En anglais

Quiet inside a conductor

Science · Electricity & Magnetism · ages 18-20

Name ______________________ Date ____________

At electrostatic equilibrium, a charge-free cavity in an ideal closed conductor is field-free, but a cavity containing a separate point charge need not be.
Circle one: True False
What allows solid copper to conduct electric current?
- Conduction electrons can move through the solid
- Electrons move only after the copper has melted
- Current carries whole copper atoms along the wire
Where is excess charge on a solid, isolated copper conductor in electrostatic equilibrium?
- Uniformly throughout the metal's volume
- On the conductor's outer surface
- Only at the center of the metal's volume
In electrostatic equilibrium, what is the direction of a nonzero electric field just outside a smooth conductor surface?
- Along the surface, toward regions with less charge
- In loops around the surface, following its curvature
- Perpendicular to the surface, with no sideways component
A point charge +q is held inside a closed metal shell without touching it. At electrostatic equilibrium, what is the total charge on the inner surface?
- -q, cancelling the enclosed point charge
- +q, matching the enclosed point charge
- Zero, because the metal itself is field-free
Why does a hard-topped car with a continuous metal frame offer protection during a lightning strike?
- Rubber tires stop the current from reaching the ground
- The metal frame routes current around the passenger space
- The cabin air absorbs the strike before it reaches any rider
An isolated, neutral metal shell surrounds a point charge +q that does not touch it. At electrostatic equilibrium, what is the total charge on the outer surface?
- +q, balancing the inner surface charge
- -q, matching the inner surface charge
- Zero, because the entire shell stays neutral
A student draws electric field lines through the conducting material of a shell in electrostatic equilibrium. Which statement identifies the error?
- Field lines cannot cross any boundary between different materials
- The field in the metal must vanish in electrostatic equilibrium
- Zero net flux forces a field to run tangentially through the metal

Answer key

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

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True · An empty closed cavity is shielded from external static fields. A charge within the cavity can create a field there, even though the surrounding metal remains field-free.
Conduction electrons can move through the solid · Mobile conduction electrons acquire a net drift under an electric field. Copper need not melt, and its atoms do not travel along the wire with the current.
On the conductor's outer surface · At electrostatic equilibrium, excess charge is absent from the metal's bulk. A solid conductor has no cavity surface, so its excess charge is on the outer surface.
Perpendicular to the surface, with no sideways component · A sideways field component would cause charge to move along the conductor. At electrostatic equilibrium that component is zero, so a nonzero outside field is normal to the surface.
-q, cancelling the enclosed point charge · A Gaussian surface within the metal wall has zero flux. It encloses the point charge and inner surface, whose charges must therefore sum to zero: +q + (-q) = 0.
The metal frame routes current around the passenger space · The conducting frame provides a route for lightning current around the passenger space. This current flow is not electrostatic equilibrium and does not guarantee complete safety during a strike.
+q, balancing the inner surface charge · The inner surface has total charge -q. The isolated shell remains neutral, so its outer surface must have total charge +q. The separate point charge is not part of the shell's net charge.
The field in the metal must vanish in electrostatic equilibrium · Field lines through the metal indicate a nonzero field there. That field would drive conduction-electron drift, contradicting electrostatic equilibrium.