Transcript Electric Circuits

ELECTRIC CIRCUITS
Students will:
• Solve problems involving current, charge and time.
• Solve problems relating potential dif ference to
energy and charge.
• Use Ohm’s Law to relate current, voltage and
resistance
• Apply Kirchhof f’s Laws to analyze current and
voltage in circuits.
SPH3U/SPH4C
Findlay
AGENDA
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Circuit Fundamentals
Current
Resistance
The Battery and Potential Dif ference
Ohm’s Law
Kirchhof f’s Laws
 Series Circuits
 Parallel Circuits
Circuit Fundamentals
 An electric circuit is a
closed loop, with a
steady flow of charged
particles provided by a
source of electrical
energy.
 A battery converts stored
chemical potential energy
into electrical energy.
 Current electricity is the flow
of charged particles along a
conductor.
 A circuit contains a source of
electrical energy, a
conductor, and a load.
 A load converts electrical
energy into light, sound,
heat, or motion.
Drawing Circuits
 Draw a circuit containing three batteries connected to each
other, a switch so that electricity can flow through it and
three light bulbs connected to each other.
Electric Current
 Electric current can be compared to the volume flow rate of
water.
a)
b)
Volume flow rate of water is a measure of the volume of water
flowing past a given point each second.
Electric current is a measure of the number of electric charges
flowing past a given point each second.
Current
• The amount of charge transferred per unit time is referred to as current.
 The movement of electrons in a
circuit is called electron flow, and is
the correct definition of current in a
circuit.
 Conventional current describes
current as positive charges moving
from positive to negative poles.
Although technically incorrect,
conventional current is often used in
circuit analysis.
 The equation for current is
current =
charge
time
Example
A battery delivers a charge of 9.00 C in 1.00 min of operation.
What amount of current is generated in mA?
The interior of a dry cell
or “battery”. Chemicals
in the dry cell react to
provide electrical
energy.
Practice Problems
1. A D-cell battery delivers a charge of 200.0 C in 65.0 s.
Determine the current produced by this battery.
3.08 A
2. A car battery provides a current of 600.0 A for 2.48 s.
Determine the charge provided by the battery.
1.49 x 103 C
3. A battery has a total charge capacity of 10 800 C. For how
long can this battery deliver a current of 450 mA?
2.4 x 104 s
DC vs AC Current
• Current can have one direction or move back and forth.
 Direct current (DC) is a steady current in one direction.
 Alternating current (AC) changes direction periodically. Charges in AC move back and
forth and do not move from one terminal to another in the circuit. AC is used in
household wiring.
Measuring Current
 Conventional Current
 When the direction of DC was first defined, scientists did not know
that it was the electrons that pushed each other along the wires. By
convention, they agreed that electric current was the flow of positive
charges.
 Electron Flow
 The confusing part is that in metal conductors it is the electrons, the
negative charges that flow.
Measuring Current
 The instrument used to measure electric current is called an
ammeter.
 It must be connected directly in the path of the moving charges. This
type of connection is called a series connection.
Resistance
• Resistance is the degree to which current is opposed in a circuit.
 A resistor is a device that resists or restricts current.
 Resistors remove energy from charges and convert it to other forms of energy, such
as heat.
 Under normal conditions, all components offer some resistance in a circuit, even if
resistance is not their primary role.
This is a magnetic levitation (maglev) train. It was
designed in Germany and has a cruising speed of
500 km/h.
Electromagnets provide the magnetic force needed
to levitate the train. The electromagnets require
large amounts of current to provide adequate
magnetic force.
Large conducting wires are needed to minimize
resistance in the circuits supplying the
electromagnets. Superconductors (substances that
conduct with zero resistance) may one day provide
economical solutions for carrying large amounts of
current.
Resistance
 Resistance is an opposition to the flow of charge, resulting in
a loss of potential energy.
 The magnitude of the electrical resistance of a conductor depends on
four variables:




The
The
The
The
length of the conductor.
cross-sectional area of the conductor.
material the conductor is made of.
temperature of the conductor.
 The type of material determines how easily it allows electrons
to move from one atom to the next.
 Conductors allow electrons to move easily; hence, they have very low
resistance.
 Insulators do not allow electrons to move easily. They have a very
high resistance.
 Semi-conductors fall somewhere in between.
Colour-Coded Resistors
 A resistor is a device that is used in an electrical circuit to
control current flow and potential dif ferences in circuits. The
resistance is measured in units called ohms ( Ω).
 Many resistors have coloured bands around them to identify
their resistance. The most common is a 4 -band scheme.
Each band stands for a number from 0-9.
Example: Colour-Coded Resistance
The Battery and Potential Difference
• Batteries increase the potential energy of charges in a circuit.
 A battery acts like a pump by increasing the energy of charges, much like a water
pump gives water potential energy by pumping it to a higher level.
 Electric potential is the electrical potential energy per unit charge, measured in volts
(V). It is mathematically expressed as:
Electric Potential Difference
 Charges have a greater electric potential before they pass through a load than after.
This change in electric potential is potential difference (∆V), which is often referred to
as voltage.
ΔV = Vfinal − Vinitial
ΔV =
Efinal
Einitial
−
q
q
Practice Problems
1. A potential dif ference of 120.0 V is measured across a light
bulb. The light bulb is left on for 30 min allowing a charge
of 900 C to flow through it. How much energy is converted to
light and heat?
1.08 x 105 J
2. A charge of 50.0 C has a change in potential energy of
1 .00 x 10 3 J as it flows through a resistor. What is the
potential dif ference across the resistor?
20.0 V
Measuring Potential Difference
 The instrument used to measure electric potential dif ference
is called a voltmeter.
 To measure potential rise, the voltmeter must be connected across
the source.
 To measure potential drop, the voltmeter must be connected across
the load.
 This type of connection is called a parallel connection.
Ohm’s law
• There is a relationship between potential difference, current, and resistance.
 German scientist Georg Ohm applied  Ohm’s law can be expressed mathematically.
different voltages across a resistor and
measured the current. He found a
linear relationship between voltage
and current.
Example
A student is asked to
determine the value of the
resistor in a
circuit as shown below.
The slope of the line resulting from plotting
voltage against current equals resistance. This
is the basis of Ohm’s law.
Ohm’s Law
Practice Problems
195.0 V
55 Ω
2.25 A
520.0 V
6.00 A