Electric Current

Lesson goal: define current as rate of charge flow and distinguish conventional current from electron flow.

The critical language repair is that current flows; voltage does not.

Current is the rate at which charge passes a point:

$$I = \frac{Q}{t}$$

Conventional current is defined as the direction positive charge would move in the external circuit. Electron flow is opposite to conventional current in a metal wire.

Quantity	Symbol	Unit	Meaning
current	$I$	A	charge per second
charge	$Q$	C	amount of electric charge
time	$t$	s	duration of flow

1. Current is measured in:

   • A. volts
   • B. amps
   • C. ohms
   • D. joules

   Answer: B.

2. Conventional current in an external circuit is from:

   • A. negative to positive
   • B. positive to negative
   • C. north to south
   • D. high resistance to low resistance only

   Answer: B.

3. If 20 C passes a point in 8 s, current is:

   • A. 2.5 A
   • B. 12 A
   • C. 160 A
   • D. 0.40 A

   Answer: A.

A current of $2.5\ \text{A}$ flows for $8.0\ \text{s}$. Calculate the charge that passes a point.

$$Q = It = 2.5\times8.0 = 20\ \text{C}$$

Final answer: $20\ \text{C}$.

Prompt: explain why current is not "used up" by a resistor in a simple series circuit.