
An ordinary light bulb has a resistance of 600 Ω. When placed in a circuit at 120 V, the light bulb draws 0.2 A and lights up as it was designed to do. A certain hairdryer has a resistance of 10.9 Ω. When connected to the same 120 V circuit, the hairdryer draws 11 A.
Engineers design the resistance of electrical devices to draw the right current when connected to the right voltage. Every electrical device has a resistance and its resistance determines how much current the device draws in a circuit at the proper voltage.

In electronic circuits resistance is created by resistors, such as the ones in the photo on the left. A resistor comes in many standard values of resistance. Resistors are used to control current, voltage, or both in electronic circuits.

The symbol for any resistance in a circuit diagram is a zigzag line. The example circuits for the light bulb and hair dryer show each as a zigzag line with the resistance labeled in Ω to the side. Potentiometers are variable resistors and have a knob that is turned to change the resistance.

Consider what happens when you connect the positive and negative terminals of a 1.5 V battery directly with a piece of wire, which has only a tiny resistance (such as 0.03 Ω).
Ohm’s law says that the current should be extremely large—50 A in this case!
Such a high current is more than enough current to melt the wire and more than the battery can deliver. That is why short circuits are dangerous! A short circuit presents a lowresistance path for current to flow, allowing too much current. Batteries can safely deliver currents up to 2 or 3 A, and the thin wires in a typical circuit used in the laboratory are chosen to handle this amount of current safely.

Why can the battery deliver only 2–3 A of current? Real batteries can be represented as a combination of a voltage source (1.5 V in this case) and a resistance (a few tenths of an ohm for a typical alkaline battery).
As a battery ages, its internal resistance increases.
As you will learn later in this chapter, the battery's internal resistance and the external resistance from the wire form a series circuit, so the total resistance of the circuit is the sum of the two resistances—or a few tenths of an ohm.
From Ohm's law, 1.5 V divided by (approximately) 0.3 Ω is 5 A, which is more than the 2–3 A safe range of current that the battery can deliver.
When a typical battery is delivering this much current it can overheat dangerously!

A simple circuit contains only wires, a light bulb with a resistance of 2 Ω, and a 1.5 V battery. How much current flows through the light bulb?
Asked: 
electric current I 
Given: 
resistance R = 2 Ω,
voltage V = 1.5 V 
Relationships: 
Ohm’s law for current: I = V/R 
Solution: 
$$I=\frac{V}{R}=\frac{1.5\text{V}}{2\text{\Omega}}=0.75\text{A}$$


Answer: 
I = 0.75 A. 
