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# Basic Electronics - Ohm's Law

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This presentation is an introduction to electrical theory and will help the reader understand some of the terminology used when we discuss controlling external devices using the GPIO port of the Raspberry Pi.

We look at Voltage, Current & Resistance and the relationship between the three known as Ohm’s Law.

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• Thanks for this - should be required reading for anyone with a Raspberry Pi who wants to mess around with the circuits. Love the analogy.

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### Basic Electronics - Ohm's Law

1. 1. BASIC ELECTRONICSVoltage, Current, Resistance and Ohm’s Lawwww.sf-innovations.co.uk
2. 2. This presentation is an introduction to electrical theory and will helpthe reader understand some of the terminology used when we discusscontrolling external devices using the GPIO port of the Raspberry Pi.We look at Voltage, Current & Resistance and the relationship betweenthe three known as Ohm’s Law.www.sf-innovations.co.uk
3. 3. VoltageVoltage is the electric pressure that causes current to flow in a circuit. For agiven circuit, the higher the pressure, the higher the current is that will flow.The basic unit of Voltage is the Volt (V) and the symbol V is used torepresent this.One can draw an analogy between the Voltage in a battery and waterstored in a tank, high above the ground. This constantly wants to flow downdue to gravity. However one needs to open the tap to allow it to flow. In thesame way, the Voltage in a battery wants to push a current round a circuit.Current flows as soon as the switch is closed.Some typical voltages are:AAA & AA battery: 1.5V DCPP3 battery: 9V DCCar battery: 12V DCMains in the UK 230V ACMains in the US 110V ACNote: DC stands for Direct Current and AC stands for Alternating Current.www.sf-innovations.co.uk
4. 4. CurrentCurrent is the flow of electrons in a circuit. The basic unit of current is theAmpere (A) and the symbol I is used to represent this.In the water analogy, current is like the water that flows once the tap isopened. Current can only flow once the switch is closed and a circuit ismade.Some typical currents taken by common devices are:100W bulb 0.44A 440mA2kW kettle 8.7ANote: 1000mA = 1Awww.sf-innovations.co.uk
5. 5. ResistanceThe resistance of a device is a measure of how much it opposes the flow ofcurrent. The basic unit of resistance is the Ohm (Ω) and the symbol R is usedto represent this.In the water analogy, the resistance is like the width of a pipe. A narrowerpipe would present a higher resistance to the flow of water, and less waterwould flow. A wider pipe would result in a larger flow of water as theresistance is lower. In the same way the higher the resistance, the lower thecurrent would be as it would find it difficult to flow. The current flow would bea lot higher in a circuit where the resistance was low.Some typical resistances are:100W bulb 529Ohm2kW kettle 26.5Ohmwww.sf-innovations.co.uk
6. 6. Water AnalogyHigher the pressure (voltage), more water (current) would flowSmaller the pipe (higher resistance) less water (current) will flowwww.sf-innovations.co.uk
7. 7. Ohm’s LawFrom the water analogy one can see that:Voltage is directly proportional to currentResistance is inversely proportional to currentThis is known as Ohm’s Law and can be used as follows.Voltage (V) = Current (I) x Resistance (R)Resistance (R) = Voltage (V) / Current (I)Current (I) = Voltage (V) / Resistance (R)www.sf-innovations.co.uk
8. 8. Ohm’s Law in useWhat is the current that will flow through a car light bulb which has aresistance of 3 ohms?The voltage of the car battery is 12V. We use the equation below to workout the current.Current (I) = Voltage (V) / Resistance (R)Current (I) = 12/3= 4 Amps.A current of 4 Amps will flow through this light bulb when the circuit isclosed.If we wanted to reduce this current to 2 Amps, we would need to doublethe resistance i.e. add a 3 ohm resistor in series with the bulb.www.sf-innovations.co.uk
9. 9. www.sf-innovations.co.ukPowerThis is a measure of how much energy is flowing in a circuit everysecond. The basic unit of Power is the Watt (W) and the symbol P is usedto represent this.In the water analogy, if the flow of water were used to turn a wheel thenthis could be used to produce energy. The higher the water tank and thefaster the flow of water, the higher the amount of power that could beproduced.Similarly, when a current flows around a circuit, it produces useful powersuch as light in an electric bulb or heat in a kettle or a fire. The higher theVoltage and Current, the higher the power produced.Some examples of power in appliances are:Electric bulb 40W, 60W, 100WKettle 2000W 2kWPC 150WNote: 1000W = 1kW
10. 10. www.sf-innovations.co.ukPowerThe higher the Voltage and Current, the higher the power produced. IePower is proportional to Voltage and Current.Watt (W) = Current (I) x Voltage (V)Then using Ohm’s law, we can also represent power as follows.Watt (W) = Voltage (V) x Voltage (V) / Resistance (R)Watt (W) = Current (I) x Current (I) x Resistance (R)So for example, the power dissipated in a car headlight lamp which has aresistance of 3 ohms isWatt (W) = 12 X 12 / 3= 48 W
11. 11. www.sf-innovations.co.ukSummaryOnce can use the behaviour of water under pressure to understand thebehaviour of electricity.Voltage is equivalent to pressureCurrent is equivalent to the flow of waterResistance is equivalent to the impedance to the flow of waterOhm’s law is a representation of the relations between these voltage,current and resistance.Power is a measure of the work being done by the flow of electricity.