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### Current is…

the rate of flow of electric charge.

Current is flow of charge

- Current of a river or stream: amount of water moving by a point in a certain amount of time. Electric current is the same idea. It’s the amount of electric charge (coulombs) moving past a point in one second.
- Unit of current is amperes (amps for short); named after Andre-Marie Ampére, a French scientist who studied electromagnetism.

Current is measured in amps

- 1 amp = 1 coulomb/second
- Abbreviation for amps as a unit is A.
- Current always flows from high voltage to low voltage (just like water naturally flows from a high spot to a low spot and never the other way around).

Charges are very small

- You can’t see current, only the effect of current.
- Electrons in the copper wire carry the current by repelling other current-carrying electrons.
- Electrons are so small they fit in between atoms.
- When there is no voltage, electrons in the wire do not flow in a current.

Measuring current

- Since current is the number of charges per second, charges must flow through meter in order to be counted.
- A meter must be a part of the circuit, not just an outside agent (like to measure voltage).
- AC means “alternating current”, which describes current coming out of the wall (from the electric company).
- DC means “direct current”, which describes current coming from a battery.

Circuit breakers and fuses

- Meters can be damaged by too much current.
- Circuit breakers and fuses protect appliances from too much current by opening the circuit preventing current from damaging appliances.
- Circuit breakers can be reset; fuses are one-use and must be replaced.
- Most modern homes have circuit breakers.
- Our meters have fuses in them.

Ground Fault Interrupters

- Circuits near sinks are wired with a ground fault circuit interrupters (GFCIs or GFIs). Electrical code requires this.
- Current goes in one prong of the plug and out the other plug. The third prong (if present) is the ground. The amount of current going in and out should be exactly the same. If it is not, some current could be going through you. That’s bad.

GFIs

- When there is a difference noticed by the GFI, it opens the circuit, stopping the flow of charge.
- A GFI breaks the circuit in 0.03 seconds if it detects a leak as small as a few thousandths of an amp.
- Our electrical outlets at each lab table are equipped with GFIs. Reset by pushing the button back in so it is flush with the outlet.

How to reset a circuit:

- If a circuit breaker trips, you must go to the electrical panel (where the electrical service comes into your house), locate the tripped circuit, turn it off, and then turn it back on. Circuit breakers look like toggle light switches. It’s easy.
- If you have blown a fuse, the fuse must be unscrewed and replaced. Some fuses have a base like a light bulb.

Alternating Current (AC)

- Direction of the current goes back and forth
- In US, current reverses 60 times per second.
- In Europe and other countries, current reverses 50 times per second (need adaptors).
- Each wall socket has three wires: one is 120 volts (hot), one is zero volts, and the third is the ground. It is attached to the ground, so excess current goes to the soil and not through you. Safety!

Direct Current (DC)

- Current in a battery only flows in one direction.
- Our experiments in school will make use of DC.
- When measuring current generated by batteries, use DCA setting on multi-meter.

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