Intro to Electrostatics & Electric Currents

Static Electricity? § Electrostatics: study of electrical

charges that can be collected and held in one place§ Electricity at rest, charged

objects

Recall: Structure of the Atom

§ Atoms are normally neutral§ Atoms that carry a charge = ions

§ Charges§ Protons = positive (+)§ Electrons = negative (-)§ Neutrons = neutral (0)

Charges§ Positively charged ion = Cation

§ Deficit of electrons

§ Negatively charged ion = Anion

§ Excess of electrons

§ Like charges repel

§ Opposite charges attract

Conservation of Charge§ Electrical charge cannot be created or destroyed§ Net amount of electrical charge produced in any process is always zero

Insulators & Conductors

Conductors§ Charge flows easily§ Ex: mostly metals

Insulators§ Charge doesn’t flow easily§ Ex: glass, plastic, rubber

Semi-conductor§ In-between§ Can allow or suppress electrical flow§ Ex: silicon, germanium, transistors

How to Move Charges§ There are three basic ways to

move charges between and/or within objects:§ Conduction§ Induction § Friction

§ Conduction: charges move between objects when they touch

§ Induction: separation of charge within an object because of the close approach of another charged object but without touching

§ Friction: electrons are physically stripped from one material and transferred to another

Electricity§ Electricity vs Electrostatics

§ Electrostatics = study of charges at rest § Electricity puts those charges in motion

§ Electric current is the flow of electric charge

§ How much charge passes through in a given amount of time§ Measured in amperes (A)

§ 1 A = Cs C = Coulomb = SI unit of electric charge

Producing Electric Current§ Electric current: a flow of charged particles§ Charges flow due to electrical potential difference

§ Potential difference provides the energy to create movement

§ Voltage: difference in electrical potential § Measured in volts (V)

§ Batteries, generators, and solar cells convert various forms of energy to electric energy

§ Conventional current: positive charges flowing from the positive plate to the negative plate

AC (alternating current)DC (direct current)

Electric Circuits

§ Electric circuit: continuous path in which electrical charges can flow§ Includes a charge pump that increases the potential energy of

the charges moving from A to B

Rates of Charge Flow & Energy Transfer§ Recall: What is the rate at which energy is transferred?

§ Power!§ Power of an electrical current can be determined by the following:

§ P = power§ I = electric current§ V = voltage; potential difference§ Unit = watts (W) or kilowatts (kW)

Electrical Power:P = I V

Sample ProblemA 6.0-V battery delivers a 0.50-A current to an electric motor that is connected across its terminals.a) What power is consumed by the motor?b) If the motor runs for 5.0 minutes, how much electric energy is

delivered?Given:§ V = 6.0 V§ I = 0.50 A§ t = 5.0 minUnknown:§ P = ?§ E = ?

Equations:a) P = IVb) P = E/t or E = Pt

Substitute & Solve:a) P = IV

= (0.50 A)(6.0 V) = 3.0 W

b) E = P t= (3.0 W)(5.0 min)(60 s/1 min)= 9.0 x 102 J

You Try!§ The current running through a lightbulb connected across the

terminals of a 120-V outlet is 0.50 A. At what rate does the bulb convert electric energy to light? § 60 W

§ A car battery causes a current of 2.0 A through a lamp while 12 V is across it. What is the power used by the lamp? § 24 W

§ What is the current through a 75-W lightbulb connected to a 120-V outlet?§ 0.63 A

Resistance & Ohm’s Law§ Resistance: opposition to the flow of electric charge; the property that

determines how much current will flow§ Ratio of potential difference to the current

§ Measured in ohms (Ω)§ 1 Ω = resistance that permits a current of 1 A to flow when a

potential difference of 1 V is applied across a resistance § Ohm’s law: states that the current through a conductor between two

points is directly proportional to the voltage across the two points

Resistance:

R=VI

Sample ProblemA 30.0-V battery is connected to a 10.0 Ω resistor. What is the current in the circuit?Given:§ V = 30.0 V§ R = 10.0 ΩUnknown: § I = ? Equation:

§ I = VR

Substitute & Solve:

§ I = VR

= 30.0 V10.0 Ω

= 3.00 A

You Try!§ An automobile headlight with a resistance of 30 Ω is placed

across a 12-V battery. What is the current through the circuit?§ 0.4 A

§ A motor with an operating resistance of 32 Ω is connected to a voltage source. The current in the circuit is 3.8 A. What is the voltage of the source? § 1.2 x 102 V

Power Revisited§ When charge moves through a resistor, its potential difference is

reduced§ We learned earlier that:

§ Electrical power can be determined by P = I V

§ We also know through Ohm’s Law that V = I R and I = VR

§ If the expression for electrical potential difference and current are substituted into the power equation, then we can derive the following:

Electrical Power Related to Current & Resistance:

P = I2 R

Electrical Power Related to Voltage & Resistance:

P = V2

R