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Electricity Guided Notes
Name:_____________________
Fundamental Forces
Physicists have identified four fundamental forces that account for the account all phenomena in the universe.
Many scientists believe that these four forces are different aspects of ONE fundamental force – this is the
search for the ________________________________.
Force
Strength
Distance of action
Description
Very strong!
Very, very Short
Holds the nucleus
together
Very weak
Short
Arise during radioactive
decay
Very weak
Infinite – but
decreases with the
square of distance
Attraction / repulsion of
___________________
Very, very,
very weak
Infinite – but
decreases with the
square of distance
Attraction between
____________________
With the exception of ___________________________, all the forces we studied up to now are caused on a
molecular level by ______________________________________________________ of objects – that is, they
are caused by the ________________________________.
Electric Charge
Electric charge is an intrinsic property of matter.
•
•
Defined by the effect (force) it produces.
Two types:
• Positive charge
• Negative charge
Electricity has its origin within the atom itself.
• __________________ have positive charge (+1)
• __________________ have negative charge (-1)
• __________________have no charge (0)
And that’s it! All charges come from protons or electrons!
Charge (q) is measured in units of ______________________
 Every electron has charge -1.6 x 10-19 C and every proton 1.6 x 10-19 C
 1C represents the charge of 6.25 billion billion (6.25x1018) electrons … this is the also amount of charge
passing through a 100-W light bulb in just over a second. A lot of electrons!
Charged and Uncharged Objects
Most objects are electrically ___________________ – that means they have _______________ number of positive
charges (protons) and negative charges (electrons).
Charged objects have _______________________of charges.
How to make an object charged?
• Protons are in the nucleus – they can’t move!
• Electrons are outside the nucleus – they can leave their atom, under the right circumstances.
• Add electrons and make the object ____________________________.
•
Remove electrons and make the object ___________________________.
Electrical conductors, insulators, semiconductors and superconductors
- distinction based on their ability to conduct (transfer between materials) electric charge.
Any material that allow charges to move about more or less freely is called ___________________. Tap water, human
body, and metals are generally good conductors.
What makes conductors conduct?

Conductors have __________________________ – this allows them to move around and distribute charge
•
Metals are great conductors due to metallic bonding – when metal atoms are close together, the valence
electrons move freely among all the metal nuclei creating a ‘sea of electrons’.

Insulators have ____________________ – their electrons are not free to move from atom to atom.
•
This makes insulators poor conductors of heat and electricity.
•
Examples: air, pure water, plastic, glass
________________________ are materials that can be made to behave sometimes as insulators, sometimes as
conductors. Eg. Silicon, germanium.
_______________________: thin layers of semiconducting materials joined together. Used to control flow of currents,
act as switches
________________________ Have zero resistance, infinite conductivity
•
Not common! Have to cool to very, very low temperatures.
•
Current passes without losing energy, no heat loss.
•
Under intense research! Many useful applications eg. transmission of power without loss, magnetically-levitated
trains…
Polarization
When an object is polarized
•
One side has __________ charge
•
The other side has _________ charge
•
The overall charge of the object is ____________
Electrostatic Force and Electric Fields
What do we already know about the electrostatic force?
Calculating Electrostatic Force: Coulomb’s Law
Coulomb’s law allows us to calculate the force between any two charged objects.
k is Coulomb’s law constant, and it has a value of 8.99 X 109 Nm2/C2
q1 and q2 are the amount of charge (measured in C) of each object
r is the distance between the objects (measured in m)
We do: Calculating Electrostatic Force
1)
A charge of +2 mC and a charge of -3 mC are separated by 0.1m. What is the force between them? If the
charges are each 0.001kg, what will be their acceleration?
2) Two charges are separated by a certain distance. If the distance between them is halved, how will the force
change?
You do: Calculating Electrostatic Force
1. A woman accumulates a charge of 2.0 x 10-5 C when sliding out of the seat of a car. A man has accumulated a
charge – 8.0 x 10-5 C while waiting in the wind. What is the force between them if they are 6 m away from each other?
2. A positive charge is 0.5 m away from a negative charge. If the size of the positive charge is doubled and the distance
is between them is also doubled, how will the force between the two charges change?
Electrostatic force when there are multiple charges
Calculating the force between two charges is easy. What do you do if there are multiple charges?
You can find the total force on any charge adding the forces from each other charge. Use Coulomb’s law to find the
force from each individual charge, and make a free body diagram!
Electric Fields
What is a ‘force’ field anyway?
Electric Field Lines
Electric field lines show the direction of the force on a positive test charge
Electric field lines point towards negative charges
Electric field lines point away from positive charges
The _______________ of lines shows the ___________________________of the electric field
This has two implications:
1) larger charges have more field lines radiating
2) As you move farther away from the charge, the strength of the field (density of lines) decreases
Net Electric Field Lines
If we have two or more charges creating an electric field, we ___________________________ from each
charge.
NOTE: Electric field lines ________________________! That would mean that a test charge would go in
two directions at once.
Questions
B
a) What is the charge of q1?
b) What is the charge of q2?
c) Which charge is larger?
d) Where will a positive charge at position A
move?
A
e) Where will an electron at position B
move?
4. What is the direction of the electric field at point C?
5. What is the direction of the electric field at point A?
6. What is the direction of the electric field at point B?
Electric Potential and Current
Potential Difference (V)
Recall Work: W = F d cos(q)
In order to bring two like charges together work must be done.
In order to separate two opposite charges, work must be done.
Electrical potential difference (V or ΔV) is the _____________________________________________________
__________________________________________________________________________________________
𝐽
Units are Volt = 𝐶
Charges will naturally move from high potential to low potential (the electric force does the work!).
There has to be work / input energy to move charges from low potential to high potential.
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 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.
a) What is the difference in height from the top to the bottom of a rollercoaster analogous to in a circuit?
b) 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
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!
Ohm’s Law and Power
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 ()
How do resistors affect circuits?
The energy of electricity comes from the movement of the electrons. So, by slowing down the electrons,
_______________________________________________________________________________________
Most of the loss of potential comes from the circuit elements, such as lights and motors (often just called
resistors) which are using the kinetic energy of the electrons to do something (make light, turn a motor, etc.).
We call this loss of electrical potential _____________________ (or potential drop).
Resistance in Wires
Most of the time, wires are designed to have as little resistance as possible, mainly by making them short,
thick, and out of a good conductor, such as copper.
So, in most cases, ________________________________________________________________________.
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 …
Ohm’s Law
V is potential difference across a resistor
I is current
R is resistance
Examples
• 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?
Electric Power
Units = Js-1 = W
Electric power is the rate at which electric energy is converted into another form such as mechanical energy,
heat, or light.
What does a 60W light bulb mean?
If we play around with the electric potential energy equation, we can derive another form of the power
equation that is more useful in circuits:
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?
A 9V battery costs $1.25. If placed in a cd player that draws a 300 mA current, it will last for 32 hours. What is
the price per kWhr?
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