Physics 4.4Physics 4.4

ChargeCharge

What is charge?What is charge? Where do you see charge around you?Where do you see charge around you? Describe the atom in terms of charge?Describe the atom in terms of charge?

Coulombs and elementary chargesCoulombs and elementary charges

1 coulomb (C) is equal to 6.25 E18 1 coulomb (C) is equal to 6.25 E18 elementary chargeselementary charges

1 elementary charge is equal to the charge 1 elementary charge is equal to the charge of one proton (+), one electron (-), or of one proton (+), one electron (-), or 1.60 E-19 C 1.60 E-19 C

Electrostatic forces between Electrostatic forces between objects near each otherobjects near each other

What does a positively charged object do What does a positively charged object do the a:the a: Positive objectPositive object

repelrepel

Negative objectNegative object attractattract

Neutral objectNeutral object alignmentalignment

Law of Conservation of ChargeLaw of Conservation of Charge

Charge cannot be created or destroyed-the total Charge cannot be created or destroyed-the total charge in the system remains constant. charge in the system remains constant.

Ex/ think about when you rub a balloon on your Ex/ think about when you rub a balloon on your hair. Your hair loses electrons and the balloon hair. Your hair loses electrons and the balloon gains negative charge!gains negative charge!

When charged objects are in contact, to find When charged objects are in contact, to find individual charges, add up the total charge and individual charges, add up the total charge and divide by the number of objects. They all have divide by the number of objects. They all have the same charge.the same charge.

Coulomb’s LawCoulomb’s Law

Electrostatic force (FElectrostatic force (Fee) is directly ) is directly proportional to the product of the charges proportional to the product of the charges (q1 x q2) and inversely proportional to the (q1 x q2) and inversely proportional to the square of the distance between the square of the distance between the charges (rcharges (r²).²).

Coulomb’s LawCoulomb’s Law

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Charge on sphere A is Charge on sphere A is +5.0 E-4 C+5.0 E-4 C. . Charge on sphere B is Charge on sphere B is +3.0 E-5 C+3.0 E-5 C. . Distance separated is Distance separated is 3.0 meters3.0 meters. . Find: electrical force FFind: electrical force Fe.e.

FFe e = kq1xq2 / r= kq1xq2 / r²²

Electric FieldElectric Field

Any place where there is an electrostatic Any place where there is an electrostatic force (attractive or repulsive) between the force (attractive or repulsive) between the two charged particles, we say the there is two charged particles, we say the there is an electric field between them.an electric field between them.

E = Fe / qE = Fe / q Electric field strength tells how strong an Electric field strength tells how strong an

electric field is. electric field is. Units are N/CUnits are N/C

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At point P in an electric field, the At point P in an electric field, the magnitude of the electrostatic force on a magnitude of the electrostatic force on a proton is 4.0 E-10 newton. What is the proton is 4.0 E-10 newton. What is the magnitude of the electric field intensity at magnitude of the electric field intensity at point P?point P?

E = Fe / qE = Fe / q

Electric fields around point charges Electric fields around point charges or spheresor spheres

An electric field (electric field lines or lines An electric field (electric field lines or lines of force) goes away from positive and of force) goes away from positive and goes toward the negative. Electric field goes toward the negative. Electric field lines never intersect (never cross) each lines never intersect (never cross) each other.other.

Draw electric field lines around:Draw electric field lines around: A positive point chargeA positive point charge A negative point chargeA negative point charge

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An object with a net charge of 4.80 E-6 An object with a net charge of 4.80 E-6 coulomb experiences an electrostatic force coulomb experiences an electrostatic force having a magnitude of 6.00 E-2 newton having a magnitude of 6.00 E-2 newton when placed near a negatively charged when placed near a negatively charged metal sphere. What is the electric field metal sphere. What is the electric field strength at this location?strength at this location?

E = Fe / qE = Fe / q

Electric field lines between two Electric field lines between two parallel platesparallel plates

Electric field lines go away from the Electric field lines go away from the positive plate and go toward the negative positive plate and go toward the negative plate.plate.

Electric field strength is the same Electric field strength is the same everywhere between the plates.everywhere between the plates.

Where would an electron accelerate Where would an electron accelerate towards? A proton? A neutron? Which towards? A proton? A neutron? Which would accelerate faster? Why?would accelerate faster? Why?

Potential DifferencesPotential Differences

Potential difference (or potential drop) is Potential difference (or potential drop) is the work or energy required to move a the work or energy required to move a charged particle towards a charged particle towards a likelike charged charged particle divided by charge in coulombs (C). particle divided by charge in coulombs (C).

V = W / qV = W / q Units = volts (V) or Units = volts (V) or joules per coulombjoules per coulomb..

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The energy required to move one The energy required to move one elementary charge through a potential elementary charge through a potential difference of 5.0 volts is:difference of 5.0 volts is:

V = W / qV = W / q

Electric currentElectric current

Number of charges that reach a point in a Number of charges that reach a point in a wire in one second.wire in one second.

I = I = ∆q / t∆q / t Units = C / s or amperes (A).Units = C / s or amperes (A).

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If 10. coulombs of charge are transferred If 10. coulombs of charge are transferred through an electric circuit in 5.0 seconds, through an electric circuit in 5.0 seconds, then the current in the circuit is:then the current in the circuit is:

I = I = ∆q / t∆q / t

ResistanceResistance

Resistance hinders the flow of charges, Resistance hinders the flow of charges, which means the flow of current (I).which means the flow of current (I).

Increased temperature means bigger Increased temperature means bigger resistance.resistance.

R = V / IR = V / I is called Ohm’s Law. is called Ohm’s Law. Units are volts per ampere or ohm (Units are volts per ampere or ohm (ΩΩ).).

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In a simple electric circuit, a 110 volt In a simple electric circuit, a 110 volt electric heater draws 2.0 amperes of electric heater draws 2.0 amperes of current. The resistance of the heater is:current. The resistance of the heater is:

R = V / IR = V / I

ResistivityResistivity

Characteristic of a material at a specific Characteristic of a material at a specific temperature.temperature.

Resistance is affected by temperature plus Resistance is affected by temperature plus several factors which are displayed in this several factors which are displayed in this equation:equation:

R = R = ρρ (resistivity) x L (length of wire) (resistivity) x L (length of wire)

A (cross-sectional area)A (cross-sectional area)

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A 12.0 meter length of copper wire has a A 12.0 meter length of copper wire has a resistance of 1.50 ohms. How long must resistance of 1.50 ohms. How long must an aluminum wire with the an aluminum wire with the same cross-same cross-sectional areasectional area be to have the same be to have the same resistance?resistance?

R = R = ρρL / AL / A

Series CircuitsSeries Circuits

I = II = I11 = I = I2 2 = I= I33 =… =… V = VV = V11 + V + V22 + V + V33 +… +… RReqeq = R = R11 + R + R22 + R + R33 +… +…

Parallel CircuitsParallel Circuits

I I = I= I11 + I + I2 2 + I+ I33 +… +… VV = V = V11 = V = V22 = V = V33 =… =… 11 1 1 1 1 11 RReqeq = R = R11 + R + R22 + R + R33 +… +…

Ammeters and VoltmetersAmmeters and Voltmeters

An ammeter is used to measure current An ammeter is used to measure current and is connected in series with the circuit and is connected in series with the circuit element.element.

A voltmeter is used to measure the voltage A voltmeter is used to measure the voltage and is connected in parallel.and is connected in parallel.

Conservation of charge in a circuitConservation of charge in a circuit

PowerPower

Electric power is the product of voltage Electric power is the product of voltage and current. and current.

P = IV = IP = IV = I²R = V²/R²R = V²/R

Magnetic Field StrengthMagnetic Field Strength

Magnetic field strength is strongest where Magnetic field strength is strongest where the field lines are closest.the field lines are closest.

Electromagnetic InductionElectromagnetic Induction

When a conductor of wire cuts across When a conductor of wire cuts across magnetic field lines, one end of the wire magnetic field lines, one end of the wire becomes more negative and one end becomes more negative and one end more positive, producing a potential more positive, producing a potential difference. difference.

This induction is greatest when the wire This induction is greatest when the wire moves perpendicular to the magnetic field moves perpendicular to the magnetic field lines at a greater speed.lines at a greater speed.