The hot carries current out to the drill. The neutral carries it back. The ground wire carries current only when something goes wrong, 250.4(A)(5).
A hand-held power tool at a home needs a ground wire unless it is double-insulated, 250.114.
During a fault the case is not at 0 volts. The ground wire has resistance, so the big fault current pushes the case above ground until the breaker opens.
Fault current goes back to the panel on the ground wire, not through the ground rod. The earth is not a path a breaker can count on, 250.4(A)(5).
Receptacles on 15 and 20 amp circuits must have a ground hole, 406.12(A). Old two-wire circuits are the exception. There, a two-slot receptacle may be replaced with another two-slot one, 406.12(D)(2)(a). In a garage the replacement also needs GFCI protection, 406.12(D)(3).
An old two-wire circuit has two allowed fixes. You can run a ground wire, 406.12(D)(1) and 250.130(C). Or you can add GFCI protection, 406.12(D)(2)(b). A GFCI does not add a ground. It shuts off fast when current leaks out.
A three-prong adapter is never a fix. Neither is a three-prong receptacle with its ground screw jumped to the neutral, called a bootleg ground.
New garage receptacles at a home need GFCI protection, 210.8(A)(2). These circuits have none, so you can see what the ground wire does by itself.
The wire is 12 AWG copper on a 20 amp breaker, 240.4(D)(6). The ground wire is 12 AWG too, Table 250.122.
The fault current is many times the 20 amp rating, so the breaker trips at once. Here it opens after one cycle, 1/60 of a second. That time is a teaching model. A real breaker's time comes from its maker's trip curve.
The model uses 120 volts and 50 feet of 12 AWG solid copper at 1.93 Ω per 1,000 feet, from Chapter 9, Table 8. The cord is 8 feet of 18 AWG stranded copper at 7.95 Ω per 1,000 feet. The drill is rated about 7 amps.
The worn spot is a solid fault of 0.01 Ω. The utility and service add 0.02 Ω. The breaker and each plug contact add 0.002 Ω. The path back through the earth is a 25 Ω ground rod working alongside the utility ground.
The skin numbers are typical figures from NIOSH (publication 98-131). With dry skin, the body's resistance may be as high as 100,000 Ω. Wet or broken skin may drop it to 1,000 Ω. Skin that is dry today can be sweaty or cut tomorrow.
The panel is the service, so neutral and ground are joined on its bar. They are kept apart everywhere past it.
Shock effects are typical figures from OSHA, not code.
The dots show how much current flows. They do not show how fast electrons move. Real electrons drift a fraction of a millimeter per second.
In the person and the earth, red diamonds show milliamps. Each one there stands for 1,000 times less current than a wire dot. They are hidden under 1 mA.
This is AC, 60 cycles a second. At Slowest the dots rock back and forth. At the faster speeds they show the flow one way.