Electricity & Magnetism

Physical Science

Static Electricity

• Atoms are made of charged particles:– Electrons: orbit the

nucleus of the atom and have a negative charge

– Protons: bound in the nucleus of the atom and have a positive charge

Static Electricity

• The accumulation of excess electric charge on an object is called static electricity

Static Electricity• Charges exert forces

on each other:– Like charges repel

–Opposite charges attract

Static Electricity• Charges do not have to

touch to exert these forces• Surrounding every electric

charge, there is an electric field.

• Any charge in an electric field feels an electrical force.

Charging Objects

• Charging by contact: transferring charge by touching or rubbing

• Just like transferring heat, transferring charges by touching is called conduction

Induction• Charging by Induction: the rearrangement of a

neutral system caused by a nearby charge

Grounding• Grounding: charging or

neutralizing an object by connecting negative charge from the earth

Warm Up

• What are the three methods of charge transfer?

Grounding

Charge by contact

Induction

Circuits

• A Circuit is a closed path through which electric charge flows.

• Excess electric charges repel and flow toward a positively charged area

Electric Flow

• Conductor: a material in which electrons are able to move easily

• Insulator: a material in which electrons are not able to move easily

Electric Current

• Electric current (I) is the net movement of electric charges in a single direction

• Measured in Amperes (A)

Electric Current and Switches• As long as the system is

closed, current flows. • A switch can open and

close the circuit to allow or prevent electric current

Electric Current

• Voltage Difference (or Potential Difference, or Voltage) is the ability to move an electric charge through a resistance

• Voltage Difference is measured in Volts (V)

Electric Current• A battery is a device

that stores chemical energy and makes it available as electrical energy

• A battery can provide voltage

Resistance

• Resistance: the tendency of a material to oppose the flow of electrons

• The SI unit for resistance is Ohms (Ω)• Resistance changes electrical energy into

thermal energy and light

Resistance

• Insulators have more resistance than conductors

• Thinner wires have more resistance

• Longer wires have more resistance

• Hotter wires have more resistance

Warm-Up

• 1. If a tow truck has a mass of 16,000 kg and is accelerating at 5m/s2 what is its pulling force?

• F=ma (16000)(5m/s2) = 80,000 N• 2. If the tow truck’s pulley can do 20,000 joules of

work in 20 seconds how much power does it have? P= W/T (20000)(20) = 1000 Watts• 3. If the same tow truck towed a car 5 km and did

100,000 joules of work how much force is applied?• W= FD 100,000= (F) (5) =20000 N

What Makes a Light Bulb Glow?The filament of a light bulb is a very thin wire.

When current flows through the wire, electrons collide with metal atoms of the wire.

Electrical energy is converted to thermal energy and the bulb becomes hot enough to glow.

Circuit Symbols

• Current:

I – measured in Amperes (A)• Voltage Difference:

V – measured in Volts (V)• Resistance:

R Measured in Ohms – (Ω)

Ohm’s Law

• Ohm’s Law is the relationship between voltage difference, current, and resistance

• I = V/RI R

V

Ohm’s Law• Example: An iron with a resistance

of 9.6-Ω is plugged into an electrical outlet with a voltage difference of 120-V. What current flows in the iron?

I = V/R

I = ? V = 120 R = 9.6

I = 120/9.6 = 12.5-A

Practice Problems

1. How much resistance does a light bulb have if a 2.8 V battery draws .35 A of current?

I= V/R .35= 2.8 V / R = .98 Ω

2. What is the voltage needed to provide a 3.2 kΩ resistor with 88.2 mA of current?

I=V/R 88.2= V/3.2 kΩ V= 282.24

Series Circuits

• Series circuits have one loop through which current can flow

Example:

Parallel Circuits

• Parallel Circuits have two or more branches through which current can flow

Parallel Circuits• In a parallel circuit: – Voltage difference is the same across all branches– More current flows through branches with lower

resistance• If one branch of aparallel circuit is cut,energy can still flow down another branch.

Example:

Parallel Circuits

• In your house: Parallel circuits with circuit breakers

• Circuit breakers have a main breaker in series with many parallel circuits. The parallel circuits each have circuit breakers. When the wires overheat, the circuit breaker switches off the current.

Warm Up

• What is the voltage in a battery that provides a 50Ω resistor 80mA?

• How much resistance does a circuit have if 30 A flows through it with a volt difference of 100 V?

I= V/R .080A= V/50ΩV= 4

I= V/R 30A= 100V/ I Ω= 3000

Electric Power

• Electric Power: The rate at which electric energy is converted into another form of energy

P = IVPower is measured in WattsCurrent in AmperesVoltage in Volts

• Appliances with heating elements usually use more electric power then things that don’t have heat

Example Problem

A 1.5 V battery provides 2A to a light bulb. How much power does the light bulb draw?

P= VI P= (1.5) (2) P= 3 watts

Electric Power• A 9 Volt battery powers a digital clock. The current through the

clock is 6.67 Amps. How much power does the clock use?

• A light bulb uses 60 Watts of Power. It is plugged into a 120 Volt electrical outlet. What current flows through the bulb?

P= VI (9) (6.67) P= 60.03 Watts

P= VI 60 W= (120 V) (I) I = 0.5 A

Warm Up

1. If a generator has 10 watts of power and can complete 50 J of work. How long would that take in minutes?

2. What is labeled on opposite ends of a magnet?

Magnets• Properties of Magnets:– Magnets have magnetic fields, which exert forces

on other magnetic materials– Magnets have a north pole and a south pole

Magnetic Fields

• Magnetic field lines travel from the magnet’s north pole to the south pole

• Magnetic fields are strongest at the magnet’s poles

Magnetic Domains

• Electric particles have magnetic properties• When different particles align their poles in similar

directions, magnets are formed• Domains are groups of atoms with particles with

aligned magnetic fields

Magnetic Domains• Different objects have different natural states of their

domains– This determines why a “magnet” is a “magnet”

• Difference between a nail and a permanent magnet

Permanent v. Induced Domains

• When a magnetic field is brought close to another non-magnet, the domains line up to the permanent angle

Electromagnets

• An electric current traveling through a wire produces a magnetic field around the wire

• An electric current traveling through a coil of wire produces a magnetic field inside the coil. This creates an electromagnet