Voltage between plates turns into energy

Science · Energy · ages 16-18

Name ______________________ Date ____________

No work is needed to move a charge along an equipotential.
Circle one: True False
In E equals V over d, what must d be measured in?
- metres
- volts
- coulombs
Which unit also means newtons per coulomb?
- joules per coulomb
- volts per metre
- coulombs per volt
A charge of 0.5 C crosses 12 V. W equals Q V. Give W in joules.
Answer: ______________
Plates 0.05 m apart hold 500 V. E equals V over d. Give E in V/m.
Answer: ______________
A pupil sketches equipotentials running parallel to field lines. What is wrong?
- They must cross the field lines at right angles
- They must never be drawn straight
- They must touch both plates
A pupil says a proton moved to higher voltage loses potential energy. Who is right?
- The pupil, since all charges lose energy uphill
- Both gain energy either way
- An electron would lose it; a proton gains it
An electron crosses a 500 V gap starting from rest. What happens to the QV energy?
- It vanishes into the plates
- It returns the electron to rest
- It becomes kinetic energy, raising its speed

Answer key

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

True · Voltage never changes there, so Q times delta V is zero.
metres · The gap between the plates is a distance in metres.
volts per metre · Volts per metre and newtons per coulomb describe the same field.
6 · Multiply 0.5 by 12 to get 6 J.
10000 · Divide 500 by 0.05 to get 10000 V/m.
They must cross the field lines at right angles · Equipotentials always meet field lines at right angles.
An electron would lose it; a proton gains it · Sign flips the story: electrons and protons behave oppositely.
It becomes kinetic energy, raising its speed · The crossing energy turns into motion, so the electron speeds up.