Learning outcomes
- Predict alpha, beta and gamma behaviour in an electric field.
- Describe qualitative magnetic deflection.
- Relate direction to charge sign.
- Relate curvature to particle mass and momentum.
7.1 Electric-field forces
A charged particle in an electric field experiences a force. Positive alpha particles accelerate towards the negative plate. Negative beta particles accelerate towards the positive plate. Gamma radiation has no charge and is not deflected.
Beta is usually deflected much more strongly than alpha because beta has far smaller mass. Although alpha carries twice the charge magnitude, its mass is thousands of times larger than an electron’s, so its acceleration and path curvature are much smaller under comparable conditions.
7.2 Interpreting an electric-field diagram
First identify the sign of each plate. Then use the radiation charge: positive moves towards negative, negative moves towards positive, neutral remains straight. A particle beam entering midway may follow a curved path because the sideways force changes its velocity continuously.
Do not decide direction from “up” or “down” without plate labels. If the diagram is rotated, the physics is unchanged.
7.3 Magnetic deflection
Moving charged particles experience a magnetic force whose direction is perpendicular to both their motion and the magnetic field. Alpha and beta bend in opposite directions because their charges have opposite signs. Gamma is not deflected.
For similar speeds and field conditions, beta follows a much tighter curve than alpha because of its much smaller mass and different momentum. Exact direction can be found using the relevant hand rule when the field direction is specified, but many O Level questions require only opposite curvature and relative amount.
7.4 What fields reveal
Deflection provides evidence about charge. A beam that bends towards the positive electric plate must carry negative charge. A beam unaffected by either electric or magnetic field may be neutral, as gamma is.
Field experiments also separate mixed radiation into different paths. The absence of gamma deflection does not mean gamma has no energy; it means it carries no electric charge.
Worked examples
Electric plates
The upper plate is positive and the lower plate is negative. Alpha bends downward, beta-minus upward and gamma continues straight.
Comparing curvature
In the same magnetic field, a beta path is more sharply curved than an alpha path because the beta particle has much smaller mass and momentum.
Practical focus
Investigation
Use a simulation showing particle beams in electric and magnetic fields. Change charge sign, mass, speed and field direction one variable at a time. Record path direction and curvature, then apply the observations to alpha, beta and gamma.
Examination guidance
- Check plate signs before choosing electric deflection direction.
- Alpha and beta bend oppositely.
- Beta bends more strongly than alpha in the standard comparison.
- Gamma is not deflected because it is uncharged.
- Do not say gamma is unaffected because it has no energy.
Check your understanding
- Which way does beta-minus move in an electric field?
- Why does alpha curve less than beta?
- What happens to gamma in a magnetic field?
Answers
- Towards the positive plate.
- Its much larger mass and momentum make its path harder to change.
- It continues undeflected because it has no charge.
