Circuits Guided Notes
Name:______________________
Electrical Potential
Just as masses can have gravitational potential energy, charges can have electrical potential energy
How can a mass or charge gain potential energy? ________________________________
Does the amount of work done by the monkey change if the charge changes? How do we know?
What does this imply about electrical potential energy?
Electrical Potential and Electrical Potential Energy
Introducing … Electrical Potential (V) … a measure that depends only on ___________________, not on ___________.
The SI unit of electrical potential is the volt
Note important difference between energy and potential:
•
A ______________________ has ________________, a _______________ placed there has
___________________________
•
Two points that are at the same distance from the charged object have the same potential.
•
So, when two charged objects are placed there, they are at the same potential, but the one with more charge on
it has higher electric potential energy. It is harder to push it there.
The quantity electric potential is defined as the amount of _____.
1. electric potential energy
2. force acting upon a charge
3. potential energy per charge
4. force per charge
What happens if a positive charge is moved from point A to point B?
Potential Difference ΔV
The potential difference between two points is exactly what the name implies – the
______________________________ between two points.
The change in electric potential energy between two points also equals the ____________________________________
in order to move it from one point to the other.
Potential Difference (ΔV) & Circuits
What is the role of a battery in a circuit?
Batteries supply the _________________________ to maintain the
________________________________ across a circuit
1) A 12V battery means that the _______ terminal has an ____________________ that is 12V higher than the –
terminal.
2) Charges flow through the external circuit (the wire) from high to low potential.
3) As the charges flow through the circuit, they ‘lose’ energy to circuit elements such as lights and motors
4) By the time the charges get to the end terminal, they have used up 12 V of potential
5) The battery supplies the _________________________________________________________________
As charges ‘lose’ energy to different circuit elements, the electric potential decreases. This is known as voltage drop.
All of the electric potential difference– i.e. all of the voltage -- is used up by the end of the circuit.
Terminology Fun
1. The variable we use for potential, potential difference, and the unit for potential difference (volts) is V.
2. Don't let that confuse you when you see V = 1.5V
3. Electric potential energy is not the same as electrical potential.
4. Electrical potential can also be described by the terms, potential difference, voltage, voltage drop, potential
drop, potential rise, electromotive force, and EMF. These terms may differ slightly in meaning depending on the
situation.
You Do Problems
1.
Compare an electric circuit to a roller coaster ride.
What is the difference in height from the top to the bottom of a rollercoaster analogous to in a circuit?
What is the motor that pulls the rollercoaster up the hill analogous to in circuit?
2.
If a battery provides a high voltage, it can ____.
a. do a lot of work over the course of its lifetime
b. do a lot of work on each charge it encounters
c. push a lot of charge through a circuit
d. last a long time
3.
Compared to point D, point A has …
a. 12 V higher potential energy
b. 12 V lower potential energy
c. Exactly the same potential energy
4.
The electrical potential energy is 0 at …
a. A
b. B
c. C
d. D
5.
Energy is required to move a charge from…
a. Point A to point B
b. Point B to point C
c. Point C to point D
d. Point D to point A
6. The energy required to move +2C of charge from D to A is
a. 1. 0.167 J
b. 2. 2 J
c. 3. 6 J
d. 4. 24 J
Electric Current
Current = _________________________
Symbol:
Unit:
1A = 1 C / s
To have an electric current, you need two things:
• A closed circuit / path for the charges
• A power supply maintain the potential difference
Remember: Current will flow from high potential to low potential, but charges need to be
pushed (by an energy source) back from low potential to high potential.
Closed vs. Open Circuits
Direction of Current
Current is defined as the direction __________________ charges would flow
• From high potential to low potential
• From + side of battery to – side of battery
Fun fact: Benjamin Franklin defined current in this manner long
before we knew much about charges.
Now, we know that positive charges stay put and negative charges
flow. So, electrons actually flow opposite current.
Calculating Current
Current is the rate of charge flow per unit time
I=
𝑄
𝑡
So, if you have 6 C of charges passing through a section of wire every 2 seconds, then your
current is: _________________
An important note about current:
Although potential decreases across a circuit, current is the same everywhere in a circuit!
Electrical Resistance (R)
• Electrons to flow due to potential difference. If the potential difference is removed,
current stops
• As electrons move, they bump into other atoms, this slows them down and impedes
their motion.
Resistance (R) is a measure of the degree to which a object __________________________.
Resistance is measured in Ohms ()
Ohm’s Law: Current, Voltage, and Resistance
Current = the potential difference applied across a circuit divided by the total resistance of the
circuit.
• If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw?
• If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the
resistance of the light bulb?
• Which of the following would cause the current in a circuit to decrease the most?
•
•
•
•
Increased voltage and increased resistance
Increased voltage and decreased resistance
Decreased voltage and decreased resistance
Decreased voltage and increased resistance
• If the resistance of a circuit were tripled, then the current through the circuit would be
____.
• one-third as much
• three times as much
• unchanged
• nonsense! There would be no way to make such a prediction.
human body resistance varies:
100 ohms if soaked with salt water;
moist skin - 1000 ohms;
normal dry skin – 100 000 ohms,
extra dry skin – 500 000 ohms.
What would be the current in your body if you touch the terminals of a 12-V battery with dry
hands?
But if your hands are moist and you touch 12 V battery, how much current would you draw?
Resistance & Power
What is resistance?
_____________________________________________________________________________________
All materials have some resistance.
Circuit elements (lights, motors, etc.) have much, much more resistance than the wires, and are often just
called ‘resistors’.
How do resistors affect circuits?
________________________________________________________________________________________
You can calculate the loss
of potential – called
______________________
– due to resistance using
Ohm’s Law.
Resistance in wires
The resistance wires that connect most circuits can be completely ignored!
But, in some cases, the wire is designed to have high resistance (and high ‘loss’ of energy to heat), usually by
making the wire very very long …
Examples: ________________________________________________________
The resistance of a conducting wire depends on four main factors:
• length
(___________________ = more resistance)
Resistance of a wire when
temperature is held constant:
• cross-sectional area (___________________ = more resistance)
• temperature (________________________ = more resistance)
•
resistivity of material (ρ – different materials have different values)
L – length,
A – cross sectional area
We do problems
1. What kind of wire is the best conductor?
2. If you double the length of a wire, how does the resistance change?
3. If you double the cross sectional area of a wire, how does resistance change?
4.
A copper wire (r = 1.72x10-8 Wm) has a length of 1.67 m and a radius of 1.00 mm. If the wire is connected to a
1.5-volt battery, how much current flows through the wire?
Electric Power
Electric power is the rate at which …
…energy is supplied to or used by a device
OR
… electric energy is converted into another form such as mechanical energy, heat, or light
Where is that energy coming from? This energy is equal to the potential energy lost by the charges as they
move through the circuit elements
Power is measured in __________________ called watts _____________.
What does a 60W light bulb mean?
It converts ____________________________________________________________________________
Equations for Power
Power We Do Problems
•
How much current is drawn by a 60 Watt light bulb connected to a 120 V power line?
•
What is the resistance of the bulb?
Power You Do Problems
1. Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a US household
outlet (120 V).
2. The sticker on a compact disc player says that it draws 288 mA of current when powered by a 9 Volt
battery. What is the power (in Watts) of the CD player?
Paying For Electricity
You pay for electricity by the Kilowatt-hour (kWh). What is a kWh? Simply another unit for energy.
Physicists measure energy in joules, but utility companies customarily charge energy in units of kilowatthours (kW h), where :
Kilowatt-hour (kWh) = 1000 W x 3600 s
1W x 1s = 1J
1 kWh = 3.6 x 106
At a rate of 14 cents per kWh, how much does it cost to keep a 100 W light bulb on for one day?
Drift Speed
When you connect a circuit, current flows IN EVERY PART of the circuit instantaneously (near the speed of light).
•
•
ALL free electrons in the circuit start moving the moment the circuit is connected / the moment the electric field
is applied.
Batteries create potential difference, they do NOT supply electrons; the electrons come from the wire.
Electrons move very fast (106 ms-1), but are not instantaneous.
Because electrons bounce around due to collisions with atoms in the wire, the
average speed of electrons opposite the flow of current – known as drift
speed – is VERY slow (0.01 cm/s)
This means it can take an electron 3 hours to travel through 1 m of
wire!
Define drift speed
______________________________________________________________________________________________
Respond to the statement: “Lights turn on so quickly after you flip the switch because electrons move very quickly.”
________________________________________________________________________________________________
DC circuits
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•
•
•
____________________________________________________
___________________________________________________
in a DC circuit the current always flows in the _________________.
Current flows from + to –
Electrons flow the opposite direction
AC circuits
•
•
•
__________________________ supply AC current
In an AC circuit ________________________ at a frequency of 50 Hz (US) or 60 Hz (Europe)
!! the source of electrons are the free electrons in wire itself !!
•
When you are shocked by AC current, the electrons that make the current come from your body. The current
makes the electrons in your body vibrate.
Why do power plants produce AC?
•
AC can be much more easily ________________________________________________________
____________________________________________________________________.
•
_____________________________________________________ because it is safer.
•
________________________________________________________________________________ –
thinner wires can be used and there is less loss to heat.
This means that AC is cheaper to transmit and the plants can be farther from the users.
Kirchoff’s Rules
1. ___________________________________________________________
The sum of the currents flowing into a node equals the sum of the currents flowing out of the node.
This is a slight alteration to our ‘current is the same everywhere rule’ that we used before.
It means that current can be split (or combined) at junctures.
2. __________________________________________________________
Emf = electromotive force = the potential difference supplied by a battery
This is exactly the same as our rule that voltage is ‘used up’ across a circuit.
Circuit Diagrams
You Do: Draw the corresponding schematic diagrams
Resistors in series



All current must ______________________________________
Each component has the ______________________ going through it
If current is disrupted through one component (e.g. a light goes out), then all components go out.
Equivalent resistance = the resistance that could replace all resistors and result in the same current
through the circuit.
Adding more resistors in series increases equivalent resistance!
Resistors in parallel





Current can _________________________________________________________
Current ____________________________________________________________ (greater current
goes through smaller resistors)
The current flowing into a node equals the current that flows out of that node
The voltage drop across each resistor is the same
______________________________________________________; if one resistor goes out, the
others keep working.
equivalent resistance is smaller than the smallest resistance.
We Do: Calculating Req
You Do: Calculating Req
Calculating current, potential drop, and power dissipated
•
To calculate current through a circuit, find the Req for all resistors in a circuit, then use Ohm’s Law (I = V/R)
Example: what is the current through this circuit?
To find potential drop across a resistor:
1.
find overall current
2. Use Ohm’s Law to find voltage drop
*Note: the voltage drop of EACH parallel resistor is equal to the voltage drop across the equivalent resistor
What is the current across resistors 2 and 3?
To find power dissipated across each resistor –
1) Calculate the voltage drop across each resistor
2) Calculate current through each resistor
3) Use the power law (P = IV)
What is the power dissipated across each resistor?
Capacitors, Ammeters, Voltmeters
Capacitors
Capacitors store ______________________________ by creating a _________________
____________________ usually on two parallel plates
Positive battery terminal pulls electrons from one plate and pushes them through
the terminal to the opposite plate.
Process continues until the ΔV of the plates = ΔV of the battery
◊ Capacitors are useful b/c they ___________________________________,
but can ______________________________________________ than a battery
◊ Ex amples: flash in a camera, amplifier in car audio system,
detonator, particle accelerator
Found in nearly all electronics!
Ammeters
Measures__________________ passing through it
• is always ______________________________________________________ with a component we want to measure in
order that whatever current passes through the component also passes the ammeter.
• has a ________________________________ compared with the resistance of the circuit so that it will not alter the
current the current being measured.
• would ideally have no resistance with no potential difference across it so no energy would be dissipated in it.
Voltmeter
Measures ______________________________ between two points

is always connected ________________________________________________
• has a ___________________________________________ so that it takes very little current from the device whose
potential difference is being measured.
• an ideal voltmeter would have infinite resistance with no current passing through it and no energy would be
dissipated in it.
Why should an ammeter have very low resistance?
Why should a voltmeter have very high resistance?
How are ammeters connected to a circuit and why?
How are voltmeters connected to a circuit and why?