Physics Unit Test Review
Definitions:
Static Electricity: a charge on a substance that stays in 1 place
Current electricity: amount of charge that passes a point in a conducting wire every second of
movement/flow of charge.
Electrostatics: study of electric charges
Electrostatic Series: the electrostatic series determines the charge on 2 substances when in contact with
another material.
(Higher = positive & lower = negative)
{Negative = more apt to capture electrons} {Positive = more apt to give up electrons}
Insulators: a substance in which electrons cannot move from atom to atom and the charge stays on the
spot where you rub the object
(ex.: cotton, wood, rubber)
Conductors: any material that allows charges to move free
(ex.: “generally metals” aluminum, copper, iron)
Discharged: if a charged object has all excess charges removed
Neutralized: if a charged object has all excess charges removed
Grounded: a way to discharge something; connect it to the earth by means of a conductor
Induction: when there is a transfer of charge without contact
Electric Potential: energy that each electron holds; known as voltage
Voltage: energy that each electron holds
Current: measure of rate electric charges move past s given point on a circuit
Resistance: ability to slow down flow of electrons in conductors
Energy: energy provided by flow of electrons (through circuit)
Series Circuit: has only 1 path to flow; the current is at any point in the circuit is exactly the same
Parallel Circuit: 2 or more different paths for current to flow; current isn’t same throughout the circuit
Electrical Power: describes the amount of power converted into heat, light, sound or motion each second ;
how fast an appliance is using up electrical energy
Renewable energy:
Non-renewable energy:
Electrical efficiency: determines how well electrical energy is changed into useful energy by the electrical
device
Sustainability: the social, economic and environmental aspects of the production and use of electrical
energy for now and the future; production of energy shouldn’t produce increased burden on environment
or require unsustainable use of non-renewable sources
The Electrical Nature of Matter:
Law of electric charges:
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Like charges repel each other
Opposite charges attract each other
Charged objects attract neutral objects
How do materials become charged? What subatomic particle is involved?
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Charged by contact
o When one substance touches another and electrons move from one to the other.
o The object that was touched (pith ball) becomes overall negative and when rod is
brought close again to ball, like charges repel.
Charged by induction
o A transfer of charge without contact
Charged by friction
o When one substance merely touches or rubs another.
o One substance becomes positively charged while the other becomes negatively charged
 Ex. Walking across a carpet
 Brushing hair in colder months
The subatomic particle involves are electrons. Protons cannot move.
Charging by friction:
For example if fur rubs on a polyethylene strip. Due to the electrostatic series, the fur has a weaker hold
on electrons, thus it is positively charged. The strip is negatively charged because it has a stronger hold on
electrons. When charging by friction one substance will become positively charged as one becomes
negatively charged.
Charging different substances by contact:
Before contact, the ball is neutral and the strip is negative. When the strip comes close to the ball, the
protons in the ball become attracted to the strip and the electrons move to the other side of the ball. When
touched by the strip, the electrons move from the strip towards the ball. This creates an overall negative
charge on the pith ball because of excess electrons. After the contact, if the strip is brought close to the
ball, the ball repels (like charges repel).
When an object is charged by contact, what kind of charge does the object have compared with that on the
object giving the charge?
When an object is charged by contact, the object has the same charge compared with that on the object
giving the charge.
Why can charging by contact be dangerous?
Insulators and Conductors:
The difference between insulators and conductors is that insulators are substances where electrons cannot
move from atom to atom and the charge stays on the spot where you rub the object. Conductors on the
other hand, are any material that allows charges to move free. An example of a conductor is aluminum
and an example of an insulator is wood.
Why are problems of static electricity more common in the winter?
Problems of static electricity more common in the winter because the air is much dryer. This dry air is
considered an insulator thus, when we touch a door knob (conductor) we receive a shock.
How can we reduce these problems?
We can reduce these problems by buying a humidifier. By doing so, there will be more water in the air
and that means more moisture. The water molecules will transfer electric charges easily.
What is an in-between conductor?
A fair conductor allows charges to move less than conductors and more than insulators. For example,
salty water or the human body.
Discharging electrically charged objects:
Grounding is when you connect something to the earth by mean of conductor to discharge it. Discharging
at a point is when conductors shaped into a point lose their charge quickly because of a buildup of
negative charge in a small surface. Thus, electrons repel out of the conductor. (ex.: airplanes)
Induction:
Induction is when there is a transfer of charge without contact. Induced charge separation is when you
bring a charged object towards a neutral charged object. The electrons shift away if they are repelled or
towards the charged object if attracted.
Positive rod:
Negative rod:
When an object is charged by induction, what kind of charge does the object have compared with that on
the object inducing the charge?
When an object is charged by induction, it is opposite to the charge on the object inducing the charge.
Lightning:
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Water and ice move around inside the cloud after forced up by warm air currents
The particles in the cloud become charged. Then the charges separate (positive moves up,
negative move down).
A build-up of positive charge on the ground is attracted to the negative charge in the bottom of
the cloud.
The positive charge from the ground connects with the negative charge from the clouds and a
spark of lightning strikes.
Interesting Insulators and conductors:
Dryer sheets release positively-charged particles to balance out loose electrons. This means there will be
less electrons moving from one shirt to another and less attraction (because they’re more positive and not
attracted to other positives).
Electricity and Electric Circuits:
Four parts of an electric circuit:
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Battery (source of electrical energy)
Light Bulb (electrical load)
Switch (Control device)
Connecting wires (electrical conductor)
Drawing schematic diagrams:
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Battery
Connecting wires
Open switch
Closed Switch
Light bulb
Ammeter
Voltmeter
Open and closed circuits:
An open circuit mean the current cannot pass through to the positive end. A closed circuit means it is a
complete circuit and the current can flow to the positive end.
Static and Current Electricity:
Static Electricity is when a charge on a substance that stays in 1 place.
Current electricity is an amount of charge that passes a point in a conducting wire every second of
movement/flow of charge.
Voltage:
The electric charges flow into the circuit from the negative terminal.
Unit: Volts
Symbol: V
Series Circuit:
In a series circuit, if yo have 2 lights in series, and bags of pretzels (batteries) were placed one after
another, you’d gain a pretzel at each bag. The voltage would be divided. There is more than one light
thus, you need to divide the voltage so you have the same amount per load.
Parallel Circuit:
In a parallel circuit, if there were 2 lights in parallel, and the bags of pretzels were placed one after
another, you’d gain a pretzel at each bag. The voltage would be equal. In a parallel, you only take one
path. That means you only pass one light bulb. So, you don’t need to divide voltage.
Equation for Voltage: V
=IxR
Example: If the current was 10 amps, and the resistance was 2 ohms, what would the voltage be?
I = 10 A.
R = 2 ohms
V=?
V=IxR
V = 10 A x 2 ohms
V= 20 V
The voltage would be 20 V.
Electrochemical cells:
Primary cell: cannot be recharged and need to be replaced
2 types of primary cells:
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What makes these 2 primary cells different?
Secondary cell: they can be recharged and reused numerous ties befre being recycled
Batteries/cells in Series and Parallel:
What happens to voltage when we connect batteries in series?
-Voltage is divided
What happens to voltage when we connect batteries in parallel?
-Voltage is equal
Advantages of battery connected in series?
-doubles voltage per load
-current stays the same
Advantages of battery connected in parallel?
-equal voltage
-doubling current
Electric Current:
Symbol = I
Unit: Amperes
Current in parallel:
-current is divided; more than one path you can take
-in the pretzel experiment, you had more than one path to take so, the current will double
Current in series
-current stays the same; only one path to take
-in the pretzel experiment, you only had one path so everyone followed the same path (the current
is equal)
Equation for Current:
I=V÷R
Example: If you had 90 V and a resistance of 9 ohms, what would the current be?
V = 90V
R = 9 ohms
I=?
I = V÷ R
I = 90V÷ 9ohms
I = 10 A
The current is 10 Amperes.
What instrument measures current?
Ammeter
How is an ammeter connected to a circuit?
An ammeter is connected in series
What happens if the ammeter isn’t connected properly?
If the ammeter is connected in parallel instead, it will only pick up current from one path. In a parallel, the
current doubles so, it won’t pick up the current from the other path.
Why is electricity dangerous?
Electricity is very dangerous. When above let-go threshold, the person wont be able to remove their hands
from the object giving the electrical shock. With only 0.05 A, your heart muscles will stop pumping and
flutter.
How should we help someone who is experiencing an electric shock?
1. Make sure you don’t receive the shock as well
2. Pull the victim from the source using a non-conducting material such as wood
Electrical resistance and Ohm’s law:
Resistance symbol: R
Unit: Ohm
Resistance in a circuit:
The resistance in the circuit is like when playing soccer. To get to the other side of the field (where the net
is) you have to get through the other team. The opposing teams acts as a resistor and slows you down.
You are the electrons in a circuit. So, when you encounter a resistor the flow of electrons slow down.’
Equation for Resistance: R
=V÷I
Example: If the voltage is 10V and current is 5 A, what is the resistance?
V= 10V
I= 5 A
R=?
R=V÷I
R = 10V ÷ 5A
R = 2 ohms
The resistance is 2 ohms.
Patterns with voltage, resistance and current:
1. When voltage remains the same, current increases
2. When current remains the same and voltage increases, the resistance also increases
3. When resistances stays the same and voltage increases, the current increases
Electric Circuits and Multiple Loads:
Series circuit:
- Only one path for current to flow
- Current at any point is the same
- If batteries were placed one after another, current would be equal and voltage would be divided
Parallel circuit:
- More than 1 path for current to flow
- Current is divided
- If batteries were placed one after another, current would be divided and voltage would be equal
Current and voltage in series when more loads added
- Current stays the same
- Voltage is divided (3 loads = 1/3 of V per light)
Current and voltage in parallel when more loads added
- Current is divided (more than one path to flow)
- Voltage stays the same
Measuring Electrical Energy:
3 factors that determine amount of electrical energy:
 Voltage drop
 Current
 Time
Equation for electrical energy: V x I x ∆ t
Example: If the voltage is 10 V, current is 2 A and time is 1 hour, what is the energy?
V = 10V
I=2A
t = 1 hour
E=VxIx∆t
E = 10V x 2A x 1 hr
E = 20 joules
The electrical energy is 20 joules.
Unit for Electrical Energy: joules (J)
Symbol: E
How do we determine the electrical energy if we are given the rated capacity?
E=VxIx∆t
I x ∆ t = Rated Capacity
E = V x Rated Capacity
The Rate at Which Energy is used:
Electrical power: describes the amount of electrical energy converted into heat, light, sound or motion
each second (how fast an appliance uses up electrical energy)
Unit: Watt (W)
Symbol: E
Equation for power: P = I x V
Example: If the current is 10 A and battery is 5 V, what is the power?
I = 10A
V = 5V
P=?
P=IxV
P = 10A x 5V
P = 50 Watts
The power is 50 W.