New
New
Year 11
OCR
Higher

The motor effect

I can describe the force acting on a current–carrying wire or loop in a uniform magnetic field.

New
New
Year 11
OCR
Higher

The motor effect

I can describe the force acting on a current–carrying wire or loop in a uniform magnetic field.

Lesson details

Key learning points

  1. The magnetic field lines around a current-carrying wire are concentric circles
  2. In a uniform magnetic field, a current carrying wire is forced in one direction
  3. Fleming’s left–hand rule can be used to predict the direction of movement of a current–carrying wire in a magnetic field
  4. The force on a current–carrying conductor at right angles to a magnetic field, $$F$$, is found using $$F = B I l$$
  5. Opposite sides of a conducting coil in a uniform magnetic field are forced to move in opposite directions

Common misconception

It is not clear how the magnetic field around a conducting wire interacts with a uniform magnetic field.

Focus on observing and predicting the direction that a conducting wire is forced in by a uniform magnetic field, and openly admit that explaining this in terms of how the magnetic field interact is too challenging to include in the syllabus.

Keywords

  • Uniform magnetic field - has evenly spaced field lines and the strength of the magnetic field is the same everywhere

  • Fleming’s left-hand rule - indicates the direction a conducting wire is forced to move in a uniform magnetic field

  • Magnetic flux density, B - is a measure of the strength of a magnetic field

  • Tesla (T) - is the unit that magnetic flux density is measured in

Apparatus is available to buy that demonstrates the force on a conductor in a magnetic field (the motor effect). Alternatively, a strip of aluminium foil can be forced to move in the uniform magnetic field between two motor magnets on a U–shaped (motor kit) yoke, when a current flows through it.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
In a drawing of the magnetic field lines around a magnet, the field lines are closer together in some areas than others. Which of the following statements is correct?
The closeness of the field lines does not mean anything.
Correct answer: The magnetic field is stronger where the field lines are closer together.
The magnetic field is weaker where the field lines are closer together.
Q2.
What do the directions of magnetic field lines represent?
The positions of a north-facing pole.
The positions of a south-facing pole.
Correct answer: The directions of the forces on a north-facing pole.
The directions of the forces on a south-facing pole.
Q3.
A compass usually points north. If the needle of a compass is deflected (changes direction) so that it no longer points north, which of the following are possible explanations?
It is near a piece of copper.
Correct answer: It is near a wire that is carrying a current.
Correct answer: It is near the north-seeking end of a bar magnet.
Correct answer: It is near the south-seeking end of a bar magnet.
Q4.
Which of the following statements are correct?
Magnetic field line arrows point towards a north-seeking pole.
Correct answer: Magnetic field line arrows point towards a south-seeking pole.
Correct answer: Current flows from the positive terminal to the negative terminal of a battery.
Current flows from the negative terminal to the positive terminal of a battery.
Q5.
Two variables have this relationship: whenever the value of one variable is multiplied by a number, the value of the other variable is multiplied by the same number. The two variables are directly...
rational.
relational.
positional.
Correct answer: proportional.
Q6.
Two equal and opposite forces act on the ends of a ruler as shown by the arrows. The ruler is initially stationary. Which of the following describes what happens to the ruler?
An image in a quiz
It does not move.
It moves to the left.
It moves to the right.
It rotates clockwise.
Correct answer: It rotates anticlockwise.

6 Questions

Q1.
Which of the following statements about magnetic flux density are correct?
Correct answer: Its symbol is $$B$$ and its unit is T.
Its symbol is $$T$$ and its unit is B.
It is always greater if the magnet is larger.
Correct answer: It is always greater if the magnetic field is stronger.
Q2.
Which of the following describes the magnetic field lines near a straight current-carrying wire?
Correct answer: They are circular, with their centres on the wire.
They have the same shape as the field lines around a bar magnet.
They are straight and point in the same direction as the current.
They are straight and point in the opposite direction to the current.
There are no magnetic field lines.
Q3.
Which of the following are characteristics of a uniform magnetic field?
Correct answer: Its field lines are evenly spaced.
Correct answer: It has the same strength everywhere.
Its field lines are perpendicular to each other.
Correct answer: The arrows on its field lines all point in the same direction.
Q4.
Fleming’s left-hand rule can be used to predict the direction of the force on a current-carrying wire in a magnetic field. Match each digit with what it represents in the left-hand rule.
Correct Answer:first finger,represents the magnetic field direction

represents the magnetic field direction

Correct Answer:second finger,represents the direction of the current in the wire

represents the direction of the current in the wire

Correct Answer:third finger,is not used in Fleming’s left-hand rule

is not used in Fleming’s left-hand rule

Correct Answer:thumb,represents the direction of the force on the wire

represents the direction of the force on the wire

Q5.
A 12 cm length of straight wire carrying a current of 1.2 A is in a uniform magnetic field with flux density 0.50 T. The size of the force on the wire is N.
Correct Answer: 0.072, 0.072 N, 0.072N, .072, .072N
Q6.
The diagram shows a current-carrying loop of wire in a uniform magnetic field. Three sides of the loop are labelled P, Q and R. Match each side to a description of the force acting on it.
An image in a quiz
Correct Answer:side P,An upwards force acts.

An upwards force acts.

Correct Answer:side Q,No force acts.

No force acts.

Correct Answer:side R,A downwards force acts.

A downwards force acts.