Electric fields: force and a positive test charge
Введено| English | Значение |
|---|---|
| test charge/test tʃɑːdʒ/ | Пробный заряд |
| electric field strength/ɪˈlektrɪk fiːld streŋθ/ | Напряжённость электрического поля |
What would explain this observation?
- A negatively charged particle moves opposite to the direction of the electric field. Field direction is defined using a positive test charge 试探电荷.
Build the model
- Electric field strength 电场强度 is force per unit positive test charge.
- Like charges repel and unlike charges attract.
- Field lines point away from positive charges and toward negative charges.
- Closely spaced lines represent a stronger field within a consistently drawn diagram.
- electric field strength: Electric force per unit positive test charge.
- test charge: A small charge used to probe an electric field.
What happens to force direction if the test charge changes sign?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
Force on a charge in a field is F = qE, so reversing the sign of q reverses the force while the field itself - defined by the source charges - is unchanged. Field direction is defined by the force on a positive test charge.
Match each technical term to its precise meaning.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- In a uniform field between ideal parallel plates, E = V/d away from edge effects.
- The force is F = qE, so its direction reverses for a negative charge.
- Coulomb force between ideal point charges decreases with separation squared.
In the lab: Sketch labelled plate polarities, field arrows and the charge before calculating. Convert separation to metres. In school, use simulations or approved low-voltage electrostatic models rather than exposed high-voltage equipment.
Which two habits make the investigation or model in this case more defensible?
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: plate potential difference is 120 V and separation is 0.020 m.
- E = V/d = 6,000 V/m.
- A +2.0 microcoulomb charge experiences F = qE = 0.012 N along the field; a −2.0 microcoulomb charge experiences the same magnitude opposite to it.
Your turn: Use E = V/d. Find E for 100 V across 0.025 m.
Use E = V/d. Find E for 100 V across 0.025 m.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
The result is 4000 V/m. Known: V = 100 V; d = 0.025 m, uniform field between plates. Use E = V/d. E = 100/0.025 = 4,000 V per m. The same field acts on a charge anywhere between the plates in this uniform-field model.
Check the conclusion and its limits
- Field strength is a vector.
- Potential difference is energy per charge and is not the same quantity.
- A field-line drawing is a model, not a set of physical threads.
Not true: Every charged particle experiences a force along the electric field direction. Field direction is defined by the force on a POSITIVE charge. A negative particle feels a force opposite to the field, so the field direction is not every particle's force direction.
- Electric field strength is force per unit positive test charge.
- In a uniform field between ideal parallel plates, E = V/d away from edge effects.
- Watch for: field strength is a vector.
Write from a field of 3000 volts per metre
- A potential difference of 60 V is across a gap of 0.020 m.
- Find the electric field strength. A negative charge feels the opposite force.
Field direction is defined by the force on a POSITIVE charge.
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
True. Field direction is defined by the force on a POSITIVE charge. A negative particle feels a force opposite to the field, so the field direction is not every particle's force direction.
Electric force per unit positive test charge: write the technical term.
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
electric field strength means Electric force per unit positive test charge.
A potential difference of 60 V is across a gap of 0.020 m. What is the electric field strength, and which way does a negative charge feel the force?
A uniform electric field is potential difference divided by the distance. The sign of the charge decides the force direction.
What is the electric field strength?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
60 divided by 0.020 is 3000 volts per metre. Other options — 4000 volts per metre (the lesson gap of 100 V across 0.025 m); 60 V (the potential difference).