Voltage, Current and Resistance

One-sentence summary

Voltage is the push that moves electrons, current is how much flow passes through, and resistance is what holds that flow back; together they decide how a circuit behaves.

Why does it matter?

Why does a robot's motor sometimes turn weakly? Why does an LED suddenly flash bright and burn out? The answer almost always hides in the same three ideas: voltage, current and resistance.

Without understanding these three quantities, we cannot build a circuit safely. Choose the wrong resistor and we may burn out an LED; apply the wrong voltage and we may damage a part. This lesson is the foundation for the real circuits you will build later. We are not connecting any wires yet; first we build a clear picture in our heads.

The good news is that these ideas work almost exactly like the water and pipes you already know from everyday life.

The water-pipe analogy

We cannot see electricity directly, so comparing it to something visible helps a lot. The most common comparison is the water-pipe analogy. Picture a water tank, some pipes, and narrow spots along those pipes.

Voltage: the push

Voltage is the force that pushes electrons. Its unit is the volt, shown with the letter V.

In the water analogy, voltage is like the height of the tank, or the pressure of the water. The higher the tank sits, the harder the water pushes through the pipe.

• A tall water tower shoots water out strongly when you open the tap. That is like high voltage.
• A small bucket on the ground trickles water out of a hose with little force. That is like low voltage.

For example, two AA batteries connected in a row (2×AA) give you about 3 V, while a USB cable carries 5 V. USB's "push" is a little stronger than the batteries'.

Current: the amount of flow

Current is how much electric charge passes a point. Its unit is the ampere (amp), shown with the letter A. In small circuits we often use milliamps (mA); 1000 mA = 1 A.

In the water analogy, current is the amount of water passing through the pipe each second. A thick, full pipe lets a lot of water through; a thin pipe lets only a little through.

• A garden hose pours out lots of water: like high current.
• A drip-irrigation tube lets only a trickle seep through: like low current.

A small LED is happy running at about 10 mA, while a tiny robot motor can draw hundreds of mA. The larger the current, the more the wires and batteries warm up.

Resistance: the barrier to flow

Resistance is what stands in the way of the flow. Its unit is the ohm, shown with the Greek letter omega (Ω).

In the water analogy, resistance is how narrow the pipe is. The narrower the pipe, the harder it is for water to pass.

• A wide pipe lets water through easily: low resistance.
• A pinhole-sized opening barely lets water through: high resistance.

In electronics there is a special part called a resistor. Its job is exactly this: to limit the flow. If we connect an LED straight to a battery, too much current passes and the LED is damaged; adding a resistor pulls the current down to a safe level.

All three together: Ohm's law

Voltage, current and resistance are not independent. A simple rule called Ohm's law links them:

Voltage = Current × Resistance
V = I × R

We can read this with the water analogy:

• Raise the pressure (voltage) and more water (current) passes through the same pipe.
• Narrow the pipe (raise the resistance) and less water (current) passes at the same pressure.

So more voltage means more current, and more resistance means less current. This one idea is often enough to explain what is happening in a circuit.

A small calculation: a resistor for an LED

Suppose we want to safely light an LED from a 5 V USB source. A typical red LED "uses up" about 2 V across itself, and we want about 10 mA (that is 0.01 A) of current for comfortable operation.

The voltage left across the resistor is:

Voltage across the resistor = 5 V - 2 V = 3 V

Rearranging Ohm's law to find the resistance:

Resistance = Voltage / Current
R = 3 V / 0.01 A = 300 Ω

An exact 300 Ω can be hard to find in shops, so we use the next standard value up, 330 Ω. This lowers the current slightly and makes the LED even safer. That is how we work out a resistor's real job in a circuit.

Mini practice

In this practice we connect no parts at all; we just match the ideas to the water analogy. Draw three columns on a sheet of paper: Voltage, Current, Resistance.

Read the sentences below and place each one in the correct column:

1. "The higher the water tower, the stronger the push."
2. "The narrower the pipe, the harder the flow."
3. "How much water passes through the pipe each second."
4. "Its unit is the volt, its letter is V."
5. "Its unit is the ohm, its letter is Ω."
6. "Its unit is the amp, its letter is A."

Then answer this: if you raise the resistance in a circuit, does the current go up or down? Explain your answer using Ohm's law.

Bonus challenge: if you wanted to light the same red LED (about 2 V, 10 mA) from a 6 V battery pack, calculate the resistor you would need. (Hint: first find the voltage across the resistor.)

Common mistakes

Mixing up voltage and current

Voltage is the "push" and current is the "amount that flows." They are different things. High voltage alone does not mean high current; if the circuit has enough resistance, the current stays small.

Connecting an LED without a resistor

Connecting an LED straight to a battery is one of the most common mistakes. Without a current-limiting resistor, too much current flows through the LED and it can flash bright and burn out in an instant.

Confusing the units

10 mA and 10 A are very different: they differ by a factor of 1000. When you calculate, do not forget to convert milliamps to amps (divide mA by 1000).

Thinking "more voltage is always better"

Every part has a limit. If you apply more voltage than a part can handle, you can damage it. The goal is to use the right value, not the highest one.

Safety note

• This lesson uses only low-voltage, educational sources: AA batteries (for example 2×AA, 3–6 V), USB, micro:bit or Arduino. Never work with wall sockets, mains electricity or exposed wall wiring. Mains electricity can be deadly and is not the subject of these lessons.
• Always work under adult supervision when handling batteries, motors, soldering, cutting tools and hot surfaces.
• Connect the polarity correctly (plus to plus). A reversed connection can damage some parts.
• Use a current-limiting resistor and avoid short circuits: joining a battery's plus and minus terminals directly with a wire overheats the battery.
• Be careful with lithium batteries: if short-circuited or crushed they can heat up, swell or even catch fire. Do not use a damaged battery, and if a battery gets hot, pause the circuit and tell an adult.

Lesson summary

• Voltage (V, volt) is the force that pushes electrons; in the water analogy it is pressure or height.
• Current (A, amp) is how much charge passes per unit of time; in the water analogy it is the amount of water flowing through the pipe.
• Resistance (Ω, ohm) is the barrier to flow; in the water analogy it is how narrow the pipe is.
• Ohm's law links all three: V = I × R; more voltage means more current, more resistance means less current.
• To light an LED safely you need a current-limiting resistor; about 330 Ω suits a red LED at 5 V.

Check questions

1. What is the unit and symbol of voltage?
2. What do we compare current to in the water-pipe analogy?
3. What is the job of a resistor, and what is its unit?
4. Write Ohm's law. What happens to the current when resistance increases?
5. On a 5 V source, how many ohms of resistance does a red LED need if it uses 2 V across itself and wants 10 mA?

Answers

1. The unit of voltage is the volt, and its symbol is V. It is the force that pushes electrons.
2. Current is like the amount of water passing through the pipe (each second). Its unit is the ampere (A), symbol A.
3. A resistor's job is to limit the flow / stand in the way of the flow. Its unit is the ohm, symbol Ω.
4. V = I × R. When resistance increases (with voltage fixed), the current decreases, because the barrier to flow grows larger.
5. The voltage across the resistor is 5 − 2 = 3 V. R = 3 V ÷ 0.01 A = 300 Ω; in practice we use the next standard value up, 330 Ω.

Source and verification note

For “Voltage, Current and Resistance”, verification focuses on whether the relationship between The water-pipe analogy and Current: the amount of flow remains consistent across examples. Activities are designed only for low-voltage direct-current circuits; mains electricity is not used. Component values must be rechecked for the actual circuit, and connections should be reviewed with an adult before power is applied.

Next lesson

Open and Closed Circuits: We will learn that electricity needs a complete path to flow, and how a switch opens and closes that path.