Lesson test: The electric field

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The electric field

1.[2p]
Why is the electric field properly defined as the limit of $F / q_{0}$ as $q_{0}$ tends to zero?

A finite test charge redistributes the charges producing the field, changing what is being measured

Correct

Otherwise the units would not come out as newtons per coulomb

The answer is: A finite test charge redistributes the charges producing the field, changing what is being measured

Because charge is quantised and $q_{0}$ must be smaller than $e$

The answer is: A finite test charge redistributes the charges producing the field, changing what is being measured

Because the force on a large charge is not proportional to that charge

The answer is: A finite test charge redistributes the charges producing the field, changing what is being measured

2.[3p]
A ring of radius 6.0 cm carries 30 nC spread uniformly. What is the field on its axis 8.0 cm from the centre, in newtons per coulomb?

CorrectNot quite: 21570

3.[2p]
At what distance along the axis is the field of a uniformly charged ring of radius $a$ strongest?

At $x = a / \sqrt{2}$

Correct

At the centre, $x = 0$

The answer is: At $x = a / \sqrt{2}$

At $x = a$

The answer is: At $x = a / \sqrt{2}$

The field increases without limit as $x$ grows

The answer is: At $x = a / \sqrt{2}$

4.[2p]
Field lines are the paths that a charge released from rest would follow.

True

The answer is: False

False

Correct

5.[3p]
A charge $+ 8.0$ nC sits at $x = 0$ and a charge $+ 2.0$ nC at $x = 12$ cm. What is the magnitude of the field at the midpoint, in newtons per coulomb?

CorrectNot quite: 14979

6.[3p]
Why does the field of a dipole fall off as $1 / r^{3}$ rather than $1 / r^{2}$ ?

The two opposite charges nearly cancel at a distance, and the near cancellation costs one power of $r$

Correct

The charges are closer together than the point of observation

The answer is: The two opposite charges nearly cancel at a distance, and the near cancellation costs one power of $r$

Because the dipole moment has units of charge times length

The answer is: The two opposite charges nearly cancel at a distance, and the near cancellation costs one power of $r$

Because a dipole is electrically neutral and so produces no field at all

The answer is: The two opposite charges nearly cancel at a distance, and the near cancellation costs one power of $r$

7.[3p]
A charged rod attracts a neutral scrap of paper because

the rod polarises the paper, and its non-uniform field then pulls the near end harder than it pushes the far end

Correct

the paper acquires a net charge of the opposite sign

The answer is: the rod polarises the paper, and its non-uniform field then pulls the near end harder than it pushes the far end

a dipole in any field feels a net force towards the source

The answer is: the rod polarises the paper, and its non-uniform field then pulls the near end harder than it pushes the far end

paper is a conductor, so charge flows onto it from the air

The answer is: the rod polarises the paper, and its non-uniform field then pulls the near end harder than it pushes the far end

8.[2p]
An electron starts from rest in a uniform field of 500 N/C. What is its acceleration, in units of $10^{13}$ metres per second squared?

CorrectNot quite: 8.79

9.[3p]
Match each charge arrangement to the field it produces.

Point charge
Dipole, far away
Very long line
Uniform ring, at its centre

falls as one over r cubed
falls as one over r
falls as one over r squared
exactly zero

Show the answer

Point charge: falls as one over r squared Dipole, far away: falls as one over r cubed Very long line: falls as one over r Uniform ring, at its centre: exactly zero

1.[2p] Why is the electric field properly defined as the limit of F/q0 as q0 tends to zero?

2.[3p] A ring of radius 6.0 cm carries 30 nC spread uniformly. What is the field on its axis 8.0 cm from the centre, in newtons per coulomb?

3.[2p] At what distance along the axis is the field of a uniformly charged ring of radius a strongest?

4.[2p] Field lines are the paths that a charge released from rest would follow.

5.[3p] A charge +8.0 nC sits at x=0 and a charge +2.0 nC at x=12 cm. What is the magnitude of the field at the midpoint, in newtons per coulomb?

6.[3p] Why does the field of a dipole fall off as 1/r3 rather than 1/r2?

7.[3p] A charged rod attracts a neutral scrap of paper because

8.[2p] An electron starts from rest in a uniform field of 500 N/C. What is its acceleration, in units of 1013 metres per second squared?

9.[3p] Match each charge arrangement to the field it produces.

1.[2p]
Why is the electric field properly defined as the limit of $F / q_{0}$ as $q_{0}$ tends to zero?

2.[3p]
A ring of radius 6.0 cm carries 30 nC spread uniformly. What is the field on its axis 8.0 cm from the centre, in newtons per coulomb?

3.[2p]
At what distance along the axis is the field of a uniformly charged ring of radius $a$ strongest?

4.[2p]
Field lines are the paths that a charge released from rest would follow.

5.[3p]
A charge $+ 8.0$ nC sits at $x = 0$ and a charge $+ 2.0$ nC at $x = 12$ cm. What is the magnitude of the field at the midpoint, in newtons per coulomb?

6.[3p]
Why does the field of a dipole fall off as $1 / r^{3}$ rather than $1 / r^{2}$ ?

7.[3p]
A charged rod attracts a neutral scrap of paper because

8.[2p]
An electron starts from rest in a uniform field of 500 N/C. What is its acceleration, in units of $10^{13}$ metres per second squared?

9.[3p]
Match each charge arrangement to the field it produces.