight, they bow
Because they are soooo hot!
How much does it cost to operate
A 100 W light bulb for 24 hours
If electrical energy costs $0.08
Per kW h? What is the cost per
Year? What is the cost if you used
A energy conserving bulb at 14 W?
Cost = $0.19/day $69/year
Cost = $ 0.03/day $9.81/year
Current moving through a resistor
Causes it to heat up because
Flowing electrons bump into the
Atoms in the resistor.
These collisions increase the
Atoms’ kinetic energy and, thus,
The temperature of the resistor.
If we rearrange Ohm’s Law, and
The formula for power, we have
3 new formulas that we can use…
P=
I2R
V2
P=
R
And for the thermal
Energy dissipated…
V2
E=
t
R
Power
Electrical Energy is usually converted to
• Electromagnetic Energy (light),
• Thermal Energy (heat),
• or Kinetic Energy (motion).
A heater has a resistance of
10 Ω. It operates on 120 V.
What is the power dissipated by
The heater? What is the thermal
Energy supplied by the heater
In 10 s?
1.44 kW
14.4 kJ
Remember that there are 2
Types of circuits…
Series
&
Parallel
2. Current Electricity
* Electrons flow in a conductor.
* Must have a closed circuit (loop).
Charge = number of transferred electrons
Measured in Coulombs (C)
Current = electron flow (I)
(like water flow) Measured in Amperes (A)
Voltage = electrical potential energy (V)
(like water pressure) Measured in Volts (V)
Resistance = ease that current can flow
(like pipe diameter)
Measured in Ohms (Ω)
Resistor
= item in circuit that causes resistance.
Resistors “use up” a circuit’s voltage.
Ex: Light Bulb
Circuit = the path that flowing electrons follow
If path is continuous,
it’s a closed circuit
and electricity can
flow.
If path is interrupted,
it’s an open circuit
and electricity can
NOT flow.
Series Circuits
• Current goes through one pathway only.
• Same amount of current goes through
each component (battery, resistor, etc.).
• The total resistance of resistors in
series = the sum of each resistance
(add them up).
Series Circuit
In a series circuit, all of the
Resistors are added together
In order to get the
Equivalent Resistance.
R = R1 + R2 + R3 + …
See that the equivalent resistance
Is greater than any individual
Resistor, therefore if the voltage
Does not change, adding more
Resistors always decreases current
Three 20 Ω resistors are connected
In series across a 120 V generator.
What is the equivalent resistance
Of the circuit? What is the current
In the circuit?
R = 60 Ω
I = 2.0 A
Parallel Circuits
• Current divides into 2 or more paths
• Voltage drop across components is
equal.
• The sum of the current in each parallel
path = the total current in (or out) of the
circuit.
Parallel Circuit
In a parallel circuit the resistance
Is always smaller than any
One resistor because there are
More branches for the current
To flow through.
Equivalent Resistance for Parallel
1/R = 1/R1 + 1/R2 + 1/R3 + …
Three 15 Ω resistors are connected
In parallel and placed across
A 30 V battery. What is the
Equivalent resistance? What is the
Current through the entire circuit?
What is the current through each
Branch of the circuit?
R=5Ω
I=6A
I=2A
Draw a circuit diagram of a
series circuit and a
Parallel circuit that each contain
The following…
Battery, motor, 3 lights, and 2
Resistors.
There are many safety devices
When dealing with electrical
Circuits.
A short circuit is when the
Resistance is very small which
Makes the current very large.
When a short happens enough
Thermal energy is produced to
Start a fire, or just melt the
Wires if you’re lucky.
A fuse is used to help stop a short
Before it gets too bad.
When a fuse is connected to the
circuit it will melt and “blow”
Before the wires do.
A circuit breaker is another kind
Of fuse.
It breaks the circuit when the
Current gets too high and a
Switch flips. In order to fix
The circuit, all you have to do is
Flip the switch back.
A ground fault interrupter (GFCI) is
a kind of electrical outlet that
Breaks the circuit at the plug
Instead of the circuit/fuse box.
An ammeter is a device used to
Measure the current in a circuit.
It is added in series and
Has a low resistance.
A voltmeter measures the
Voltage of a circuit.
It is added in parallel and has
A high resistance (10 kΩ).
A multimeter measures both
And more.