3.4 Resistance and Ohm's Law
Learning Objectives:
After this lesson, students can
- understand voltage, current, resistance and Ohm's law;
- use multimeter to measure voltage, current and resistance;
- use potentiometer;
- use Tinkercad for circuit simulation.
1. CURRENT, VOLTAGE AND OHM'S LAW
- Ohm’s Law was discovered by Georg Simon Ohm and published in his paper in 1827. The law is the first, and perhaps most important law in electricity, that describes the relationship between voltage, current and resistance.
A. Current
- Current is the flow of electrons (電子) in a circuit (電路), like water current.
- Usually, we use I to represent current.
- Unit: Ampere (Amps) (A) (安培)
- In a conductor (導電體), like copper (銅) wire, electronics can easily move from a copper atom (原子) to other atoms.
- To make a current, we need to apply a voltage (電壓) to the circuit to push electrons, for example, by a battery.
- Current flow in ONE direction is called direct current (DC) (直流電), e.g. from battery.
- Current flow in BACKWARD AND FORWARD directions is called alternating current (AC) (交流電), e.g. from a power station (發電廠).
- AC is more efficient than DC for transmitting electricity along a long distance.
- Current is measured by an ammeter.
- Heat produced by high current may burn out a conductor. We can use resistor (電阻) to limit a current.
- 0.001 A ~ You can feel the flow of current in your body.
- 0.005 A ~ It is painful.
- 0.01 A ~ It causes muscle contraction.
- 0.015 A ~ Muscles will be paralyzed.
- 0.07 A ~ It affects the heart. Larger currents are fatal. (Conceptual physics 7th Edition – Paul G. Hewitt)
B. Voltage
- Voltage is the force pushing electrons to move in a circuit. It is what pushes electrons around a circuit, like water pressure.
- Usually, we use V to represent voltage.
- Unit: Volts (V) (伏特)
- Higher voltage --> higher current --> brighter lamp
- Voltage can exist without current, as if water pressure can be measured without water flowing. So we can measure a voltage in an open circuit (no current).
- Voltage is also called potential difference.
- If two 1.5V batteries are connected in series (串聯), the overall voltage is 3V. (--> brighter than using single 1.5V battery)
- If two 1.5V batteries are connected in parallel (並聯), the overall voltage is 1.5V. (--> same brightness as using single 1.5V battery but lasts for longer time)
- Voltage ≠ Volts. Voltage is the "pressure" but volt is a unit.
- Voltage is measured by a voltmeter, connected in parallel to measure the potential difference between 2 points in a circuit.
C. Ohm's Law
- Ohm's law is a relationship between voltage, current and resistance (電阻).
- Usually, we use R to represent resistance.
- Unit of resistance: Ω (Ohm)
- Resistance is the opposition to the flow of electrons. It tries to prevent electrons from flowing.
- Definition: Resistance (R) = Voltage (V) / Current (I)
V = IR
Voltage (V) = Current (I) x Resistance (R)
- For a fixed resistance, the current (I) is directly proportional (正比) to voltage (V). If we double the voltage, we double the current.
- For a fixed supply voltage (V), if we double the resistance (R), the current (I) will be halved.
Simulation:
- Current is the flow of electrons (電子) in a circuit (電路), like water current.
2. MULTIMETER AND BREADBOARD
A. Multimeter
DON'T MEASURE THE AC VOLTAGE ON WALL AS THE VOLTAGE IS HIGH!
B. Breadboard
A breadboard is a rectangular plastic board with a bunch of tiny holes in it. These holes let you easily insert electronic components to prototype.
- Modern breadboard is a rectangular piece of plastic with a grid of holes that allows you to easily and quickly build electronic circuits by pushing electronic components into the holes.
- Commonly used breadboards are: full-sized, half-sized and mini breadboards.
- The metal legs of electric components are also called leads.
- Power buses or rails are used to deliver power to your circuit. The long strips on either side of your breadboard that is usually labeled with red and black, or red and blue lines, and also a plus (+) or minus sign (-). The (+) labeled one is for connecting to positive voltage, and the (-) labeled one is for connecting to the ground (or 0V).
- Some of the holes on a breadboard are connected by metal clips. Some holes are not connected by metal clips. See the following diagram.
- Some common mistakes in breadboarding:
- Insertion to wrong rows
- Loose jumper wires or component leads
- Wrong polarity of component, e.g. LED can only light in forward bias, not in reverse bias.
- Solid-core wires are better than stranded wires in breadboarding.
- If you want to make your own jumper wires, you may need a wire stripper to strip off some of the insulation at the ends of the wires.
- Troubleshooting (not in the video):
- Check the connections visually. Check the circuit is an open circuit, closed circuit or short circuit. Use some colour convention of using wires can help.
- Use multimeter to check the voltages in a live circuit.
- Some components may be damaged or wrong. Remove the questionable components from the circuit and check them by multimeter.
- Check the running current of the components and the current limit stated in their specification.
Simulation: Which bulb(s) will be turned on?
3. POTENTIOMETER
If the output pin (2) of the potentiometer is used for Wemos D1 analog input (A0), Vcc should be connected to 3.3V so that the output (2) will vary from 0V to 3.3V.
Simulation: Potentiometer - JavaLab
Simulation:
https://www.tinkercad.com/things/jt6lcwh8oyJ
REMARK
In our daily life, you should know how to use a hot / cold water tap, which is quite similar to a potentiometer.
For example:
- Input hot water: High temperature 40 degrees Celcius
- Input cold water: Low temperature 30 degrees Celcius
- Output warm water: 30 to 40 degrees Celcius (a mixture of hot and cold water)
3. HOMEWORK
- CW3.4.1 Arduino + Potentiometer + Console (Tinkercad) 网页
Arduino / breadboard / Potentiometer / Multimeter / Serial Monitor in Tinkercad
- Make the following circuit in Tinkercad. Use two multimeters to measure the input and output voltage of the potentiometer. Use red wire for 5V and blue/black wire for GND (0V).
- Write the given program code.
- Start simulation and use the serial monitor to check the readings and graph.
- Pin A0 is the analog input pin of Arduino UNO board.
Circuit (https://www.tinkercad.com/things/1RbeX71MQDa)
Program Code
- Make the following circuit in Tinkercad. Use two multimeters to measure the input and output voltage of the potentiometer. Use red wire for 5V and blue/black wire for GND (0V).