MYP Physics 3.8 AC & DC Notes

How Does The Way Electric Current Flows Affect How Electricity Is Used?

1. Electric current refers to the movement of electric charge through a circuit.
2. The behaviour of electric current depends on how charges move over time.
3. Electrical systems are designed based on the type of current they use.
4. There are two main types of current used in electrical systems: direct current (DC) and alternating current (AC).

DC Means A Constant Direction (And Usually A Constant Voltage)

1. In a DC circuit, electrons move in one continuous direction.
2. The direction of charge flow does not change with time.
3. As long as the power source remains connected, the current remains steady.

Sources of Direct Current

1. Batteries and cells produce DC due to chemical reactions inside them.
2. Solar cells produce DC when light energy is converted into electrical energy.
3. Laboratory DC power supplies provide adjustable direct current.

Voltage Behaviour in DC Circuits

1. The voltage supplied by a DC source is constant.
2. Because the voltage does not change, the current in a simple DC circuit remains constant.
3. This stability makes DC suitable for precise electronic applications.

AC Means The Direction Reverses Regularly

1. In an AC circuit, electrons do not travel steadily in one direction.
2. Instead, they move back and forth around fixed positions.
3. The direction of current changes repeatedly over time.

Voltage Behaviour in AC Circuits

1. The voltage in an AC supply increases and decreases continuously.
2. The voltage passes through zero and then reverses direction.
3. This repeating change creates a regular pattern.

AC in Everyday Electricity Supply

1. Electricity supplied to homes is alternating current.
2. Wall sockets provide AC power to appliances.
3. Household devices are designed to operate safely with AC input.

DC on a Voltage–Time Graph

1. A DC voltage remains constant over time.
2. On a voltage–time graph, DC appears as a straight horizontal line.
3. The line does not cross the time axis.

AC on a Voltage–Time Graph

1. AC voltage changes continuously with time.
2. The graph shows a repeating wave shape.
3. The voltage crosses zero whenever the current reverses direction.

How A Generator Produces AC (Electromagnetic Induction)

1. A motor run "in reverse" becomes a generator.
2. Instead of using electrical energy to produce rotation, you use rotation (from a turbine, windmill, waterwheel, etc.) to produce electrical energy.
3. The core idea is electromagnetic induction: a voltage is induced when a conductor experiences a changing magnetic field, or when it cuts magnetic field lines.
4. A generator can work in more than one way:
   1. the coil moves through the magnetic field (common in simple models), or
   2. the magnets move relative to a stationary coil (also common in real devices), or
   3. a rotating disk of magnets creates a changing field near coils.

How is alternating current produced?

1. An AC generator produces electricity by combining motion, a magnetic field, and a conductor.
2. When a coil of wire rotates in a magnetic field, the magnetic field through the coil changes.
3. This changing magnetic field causes charges in the wire to move.
4. The direction of induced current changes as the coil rotates, producing alternating current.

Why the current alternates

1. As the coil turns, each side of the coil moves in opposite directions through the magnetic field.
2. After half a turn, the direction of motion reverses.
3. This reversal causes the current direction to change.
4. Continuous rotation results in a continuously changing current direction.

Why The Output Reverses Every Half Turn

1. When the coil rotates, each side of the coil alternately moves "up" and "down" through the magnetic field.
2. That reverses the direction of the induced voltage across the coil, so the current reverses. The result is AC.

Why AC Can Be Transformed (And DC Cannot, In The Same Simple Way)

1. A major advantage of AC is that it can be changed easily using a transformer.
2. A transformer has:
   1. a primary coil connected to an AC supply,
   2. an iron core (usually a closed loop) that guides magnetic field lines,
   3. a secondary coil where a new voltage is induced.
3. Because the primary current is constantly changing (AC), the magnetic field in the core is also constantly changing.
4. This changing field induces a voltage in the secondary coil.

Step-Up And Step-Down Transformers

1. Step-up transformer: the secondary coil has more turns than the primary coil, so the induced voltage in the secondary is larger.
2. Step-down transformer: the secondary coil has fewer turns, so the secondary voltage is smaller.

Safety And "Which Is More Dangerous?" Depends On Context

1. In everyday life, mains electricity is often described as more dangerous than a small battery because it typically provides:
   1. much higher voltage, and
   2. the ability to deliver a large current through your body.
2. However, "AC vs DC" is not the only factor.
3. The danger depends on the size of the voltage and current, the pathway through the body, contact time, and conditions like wet skin.