IV. Outcomes 4: Circuits
A. Electric current = continuous flow of electric charge. 1. caused by the flow of electrons
Which direction? Electrons flow from negative to positive
2. Direction of CURRENT is defined as from + to – (the way positive charges would move)
= opposite to the flow of electrons
3. Materials and electricity
a. Electrical conductors = electrons are easily released, so charge flows easily
Ex. Metals, water, body
***The electrons that move come from the atoms in the wire, not the BATTERY!!!***
b. Electrical insulators = electrons tightly held, charge cannot flow easily.
Ex. Paper, plastic, rubber, etc.
B. Electric circuit = a complete path through which charge can flow 1. Circuit diagrams or schematic diagrams
a. Use symbols to represent parts of a circuit
b. Neat and clear: easy to understand (if you know something about electricity)
c. Easier to plan/design circuits
2. Open circuit = charge cannot flow (switch is off)
3. Two types
a. Series circuit = charge has only ONE path through which it can flow
- What happens if one part of the circuit stops working? o No current can flow – open circuit – nothing works
- What happens if you add bulbs or other energy users to the circuit? o More resistance = less current (bulbs are dimmer)
b. Parallel circuit = 2 or more paths through which charges can flow
- What happens if one part of the circuit stops working? o Current can still flow along other path(s)
- What if you add bulbs or other energy users to the circuit? o Current still flows equally to all – all equally bright 4. Safety
a. Short circuit: Electric current taking the path of least resistance. - a short circuit is one with no energy user (no resistance)
- when this happens energy is converted to heat and can start a fire b. Fuse (and circuit breakers) = Used to prevent current overload by
V. Outcomes 5: Ohm’s Law – Current, Voltage,
Resistance.
A. Ohm’s Law
1. Current = continuous flow of electric charge, measured in units of amperes (amps, A).
**Amount of current leaving battery must be the same as the amount returning to battery**
2. Voltage = a measure of electrical potential energy, caused by a difference in electrical charge.
a. Voltage difference causes current to flow. b. Measured in units of volts (V).
c. Battery = uses chemical energy to create a voltage difference between two ends. d. Amount of voltage decreases to zero as current flows through a circuit.
e. How does voltage affect current?
The higher the voltage, the more current.
3. Resistance = how strongly an object resists current flowing through it (how difficult it is for current to flow through)
~high resistance = it’s difficult for current to flow = Can carry a tiny current ~low resistance = it’s easy for current to flow = Can carry a large current.
a. Measured in units called ohms (Ω) b. How does resistance affect current?
The lower the resistance, the more current. 4. Ohm’s Law
Voltage = Current x Resistance V = IR
Example 2. A motor in a toy car needs 2 amps of current to work properly. If the car runs on four 1.5-volt batteries, what is the motor’s resistance?
Example 3. A television uses 15 amps of current and works against 8 ohms of resistance. Calculate voltage?
B. Current, Voltage, Resistance, and Ohm’s Law: Series vs. Parallel Circuits
Series Circuit Parallel Circuit # of paths
Current
Voltage Resistance
a What is the total voltage across the bulbs? (equal to voltage from battery)
b What is the total resistance of the circuit? (add them up!)
c What is the current in the circuit? (Use Ohm’s law and answers from a and b)
d What is the voltage drop across each light bulb? (Remember that voltage drop is calculated by multiplying current in the circuit by the resistance of a particular resistor: V = IR.)
(If resistances are different, use Ohm’s law to calculate EACH voltage drop SEPARATELY. Here they’re the same so we’ll only calculate it once. For current, use your answer from c – same everywhere in the circuit, for resistance look at the diagram.)
e Draw the path of the current on the diagram. (Flows from positive to negative)
2. PARALLEL CIRCUIT Practice Problem
a What is the voltage across each bulb? (same across each branch, equals voltage of battery)
b What is the current in each branch?
(Use Ohm’s law, voltage from a resistance from diagram. If different resistance, need to calculate EACH one SEPARATELY).
c What is the total current provided by the battery? (Add current from each branch that you found in b)
IV. Outcomes 4: Electric
Circuits
Preview: Voltage, Current, Electricity, Magnetism (11:30):
http://www.youtube.com/watch?v=XiHVe8U5PhU
A. Electric
current
= continuous
flow
A. Electric current = continuous flow of electric charge
1. Caused by the flow of
electrons.
Which direction? Electrons flow
from
negative
to
positive
.
1. caused by the flow of electrons
Which direction? Electrons flow from negative to positive
2. Direction of
CURRENT
is defined as
from + to – (the way
positive charges
would move)
=
opposite
to the flow of electrons
= opposite to the flow of electrons
3. Materials and electricity
a. Electrical
conductors
= electrons
are
freely moving around
, so
3. Materials and electricity
a. Electrical conductors = electrons are easily released, so charge flows easily
Ex.
Metals, humans
Ex. Metals, water, body
***The electrons that move come from the atoms in a wire, not the BATTERY!!!***
b. Electrical
insulators
= electrons
are
held tightly in the atom
so
current cannot flow easily.
b. Electrical insulators = electrons tightly held, charge cannot flow easily.
Ex.
Paper, plastic, rubber, glass,
wood
Conductor, Semiconductor, Insulator
Conductor (free electrons)
Semiconductor (some free electrons)
Insulator (no free electrons) Battery
+
Superconductors:
Some materials give
zero resistance
to current when they are
cooled to
very low temperatures
.
Example: Niobium-Zirconium alloy
Superconductors:
Some materials give zero resistance to current when they are cooled to very low temperatures.
Example: Niobium-Zirconium alloy
B. Electric
circuit
= a complete
path
through which charge can flow.
B. Electric circuit = a complete path through which charge can flow.
1. Circuit diagrams or
schematic
diagrams:
a. Use
symbols
to represent parts
of a circuit.
b. Neat and clear: easy to
understand (
if you know something about
electricity
).
1. Circuit diagrams or schematic diagrams: a. Use symbols to represent parts of a circuit.
b. Neat and clear: easy to understand (if you know something about electricity). c. Easier to plan/design circuits.
2.
Open circuit
= charge cannot
flow (switch is
off
).
2. Open circuit = charge cannot flow (switch is off).
Closed circuit = path is complete, charge flows (switch is on).
3. Two types of circuits:
Two circuit types (Bozemanbiology …8:00): http://www.youtube.com/ watch?v=x2EuYqj_0Uk
a.
Series
circuit = current has only
3. Two types
a. Series circuit = charge has only ONE path through which it can flow
What happens if ANY ONE part of
the circuit stops working?
What happens if one part of the circuit stops working? No current can flow – open circuit – nothing works
What happens if you add bulbs or
other energy users to the circuit?
What happens if you add bulbs or other energy users to the circuit? More resistance = less current (bulbs are dimmer)
b. Parallel circuit = 2 or more paths through which charges can flow
What happens if one part of the
circuit stops working?
What happens if one part of the circuit stops working?
Current can still flow along other path(s)
What if you add bulbs or other
energy users to the circuit?
What if you add bulbs or other energy users to the circuit? Current still flows equally to all – all equally bright.
4. Safety
a.
Short circuit
: electricity taking
a. Short circuit: electricity taking the path of least resistance.
- a short circuit is one with no
- a short circuit is one with no energy user (no resistance).
- When this happens energy is
- a short circuit is one with no energy user (no resistance).
- when this happens energy is converted to heat and can start a fire.
b. Fuses (and circuit breakers) =
Used to prevent
current overload
b. Fuses (and circuit breakers) = Used to prevent current overload by opening the circuit if too much current passes through (“blowing a fuse”).
V. Outcomes 5: Ohm’s Law –
Current, Voltage, Resistance.
Preview: Voltage, Current, Electricity, Magnetism (Sullivan…8:45):
http://www.youtube.com/watch?v=zYS9kdS56l8
A. Ohm’s Law
1. Electric Current = continuous flow
of electric charge, measured in
1. Current = continuous flow of electric charge, measured in units of amperes (amps, A).
**Amount of current
leaving a
**Amount of current leaving battery must be the same as the amount returning to battery**
2.
Voltage
= a measure of
electrical
potential energy
, caused by a
2. Voltage = a measure of electrical potential energy, caused by a difference in electrical charge.
a. Voltage
difference
causes
current to flow
.
a. Voltage difference causes current to flow. b. Measured in units of volts (V).
c. Battery = uses
chemical energy
to create a
voltage difference
c. Battery = uses chemical energy to create a voltage difference between two ends.
d. Amount of voltage
decreases to
zero
as current flows through a
d. Amount of voltage decreases to zero as current flows through a circuit.
e. How does voltage affect current?
The higher the voltage, the
more
So, the higher the voltage, the more current.
3. Resistance = how strongly an object resists current flowing through it (how difficult it is for current to flow through)
~high resistance =
it’s difficult
for
current to flow =
Can carry a tiny
current.
~low resistance =
it’s easy
for
~low resistance = it’s easy for current to flow = Can carry a
large current
.~low resistance = it’s easy for current to flow = Can carry a large current. a. Measured in units called ohms (Ω)
b. How does resistance affect
current?
The lower the resistance, the
more
So, the lower the resistance, the more current.
4. Ohm’s Law
4. Ohm’s Law
Voltage = Current x Resistance V = IR
Example 1. If a toaster produces 12 ohms of resistance in a 120-volt circuit, what is the amount of current in the circuit?
Example 2. A motor in a toy car needs 2 amps of current to work properly. If the car runs on four 1.5-volt batteries, what is the motor’s resistance?
D. Current, Voltage, Resistance, and Ohm’s Law: Series vs. Parallel Circuits
Series Circuit Parallel Circuit
# of paths 1 path 2 or more
Current Same everywhere in the
circuit
Drops as resistors are added.
- Total current:
1) calculate current in each branch,
2) add them together - Increases as resistors are
added. Voltage - Calculate the voltage drop
for each resistor
- Total voltage drop = voltage from batteries
- Voltage across each branch = the total voltage of
battery.
Resistance Total resistance = sum of all
1 SERIES CIRCUIT Practice Problem ANSWERS
a What is the total voltage across the bulbs?
(equal to voltage from battery)
Answer = 6 V
b What is the total resistance of the circuit?
(add them up!)
Answer: 1 + 1 = 2 ohms
c What is the current in the circuit?
(Use Ohm’s law and answers from a and b)
Answer: V = IR 6 = I (2) I = 3 amps
d What is the voltage drop across each light bulb? (Remember that voltage drop is calculated by multiplying current in the circuit by the resistance of a particular resistor: V = IR.)
(If resistances are different, use Ohm’s law to calculate EACH voltage drop SEPARATELY. Here they’re the same so we’ll only calculate it once. For current, use your answer from c – same everywhere in the circuit, for resistance look at the diagram.)
Answer: V = IR V = (3 amps)(1 ohm) = 3 V
e Draw the path of the current on the diagram. (Flows from positive to negative)
2. PARALLEL CIRCUIT Practice Problem
a What is the voltage across each bulb?
(same across each branch, equals voltage of battery)
Answer: 12 V
b What is the current in each branch?
(Use Ohm’s law, voltage from “a” and resistance from diagram. If different resistance, need to calculate EACH one SEPARATELY).
Answer: V = IR 12 = I (2) I = 6 amps in each branch (same since same resistance here)
c What is the total current provided by the battery?
(Add current from each branch that you found in b)
Answer: 6 + 6 = 12 amps
d Use the total current and the total voltage to calculate the total resistance of the circuit.
(Use Ohm’s Law and your answers from c and a)
~high resistance = it’s difficult for current to flow = Can carry a tiny current
~low resistance =
it’s easy
for
~low resistance =
it’s easy
for current to flow =
Can carry a
large current
.
a. Measured in units called ohms (Ω)
b. How does resistance affect
current?
Back to the Demo:
How does
tube size
affect
flow rate
?
Increasing the tube size represents
How does tube size affect flow rate?
Increasing the tube size represents less resistance.
What happens? Does the bucket
fill more or less quickly?________
So, the lower the resistance, the
What happens? Does the bucket fill more or less quickly? more So, the lower the resistance, the more current.
5. Ohm’s Law
Voltage (V) =
Current (
I
)
x
Resistance(R or
Ω
)
5. Ohm’s Law
Voltage = Current x Resistance V = IR
Example 1. If a toaster produces 12 ohms of resistance in a 120-volt circuit, what is the amount of current in the circuit?
Example 2. A motor in a toy car needs 2 amps of current to work properly. If the car runs on four 1.5-volt batteries, what is the motor’s resistance?
B. Current, Voltage, Resistance, and Ohm’s Law: Series vs. Parallel Circuits
Series Circuit Parallel Circuit
# of paths 1 2 or more
Current Same everywhere in the circuit - Total stays the same, but current divides between branches:
- To calculate total current, 1st calculate current in each branch with Ohm’s Law, 2nd add them together to get total current provided by the battery
Voltage - Decreases to zero throughout circuit
- Use Ohm’s Law to calculate the voltage drop for each resistor
- Total of the voltage drops for all resistors should equal voltage provided by batteries
- Still decreases to zero throughout circuit - Voltage across each branch is the same as the total voltage (regardless of resistance on the branch)
Resistance Total resistance = sum of the resistances of all objects in the circuit (add them up!)
- Total resistance decreases the more paths that you add for current (more branches) - If you have a parallel circuit with two branches and each branch contains equal resistance, the total resistance of the circuit will equal HALF of the resistance on one branch.
1 SERIES CIRCUIT Practice Problem
a What is the total voltage across the bulbs?
(equal to voltage from battery) Answer = 6 V
b What is the total resistance of the circuit? (add them up!)
Answer: 1 + 1 = 2 ohms
c What is the current in the circuit? (Use Ohm’s law and answers from a and b) Answer: V = IR 6 = I (2) I = 3 amps
d What is the voltage drop across each light bulb? (Remember that voltage drop is calculated by multiplying current in the circuit by the resistance of a particular resistor: V = IR.)
(If resistances are different, use Ohm’s law to calculate EACH voltage drop SEPARATELY. Here they’re the same so we’ll only calculate it once. For current, use your answer from c – same everywhere in the circuit, for resistance look at the diagram.)
Answer: V = IR V = (3 amps)(1 ohm) = 3 V
2. PARALLEL CIRCUIT Practice Problem
a What is the voltage across each bulb? (same across each branch, equals voltage of battery)
Answer: 12 V
b What is the current in each branch?
(Use Ohm’s law, voltage from a and resistance from diagram. If different resistance, need to calculate EACH one SEPARATELY).
Answer: V = IR 12 = I (2) I = 6 amps in each branch (same since same resistance here) c What is the total current provided by the battery?
(Add current from each branch that you found in b) Answer: 6 + 6 = 12 amps
d Use the total current and the total voltage to calculate the total resistance of the circuit. (Use Ohm’s Law and your answers from c and a)
