A circuit is a path from one side of a power source (e.g. a cell, battery or power pack) to the other. The four basic parts of a simple circuit are:
• an energy source, such as a cell or battery. A cell or battery can be thought of as a charge pump.
• a conducting path (wires) for the electricity to flow through
• an energy user or load, such as a globe, motor, buzzer, heating element or resistor
• a switch to turn the current on and off. A major blackout
On 14 August 2003 an electrical failure suddenly hit the United
States and Canada. About 50 million people in cities from New
York to Toronto had no power. People were trapped in subway
trains and elevators for hours. The loss related to the blackout was estimated at $6 billion. One
month later, Italy’s 57 million people also were affected by a blackout. Luckily it occurred on
a weekend so its initial impact was less dramatic and caused less economic damage. Some developing countries have regular ‘brownouts’ because their need for electricity exceeds their ability to generate it. Electricity supply must
be ‘rationed’, and so suburbs and towns have times each day when
no electricity is available.
Fig 3.1.2 Imagine this scene without electricity. What problems would it cause?
A simple circuit and its equivalent circuit diagram
connecting wire cell 1.5 V globe circuit diagram switch circuit 1.5 V cell ++ –– Fig 3.1.1
3.13.1
Unit
Unit
Water in pipe Units Electricity in wire Units
Pressure (P) Pascals Voltage (V) Volts Flow rate (F) Litres/second Current (I) Amps Resistance to flow (W) Newtons Resistance (R) Ohms A V Conductor/lead Globe Closed switch Open switch Ammeter Voltmeter Leads connected Cell Battery Fixed resistor Variable resistor Leads crossing Fig 3.1.3 Common components in simple circuits
3.1
3.1
Inside a circuit
There are three very important values in circuits that we can measure and calculate.
• Whenever charge moves, we have a current. In most circuits the moving charges are electrons and current is defined as the rate of flow of those electrons. Current is measured in amperes (A) or amps for short. Sometimes in a circuit there will be more than one path that the current can take. More current will flow down the easier path and less down the harder one. In mathematical formulas, current is given the symbol I.
• Depending on what part of the circuit we are talking about, voltage is a measure of how much energy: – is available from the battery or power pack
to push current through the circuit. It may be thought of as the size of the ‘push’.
– is used when current passes through a load. Voltage is measured in volts (V) and is sometimes referred to as potential difference. Voltage is given the symbol V in mathematical formulas.
• Resistance is a measure of how much a load (e.g. globe, motor, resistor) restricts and reduces the flow of current. Resistance is measured in ohms, or Ω for short. In mathematical formulas, resistance is given the symbol R.
To help you understand these terms we will use the analogy of a water pump circuit.
In a water circuit, the pressure supplied by the pump (P) drives the water around the closed loop of a pipe at a certain flow rate (F). The waterwheel (W)
The water pump and electrical circuits water reservoir current (I) switch battery high voltage + low voltage – ground water wheel valve resistance high pressure pump low pressure Fig 3.1.4
restricts the flow, slowing down the water, using up its energy. The valve turns the flow of water on and off.
In an electrical circuit, the energy or voltage (V) supplied by the battery drives the electrons around the circuit, causing an electric current (I). The resistance (R) slows the electrons, using up their energy. A switch turns the flow of electricity on and off.
Voltage
A battery or power pack is the ‘pump’ of an electrical circuit. A water pump takes in water at low pressure, supplies energy to it and ejects it at high pressure. A battery or power pack takes in charge at low voltage, adds energy to it and ejects it at a higher voltage.
>>>
low voltage
valve
If closed, pressure is behind valve but no flow of water. high pressure low pressure high voltage – + A switch has voltage behind it, but no current if not switched on.
Fig 3.1.5 Voltage can be compared to the
pressure of water in a pipe.
Current
When current flows through a wire it moves freely, losing almost no energy. This is just like water in a pipe where there is little resistance to slow the water down. A higher current means more electrons flow
past a point in a circuit every second.
A current of 1 ampere means that 1 coulomb of charge passes
by a point in the circuit each second. A coulomb is an amazing 6 250 000 000 000 000 000 electron- sized charges!
Electricity
Electricity
Fatal currents A current as small as 0.1 to 0.2 amps can kill! Most deathsassociated with electric shock happen because the electricity interrupts the heartbeat, which is controlled by small electrical currents in your body. High voltages are more dangerous than low ones because they can
drive a higher current through your body. The 240 volts in our
home power supplies is easily enough to drive a deadly current
through your body.
Current can be compared to the rate of flow of water through a pipe.
Thick wire offers little resistance to flow of electrons.
A large pipe offers little resistance to flow of water.
Fig 3.1.6
Resistance
A waterwheel restricts the flow of water, slowing the water down and taking away its energy. Light globes, buzzers, motors, heating elements and resistors are loads that restrict the flow of current and remove energy from the electrons. These loads change the electrical energy into other forms such as sound, light, heat and kinetic (moving) energy.
The filament of a light globe is a very thin wire. As the current tries to squeeze through, it encounters resistance and uses up some of its energy. In a thick wire, electrons move more freely and with little resistance. Little energy is lost.
Increasing the resistance of the circuit will cause a decrease in the current, and results in more energy being used up by the load.
Resistance in a circuit can be compared to a water wheel.
A resistor acts as a load, converting electrical energy to heat and light.
A water wheel is like a load in the circuit. It converts kinetic energy of water to movement of the wheel.
Fig 3.1.7
Fig 3.1.8
Heat energy being released in a glowing resistor of an electric bar heater