Electric charge, electric force, current and circuits Electricity Electric charge, electric force, current and circuits

Electrical Charge and Force When you speak into a phone the microphone changes your sound waves into electrical signals When you walk across a rug on a dry day you can be shocked when you touch something This happens because your body picks up an electrical charge as your feet move across the carpet. You don’t feel this until you notice them pass from your body Electrical Charge: matter that creates a electric and magnetic forces

Like repels like and opposites attract If you rub two balloons across your head, each would be attracted to your head, but repel each other. The charge on your head is positive because you gave away electrons. The charge on the balloon is negative Remember….. Protons: positive, found in the nucleus Neutrons: neutral, found in the nucleus Electrons: negative, found in cloud surrounding the nucleus.

Electrons are on the outside of atoms, so only they are available to be lost or gained. For every atom, the number of protons equal the number of electrons. This makes atoms neutral When electrons are lost, there is an imbalance of charge. Less electrons, means the protons have a greater affect on the charge, making it positive When electrons are gained, there is also an imbalance of charge. More electrons, means the electrons have a greater affect on the charge, making it negative. This is called charging by friction

Coulomb (C): The standard unit for charge. The direction of the flow of electrons depends on the material involved Synthetic material, plastics, and rubber tend to form the negative charges Al, cotton, silk, steel, wool, glass, paper, tend to form positive charges Coulomb (C): The standard unit for charge. Electron charge = - 1.6 x 10-19 C (negative) Proton charge = + 1.6 x 10-19 C (positive)

Conductors and Insulators Electrical Conductors: allow charges to flow and allow current to transfer Cords and electrical outlets are made up of copper wire. Electrical Insulators: do not transfer current The electrical cords are covered in plastic because the plastic is an insulator and does not allow current to transfer. This is why we use it to cover the metal, because it is safe.

When your skin is wet it is a good electrical conductor When your skin is wet it is a good electrical conductor. When it is dry, it is more of an insulator. This is because water with IONS in it, will allow current to transfer. This is because IONS are charged particles, both positive and negative Pure water does not conduct electricity, but tap water will conduct electricity due to the IONS it contains. Your body has salt all over your skins, due to sweat. Salt when mixed with water breaks into sodium ions (Na+) and chloride ions (Cl-)

Objects can be charged without friction This can be done by touching the object with a negatively charged rod. The electrons will transfer from the negatively charged rod through into the object This can also be done by touching the object with a positively charged rod. The electrons then flow from the object into the positively charged rod (because of the attraction of opposite charges) Objects charged this way are charged by contact

Induced charge: occurs when a charged rod is brought NEAR but not touching the object. If the rod is negative it will attract the positive charges already in the object thus inducing a charge. The total charge does not change, it has the same number of positive and negative charges, but since the charges are redistributed, they create a temporary imbalance This process allows insulators to have a temporary induced charged, even though they do not like current and do not transfer current VERY weak charge is created Called polarization

Electrical Force The force created from attraction or repulsion between charge This is what causes clothes to cling when they come out of the dryer Life would not be possible without electrical force that hold atoms, molecules, and ions together Depends on distance – the greater the distance between objects the weaker to charge Also depends on strength of charge DOES NOT require objects to touch!

Electrical fields: a region in space around a charged object that causes an unmoving object nearby to experience an electrical force. Electric force is similar to gravitational force in that they both are affected by mass and distance between objects However gravitational force is always attractive, but electric force is both attractive and repulsive Knowledge of electric force has allows scientist to come up with a way to repel sharks for deep sea divers by creating a repulsive electric field

Current Remember gravitational potential energy? A ball rolling down a hill moves from a higher gravitational potential energy to a lower gravitational potential energy as it gets closer to the earth. We also have electrical potential energy The closer two negatively charges particles are the greater the electric potential energy The farther away the negative charges get the less electric potential energy there is, because it is using that energy to push away the other negative object

Electrical potential energy: the ability to move an electric charge This electrical PE depends on if the forces present are attractive or repulsive Attractive forces have greater electrical PE when they are father apart (like how gravitational PE works between an object held high above the earth) Repulsive forces (the same charge) have greater electrical PE when they are closer together …in other words….they have greater repulsive forces the closer they are to one another

Rather than using electrical potential energy, we use potential difference Potential difference: the work needed to fight against electrical forces The unit for potential difference is a volt, V 1 Volt = 1 joule/coulomb Potential difference is often called voltage

Batteries Voltage across terminal of a battery vary from 1.5 V to 12 V Most batteries are an electric cell that converts chemical energy into electrical energy Batteries have a positive terminal and a negative terminal and are often called cells Electrochemical cells contain an electrolyte These cells can be dry or wet Dry cells, like in a flashlight, use a paste-like electrolyte Wet cells, like in a car battery, contain a liquid electrolyte

Voltage sets charge in motion When a flashlight is switched on, the electrons move through the light bulb from the negative terminal to the positive terminal Current is the rate at which these charges move through a conductor The unit for current is an ampere, A 1 amp = 1 C of charge moving past a point in 1 sec Direction of current in a wire is opposite the direction that the electrons move in that wire

Types of electric cells Electrochemical- electrons transfer through different metals immersed in an electrolyte Common batteries, automobile batteries Photoelectric- electrons are released from a metal that has been struck by a high energy light Calculators, streetlight, satellites Thermoelectric-two different temperature metals are held together to cause electron transfer Thermostats for furnaces and ovens Piezoelectric- crystals that become charged under pressure Computer keypads, microphones

Electrical Resistance Most electrical appliances are designed for the same voltage- 120V However, light bulbs come in many varieties. These bulbs have different amount of current in them, due to resistance The filament inside a 40W bulb has greater resistance Resistance is the opposition to current Caused by internal friction, which slows the movement of charges

Resistance = voltage/current, units are ohm, Ω Electrical devices get hot after a period of time because as the electrons collide they lose some of their kinetic energy Resistance = voltage/current, units are ohm, Ω R = Voltage R = V Current I An ohm is volts per ampere Ex: The current in a handheld video game is 0.5 A. If the resistance of the game’s circuitry is 12 Ω, what is the voltage produced by the battery? Ex: a 1.5 V battery is connected to small light bulb with a resistance of 3.5 Ω. What is the current of the bulb?

Insulators do not like current and therefore have a high resistance. Most electronic devices contain conductors, insulators, and semiconductors Superconductor: some metals and metallic compounds become superconductors at really low temperatures, meaning they really transfer electric current well Insulators do not like current and therefore have a high resistance. Semiconductors are somewhere in between Silicon is a semiconductor found in a lot of complex electronic devices

Circuits What are circuits? A set of electrical components that provide a pathway for the movement of charges A path for charges! Electrical circuits are the bulb, battery and wires all put together The charge moves from positive terminal of battery through the wires and into the light bulb, then back through the wires to the negative terminal of the battery This creates a closed circuit

A switch interrupts the pathway of a circuit It allows you to open and close the circuit Schematic diagrams are used to represent a circuit

Series Circuits: a single pathway for charges to flow, but allows for multiple devices connected in a series The current is the same in each device, but the resistance may be different, therefore the voltage could be different Voltage is added up Parallel Circuit: charges have more than one path to follow, even if one light goes out, then the circuit is still complete twinkle lights that don’t go out when one stops working The voltage is the same, but the current in each may be different Voltage is the same because only one pathway is actually taken- the one with the least resistance

Electric Power and electrical energy Electric power is the rate at which electrical energy is used in a circuit When a charge moves in a circuit, it loses energy This energy is transformed into useful work, such as the turning of a motor, and is lost as heat. Electrical power (P) is current times voltage P = IV The unit for power is watt (W) If V = IR, then P = V2 R

Ex: a space heater requires 29 A of 120 V current to adequately warm a room. What is the power rating of the heater? A color television has a power rating of 320 W. How much current is in the television when it is connected across 120 V?

Fuses and Circuit Breakers Power companies use kilowatt-hours to track consumption of energy (kW*h) 1 kW*h = 3.6 x 106 J When too many appliances, lights, etc are connected across a 120V outlet, the overall resistance is lowered and the wires carry more than the safe level of current and is thus overloaded Fuses melt to prevent overloads, they are connected in a series Circuit breakers respond to overload by opening circuits, by acting like a switch Fuses and Circuit Breakers