OHM S LAW AND CIRCUITS Mr. Banks 8 th Grade Science
Ohm s Law Ohm s law describes the relationship between current, voltage, and resistance. Ohm created a circuit and measured the resistance of the conductor and the current flowing through it. He would then change the voltage and took new measurements. Ohm found that resistance doesn t depend on the voltage. Current however does depend on voltage and resistance. Georg Ohm
Ohm s Law Through these findings, Ohm created an equation to describe the relationship. He concluded that the resistance is equal to the voltage divided by the current. Resistance = Voltage Current Resistance = R Voltage = V Current = I If you know two values, then the other can be found. R = V I V = I x R I = V R
Using Ohm s law Find resistance V = 12 V I = 0.50 A R = 24 Ω Find current V = 9 V R = 2 Ω I = 4.5 A Find voltage I = 10 A R = 60 Ω V = 600 V R = V I I = V R V = I x R
Basic circuit A basic circuit consists of three things An electrical energy source Something that runs on electrical energy (resistor) Conductive wires connecting the two
Circuit diagrams Diagramming makes it much easier to track what is going on in a circuit. Simple symbols are used to represent the components of the circuit. Note: a resistor symbol is used to represent essentially any device that runs off of electricity. + -
Example Battery Switch Light
Types of circuits There are two types of basic circuits Series circuits Parallel circuits
Series circuits The most basic type of circuit. A series circuit is one where everything in the circuit is connected one after another in a single path. Series circuits are easy to build. There are disadvantages to them Since there is one path for electricity to follow, any sort of break in the path means that no electricity will flow. Resistance goes up for every resistor on the circuit. So, for every resistor added the current goes down. Eventually the circuit could become unusable.
Parallel circuit A parallel circuit is one which there are several paths for current to take. There are several advantages to parallel circuits. Since there are several paths, even if one part of the circuit is broken then electricity can still reach the other parts of the circuit. The resistance goes down as more branches are added to the circuit.
Independent work Chapter 2.4 (Page 60) Vocab Ohm s law Series circuit Parallel circuit End of section assessment 1a, 1b, 1c 2a, 2b, 2c 3a, 3b
ELECTRICITY PT. 4 Mr. Banks 8 th Grade Science
Electrical power Electrical devices transform electrical energy into other, useful forms. The rate at which energy is transformed between different forms is known as power. The unit of power is the in watt (W).
Electrical power Appliances vary greatly in their power ratings. Do any of these surprise you?
Calculating power Electrical power depends on two things, current and voltage. Power = voltage x current P = V x I So, as current goes up. Power goes up And as voltage goes up Power goes up What about resistance? If voltage = 120 V And current = 0.5 A What is the power? Power = 60 W
Energy use Electric companies charge for energy use and not power. Energy is calculated as the power used by the appliance for a given amount of time. Energy = power x time Electric companies usually measure power in thousands of watts (kilowatt) and time in hours. So, ten 100 watt light bulbs on for one hour is 1 kilowatt-hour
Electrical safety Electricity can be very useful, but it can also be incredibly dangerous.
Electric shocks and short circuits Electrical impulses in the human body control breathing, heartbeats, and muscles. Electrical shocks can interfere with these signals. Electric current always takes the path of least resistance. Which can include through the human body.
Electric shocks and short circuits The severity of an electric shock depends on the current. Voltage hurts, but current kills. A current of more than 0.2 A can be dangerous. Shocks can cause severe burns or even stop your heart.
Safety devices Since electricity takes the path of least resistance, by providing a path better than through a person you can prevent shocks. Ground wire A wire that runs directly through the house s circuits to the earth.
Short circuits Electricity always takes the path of least resistance. A short circuit is when the current follows an unintended path with less resistance than the normal one. Due to the lowered resistance, the electrical current increases to very high levels. This causes the wires to overheat and can lead to fire or damage to the circuit.
Safety devices As current increases in a circuit the wire heats up, this can be a major fire danger. This can be prevented using certain devices. A fuse is a thin strip of metal that will melt if too much current flows through it. A fuse that melts through is known blowing a fuse. Older houses, appliances, and cars use fuses. Fuses must be replaced after they blow out.
Safety devices A Circuit breaker is a reusable safety switch that breaks the circuit when current gets too high. High current causes a metal band in the breaker to heat up, bending the band. Eventually it bends so far it breaks the circuit. The breaker then can easily be reset to reconnect the current.
Lab materials To start with you will need 1 battery 2 bulbs 5 pieces of wire
ELECTROMAGNETISM Mr. Banks 8 th Grade Science
Electricity and magnetism Recall EM waves, where a magnetic field creates an electric field and vice versa over and over as they travel through space.
Electromagnetism Electricity and magnetism are closely related. One cannot exist without the other. Where there is electricity, there is magnetism. Electric current (or moving electric charge) produces a magnetic field. Likewise, movement in a magnetic field produces an electric current. This relationship is known as electromagnetism. Magnetic field
Electromagnetism In a straight wire, a magnetic field will form in circles around the wire perpendicular to the electric current. The magnetic field will only form when there is current flowing through the wire.
Electromagnetism Magnetic fields produced by electric current are different from the magnetic fields produced by magnetic domains. 1. It can be turned on or off. 2. Its direction can be reversed by reversing the electric current. 3. The strength of the field can be changed.
Solenoid You can increase the strength of the magnetic field by twisting the wire into loops. Looping the wire pushes the magnetic field lines together in the loop. The more loops there are, the stronger the magnetic field is. A solenoid is a coil of wire with a current that acts like a magnet. The two ends of the solenoid act like the poles of a magnet. The poles can change based on the direction of the current.
Electromagnets A solenoid with a core made out of a ferromagnetic material is called an electromagnet. The field of an electromagnet is produced by both the wire and the magnetized core. This increases the strength of the field. Electromagnets are very useful as they are essentially a strong magnet that can be turned on or off.
Electromagnets The strength of an electromagnet can be changed in several ways. 1. You can increase the current flowing through the wire. 2. You can add more loops of wire. 3. You can wind the coils closer together. 4. You can use a stronger ferromagnetic core.
Electromagnets Electromagnets are used in many things