MYP Physics Current, Voltage, Power Notes

How Does Current, Voltage and Power Explain Electricity?

1. Electric current is used to describe how electric charge moves through a conductor such as a metal wire.
2. It does not describe how much energy is transferred; instead, it describes how much charge passes a point in a given time.
3. In metal conductors, the charges that move are electrons, which drift through the lattice of metal ions.
4. Although individual electrons move slowly, the effect of their movement allows energy to be transferred quickly throughout a circuit.
5. Current is therefore a measure of charge flow rate, not energy flow.

Electric Current Is The Rate Of Flow Of Charge

Current is defined by the relationship:

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

where:

1. $I$ is current (in amperes, A)
2. $Q$ is charge (in coulombs, C)
3. $t$ is time (in seconds, s)

How is the current connected to charge and time?

1. Electric charge is measured in coulombs (C).
2. Current depends on both:
   1. The amount of charge that moves, and
   2. The time taken for that charge to move.
3. A larger current means that a larger amount of charge passes a point every second.
4. A smaller current means that less charge passes a point every second.
5. Current allows scientists to quantify how intense the flow of charges is.

What does the direction of current mean?

1. Scientists use a standard direction called conventional current.
2. Conventional current is defined as the direction that positive charges would flow.
   1. Electron flow: from $-$ to $+$
   2. Conventional current: from $+$ to $-$
3. In metal conductors, electrons are negatively charged and move in the opposite direction.
4. Using a fixed convention allows circuits and systems to be described consistently.

Voltage Is Energy Transferred Per Unit Charge

1. Charges require energy to move through a conductor.
2. Voltage provides this energy to the charges.
3. Voltage does not push charges mechanically; it transfers energy to each charge, enabling motion.
4. Without voltage, charges remain randomly distributed and no net current flows.

$$V = \frac{E}{Q}$$

where:

1. $V$ is potential difference (in volts, V)
2. $E$ is energy transferred (in joules, J)