In the previous article about circuits, we discussed circuits and conductance. We learned that a simple circuit has a power source, a conductor, and a load. The electrons come from the battery's negative terminal, which will move to the positive terminal, where there is an electron deficit. We also defined the driving force of electron flow as a potential difference. This article will discuss potential differences or voltage and how they relate to Ohm’s law.
I. Redox Reactions
Before anything else, let’s go back and look at the mechanism of electron flow. If we look at the battery in our simple circuit model, we see that our power source has a positive terminal (or cathode) and a negative terminal (or cathode). Both ends have a wire that carries electrons, and the arrows show that electrons come out of the anode and into the cathode. A battery supplies energy through redox (reduction-oxidation) reactions happening inside it. At the anode, oxidation occurs, and it loses electrons, whereas at the cathode, reduction occurs, which means it gains electrons. Due to these redox reactions inside the battery, the potential difference between the anode and the cathode is known as the voltage.
II. Voltage, Current, and Resistance
A. Voltage
Think of voltage as a pressure built up inside a syringe; at the anode, there’s a surplus of electrons pushing against each other, and this is connected to the cathode, where there is space for electrons to move around. This “pressure” is what sets the stage for electron flow to occur. Mathematically, 1 volt is equivalent to 1 joule per coulomb. Joule is the SI unit for work, and coulomb is the SI unit for electric charge.
We can then visualize this mathematically as the amount of work (in Joules) that 1 charge (coulomb) can do. Voltage tells us the amount of energy that the battery gives to each coulomb of charge. It’s the difference in pressure between two terminals.
B. Current
We can think of current as the water flow rate inside a pipe. In a circuit, a current describes how many electric charges pass through per unit of time. Current (symbolized as I) is measured in Amperes or Amps. There are two types of current:
a. Direct Current (DC)
The flow of electric charge in a direct current has one direction. The current supplied by a battery is an example of DC.
b. Direct Current (DC)
The flow of electric charge in an alternating current changes direction periodically. The current used in households is an example of an alternating current.
We won’t dive deep into these types of currents. What’s important right now is that we know that current is the amount of charge that passes through a circuit over a period of time.
C. Resistance
We defined resistance as the measure of how well something resists electricity. Previously, we also defined conductance as the reciprocal of resistance, measured in ohms (Ω).
III. Ohm’s Law
Voltage, current, and resistance are neatly defined in one well-known law, Ohm’s law. This law describes the relationship between voltage, current, and resistance in an electrical circuit. Ohm’s law indicates that at a given resistance, the voltage increases as the current increases and vice versa. This correlation also describes other ways that we can calculate current, resistance, and voltage. Look at the image above to see how you can remember Ohm’s law better.
Electrical power measures how fast energy is transformed into another form. We define power as the amount of energy transferred by current. The SI power unit is watts, equal to 1 volt-amp.
Once you remember the formulas above, it will be easy to calculate for other quantities by transposing the value we need on one side.
IV. Conclusion
Voltage or potential difference serves as the trigger for electron flow. This tells us how the voltage between the two terminals in a circuit differs. The rate of charge flowing through a circuit over a period of time is known as the current and is measured in Amperes (A). The resistance of a material for electricity to flow through it. All these three are correlated into one line of the equation known as Ohm’s law. This law states that voltage and current are directly proportional to each other. The next article will discuss resistance and circuits and how resistors affect certain circuits.
V. Key Terms
- Voltage - the potential difference between two terminals that is measured in volts (V)
- Current - rate of flow of charges that is measured in amperes (A)
- Resistance - the ability of a material to resist electricity that is measured in ohms (Ω)
VI. Practice Questions
Sample Practice Question 1
Typically, household light bulbs use 120 Watts of power in a 120-volt socket. Calculate the resistance at the bulb.
A. 120Ω
B. 1Ω
C. 12Ω
D. 10Ω
Ans. A
Sample Practice Question 2
All of the following are formulas for voltage EXCEPT:
A.
B.
C.
D.
Ans. C
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