Unit 6 Electricity & Magnetism 11/15/2018

Electric Charge & Force (13.1) An electrical property of matter that creates a force between 2 objects. Like charges repel; opposite charges attract. 11/15/2018

Rub balloons on your hair and your hair is attracted to the balloons. The balloon sticks to the wall. Opposites attract The two balloons repel each other. Like charges repel The 2 balloons have the same charge; while the hair has the opposite charge. 11/15/2018

Atomic Structure Review Parts of an Atom 1. Nucleus-composed of protons & neutrons. Protons- positively charged particles (+) Neutrons- particles equal in mass to proton, but with no charge.(neutral) 2. Electron cloud- Electrons- move at high speed around the nucleus. Electrons are negatively charged (e-), have no mass. 11/15/2018

What causes an electrical charge? Neutral atoms have equal numbers of protons and electrons. (equal + and -). Gaining or losing electrons will cause an atom to become charged. (ionized) 11/15/2018

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Electrons (e-) can move through matter. When objects are rubbed together, electrons can move between the objects. If an object gains electrons, the object becomes negatively charged(-). If an object loses electrons the object becomes positively charged. 11/15/2018

SI unit for electrical charge: Coulomb (C): Each electron and proton have a charge of 1.6 x 10-19 C. 6.3 x 1018 e 11/15/2018

Conductors and Insulators Conductor- a material that transfers charge easily. Insulator- an object that is a poor conductor. Does not transfer charge easily. 11/15/2018

Charge is acquired by transfer of electrons: Charge by friction: Electrons transfer from one material to another. Charge by contact:The Physics Classroom When objects contact each other electrons can be transferred. Charged object contacts a neutral object. 11/15/2018

Induced charges A neutral object coming close to a charged object can have its charges moved without taking on electrons. The object is still neutral while it has a + and – region. (polar molecules in chemistry) 11/15/2018

Static Electricity Combs, balloons, hair, etc. Electrical charge built up in one place. When an object accumulates positive or negative charge, this is static electricity. In what type of objects have we observed static electricity? Combs, balloons, hair, etc. These are all non-metals, or poor conductors. Charges do not move very well or very far on poor conductors. 11/15/2018

Allows an object to lose its charge. Grounding Allows an object to lose its charge. The charge of an object is absorbed upon contact with a much larger object. Most often this means contacting with the earth. 11/15/2018

Electrical Force Force depends on charge and distance. The force of attraction or repulsion between objects due to charge. Force depends on charge and distance. 11/15/2018

Electric Force The electrical force at an atomic level is responsible for many things such as friction, atoms being attracted to form compounds and molecules. Without the electrical force life itself would not be possible. 11/15/2018

Electrical Field The region around a charged object in which other charged objects experience an electric force. This is often shown by diagramming electric field lines. 11/15/2018

Electric Field Lines Inward lines represent negative charge. Outward lines represent + charge. Increase in charge is shown by increasing number of lines. 11/15/2018

Detecting Charge Electroscope- used to detect electrical charge. If a charged object comes in contact with the electroscope, the leaves of the electroscope push away from each other. (Like charges repel each other) IMSS - Multimedia Catalogue - Animations - Gold-leaf electroscope 11/15/2018

Voltage & Current Current 13.2 Electrical Potential Energy Gravitational PE is due to position of an object Electrical Potential Energy PE of a charged object due to its position in an electric field. Negative charge will move away from other negative charges. As the distance between two oppositely charge objects increases the potential energy decreases. 11/15/2018

Two negative charges have an increase in electrical PE as they are moved closer together. As a + charge is moved farther from a – charge the electrical PE increases. Electrical PE depends on position and attractive and repulsive forces. 11/15/2018

Potential difference Referred to as voltage. Rather than using Electrical potential we use the potential difference. The change in the electrical potential energy per unit charge. The SI unit for Measuring potential difference is volts (V), equivalent to 1 J/ C. Referred to as voltage. 11/15/2018

Voltage across battery terminals Batteries range from 1.5V to 12V. Batteries are electrochemical cells that contain an electrolyte and 2 electrodes. These cells can be dry cells or wet cells. Dry-electrolyte is paste Wet- electrolyte is a liquid 11/15/2018

Voltage (potential difference) sets a charge in motion Electrons from the negative terminal move through a conductor to the positive terminal. The movement of electrons to a position of lower PE a current is produced. 11/15/2018

Voltage Sources Electrochemical (chemical reaction  V) batteries Thermoelectric (∆T  V) Thermoelectric generators Photoelectric (light  V) Solar panels Piezoelectric (stressing a crystal  V) Tweeters (speakers), truck weigh stations, microphones, guitar pickups 11/15/2018

Electrochemical Cells Two electrodes in an electrolyte cathode = negative terminal anode = positive terminal A chemical reaction separates charges and produces a voltage. 11/15/2018

Can YOU make a voltage… From a Lemon? Activity: Voltage sources in series and parallel… 11/15/2018

Current The Rate at which electric charges move through a conductor. SI unit for current (I) is ampere (A). Batteries are a direct current (DC), because the charges move from one terminal to the other in the same direction. No change in direction of flow occurs. 11/15/2018

Current can be made of + , - , or a combination of both. In metals, moving electrons make up the current. In gases and chemical solutions, both positive and negative particles move. Nerves operate on potential across cell membranes of nerve cells. This is mostly positive charge movement. 11/15/2018

Conventional Current Defined as movement of positive charges. In physics, the direction of current is always given as the direction of a positive charge movement that is equivalent to the actual motion of charges in the material. This means that the current in a wire is shown as positive to negative rather than the actual movement that occurs of negative to positive. 11/15/2018

Electrical Resistance The ratio of the voltage across a conductor to the current it carries. Resistance is caused by internal friction. Light bulbs filaments vary in resistance. 40W light bulbs have a higher resistance than the 100W bulbs. 11/15/2018

Ohm’s Law Resistor: conductor used to control current. Google Image Result for http://www.darvill.clara.net/hotpots/ohm0.gif Current is the rate that electric charges move through a material. 11/15/2018

Calculating Resistance What current would run through your body if you touch the terminals of a 24V battery and your dry skin has a resistance of 100,000. I =2.4 x 10-4 A 11/15/2018

Conductors have low resistance. Insulators Superconductor: have high resistance. Used to prevent electrical leakage. Superconductor: Have no resistance below critical temperature. 11/15/2018

Electric Circuits 13.3 A path through which charges flow or can be conducted. Due to the voltage in the battery, electrons will flow through the conductor back to the battery(from – to + terminal). The battery then adds energy to the charges as they move through the battery. Switch is added to a circuit to allow opening and closing of the circuit. 11/15/2018

Closed loop for electrons to flow. The circuit is complete Open & Closed Circuits Closed Circuit: Closed loop for electrons to flow. The circuit is complete Open Circuit: incomplete path, there is no charge flow, no current. 11/15/2018

Switches allow circiuits to be opened & closed. Switches are devices added to circuits to allow them to be opened and closed as needed. 11/15/2018

Schematic Diagrams Wire or conductor Battery or source + - Resistor Bulb or lamp Switch (open or closed) 11/15/2018

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Series and Parallel circuits Single pathway for current. Current is the same for all devices in a series circuit. Voltage may vary if resistance is not the same. 11/15/2018

The following rules apply to a series circuit: 1.) The sum of the potential drops equals the potential rise of the source. VT = VR1 + VR2 + VR3 + VR…. 2.) The current is the same everywhere in the series circuit. IT= I1= I2 = I3 = I….. 3.) The total resistance of the circuit (also called effective resistance) is equal to the sum of the individual resistances. RT= R1 + R2 + R3 + R…. 11/15/2018

ET = E1 + E2 Calculating individual voltage drops in a series circuit. Voltage drop across R1 is 10V ET = E1 + E2 R2 is the same ohmic value as R1 and carries the same current. The voltage drop across R2 is therefore also equal to 10 volts. Adding these two 10-volt drops together gives a total drop of 20 volts, exactly equal to the applied voltage 11/15/2018

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Parallel: Two or more separate conducting pathways available. Current does not have to be the same for all devices. Voltage across the devices is constant. Disruption in one pathway does not disrupt the other pathway. IN parallel if one bulb has less resistance than another, more current will flow through that pathway than the other pathway. 11/15/2018

The following rules apply to a parallel circuit: 1.) The potential drops of each branch equals the potential rise of the source. VT=V1=V2=V3=V….. 2.) The total current is equal to the sum of the currents in the branches. IT= I1 + I2+ I3 + I….. 3.) The inverse of the total resistance of the circuit (also called effective resistance) is equal to the sum of the inverses of the individual resistances. 11/15/2018

One important thing to notice from this last equation is that the more branches you add to a parallel circuit (the more things you plug in) the lower the total resistance becomes. Remember that as the total resistance decreases, the total current increases. So, the more things you plug in, the more current has to flow through the wiring in the wall. That's why plugging too many things in to one electrical outlet can create a real fire hazard. 11/15/2018

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Calculating current in a parallel circuit 11/15/2018

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Series or parallel? 11/15/2018

Instruments for Measuring Current and Voltage 11/15/2018

Electrical Power The rate at which electricity does work or provides energy. Appliances use electrical energy to do useful work. Electrical power is the product total current in and total voltage across a circuit. Watts- The unit in which electrical power is measured. 11/15/2018

60 Watts Power = voltage x Current P = V (I) Electrical power can be calculated with the following formula: Power = voltage x Current Watts = Volts x Amperes P = V (I) 60 Watts Homes operate on 120Volts. Light bulbs operate at 0.5amps. According to these figures what is the lights power? 11/15/2018

Electric Energy Electric meters are used to measure usage of electricity in your home. Meters in homes measure kilowatt-hours(kW•h) One kilowatt-hour is equal to 1000 watts of power used for one hour of time. 11/15/2018

Formula for Electrical energy: Energy = power x time E = P (t) kWh = kW·(h) A lamp uses 150watts of electrical power. If it is on for 4.0 hours, how many kilowatt hours of energy is converted into light and thermal energy? E = P · t P = 150 W or 0.15kW t = 4.0 hours E = 0.60 kW•h 11/15/2018

Fuses and Circuit Breakers When too many appliances are on one circuit, resistance is lowered. The wires may carry more current than they are designed to carry. Overloaded Circuit: Carries more current than designed to carry. Fuses and circuit breakers are designed to detect circuit overload and stop the flow of current. 11/15/2018

Short Circuit: If wire insulation is worn down, two wires may touch creating a new alternate pathway for current. This decrease in resistance causes and increase in current. Both short circuit and overloaded circuits are fire hazards. 11/15/2018

Normal and short circuit schematic diagram. 11/15/2018