Capacitors hold charge, then pay it back

Science · Energy · ages 16-18

Name ______________________ Date ____________

The energy stored in a capacitor equals QV.
Circle one: True False
In C equals Q over V, what is C measured in?
- farads
- volts
- coulombs
Which prefix means ten to the minus six?
- nano
- micro
- milli
Why is the stored energy half of QV?
- Half the charge leaks away as heat
- The second plate holds negative energy
- Charging starts at zero volts, so the average is V over 2
A capacitor holds 200 microcoulombs at 10 V. C equals Q over V. Give C in microfarads.
Answer: ______________
A 100 microfarad capacitor is charged to 12 V. E equals half C V squared with C as 100 times 10 to the minus 6. Give E in joules.
Answer: ______________
A pupil uses E equals QV for a charged capacitor. What went wrong?
- The pupil missed the half from the charging ramp
- Nothing, QV is exact
- The pupil forgot the dielectric
The voltage on a capacitor doubles with C fixed. What happens to the stored energy?
- It doubles
- It stays the same
- It quadruples

Answer key

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

False · It is half of QV, since charging starts at zero volts.
farads · Charge per volt is capacitance, in farads.
micro · Micro means ten to the minus six; nano is ten to the minus nine.
Charging starts at zero volts, so the average is V over 2 · The voltage ramps from zero, so the mean voltage is V over 2.
20 · Divide 200 by 10 to get 20 microfarads.
0.0072 · Half of 100 times 10 to the minus 6 times 144 is 0.0072 J.
The pupil missed the half from the charging ramp · QV is the rectangle; charging only fills the triangle beneath.
It quadruples · Energy goes as V squared, so doubling V gives four times.