ac and dc (AQA GCSE Physics Combined Science): Revision Notes
ac and dc
What is electric current?
Electric current is simply the flow of tiny particles called electrons through a wire. Think of it like water flowing through a pipe - but instead of water, we have electrons moving along.
The water-pipe analogy is very helpful for understanding electricity. Just as water needs a complete pipe system to flow from one place to another, electrons need a complete path to move through a circuit.
For current to flow, you need a complete circuit. This means the electrons must have a full path to travel around from one end of a battery or cell to the other end.
Direct current (dc)
Direct current means electrons flow in just one direction around the circuit. They all move the same way and don't change direction.
Sources of dc:
- Batteries
- Cells
- Solar panels
When you look at dc on an oscilloscope (a device that shows electrical patterns), you see a straight horizontal line. This shows the voltage stays constant and doesn't change.
Example: Torch Circuit
A torch uses dc from batteries. The electrons flow from the negative end to the positive end in one steady direction. This creates a constant flow of electricity that keeps the bulb glowing steadily.
Alternating current (ac)
Alternating current means electrons keep changing direction. They flow one way, then switch and flow the other way, over and over again.
The mains electricity in your home is ac. It has:
- Voltage: 230V (average working value)
- Frequency: 50Hz
What does 50Hz mean? The current changes direction 50 times each second. Since it goes forwards then backwards, that's actually 100 direction changes every second!
The calculation for total direction changes: 50Hz × 2 directions = 100 changes per second. This constant switching is why ac can be easily transformed to different voltages using transformers.
On an oscilloscope, ac looks like a wavy line (sine wave) going up and down. This shows the voltage constantly changing from positive to negative values.
Dc and ac in the home
Both types of current are used in homes:
DC is used for:
- Mobile phones
- Laptops
- Torches
- Remote controls
AC is used for:
- Kettles
- Washing machines
- Televisions
- Most plugged-in appliances
Power ratings
Different electrical appliances have different power ratings. The power rating tells you how much energy the device uses each second.
Power Rating Examples:
- Electric oven: 3000W (uses lots of energy)
- Kettle: 2200W
- Hairdryer: 1500W
- Microwave: 950W
- Electric shaver: 15W (uses very little energy)
Higher power rating = more energy used per second = costs more to run.
The relationship is direct: double the power rating means double the electricity cost when running for the same amount of time.
Key Points to Remember:
- DC flows in one direction - like a one-way street for electrons
- AC changes direction regularly - electrons go back and forth
- DC comes from batteries and cells - gives a straight line on oscilloscope
- AC comes from mains electricity - gives a wavy line on oscilloscope
- UK mains supply is 230V, 50Hz - changes direction 100 times per second