Electricity & Magnetism

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Electricity & Magnetism
Chapters 17, 19, 20, 21 and 22-2

Chapter 17 - Charge The two different kinds of Electric charges are positive and negative Like charges repel – unlike charges attract Protons and neutrons are relatively fixed in the nucleus of the atom but electrons are easily transferred from one atom to another.

What causes charge? All charge is a result of the movement of electrons. All atoms begin as neutral- with no charge. If you take away negative electrons then the atom has a positive charge. If you add negative electrons then the atom becomes negatively charged. All atoms with a charge are called ions.

How do we charge objects?
What causes the electrons to move? Friction! When objects rub together electrons are moved from one object to the other. This causes one object to be positively charged and the other to be negatively charged and the process is called charge by contact.

Calculating charge 1 electron contains 1.6 X 10-19 coulombs of charge
C (coulomb) is the SI unit of electric charge 1.0 C contains 6.2 X 1018 electrons

Example problem How many electrons are in 0.85 C of charge?

Types of Materials Materials in which electric charges move freely are called conductors. Ex: Copper, Aluminum, most metals Materials in which electric charges do not move freely are called insulators. Ex: Wood, glass, styrofoam Semiconductors are materials between conductors and insulators. Ex: silicon, germanium

More Terms to Know Grounding is when a conductor is connected to the Earth by another conducting object such as copper wire. Many times it is a safety precaution in electrical devices. Induction is the process of charging a conductor by bringing it near another charged object and grounding the conductor.

More Terms to Know Electric Force – two or more charged objects near one another may experience motion either toward or away from each other because each object exerts a force on the other objects. Electric force is an example of a field force (a force which does not require physical contact to act).

Coulomb’s Law F = Electric Force (N) q = charge (C)
r = distance between charges (m) k = 8.99 X 109 Nm2/C2

Electric Field Electric field – a region in space around a charged object in which a stationary charged object experiences an electric force because of its charge. No contact needs to take place for this to occur

What is the electric force between a proton and an electron if they are separated by 2 cm?
q (proton) & q (electron) = 1.6 x C r = 2 cm = 0.02 m k = 8.99 x 109 F = ?

Current, Resistance & Voltage
Chapter 19

Electric Current Current is the rate at which electric charges move through a given area. SI unit is the Ampere or Amp. 1 A = 1 C/s I = ΔQ/t Current = charge / time

Example problem The current in a light bulb is A. How long does it take for a total charge of 1.67 C to pass a point in the wire? ΔQ = 1.6 C I = A t= ? I = ΔQ/t t = ΔQ/I t= 1.6C/0.835A t= 2.00s

Electric Current Batteries maintain electric current by converting chemical energy into electrical energy. Generators convert mechanical energy into electrical energy.

AC/DC There are two kinds of current:
Direct current is where charges are always moving in the same direction. Batteries produce direct current because the positive and negative terminals always stay the same.

AC/DC Alternating current is where the charges change the direction of flow constantly. Power plants supply alternating current to homes and businesses by using giant electromagnets to change positive and negative terminals. In the US current alternates (changes direction) 60 times every second while in Europe, current alternates 50 times every second.

Resistance Resistance- The opposition to the flow of current in a conductor R = V/I Resistance = Potential difference/Current SI unit – ohm Symbol-  (omega)

Resistance Resistance depends on length, cross-sectional area, material and temperature. Length: short = ↓ R; long = ↑ R Area: skinny = ↑R; wide = ↓R Material: insulator = ↑R; conductor = ↓R Temperature: hot = ↑R; cold = ↓R

Resistance Resistance is important in controlling the amount of current in a circuit. If the voltage is constant, resistance is the only way to adjust the current. Change the material of the wires, or add resistors to the circuit.

Example Problem The resistance of a steam iron is 19.0 Ω. What is the current in the iron when it is connected across a potential difference of 120V? R= 19.0 Ω V= 120V I= ? R=V/I I=V/R I=120V/19.0 Ω I= 6.32 A

Potential Difference The electric potential is the amount of energy contained in each unit of charge. Only differences in electric potential from one point to another are measured and used in calculations. Potential Difference is the change in energy per unit of charge. Potential Difference is also known as VOLTAGE, and is measured in volts (V).

Potential Difference

Potential Difference The potential difference between the positive and negative ends of batteries: All AA, AAA, C, D Cell Batteries = 1.5 V The only difference is how long they produce the 1.5 V. Car battery = 12 V Positive and Negative slots of an electrical outlet = 120 V

Electric Power Electric power is the rate of conversion of electrical energy Formula for Electric Power: P = IV Electric power = current X potential difference

Electric Power Because P= IV and V=IR we can also say;
P= IV = I(IR) = I2R P = I2R Or, because I = V/R, we can also say: P = IV = (V/R)V = V2/R P=V2/R

Electric Power An electric space heater is connected across a 120 V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater. P = V2/R R = V2/P R = 1202/1320 R = 10.9 Ω

Electric Power Power companies measure energy not power, using the kilowatt-hour as the unit One kilowatt-hour = the energy delivered in 1 hour at the constant rate of 1 kW. To convert between kWh and the SI unit of Joule: 1 kWh = 3.6 X 106 J

Example Problem How much does it cost to operate a W light bulb for 24 h if electrical energy costs \$0.080 per kWh? P= 100W = kW; t= 24 h Energy = Pt = kW*24 h = 2.4 kWh Cost = 2.4 kWh*\$0.080 = \$0.19

Circuits Chapter 20

Schematic Diagrams and Circuits
Schematic Diagram or Circuit Diagram: diagram which depicts the construction of an electrical circuit.

Symbols

Load- energy user of a circuit
Since bulbs have internal resistance, sometimes bulbs are drawn as resistors in circuit diagrams and treated as resistors in calculations. Electric circuit- a set of electrical components connected so that they provide one or more complete paths for the movement of charges. Load- energy user of a circuit All complete circuits must contain a source of potential difference and a load.

Closed vs. Open Closed circuit- there is a closed-loop path for the electrons to follow Open circuit- no complete path, no charge flow, no current.

Resistors in series Series- describes a circuit or portion of a circuit that provides a single conduction path without junctions. If any one bulb burns out, all of the bulbs go out because the broken filament becomes a break in the circuit.

Resistors in series When connected in series, the current is the same in all bulbs (or resistors). The equivalent resistance (Req) in a series circuit is the sum of all resistances. V = I/R can be used to find current and potential difference in a series circuit.

Resistors in parallel Parallel- describes two or more components in a circuit that are connected across common points or junctions, providing separate conduction paths for the current Because of this, a bulb can burn out and will not effect any other bulbs.

Resistors in series vs. parallel
Circuit Series Parallel Current I = I1= I2= I3 … Current is the same for each resistor and the same as total For Total Current: I = V/Req I = I1 + I2 + I3 … Sum of currents = total current Current across a resistor: I1=V/R1 and I2=V/R2 ,etc. Potential Difference V = V1 + V2 + V3 … Sum of potential differences = total potential difference. Potential difference across a resistor: V1 = IR1 and V2 = IR2 ,etc. V = V1 = V2 = V3 … Same for each resistor and same as total Equivalent resistance Req = R1 + R2 + R3 … Sum for each resistor 1/Req = 1/R1 + 1/R2 + 1/R3 … Reciprocal sum of resistances

Req = R1 + R2 + R3 + R4 Req = 2Ω+4Ω+5Ω+7Ω = 18Ω I = V/R
A 9V battery is connected to four light bulbs. Find the equivalent resistance for the circuit and the current in the circuit. Req = R1 + R2 + R3 + R4 Req = 2Ω+4Ω+5Ω+7Ω = 18Ω I = V/R I = 9V/18Ω = 0.5 A

1/Req = 1/R1+1/R2+1/R3+1/R4 1/Req = 1/2Ω+1/4Ω+1/5Ω+1/7Ω = 0.92Ω
A 9V battery is connected to four resistors. Find the equivalent resistance for the circuit and the total current in the circuit. 1/Req = 1/R1+1/R2+1/R3+1/R4 1/Req = 1/2Ω+1/4Ω+1/5Ω+1/7Ω = 0.92Ω I = V/R I = 9V/0.92Ω = 9.8 A

Magnetism Chapter 21

Magnets Every magnet has “poles” which contain opposite charges.
Like poles repel each other, and unlike poles attract each other due to their magnetic fields.

Magnetic Fields Magnetic Field (B) – region around a magnet with magnetic force Magnetic Fields are measured in Teslas (T) The direction of the magnetic field at any location is the direction in which the north pole of a compass needle points at that location

Earth’s Poles A compass is a magnet
Its north pole points north with regard to the Earth That means the magnetic South pole of the Earth is near the geographic North pole and the magnetic North pole of the Earth is near the geographic South pole!

Electromagnetism When a wire is carrying a current it creates a magnetic field of concentric circles around the wire. We use the “right hand rule” to describe the direction of the field around the wire. If the current changes direction the magnetic field changes direction.

Electromagnetism Right hand rule: Pretend the wire is grasped in your right hand with your thumb pointing in the direction of the current. Your fingers curl around the wire in the direction of the magnetic field.

Solenoids When wires are looped, the magnetic field works the same way. Several closely spaced loops create a device called a solenoid. Solenoids generate a strong magnetic field The more loops, the stronger the magnetic field The magnetic field can also be increased by inserting an iron rod through the center of the loops

Solenoid

Electromagnetism