IB MYP 4-5 Physics- Understanding AC vs DC current- Study Notes - New Syllabus
IB MYP 4-5 Physics-Understanding AC vs DC current- Study Notes
Key Concepts
- Understanding AC vs DC current
Alternating Current (AC) vs Direct Current (DC)
Alternating Current (AC) vs Direct Current (DC)
Direct Current (DC)
Electric charges flow in one direction only.
- Voltage is constant (does not change with time).
- Supplied by batteries, solar cells, and DC generators.
- Symbol in circuits:
.
Alternating Current (AC)
Electric charges change direction periodically.
- Voltage alternates between positive and negative values in a sinusoidal pattern.
- Supplied by power stations through the mains supply.
- Frequency: In most countries, \(50 \, Hz\) (50 cycles per second); in the USA, \(60 \, Hz\).
- Symbol in circuits:
.
Key Differences
| Feature | Direct Current (DC) | Alternating Current (AC) |
|---|---|---|
| Direction of flow | One direction | Changes direction periodically |
| Voltage | Constant | Alternates (positive & negative) |
| Source | Batteries, solar cells | Power stations, generators |
| Transmission | Not suitable for long distances | Efficient for long-distance transmission (with transformers) |
| Applications | Electronics, laptops, cars | Homes, factories, appliances |
Real-Life Applications
- DC: Used in phones, laptops, cars, LED circuits, solar panels.
- AC: Used in household power supply, factories, power transmission.
Example:
Why is AC used for transmitting electricity over long distances instead of DC?
▶️ Answer/Explanation
Step 1: Power loss in wires is \(P_{loss} = I^2 R\).
Step 2: To reduce power loss, we must reduce current while keeping power constant.
Step 3: With AC, transformers can step up voltage → reduce current → reduce losses.
Step 4: DC cannot be stepped up/down easily with transformers.
Final Answer: AC is more efficient for transmission because voltage can be increased or decreased using transformers, reducing power loss.
Example:
A mobile phone battery provides \(5 \, V\) DC. If it is charged using a \(230 \, V\) AC supply, explain how this is possible.
▶️ Answer/Explanation
Step 1: The charger first uses a step-down transformer to reduce voltage from \(230 \, V\) AC to a lower AC value.
Step 2: A rectifier circuit converts AC to DC.
Step 3: Voltage regulators stabilize the output to \(5 \, V\) DC.
Final Answer: The charger transforms high-voltage AC into low-voltage DC suitable for charging the battery.
Example:
Sketch and describe the shape of voltage-time graphs for DC and AC.
▶️ Answer/Explanation
Step 1: For DC, the graph is a straight horizontal line (constant voltage with time).
Step 2: For AC, the graph is sinusoidal, alternating between positive and negative values.
Final Answer: DC = flat line; AC = sine wave curve.