OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the equation that links the force on a conductor to the magnetic…
Full specification wording
Uses of magnetism: apply the equation that links the force on a conductor to the magnetic flux density, the current and the length of conductor to calculate the forces involved (Higher tier only)
The force on a straight conductor carrying current at right angles to a uniform magnetic field is given by F = B × I × L, where F is the force in newtons (N), B is the magnetic flux density in…
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 5 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The force on a straight conductor carrying current at right angles to a uniform magnetic field is given by F = B × I × L, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and L is the length of conductor in the field in metres (m). The equation applies when the conductor is perpendicular to the field; if it lies parallel to the field the force is zero. To calculate, convert length to metres and current to amperes, then multiply the three values, keeping units consistent.
For example: B = 0.20 T, I = 3.0 A and L = 0.50 m give F = 0.20 × 3.0 × 0.50 = 0.30 N. Rearranging gives B = F ÷ (I × L), I = F ÷ (B × L) and L = F ÷ (B × I).
What earns the marks
- Selects the equation F = B × I × L and identifies each symbol with its quantity.
- States the units: force in newtons, flux density in tesla, current in amperes and length in metres.
- Substitutes consistent SI values, converting length to metres, and multiplies to obtain the force.
- Rearranges the equation to find flux density, current or length when the force is known.
- Recognises that the force is zero when the conductor is parallel to the field, since the equation applies at right angles.
Common mistakes and how to put them right
Leaves the length in centimetres before multiplying.
Put it right: Convert to metres so the force comes out in newtons.
Adds the three quantities instead of multiplying them.
Put it right: Apply F = B × I × L as a product.
Applies the full equation when the conductor is parallel to the field.
Put it right: Recognise the force is then zero.
Mixes units such as milliamperes with tesla and metres.
Put it right: Convert all values to SI units first.
Exam tips
- Write the equation, then substitute values with units before calculating to catch conversion slips.
- When asked for a rearranged quantity, divide the force by the product of the other two values.
- Sanity-check the magnitude: a small flux density and modest current give a small force in newtons.
Try the lesson
The start of the Uses of magnetism: apply the equation that links the force on a conductor to the magnetic flux density, the current and the length of conductor to calculate the forces involved (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A wire of length 0.80 m carries a current of 2.5 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force on the wire.
Listen
The force on a straight conductor carrying current at right angles to a uniform magnetic field is given by F = B × I × L, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and L is the length of conductor in the field in metres (m). The equation applies when the conductor is perpendicular to the field; if it lies parallel to the field the force is zero. To calculate, convert length to metres and current to amperes, then multiply the three values, keeping units consistent. For example, B = 0.20 T, I = 3.0 A and L = 0.50 m give F = 0.20 × 3.0 × 0.50 = 0.30 N. Rearranging gives B = F ÷ (I × L), I = F ÷ (B × L) and L = F ÷ (B × I). A strong answer earns marks for these points. Selects the equation F = B × I × L and identifies each symbol with its quantity. States the units: force in newtons, flux density in tesla, current in amperes and length in metres. Substitutes consistent SI values, converting length to metres, and multiplies to obtain the force. Rearranges the equation to find flux density, current or length when the force is known. Exam tip: Write the equation, then substitute values with units before calculating to catch conversion slips.
Key words
Say what each word means before you turn the card.
Magnetic flux density
Try answering aloud or in your head before turning the card.
Current and length in a field
Try answering aloud or in your head before turning the card.
Rearranging equations and SI units
Try answering aloud or in your head before turning the card.
See every definition
- Magnetic flux density
- B in the equation, measured in tesla (T), and found by dividing the force by the product of current and length.
- Current and length in a field
- I is the current in amperes and L is the length of conductor in the field in metres, multiplied with B to give force.
- Rearranging equations and SI units
- Dividing the force by the product of the other two values gives the rearranged quantity, after converting all values to SI units first.
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