Topic2: Current and Circuits

Definitions/Important terms

• Current - The movement of electrons in a circuit

  • *e- flowing through a coil produces a magnetic field

  • In depth: The amount of charge or e- that passes a point per second

  • Current(I) is measures in amperes(A) using an ammeter connected in series circuit

• Electrical Energy - Energy due to the movement of charge(e-)

• Mechanical Energy - Energy due to movement or position of an object

• Chemical Potential Energy - Energy stored in the bonds of a molecule

  • Ex) battery acid

• Energy Sources(batteries)

  • Batteries create voltage

    • Voltage - which is the amount of electrical potential energy per unit charge available for doing work

  • Voltage is what causes the movement of e- and creates a current

  • Measures in volts(V) with a voltmeter which is connected parallel in the circuit

• Resistor - Any device in a circuit that disrupts e- flow, generating heat(thermal energy) or light

  • Ex) light bulb, toaster stereo, and even a wire

  • Resistance is measured in ohms with an ohmmeter

Relationship between elections in a coil(electric field), magnetic fields(magnets) and motion

*look at image in notes, pg 13

• As long as we have 2/3, we’ll get the 3rd

Simple motors

• Function - Converts electrical energy into mechanical energy

*Look at diagram in notes, pg 13

1. As current flows through the coil, a magnetic field is generates(induced magnetic field)

2. This magnetic field interacts with the permanent magnet, causing the coil to rotate

3. When there is no current, there is no magnetic field, but the coil will continue to spin due to momentum

*Look at diagram in pg14

• How to strengthen simple motors:

  • Increasing the # of coils

  • Increasing strength of magnets

  • Changing shape of magnets

  • Increasing the voltage supplied

  • Wrapping current carrying wire around a piece of metal

Generators

• Function - Convert mechanical energy into electrical energy

  • opposite of a motor

*Diagram of pg 13

AC vc DC

• Alternating current(AC) - e- flow in both directions in a circuit

  • Anything that plugs into a wall, a generator

• Direct current(DC) - e- flow in one direction in a circuit

  • Batteries, solar panels

Circuits

Symbols

*Refer to notes for symbols

Summary of circuit characteristics

Ohm’s Law - V=IR

• If resistances stays the same, current and voltage both increase

• If voltage stays the same, current increases and resistance decreases

Simply: current is directly proportional to voltage(linear increase) and inversely proportional to resistance(exponential decrease)

Power and Energy

• Power - can be defined as the rate of doing work or transforming energy

  • measures in Watts(W) or J/s

Transmitting Electrical Energy

• The current passing through the transmission lines is usually AC

  • Why?

    • To help conserve power, but to reduce energy loss over long distances

    • We want to decrease resistance and current(e- create heat)

    • And increase voltage during transmission

      • Voltage in power liners must be high to be efficient, but is too high for most household appliances

  • *see diagram pg 22

• Transformers - used to change voltages(and current) of electricity being delivered to our homes

  • Transformers may be:

    • Step us - increasing voltage(at generating plant)

    • Step down - decrease voltage(at home)

*See diagram in pg 23

• Primary coil receives an input voltage

  • Where electricity comes from

• Secondary coil supplies output voltage

  • What uses electricity

1. An alternating(AC) entering the primary coil will produce a changing magnetic field

2. This changing magnetic field produced by the primary coil is magnified by the iron core

3. The changing magnetic field induces an alternating(AC) in the second coil

*Electron DO NOT travel through the iron core

Transformers: Step up vs Step down

• Both conserve power