Electricity and Magnetism

Flashlight

• Why do the batteries have to be facing the same way in order for the flashlight to work?

Electric charge

• Electric charge causes subatomic particles to attract or repel each other – Subatomic particles are protons, electrons– Protons are +– Electrons are –

Ex: static cling, lightning

Electric Charge

• Electric charge is measured in Coulombs (C)• It takes about 6.24 x 1018 electrons to produce

1 coulomb• A lightning bolt is about 10-20 coulombs of

charge• A flash camera uses the energy from 0.025

coulombs of charge to produce each flash

Electric Forces

• Like charges repel• Opposite charges attract • Electric force – the force of attraction or

repulsion – The amount of electric force depends on the

charge and distance

Electric fields

Electric field

• Electric field- the effect an electric charge has on other charges in the space around it is the charge’s electric field – The strength of this field depends on the amount

of charge that produces the field and the distance from the charge

– The more charge the greater the force

Static Electricity

• Static electricity is the study of electric charges – Charges can be transferred by• friction, • contact, • induction

Static electricity

• Charging by friction- electrons move from your hair to the balloon– Atoms in the balloon have greater attraction for

electrons than the atoms in your hair

Static Electricity

• Charging by contact – Van de Graaff generator is a charged metal sphere, when her hand touches the sphere, she acquires a charge

Static Electricity

• Charging by induction- transfer of charge without contact between materials

• ex:rubbing your feet on a carpet and then touching a doorknob– You picked up extra electrons from the carpet so

your hand is negatively charged, Electrons move from the doorknob

Static discharge

• When a pathway through which charges can move forms suddenly – Shock from the doorknob– Lightning

Electric Current

Electric current

• Electric current – the continuous flow of electric charge – Unit of electric charge is the amp (ampere)

1 amp = 1 coulomb per second

Electric current

• Current is defined as the direction the positive charges would flow

• Two types of current – Direct – DC- battery operated devices

– Alternating current – AC- flow of electric charge that reverses direction

Conductors and Insulators

• Conductor- charge can flow easily– Metals - have loosely held electrons that can

conduct a charge

• Insulator – charge cannot flow easily– Wood, plastic, air

Resistance

• Resistance- opposition to the flow of charges in a material– Measured in ohm– A material’s thickness, length, and temperature

affect its resistance– As temperature increases a metal’s resistance

increases because electrons collide more often

Superconductor

• A superconductor has almost zero resistance when it is cooled to low temperatures

• superconductor

Voltage

• In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy

• Charges always flow from high to low potential energy

Potential Difference

• Potential difference- the difference in electrical potential energy between two places in an electric field– Measured in joules per coulomb also known as

volts –Potential difference is sometimes called

voltage

Voltage sources

• Battery – a device that converts chemical energy to electrical energy

Ohm’s Law

• Ohm found a mathematical relationship between voltage, current, and resistance- became Ohm’s Law

Voltage = current x resistance

V = I x R

Ohm’s Law

• Increasing the voltage increases the current

• Keeping the same voltage and increasing the resistance decreases the current

Electric Circuits

Circuit diagrams

• Electric circuit- a complete path through which a charge can flow

• Circuit diagrams use symbols to represent parts of a circuit including a source of electrical energy and devices that run by electrical energy

Electric circuits

• Circuit diagram– Switch – places where the switch can be opened • If a switch is open, the circuit is not complete and

current stops

– the + and – on the diagram indicate the positive and negative terminals on the battery

– Arrows show the direction in which positive charges flow – electrons actually flow in the opposite direction

Circuit Diagram

Series Circuit

• Series circuit – charge has only one path to flow– If one part of a series circuit, none of the parts can

work – Adding bulbs to a Series circuit increases the resistance and decreases the current

Parallel Circuits

• Parallel circuit- an electric circuit with two or more paths through which charges can flow– If one bulb burns out, the other stay lit

Power and Energy Calculations

• Power- the rate of doing work

• Electric power- the rate at which electrical energy is converted to another form of energy

Power (watts) = Current (Amps) x voltage (V)P = I x V

Example

• An electric oven is connected to a 240 Volt line, and uses 34 amps of current. What is the power used by the oven?

• 8200 Watts

Electrical energy

• E= P x t

• Electrical energy = power x time

• Electric bill is in kilowatt hours

Electrical Safety

• GFCI – ground fault circuit interrupter

• Monitors current flowing to and from the outlet or appliance. If these two currents are not equal, it means that current is escaping so the GFCI opens the circuit and stops the current