Physics / Unit 9: Electricity and Circuits
9.3 Current and Voltage
Notes
Electric current is the rate at which charge flows: $\displaystyle I = \frac{Q}{t}$, measured in amperes (A), where $1 \text{ A} = 1 \text{ C/s}$.
- Voltage (potential difference) is the energy given to each coulomb of charge: $\displaystyle V = \frac{E}{Q}$, measured in volts ($1 \text{ V} = 1 \text{ J/C}$).
- A battery provides voltage; the voltage pushes current around a closed circuit.
- Conventional current flows from + to $-$; electrons actually move the opposite way.
- DC (direct current) flows one way (batteries); AC (alternating current) reverses direction (wall outlets).
- Example: 30 C passes a point in 10 s: $\displaystyle I = \frac{30}{10} = 3 \text{ A}$.
Practice
A charge of 120 C flows through a wire in 1 minute. What is the current?
$\displaystyle I = \frac{120 \text{ C}}{60 \text{ s}} = 2 \text{ A}$.
A 9 V battery moves 2 C of charge through a circuit. How much energy does it give the charge?
$E = VQ = 9 \times 2 = 18 \text{ J}$.
A flashlight bulb does not light when the switch is off. Why?
Current needs a complete, closed path.
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