Transcript Chapter 16
Chapter 16
Electricity
The Electric Motor
• Electric motors convert electricity into motion!
How a Motor works!
• Electric motors operate according to the law of magnetic poles • Unlike poles attract!
• Like poles repel!
• A simple motor has an electromagnet mounted on a shaft to freely rotate!
How an Electric Motor Works!
• The electromagnet and shaft are placed between poles of a field magnet used to provide a stationary magnetic field!
• The force of attraction and repulsion cause the armature (electromagnet) to rotate and spin!
Electric Current
• Electrons are in motion!
• When a lightning strike hits the earth, a massive current of electricity flows to the earth!
• The flow of electrons is electricity!
• If the electrons are at rest, it is static electricity!
DC and AC Current
• Current electricity can be subdivided into either of two types depending on how it flows!
• If it flows in one direction, it is direct current (DC).
• If it flows in one direction and then another, it is alternating current (AC).
• AC is used in our homes because it can be transmitted over long distances.
Causes of Electric Flow
• Electrons move from one place to another!
• This is a result of attraction and repulsion.
• They will flow from negative to positive…like with a battery!
Voltage
• The greater the difference between two charges – the more negative and positive they are – the stronger the force that drives the flow!
• The difference between charges determines voltage.
• Voltage is also called “electric potential”.
• The SI unit of voltage is Volt (V) • A device that measures voltage is a voltmeter!
Amperage
• Amperage refers to the volume of flow of current! • The unit of electric current is ampere (A) • It is one coulomb per second!
• Remember – it is a RATE of flow!
• A device that measures current is an ammeter!
Power
• Power is the amount of voltage times the amount of amperage.
Power = voltage X amperage P=VI “I” represents amperage • The unit for power is the Watt (W).
Resistance
Conductors and Insulators Electric current flows through some materials much more easily than it flows through others.
Materials through which current flows easily are called conductors.
Resistance
Conductors and Insulators Materials through which current does not easily flow are called insulators.
They are called insulators because they can be used to insulate objects from electric current.
Resistance
• The amount by which a material hinders the flow of electrons is called resistance.
• The unit for resistance is the ohm!
factors which increase resistance
1. Type of material 2. Diameter of the conductor – thick wire has less resistance 3. Length of the conductor – the longer the more resistance 4. Temperature – higher temperature creates more resistance
Ohm’s Law
• There is a mathematical relationship between current and resistance.
• This relationship is called Ohm’s law.
• Current is directly proportional to voltage and inversely proportional to resistance.
V = I X R
Ohm’s Law
• • • •
V
is the voltage in volts
I
is the current in amperes or amps
R
is the resistance in ohms
IR
is
I
times
R
• The unit for resistance is the Ohm (Ω)
Ohm’s Law
Problem #1 A small electrical pump is labeled with a rating of 3 amps and a resistance of 40 ohms. What voltage was it meant to run at?
Ohm’s Law
Problem #2 A nine volt battery is hooked up to a light bulb with a rating of three ohms. How much current passes through the light?
Ohm’s Law
Problem #3 A lamp is plugged into the wall outlet, which is providing 110 volts. An ammeter attached to the lamp shows 2 amps flowing through the circuit. How many ohms of resistance is the lamp providing?
Ohm’s Law
Current and voltage are directly related.
↓↓ ↑↑
Current and resistance are inversely related.
↓↑
Limiting Current
• A resistor is a device used to limit the flow of electricity.
• Resistors are used to protect delicate electrical components that cannot handle large currents!
Electric Circuits
• The path current follows is the electric circuit.
• A simple electric circuit contains three parts: 1. battery or generator 2. conductor (wire) 3. load (uses the current – ex: bulb)
Closed and Open Circuits
• Electric current requires a complete circuit in order to flow!
• A circuit is a closed circuit if the electrons can flow in a complete path back to the source.
Closed and Open Circuits
• An open circuit has a gap and so the current cannot flow!
• Often times the gap is created with the use of a switch!
Circuit Arrangements
• Simple circuits contain only one load (one Bulb).
• More complex circuits have multiple loads.
• For this reason, a circuit can be arranged in two basic ways: – In series – In parallel
Series Circuit
• In series circuits, the loads are arranged so that the electric current flows through each load one after the other.
• The current must pass through each bulb to get to the next.
• If one bulb (load) blows out, the circuit is now open and the current cannot flow.
• Also, due to the resistance being cumulative, the current is reduced from one bulb to the next and so they do not burn as brightly.
Series Circuit
• If switch 1 and 2 are closed, the bulbs will light up. • If switch one is open. Both bulbs will go out.
• If switch 2 is open, both bulbs will go out.
Parallel Circuits
• In parallel circuits, loads are arranged in separate branches of the circuit and the current is divided among them.
• Parallel circuits have the same amount of current flowing through each load!
• If one load blows out the rest of the loads will continue to receive current and remain active!
Parallel Circuits
• If all three switches are closed then the bulbs will light up. • If switch 2 is open then only bulb
a
will go out. Current is still flowing through bulb
b
so it will stay on.
• If switch 3 is open then only bulb
b
will go out. Current is still flowing through bulb
a
so it will stay on.
• If switch 1 is open, all the bulbs will go out.
Drawing Circuits
• When drawing circuits, symbols are used to signify various components.