2 Chapter 17 - ChargeThe two different kinds of Electric charges are positive and negativeLike charges repel – unlike charges attractProtons and neutrons are relatively fixed in the nucleus of the atom but electrons are easily transferred from one atom to another.
3 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.
4 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.
6 Calculating charge 1 electron contains 1.6 X 10-19 coulombs of charge C (coulomb) is the SI unit of electric charge1.0 C contains 6.2 X 1018 electrons
7 Example problemHow many electrons are in 0.85 C of charge?
8 Types of MaterialsMaterials in which electric charges move freely are called conductors.Ex: Copper, Aluminum, most metalsMaterials in which electric charges do not move freely are called insulators.Ex: Wood, glass, styrofoamSemiconductors are materials between conductors and insulators.Ex: silicon, germanium
9 More Terms to KnowGrounding 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.
10 More Terms to KnowElectric 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).
11 Coulomb’s Law F = Electric Force (N) q = charge (C) r = distance between charges (m)k = 8.99 X 109 Nm2/C2
12 Electric FieldElectric 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
13 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 Cr = 2 cm = 0.02 mk = 8.99 x 109F = ?
15 Electric CurrentCurrent is the rate at which electric charges move through a given area.SI unit is the Ampere or Amp.1 A = 1 C/sI = ΔQ/tCurrent = charge / time
16 Example problemThe 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/tt = ΔQ/It= 1.6C/0.835At= 2.00s
17 Electric CurrentBatteries maintain electric current by converting chemical energy into electrical energy.Generators convert mechanical energy into electrical energy.
18 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.
19 AC/DCAlternating 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.
20 ResistanceResistance- The opposition to the flow of current in a conductorR = V/IResistance = Potential difference/CurrentSI unit – ohm Symbol- (omega)
21 ResistanceResistance depends on length, cross-sectional area, material and temperature.Length: short = ↓ R; long = ↑ RArea: skinny = ↑R; wide = ↓RMaterial: insulator = ↑R; conductor = ↓RTemperature: hot = ↑R; cold = ↓R
22 ResistanceResistance 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.
23 Example ProblemThe 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/II=V/RI=120V/19.0 ΩI= 6.32 A
24 Potential DifferenceThe 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).
26 Potential DifferenceThe potential difference between the positive and negative ends of batteries:All AA, AAA, C, D Cell Batteries = 1.5 VThe only difference is how long they produce the 1.5 V.Car battery = 12 VPositive and Negativeslots of an electrical outlet = 120 V
27 Electric PowerElectric power is the rate of conversion of electrical energyFormula for Electric Power:P = IVElectric power = current X potential difference
28 Electric Power Because P= IV and V=IR we can also say; P= IV = I(IR) = I2RP = I2ROr, because I = V/R, we can also say:P = IV = (V/R)V = V2/RP=V2/R
29 Electric PowerAn 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/PR = 1202/1320R = 10.9 Ω
30 Electric PowerPower companies measure energy not power, using the kilowatt-hour as the unitOne 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
31 Example ProblemHow 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 hEnergy = Pt = kW*24 h = 2.4 kWhCost = 2.4 kWh*$0.080 = $0.19
35 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 circuitAll complete circuits must contain a source of potential difference and a load.
36 Closed vs. OpenClosed circuit- there is a closed-loop path for the electrons to followOpen circuit- no complete path, no charge flow, no current.
37 Resistors in seriesSeries- 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.
38 Resistors in seriesWhen 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.
39 Resistors in parallelParallel- describes two or more components in a circuit that are connected across common points or junctions, providing separate conduction paths for the currentBecause of this, a bulb can burn out and will not effect any other bulbs.
40 Resistors in series vs. parallel CircuitSeriesParallelCurrentI = I1= I2= I3 …Current is the same for each resistor and the same as totalFor Total Current:I = V/ReqI = I1 + I2 + I3 …Sum of currents = total currentCurrent across a resistor:I1=V/R1 and I2=V/R2 ,etc.Potential DifferenceV = 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 totalEquivalent resistanceReq = R1 + R2 + R3 …Sum for each resistor1/Req = 1/R1 + 1/R2 + 1/R3 …Reciprocal sum of resistances
41 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 + R4Req = 2Ω+4Ω+5Ω+7Ω = 18ΩI = V/RI = 9V/18Ω = 0.5 A
42 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/R41/Req = 1/2Ω+1/4Ω+1/5Ω+1/7Ω = 0.92ΩI = V/RI = 9V/0.92Ω = 9.8 A
44 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.
45 Magnetic FieldsMagnetic Field (B) – region around a magnet with magnetic forceMagnetic 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
46 Earth’s Poles A compass is a magnet Its north pole points north with regard to the EarthThat 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!
47 ElectromagnetismWhen 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.
48 ElectromagnetismRight 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.
49 SolenoidsWhen 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 fieldThe more loops, the stronger the magnetic fieldThe magnetic field can also be increased by inserting an iron rod through the center of the loops