#### Unit 12 Basic electricity and magnetism

What is an electron?

part of the atom that moves from one atom to another

when an electron moves to another atom, the losing atom will have what kind of charge?

positive

What is the difference between AC and DC

AC continual and rapidly reversing current. DC travels in one direction

How would a meter be connected in a circuit to measure the potential difference across a light bulb

Voltmeter across the terminals of the bulb, without interrupting the circuit. Ammeter directly into the circuit so all current flows through it.

How is amperage measured

Ammeter in series .... clamped around one wire (single conductor)

Describe how the total resistance of a series circuit is determined ?

In series , the voltage is divided across the resistance the total current flows thru each resistance , and the resistance are added together to obtain the total resistance

Ohms law for determining voltage is?

amps X resistance = Volts

Ohms law for determining current

volts/resistance = Amps

Ohms law for determining resistance

volts/amps = resistance

Describe the characteristics of the voltage , amperage and resistance when there is more than one load in a parallel circuit

The total voltage is applied across each resistance.
The current is divided between loads according to their individual resistance.
The total resistance is less than the value of the smallest resistance

Electrical power is measured in

watts

Ohms law for determining power

watts = volts X amps

Whats the difference between a step up and step down transformer?

A step up transformer increases the voltage and decreases the current (more windings in secondary).
Step down transformer decreases the voltage and increases the current (more windings in the primary).

What are the 3 types of opposition to current flow that impedance represents?

Pure resistance. Inductive resistance. Capacitive resistance

Why is it important to size a wire correctly in a parallel circuit?

An undersized wire may overheat, burn the insulation, star a fire

What does forward bias on a diode mean?

current will flow

The unit of measurement for the charge a capacitor can store is?

Series Circuit

- voltage is divided across the resistance
- total current flows through each resistance
- resistances are added together = total resistance

Parallel Circuit

- total voltage is applied across each resistance
- current is divided between the resistances
- total resistance is less than the smallest resistance

How can electricity be produced by using magnetism?

A conductor cutting through magnetic lines of force produce electricity

DC current is .....

direct current = moving in one direction

AC current is .....

alternating current = continually reversing

Volt

electrical force or pressure

Ampere

quantity of electron flow

Ohm

resistance to electron flow

Voltage formula ....

Volts (E) = Ampere (I) x Resistance (R)

Electrical power is measured in ......

Watts (P) = Volts (E) x Ampere (I)

Inductive reactance is ....

the resistance caused by the magnetic field surrounding a coil in an AC circuit

Impedance is ....

the opposition to current flow in an AC circuit from the combination of pure resistance, inductive reactance and capacitive reactance

Step up transformer vs Step down transformer

Step up transformer = increases the voltage and decreases the current (more windings in the secondary)
Step down transformer = decreases the voltage and increase the current (more windings in the primary)

A capacitor in an AC circuit will continually ....

charge and discharge as the current in the circuit reverses

What is capacitance?

the amount of charge in a capacitor

A multimeter often used is the .....

VOM (volt-ohm-milliammeter)

A 40-VA transformer is often used in the control circuit of combination heating and cooling systems. These transformers produce 24V and carry a max of 1.66A

Output in amperes = VA rating/Voltage
(I) = VA/V
40VA/24V = 1.66A

Sizing wire is determined by its .....

diameter or cross section

Good conductors have ....

fewer electrons in their outer ring. These are considered free electrons because the move easily from one atom to another = copper, silver, gold

Like charges ..... Unlike charges .......

like = repel
unlike = attract

Wire size is identified by AWG

American Standard Wire Gauge

Insulators have .....

several electrons in their outer ring. These electrons are difficult to free = glass, rubber, plastic

An electric circuit must have three things

power source, conductor (carry the current), load (device to use the current)

As wire size numbers increase their actual diameter ...

decreases = 12 AWG smaller than 10 AWG

What is a fuse?

a device used to protect circuits from overloading or overheating. Fuses contain a strip of metal (low melting point) that has a higher resistance than the conductor in the circuit. When the current exceeds the rating on the fuse, the strip melts and open

Circuits must be protected from current overloads. They are normally protected by ..... and ......

fuses
circuit breakers

compare smaller vs larger wire resistance

smaller = more resistance
larger = less resistance

Ferrule cartridge fuses are used for 230V to 600V service up to .... amps

60A

Knife blade cartridge fuse can be used from 60A to ....

600A

Silicon Controlled Rectifier (SCR) are used for .....

control speed of motors or brightness of lights

Transistor

may be used as a switch or as a device to amplify or increase an electrical signal.

Thermistor

type of resistor sensitive to temperature
- Positive Temperature Coefficient (PTC)
temp increase = resistance increase
- Negative Temperature Coefficient (NTC)
temp increase = resistance decrease

Diode forward bias

current will flow

Diode reverse bias

current will not flow

How to check a diode?

- remove diode from circuit
- connect ohmmeter across it
- negative probe touched to cathode
- positive probe to anode
- with selector switch on R x 1 the meter should show a small resistance indicating continuity
* if a diode shows continuity in both dir