Electric Charge

Positive and Negative charges Three subatomic particles: Protons – positively charged Electrons – negatively charged Neutrons – NO charge In an atom the (+) charges or protons and (-) charges or electrons are equal, so they atoms are naturally neutral

Protons and neutrons are tightly bound in the center of the atom Atoms cannot lose protons or neutrons! Atoms can lose electrons though if an atom loses an electron it becomes (+) If an atom gains an electron it becomes (-) Some atoms hold their electrons more tightly than others

Transferring charges Some materials hold their electrons very tightly. Electrons do not move through them very well. These things are called insulators. Plastic, cloth, glass and dry air are good insulators.

Other materials have some loosely held electrons, which move through them very easily. These are called conductors. Most metals are good conductors.

Static electricity The accumulation of excess electric charge on an object The imbalance of (+) and (-) charges

Law of Conservation of Charge Charges cannot be created or destroyed – only transferred between objects!

Charges Exert Forces Like charges repel Unlike charges attract Forces decrease as the distance between atoms increases As the amount of charge increases the electrical force increases

Electric Fields Surround any electrically charged particle Exerts a force that attracts/repels other charged particles Represented by arrows drawn in the direction that a (+) charged particle would move in thee field

Electrical vs Gravitational Forces Even though there are gravitational forces between ALL objects that have mass, even subatomic particles – this force is weak due to the extremely small mass of the particles Electrical forces are stronger and are responsible for: ~ holding atoms together ~ chemical bonds between atoms

Electrical forces between OBJECTS is less than the gravitational forces. Since MOST all objects are electrically neutral, there is little if any electrical force between the objects

Conductors and Insulators When you walk on carpet the electrons transfer from the carpet to your shoes – your shoes become (-) charged Why does it shock you when you touch the door knob? Some electrons transfer from your shoes to your skin – skin is a good conductor!

Conductors The best electrical conductors are metals Metals have free moving electrons that can move between atoms This allows for electrons to flow easily through the material

Insulators A material that holds it’s electrons tightly, electrons do not move around easily

Charging Objects Charging by contact – transferring charges between objects by touching One object becomes (+) charged and the other (-)

Charging by Induction- the rearrangement of electrons in a neutral object caused by a nearby charged object

Lightning A large static discharge – a transfer of charge between 2 objects due to build up of static electricity Water molecules in clouds collide and may lose electrons creating charged particles Top of the clouds become (+) and the bottom (-) The negative charge of the cloud causes the ground below to become (+) charged (attracts (+) charges to the surface)

When enough (-) charge builds in the bottom of the cloud, it starts to move toward the (+) charged ground When the electrons bet close enough, the (+) particles move upward When they meet and complete the connection The light is due to the extreme heat The lightning travels up, but the (-) electrons are moving downward

Thunder The extreme heat produced by lightning causes the surrounding air to rapidly expand producing sound waves

Grounding Connecting objects to the Earth with a conductor – Grounding Since the earth is a large neutral object that is a good conductor; grounding provides a path for excess electrical charges before they can build enough to do damage

Electric Current Electric Current: the NET movement of electric charge in a single direction In any condutor, electrons are in constant motion – random, and typically not in any one direction When a current flows through a conductor, electrons still move randomly – they just drift in the direction of the current

Electric Current is measured in AMPERES 1 ampere = 6,250 million electrons flowing past one point per second Voltage difference – related to the force at which electrons flow Electrons will ALWAYS flow from high pressure to low pressure

Circuits Electric current requires a closed path in order to flow This path is called a circuit If any part of the circuit is broken, the current will stop flowing

Batteries In order to keep an electric current flowing in a circuit; a voltage difference must be maintained This voltage different can be created by a battery As long as there is a closed path between the two battery terminals, the current will flow

Dry Cell Batteries Convert chemical energy to electrical energy An electrolyte paste in the battery reacts with the zinc in the battery to create an electrical charge The carbon rod in the battery becomes (+) The zinc becomes (-) charged This voltage difference can cause current to flow through a closed circuit

Wet Cell Batteries Contains two connected metal plates (different metals) that are in a liquid electrolyte solution A wet cell battery usually contains several cells connected together

Lead-Acid Batteries Car batteries The metal plates are lead, and lead acetate The electrolyte solution is a high concentration of sulfuric acid Each cell provides a voltage difference of 2V, put 6 cells together and you make a 12V battery

Voltage differences can also be created by electrical outlets, such as a wall outlet The voltage difference between the two holes in a wall socket is usually 120V

What makes a light bulb glow What makes a light bulb glow? Part of the closed circuit in a light bulb is a metal structure called a filament When the electrons flow through the filament then collide with the metal particles The filament heats up and begins to glow

Resistance The tendency for a material to resist the flow of electrons, changing electrical energy into thermal energy or light All materials have some resistance Conductors have much less resistance that insulators Resistance in measured in Ohms

Electrical resistance increase as Temp increases Resistance can also depend on the length and diameter of the object, such as a wire Resistance increases with wire length Resistance increased when a wire becomes thinner

Simple Circuits A simple circuit contains: a source of voltage difference, such as a battery A device, such as a light bulb Conductors that connect the battery to the device

Increasing Current in a circuit Increase the voltage difference Reduce resistance

Ohm’s Law The current in a circuit equals the voltage difference divided by the resistance Electric current (I) = Voltage difference (volts) Resistance (Ohms)

To measure resistance of a material just rearrange the equation: R = V/I

Series Circuits All circuits must contain: a source of voltage difference conductor device that uses electrical energy Series Circuit: only has one loop of flow for electrons

Open circuit All parts of a circuit are wired one after the other, the amount of current through each device is the same When one part of the series is disconnected the current flow stops EX: some Christmas tree lights

Parallel Circuits Contain 2 or more branches for the current to move through Can turn off one device and the current still flows through the other branches

In a parallel circuit, the more devices that operate, the great the current in the circuit Too many devices on the same circuit will cause the circuit to overheat Fuses and circuit breakers prevent circuits from getting too hot

Electric Power The rate at which electrical energy is converted into other forms of energy Devices that convert electrical energy into thermal energy (heating coils) tend to use more power Power (watts) = current (amperes) X voltage (volts) P = IV