Foundations of Physical Science Unit 3: Electricity and Magnetism

Chapter 7: Measuring Electricity 7.1 Voltage 7.2 Current 7.3 Resistance

Learning Goals Measure volts with an electrical meter. Describe the role of a battery in a circuit. Describe the transfer of energy in a circuit. Explain the relationship between voltage and energy in a circuit. Describe current as a flow of electric charge. Measure amperes with an electrical meter. Classify materials as conductors, semiconductors, or insulators. Differentiate between electrical conductivity and resistance. Explain why metals are good electrical conductors. Measure ohms with an electrical meter.

Vocabulary alternating current ampere battery current direct current electrical conductivity electrical insulator electrical conductor ohm resistance semiconductor volt voltage

Voltage Voltage: electrical pressure Directly proportional to electrical PE Electrons flow only when a difference in electrical pressure exists Pushing a spring gives it PE, for the compressed spring can do work on something when released Pushing a charged particle toward another charge can increase its electrical PE

Voltage Voltage: potential energy charge Volt: the unit of electric potential A potential of 1 volt equals 1 joule of energy per 1 coulomb of charge; 1 V = 1 J/C

Voltage Battery: uses chemical energy to move charges; an energy source for charges that flow in circuits

Voltage Drops When Energy is Used If we connect anything that uses energy, like a light bulb, we reduce the voltage Voltage is made of energy!

Electric Charge Loose electrons in metals are responsible for good – Heat conduction – Electrical conduction Conduction electrons: loosely held outer electrons in atoms of metals

Current How do charges move through a circuit? Current: the flow of electric charges Amperes: (amps, A) measurement of current 1 ampere = 1 coulomb per second (1 A = 1 C/s) The flow of 6.25 x electrons per second

Which way does current flow?

Where Does Electric Current Come From? You can’t see current looking at a wire Particles that carry charge are electrons Current occurs because electrons in the battery repel electrons in the wire

Measuring Current Circuit breaker: can be reset Fuse: must be replaced

Electricity in your House: AC AC Current: the direction of the current goes back and forth In the USA the current reverses direction 60x/second Easier to transmit and generate DC Current: current does not alternate; flows in one direction (example: a battery)

Electric Current

Named after Georg Simon Ohm, German physicist 1826 Discovered the relationship between voltage, current and resistance Resistance: measures how difficult it is for current to flow Electrical Resistance 

Resistance Measures how easily charge flows through an object Low resistance: easy for charge to flow High resistance: hard for charge to flow Electrical resistance restricts the flow of current

Resistance Devices that use electrical energy have resistance The more devices in a circuit, the greater the resistance and the lower the current

Conductors and Insulators Conductor: a material in which charge flows through very easily – metals Electrical conductor: a material that can carry electrical current Insulator: a material that does not allow charge to flow (a poor conductor) – Wood, Styrofoam, etc.

Conductors and Insulators Semiconductor: a material that is not as easy-flowing as conductors, but not quite insulators either – Computer chips, LED’s, some kind of lasers

Conductivity