Download Electric Current and Circuits

Electric Current
and Circuits
Can you get a light bulb to light?
•Using the equipment given to you, try to find as many ways as
possible to get the light bulb to light.
•Diagram four ways in which you were able to get the light bulb
to light. Label the battery, the wire, and the bulb.
•Diagram at least three ways in which you were not able to get
the bulb to light.
Question:
1. From your observations, what conditions seem to be
necessary in order for the bulb to light?
Successful Attempts
Potential Energy review
Movement of a positive test charge within an electric
field is accompanied by changes in Potential Energy:
 Moving the charge AGAINST the direction of an electric
field is like moving a mass upward within Earth’s
gravitational field. (requires work by an outside force)
 Would increase the PE of the object
Potential Energy review
 Moving the charge in the SAME direction of an electric
field is like a mass falling downward within Earth’s
gravitational field. (occurs without an outside force)
 Would decrease the PE of the object
Electric Potential
Electric Potential = the amount of Potential energy per
amount of charge
A large amount of potential energy at a place where large
amounts of charge are present results in a LARGE electric
potential
A small amount of energy for a given amount of charge results
in a SMALL electric potential
Electric Potential
The difference between two such points (one high potential
and the other low) is called Electric Potential difference.
It is measured in volt, so it is also called “voltage”
Potential Difference
When the positive charge is moved from A to B, work is being
done on the charge and that work becomes Potential Energy.
There is now a difference in electrical potential between
locations A and B.
Potential Difference
Ex: If the potential difference between 2 locations is 1 volt,
then 1 Coulomb of charge will gain 1 joule of PE when moved
between those 2 locations.
Ex: if the potential difference is 2…?
Batteries
The first battery was created by physicist Alessandro Volto by
stacking alternating layers of zinc, brine-soaked cloth, and silver.
It was the first device to have a lasting current (flow of charge)
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Basic Structure of a Circuit
The internal circuit is the battery with electrochemical cells
The external circuit is the wire and any “load” or circuit
element (fan, light bulb, heater)
Basic Structure of a Circuit
Each battery has 2 terminals – positive and negative
A cathode connects to the positive terminal
An anode connects to the negative terminal
Collectively, these are called electrodes and are where
chemical reactions take place
Basic Structure of a Circuit
There is an electric field between the 2 terminals, directed
from the positive towards the negative terminal
Movement of a positive test charge from negative to positive
terminal would require work, thus increasing the PE of any
charge that moves along this path
Basic Structure of a Circuit
The positive terminal is referred to as the “high potential
terminal”
The negative terminal is referred to as the “low potential
terminal”
Basic Structure of a Circuit
In battery cells, chemical energy does work on a positive test
charge to move it from the negative terminal to the positive
terminal
The chemical energy is transformed into electrical potential
energy (within the battery)
Once at the positive terminal, a test charge will then move
through the external circuit
Basic Structure of a Circuit
As the charge travels, it will lose PE as it transforms its PE into
other forms of energy (light, thermal…) at the light bulbs, fans…
The test charge returns to the negative terminal at low energy
and low potential – it is ready to repeat the cycle again
Basic Structure of a Circuit
If a 12V battery is used in the circuit, then every C of charge
gains 12 J of PE as it moves through the battery.
Then every C of charge loses 12 J of electrical PE as it passes
through the external circuit (lost due to other types of energy)
The loss in electrical potential while passing through a circuit
element is called “voltage drop”
Producing Electric Current
This flow of charged particles is an electric current.
The flow stops when the electric potential difference between
the two plates is zero.
If the charge particles are pumped back to the original
conductor (plate) by an outside energy source (battery), the
flow of charges will continue.
Requirements to establish an
electric current
There must be a closed conducting path that extends from the
positive terminal to the negative terminal.
It is not enough that there is simply a closed conducting loop;
the loop itself must extend from the positive terminal to the
negative terminal of the electrochemical cell.
Requirements to establish an
electric current
The second requirement is that there must be an electric
potential difference across the two ends of the circuit.
Some energy source, such as a battery or charge pump, is
required
Electric Circuits
A complete conducting path through which electrons can flow
For a current to flow it must include a charge pump and be
closed
This is an open circuit.
No current would flow while open
Electric Current
Current is the rate at which charge flows past a point on a circuit.
Current flows from the positive terminal to the negative terminal.
Current (I) = q/t = Coulombs/sec=amperes (A)
Example: A 2 mm long cross section of wire is isolated and 20 C of
charge is determined to pass through it in 40 s. I = _____amperes
Example: A 1 mm long cross section of wire is isolated and 2 C of
charge is determined to pass through it in 0.5 s. I =
________amperes
Fact or fiction?
 When you turn on a light switch, electrons
move rapidly from the socket to the lamp
to the bulb.
What electrons really do
Electrons move slowly bouncing around in many directions as
they move in one direction overall.
“Electron Drift”
Power
Power (measured in watts, W) measures the rate at which energy is
transferred. (1 W = 1J/s)
The energy carried by an electric current depends on the charge
transferred and the potential difference across which it moves.
Current is the rate of charge flow (I=q/t). A flow of 1 C/s is called an
ampere (A).
Power is equal to the current times the potential difference.
P = I V
Total energy transferred = P x t (W.s or J)
Practice:
A 6.0 V battery delivers a 0.50 A current to an electric motor
connected across its terminals.
What power is delivered to the motor?
If the motor runs for 5.0 min, how much electric energy is
delivered?
The Kilowatt- Hour
The electric company (power company) provides energy rather
than power.
When a bill is paid, the customer is paying for the electric
energy used.
Because of the large amounts of energy consumed, electric
companies measure sales in a units of kilowatt-hours.
One kilowatt hour is equal to 1000 W being delivered for 3600
s (1 hr) or 3.6 x 106 J.
Power is measured in units called
1.
amperes
2.
joules
3.
kilowatt-hours
4.
watts
Current is measured in units called
1. Amperes
2. Coulombs
3. Volts
4. Ohms
The direction that conventional
current flows in the external circuit is
from the ___ terminal to the ____
terminal.
1. negative, positive
2. positive, negative
3. negative, negative
4. positive, positive
Electrons move very quickly
through an electric circuit.
1. True
2. False
If the current is measured as 6 A, then
____ C of charge flows past any point
in the circuit in 3 s
1. 0.5
2. 2
3. 3
4. 18
If a 9.0 V battery delivers 0.50 A of
current to a heater, the power
consumed by the heater is
1.
0.056 W
2.
4.5 W
3.
9.0 W
4.
18 W
Resistance
Resistance (R) is the opposition to the flow of
charge.
Resistance is measured in units called ohms
(Ω).
Factors Impacting Resistance
Length:
Resistance increases as length of wire increases.
Cross-sectional area (thickness):
Resistance increases as cross-sectional area
decreases.
Temperature:
Resistance increases as temperature increases.
Material:
Resistance varies with the material used.
silver<copper<gold<aluminum<iron<platinum
(in order of increasing resistance)
The most common materials used in wire are copper and
aluminum. Why not silver if it has such low resistance?
Ohm’s Law
Resistance is equal to potential voltage divided by current.
V=IR
The current in a circuit is directly proportional to the voltage
(potential difference)
The current is indirectly proportional to the resistance.
This means that the greater the voltage, the greater the
current, and the greater the resistance the lower the current
1. For a wire that obeys Ohm’s Law,
the resistance of a wire depends on
the ___ the wire.
A.
B.
C.
D.
current in
length of
power delivered by
voltage across
2. A battery with a voltage of 9.0 V is
connected to a lamp. The current
flowing in the circuit is 0.30 A. The
resistance of the lamp is
A.
B.
C.
D.
0.030 Ω
2.7 Ω
27 Ω
30 Ω
3. What factor impacting resistance
explains why light bulbs tend to burn
out more frequently when they are
first switched on?
A.
B.
C.
D.
Length
Cross-sectional area
Temperature
Material
4. Which wire conducts electricity
with the least resistance?
A. One with a large cross-sectional area
B. One with a small cross-sectional area
Resistors
Wires used to complete circuits have low resistance.
A wire 1 m in length that is typically used in a physics
lab has a resistance of 0.03 Ω.
Wires used in home wiring offer as little as 0.004 Ω of
resistance for each 1 m of length.
Resistors are devices with specific resistances that are
used to control current in circuits or parts of circuits.
A variable resistor (potentiometer) is used when a
smooth, continuous variation of the circuit is desired
(example: volume, brightness, contrast, hue, and tone
controls on a TV)
The human body acts as a variable resistor. (Dry skin
has high resistance-wet skin has lower resistance)
Effects of current on the body
0.001a (1 ma) – barely felt
0.005a (5 ma) – painful
0.010a (10 ma) – muscles contract
0.015a (15 ma) – loss of muscle control
0.100a (100 ma) – can be fatal if the current goes through
the heart
Bird on a Wire
If your pet Peety the parakeet perches on an electric wire.
1. Why isn’t the bird electrocuted?
2. If you fell from a tree onto the same wire, would you
be electrocuted?
3. Should you reach up and touch the bird to rescue him?
Bird on a Wire
1. Why isn’t the bird electrocuted?
No potential difference between feet = no current
2. If you fell from a tree onto the same wire, would you be
electrocuted?
No potential difference between hands = no current
3. Should you reach up and touch the bird to rescue him?
No, you would be the path from the bird to the ground and
both of you would be toast. There would be potential
difference in this case.
5. A device that can be used to
change the current in a circuits in a
continuous way is a
A.
B.
C.
D.
potentiometer
battery
motor
lamp
6. If a 200 Ω resistor is connected to a
5 V battery, the current in the circuit
will be
A.
B.
C.
D.
0.025 A
2.5 A
40 A
1000 A
7. If you double the voltage what
happens to the current (all others kept
equal)?
A. it doubles
B. it halves
8. If you double the resistance what
happens to the current (all others kept
equal)?
A. it doubles
B. it halves
Types of Current
Direct Current (dc)
Electrons move in one direction
Alternating Current (ac)
Electrons change direction
Types of Circuits
There are three types of circuits:
•Series circuits-all current travels through each device.
•Parallel circuits-a circuit in which there are several different
current paths
•Combination series-parallel circuit- circuits which includes both
series and parallel branches.
Series Circuits
In a series circuit, the current is the same throughout.
Itotal = IA =IB = IC = …
The equivalent resistance (total resistance) of the
circuit equals the sum of the individual resistances of
the resistors.
Rtotal = RA + RB + …
Series Circuits
The total voltage is the sum of the individual voltages
across each device.
Vsource = VA + VB + …
If the voltage source does not change, adding more
devices in series will decrease the current.
Because the current flows through one pathway, if one
light goes out, they all go out.
The current is ___ in a series
circuit.
1.
higher at the beginning of
2.
the same everywhere
3.
lower at the beginning
4.
variable in
In a series circuit, the equivalent
resistance is ___ any single resistance
1.
larger than
2.
determined by
3.
equal to
4.
smaller than
If the battery voltage does not change,
adding more devices in the series ___
the current.
1.
sometimes decreases
2.
always decreases
3.
sometimes increases
4.
always increases
To find the current through a series
circuit, first calculate the ____.
1.
voltage
2.
equivalent resistance
3.
power
4.
equivalent voltage
Parallel Circuits
A parallel circuit is one in which there are several paths.
The total current is the sum of the currents through each path.
Itotal = IA + IB + Ic + …
Placing two or more resistors in parallel always decreases the
equivalent resistance of a circuit.
1=1 + 1 + 1 + …
R RA
RB
RC
The potential difference across each path is the same.
Vtotal = VA = VB = VC = …
Safety Devices
A short circuit occurs when a circuit with very low resistance is
formed.
Parallel circuits are susceptible to short circuits.
Safety devices are incorporated into the circuit to prevent a circuit
overload.
1. Fuse-a piece of metal melts when too much current flows and
breaks the circuit.
2. Circuit breaker-an automatic switch that opens when the current
reaches a value greater than the rated value in the circuit.
3. Ground-fault interrupter-an outlet that prevents injuries that are
caused when an electric appliance falls into water. It contains an
electric circuit that detects an extra current path and opens the
circuit.
The current is the same everywhere
throughout the circuit
1.
Parallel
2.
Series
The total resistance is equal to the
sum of the individual resistances.
1.
Parallel
2.
Series
The voltage drop across each
resistor in the circuit is the same
1.
Parallel
2.
Series
The voltage drop in the circuit is
proportional to the resistance.
1.
Parallel
2.
Series
Adding a resistor to the circuit
decreases the total resistance.
1.
Parallel
2.
Series
Adding a resistor to the circuit
increases the total resistance.
1.
Parallel
2.
Series
If the current through one resistor
goes to zero, there is no current in the
entire circuit.
1.
Parallel
2.
Series
If the current through one resistor in
the circuit goes to zero, the current
through all of the other resistors
remains the same.
1.
Parallel
2.
Series
This form is suitable for household
wiring.
1.
Parallel
2.
Series
An ammeter is connected in ____
in a circuit.
1.
Parallel
2.
Series
A voltmeter is connected in ___ in
a circuit.
1.
Parallel
2.
Series
The basic unit of resistance is the
1.
volt
2.
ohm
3.
amp
4.
coulomb
A short thick wire has more
resistance than a long thin wire.
1.
True
2.
False
In a DC circuit, current flows from the
___ electrode to the ___ electrode.
1.
negative, positive
2.
positive, negative
3.
positive, positive
4.
negative, negative