MYP Physics 3.3 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)
3. $Q$ is charge (in coulombs, C)

How does voltage describe energy transfer?

1. Voltage tells us how much energy each charge gains or loses.
2. A higher voltage means each charge carries more energy.
3. When charges pass through a component, this energy is transferred into other forms, such as:
   1. Thermal energy in heaters
   2. Light energy in lamps
   3. Kinetic energy in motors
4. Voltage therefore links electrical systems to energy transformations.

Why is voltage measured between two points?

1. Voltage compares the energy of charges at two different locations.
2. It describes the energy change as charges move from one point to another.
3. A larger voltage difference means a greater energy transfer.
4. Voltage cannot exist at a single point
5. It always involves a comparison.

How Meters Are Connected

1. An ammeter measures current, so it must be placed in series (so the same current passes through it).
2. A voltmeter measures potential difference, so it must be placed in parallel across the component (so it compares the potential at two points).

What Is Resistance?

1. Resistance describes how difficult it is for electric charges to move through a material.
2. Materials with high resistance slow down the movement of charges.
3. Materials with low resistance allow charges to move more easily.
4. Resistance affects how much current flows for a given voltage.

What causes resistance in a conductor?

1. Charges collide with atoms inside the material.
2. These collisions transfer energy as thermal energy.
3. More collisions mean greater resistance.
4. Resistance depends on:
   1. Type of material
   2. Length of the conductor
   3. Thickness of the conductor
   4. Temperature

How does resistance affect current?

1. For the same voltage:
   1. Higher resistance results in smaller current.
   2. Lower resistance results in larger current.
2. Resistance controls how easily charges can flow.
3. This is why different components allow different currents to flow.

How Fast Is Electrical Energy Transferred?

1. Electrical power describes the rate of energy transfer in an electrical device.
2. It tells us how quickly electrical energy is converted into other forms.
3. A device with higher power transfers energy more rapidly.
4. Power depends on both:
   1. The voltage supplied
   2. The current flowing

Measuring power

1. Power is measured in watts (W).
2. One watt means one joule of energy is transferred each second.
3. Power ratings on devices indicate how fast energy is used.
4. These ratings help compare how quickly different devices consume energy.

How do voltage and current affect power?

1. Power increases if:
   1. Voltage increases while current stays the same, or
   2. Current increases while voltage stays the same.
2. A device may have high power because it uses:
   1. A high current, or
   2. A high voltage.
3. Different designs are chosen depending on the purpose of the device.

Power vs energy

1. Power tells us how fast energy is transferred.
2. Energy tells us how much energy is transferred in total.
3. Energy depends on both power and time.
4. A low-power device used for a long time can use the same energy as a high-power device used briefly.

The Relationship Between Voltage, Current, and Resistance (Ohm's Law)

1. When voltage increases and resistance stays the same, current increases.
2. When resistance increases and voltage stays the same, current decreases.
3. This relationship is summarized using a simple equation.

$$V = I \times R$$

where:

1. V = voltage (volts, V)
2. I = current (amperes, A)
3. R = resistance (ohms, Ω)