Direct current flows in one direction only. Cells, batteries and solar cells are d.c.
1) circuit drawings use arrows to show the direct current flowing from + to -.
2) On a cathode ray oscilloscope a dc would look like a straight line
Alternating current (a.c.)
Alternating current oscillates (reverses its direction) continuously. Mains electricity is a.c. (it has a frequency of 50 hertz).
1) 50 hertz means it oscillates 50 times per second. because it changes direction, you cannot show the direction of a.c. using arrows
2) On a cathode ray oscilloscope a.c. would look like a wave.
Cells and batteries
Cells and batteries are sources of direct current. a single cell normally gives 1.5 volts. A battery contains two or more single cells (although single cells are commonly referred to as batteries). There are three main types of battery/cell:
- Wet cell rechargeable: which contains lead and acid and is used for cars and industry.
- Dry cell non rechargeable: which contains zinc, carbon, manganese or mercury, lithium. It is used for torches, clocks, radios, hearing aids and pacemakers.
- Dry cell rechargeable: contains nickel, cadmium, lithium. Can be used for Mobile phones, power tools.
Non-rechargeable batteries are not beneficial to the environment because...
- the energy needed to make a cell is 50 times greater than the energy it produces
- less than 5% of dry cells are recycled
- the UK produces about 30 000 tonnes of waste dry cells every year
- toxic chemicals like mercury, cadmium and lead can leak into the ground causing pollution.
Governments are starting to tackle this problem and various safe disposal (used batteries should not be placed in dustbins) and recycling schemes are being discussed. Rechargeable batteries are one alternative.
advantages and disadvantages of rechargeable Batte
Advantages of rechargeable Batteries
- Less expensive in the long run
- Can be used many times over
- fewer batteries are discarded into the environment
- Less of a drain on energy resources
Disadvantages of rechargeable Batteries
- Need to buy a charger
- More expensive
- most contain carcinogen chemicals
- Can go flat without warning (unsuitable for smoke alarms)
Capacity(Ah) = Current (A) x Hours (h)
Electric current needs a complete circuit to flow. It will then flow continuously until the circuit is broken, e.g. a switch is opened 9turned off).
Current is the rate of the flow of electrons in a component. Electrons have a negative charge. In a complete circuit they leave the negative terminal because they are attracted towards the positive terminal. The greater the flow of electrons (i.e the more electrons per second) the greater the current.
When the current is flowing, energy is transferred from the cell to the circuit componenets (devices). The electric current will flow through an electrical component if there is a voltage (potential difference p.d) across the ends of the component.
Voltage (potential Difference, p.d.)
The atoms of all materials contain electrons but they are normally strongly bound by attraction to the positive nucleus of the atom. In the metals (e.g. copper) some of the electrons are less tightly bound (free electrons) and are able to move between thte atoms witihin the metal, making metals good conductors.
When a conductor (e.g. a piece of copper wire) is connected to a d.c.