Electricity and Magentism Day 6

2. Electricity and Magentism Day 6 Feb 21

3. Light a bulb-inquiry • Materials • Light bulb • One wire • D Battery • Use the simple materials provided (and nothing else) to light the bulb • What caused the battery to light? • Moving Electrons!!!

4. Simple Cirucuit • Congratulations, you’ve just built a simple circuit • Three things at a minimum are required: • A source of electrical potential difference or voltage. (typically a battery or electrical outlet) • A conductive path which would allow for the movement of charges. (typically made of wire) • An electrical resistance (resistor) which is loosely defined as any object that uses electricity to do work. (a light bulb, electric motor, heating element, speaker, etc.) • Real-life examples: • flashlights, doorbells, and many kitchen appliances.

5. What caused the light bulb to light? • Yeah, electricity but… • How? • The things that move through wires are negatively charged electrons as you might expect • (because metals have a “sea of electrons” – meaning the electrons are free to move) • However, the direction of an electric current is defined as the direction that positive charges move • Say what?! • Ben Franklin had to make an arbitrary choice as to whether positive charges or negative charges carry the current (based on what he knew at the time. He chose wrong! Should physicists change the definition of the direction of electric current – sorry but tradition rules over and so no. • But what does this REALLY mean? • The direction of current is opposite to the direction that electrons move!

6. Current • Write what’s underlined: • Current is: • The flow of electric charge • In electric circuits, this charge is often carried by moving electrons in a wire. • It can also be carried by ions in an electrolyte • Or by both ions and electrons in a plasma • The unit of measurement is the ampere • 1 ampere = the flow of electric charges through a surface at the rate of 1 coulomb per second. • The direction of current is opposite to the direction that electrons move.

7. Speed of current • Regardless of the direction of electric current, electrons DO move around in the wires… • Do they speed along so they get from the wall switch in your home to the overhead light in a fraction of a second? • No, they actually move slow. • Electrons in a circuit move at a speed of about four tenths of a millimeter per second. • For you standard system users, that’s 1 foot in 14 seconds. • Slow, right? • Why do the electrons move so slow? • Well, there’s stuff for them to bump into in their path. • What stuff? • Other atoms. (we say they have collisions with other atoms) • What does this do? • These collisions generate heat, which is why wires carrying electricity can be quite hot. • They can generate heat, what else can they generate?

8. How do your lights turn on almost instantly? • If electrons move slow, how do the lights in your house turn on so quickly? • The answer is the effect of turning on the switch gets the electrons to the lights quickly (at almost the speed of light). • It’s kind of like turning on a water hose when it’s already filled with water. The water at the end of the hose comes out right away because the water coming from the faucet increases the pressure in the hose and “pushes” the water out the end. • The water that initially came out of the hose didn’t have to travel very far from because it was already at the end. - That’s why you may have heard that appliances, even when they’re off, use electricity.

9. Series Circuit • In a series circuit, electricity has only one path it can travel.

10. Voltage • In an electric circuit, electric forces cause the charges to move through an electric circuit, and we keep those charges moving by using a battery that takes the charges from one place to another. • It’s fine to think of a battery as moving charges through a “height difference” where they can now “drop” back down to the lower height. • The proper term to use for this height difference when dealing with electric forces is a Potential difference • Voltage across a device = • (current through the device)(resistance of the device) • V = I R • This is known as ohms law.

11. Resistance • The units of resistance are ohms. • The resistance determines the extent to which the object limits the flow of current in a circuit. • The electrical resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor; the inverse quantity is electrical conductance, the ease at which an electric current passes. • How easy or difficult it is for the electrons to flow in a circuit. • If they flow easily, it has low resistance, • if it is difficult for them to flow, it has high resistance.

12. The end of Day 6 • Homework is to finish the homework packet. • We’ll finish up the lab on Tuesday.