Schedule for the next few classes...
- Today: New Unit
- Tomorrow: ISP assigned, work period, bring laptops/tablets.
- Friday: Lab Due.
- Jan. 5: Welcome back.
- Jan. 6: Quiz on Circuits
Unit 5: Electricity and Magnetism
What is charge?
- can be positive, negative, or neutral.
- a source of electric field (force) and also reacts to that force. (ie. charges exert forces on each other).
- the smallest charge is 1 electron (or proton) = 1.602 x 10 ^-19 C
- C is Coulombs, units of charge.
- Unlike gravity, charges attract or repel.
Electric Potential
Electric forces can do work. Therefore, there is a form of electric energy. We call it electric potential energy.
The electric potential energy per Coulomb is called “voltage”. Also called electric potential, or potential difference.
V = ∆E/q
Units are "volts", 1 V = 1 J / 1 C
Like gravitational potential, zero can be at any point, only differences matter. Often written as ∆V.
Current
The movement of charges.
I = Q/∆t
Units are "amperes", 1 A = 1 C / 1 s
Ohm’s Law
Potential difference makes charge flow, but the flow is restricted by resistance.
Example:
A flashlight requires 4 AA batteries (1.5 V each). If the bulb has 500 Ω of resistance, what current goes through the bulb?
I = V ÷ R = (1.5 A x 4) ÷ 500 Ω = 0.012 A = 12 mA
b) If the same bulb is plugged into a wall socket (120 V), what current is drawn?
I = V ÷ R = 120 V ÷ 500 Ω = 0.24 A = 240 mA
Most houses have 15 A “breakers” for safety.
Example:
A TASER can deliver up to 50 000 V through the air. Once it contacts you, the voltage drops to 1200 V. A wall socket provides 120 V. Which is more dangerous?
The resistance of a human depends on the point of entry, moisture, tissue, etc.
Dry skin, R = 100 000 Ω
Broken skin, R = 1000 Ω
Use broken skin and a wall socket.
I = V/R I = 120 mA
Taser.
I = V/R I = 1.2 A
Looks like a Taser is more dangerous… why doesn’t it kill you? Remember, current is charge/second, if you have limited charge, you have limited current.
 |
| A taser has limited charge in the battery. |
Taser claims to deliver 0.002 A up to 0.03 A.
Wall socket has unlimited supply of charge.
No comments:
Post a Comment