How Do Voltage and Current Differ?

• Battery
• Direction of current
• Lamp

What you will learn

• Tell voltage, current and resistance apart.
• Explain why a simple battery circuit needs a closed loop for sustained current.
• Describe how resistance changes the current.

Before you start

No prior knowledge needed. Having seen a flashlight or a battery is enough.

What are voltage and current?

Voltage is the difference in electrical potential energy per unit charge between two points, and current is the rate at which charge passes a point in a circuit.

A picture helps: think of a water tank. The amount of water is the charge, the water pressure is the voltage and the water flow is the current. The tank is like a battery, a place where a certain amount of energy is stored and then released.

• Water tank, like a battery
• Pipe
• Flowing water

In this analogy, lower water pressure goes with less water flow. In a flashlight, a drop in voltage can make the light dim.

Why does a battery circuit need a closed loop?

In a simple battery-and-lamp circuit, a closed loop provides a continuous path for sustained current. When the loop is open, the lamp does not stay lit.

Components in the circuit, such as a lamp, let us control the charge and use it to do work. A lit lamp is one such job.

• Battery
• Closed wire
• Lamp
• Direction of current

How does resistance change the current?

Resistance is a material's tendency to resist the flow of charge. In the tank picture, opposition to water flow corresponds to resistance; a narrower hose offers more opposition when other conditions stay the same.

Take two tanks with the same amount of water and the same pressure, but one hose is narrower. The circuit with the higher resistance allows less charge to flow, so it has less current.

• Wide pipe, low resistance
• Narrow pipe, high resistance
• Large stream
• Small stream

For a resistor that follows Ohm's law, the formula is V = I × R: V is voltage in volts, I is current in amperes, and R is resistance in ohms. If two values are known, the third can be worked out.

Real-world example

With an adult, look at a flashlight. The battery is the source of voltage, the wires and bulb form a closed loop and current running through the bulb makes it glow. Ask yourself: if the loop is cut at one spot, does the bulb still light?

Common misunderstandings

Voltage and current are the same thing. Voltage is the difference in electrical potential energy per unit charge between two points, while current is the rate at which charge passes a point in a circuit.

Higher resistance gives a bigger current. At the same voltage, the circuit with higher resistance has less current.

Frequently asked questions

What is voltage?

Voltage is the difference in electrical potential energy per unit charge between two points. In the water picture it matches the pressure of the water.

What is current?

Current is the rate at which charge passes a point in a circuit. In the water picture it matches the flow of water.

What is resistance?

Resistance is a material's tendency to resist the flow of charge. A narrower pipe is like a higher resistance.

What is Ohm's law used for?

Ohm's law lets you work out one of the three quantities when the other two are known.

Summary

• Voltage is the difference in electrical potential energy per unit charge between two points; current is the rate at which charge passes a point.
• A battery-and-lamp circuit needs a closed loop for sustained current; components in the circuit help do work.
• At the same voltage, higher resistance gives a smaller current, and Ohm's law links the three quantities.