Why accelerating charges glow

Science · Electricity & Magnetism · ages 23-24

Name ______________________ Date ____________

You double the acceleration with the same charge. By what factor does radiated power grow?
- 2
- 4
- 8
Which charge radiates?
- One that accelerates
- One sitting still
- One gliding at steady speed
A charge moving at constant velocity radiates.
Circle one: True False
Why does a radio antenna emit waves?
- Its electrons sit perfectly still
- Its electrons are pushed back and forth, always accelerating
- Its protons do all the moving
How is the radiation arranged around the acceleration?
- Strongest along the line, zero sideways
- Equal in every direction
- Strongest sideways, zero along the line
Why do electron rings lose far more energy to light than proton rings?
- Electrons refuse to bend
- Electrons are far lighter, so the bend needs bigger acceleration
- Protons carry no charge
Ned expects peak radiation along the motion and none sideways. What is wrong?
- He used the wrong charge sign
- He picked too low a speed
- He reversed the pattern: peak is sideways, zero along
Which of these is a synchrotron radiation source?
- The Crab Nebula glow
- A steady battery current
- A parked bar magnet

Answer key

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

4 · Power follows acceleration squared: 2 squared is 4.
One that accelerates · Only changing motion sends out field ripples.
False · Steady motion stays quiet; only acceleration radiates.
Its electrons are pushed back and forth, always accelerating · Repeated pushes mean repeated acceleration, hence waves.
Strongest sideways, zero along the line · The pattern peaks across the motion and vanishes on axis.
Electrons are far lighter, so the bend needs bigger acceleration · Lighter mass at the same bend means enormous acceleration, squared in power.
He reversed the pattern: peak is sideways, zero along · The axis is the null, not the peak.
The Crab Nebula glow · Fast electrons bent by cosmic magnets radiate there.