Current, drift and the material law

Science · Electricity & Magnetism · ages 19-20

Name ______________________ Date ____________

For a uniform wire, how is current density J found?
- Current times area
- Area divided by current
- Current divided by area
What does one ampere mean?
- One volt pushing charge each second
- One coulomb passing a point each second
- One ohm blocking charge each second
In a metal, resistivity rises with temperature because collisions get more frequent.
Circle one: True False
The current in a fixed wire doubles. What happens to the drift speed?
- It doubles
- It halves
- It stays the same
A wire carries 3 A through a cross-section of 1.5 square cm. Give J in A per square cm.
Answer: ______________
A copper wire is heated while the field stays fixed. What happens to J?
- It stays fixed, since the field is fixed
- It falls, because collisions raise resistivity
- It rises, because electrons move faster
The same current flows in a thin and a thick copper wire. Which is true?
- The thin wire has larger J and faster drift
- Both wires share the same J and drift
- The thick wire has larger J and faster drift
A student says the lamp lights fast because electrons sprint from the switch near light speed.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

Current divided by area · Density spreads current over area, so J equals I over A.
One coulomb passing a point each second · An ampere counts charge flow, one coulomb per second.
True · A hotter lattice shakes harder and collides with electrons more often.
It doubles · Drift speed grows in step with current when n, q and A are fixed.
2 · J equals I over A, so 3 divided by 1.5 is 2.
It falls, because collisions raise resistivity · Heating raises resistivity, so the same field drives less density.
The thin wire has larger J and faster drift · The same current squeezed into less area gives larger J and faster drift.
False · Electrons creep along, and it is the field signal that races.