OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(ii) techniques and procedures used to determine the uniform magnetic flux…
Full specification wording
Magnetic fields: (ii) techniques and procedures used to determine the uniform magnetic flux density between the poles of a magnet using a current-carrying wire and digital balance
To measure the flux density between the poles of a magnet, mount a rigid straight wire horizontally between the poles so it is perpendicular to the field, and support the magnet on a digital balance.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To measure the flux density between the poles of a magnet, mount a rigid straight wire horizontally between the poles so it is perpendicular to the field, and support the magnet on a digital balance. With no current, record the balance reading. Pass a known current I through the wire and record the new reading; the change in reading gives the force F on the wire (using the balance's force reading, F = mg where needed). Measure the length L of wire between the poles. Since the wire is perpendicular to the field, F = ILB, so B = F / (IL). Repeat for several currents and plot F against I; the gradient is LB, so B = gradient / L. This averages random errors and improves reliability.
What earns the marks
- Describe the apparatus: a straight wire clamped perpendicular to the field between the poles of a magnet, with the magnet resting on a digital balance.
- Explain that the change in balance reading when current flows gives the magnetic force on the wire.
- Measure the length L of wire within the field and the current I, then use B = F / (IL) because the wire is perpendicular to the field.
- Repeat for several values of current and plot a graph of force against current; the gradient equals LB, so B = gradient / L, reducing random error.
Common mistakes and how to put them right
Using the total length of the wire instead of the length between the magnet poles.
Put it right: Only the length inside the uniform field contributes to the force.
Ignoring the zero-current balance reading.
Put it right: Subtract the initial reading so only the magnetic force is used.
Assuming the wire is perpendicular without checking.
Put it right: Align the wire at 90° to the field lines, or include sin θ if it is not.
Exam tips
- State clearly that the balance reading changes by the magnetic force, and explain how you isolate that change.
- Describe taking repeat readings and plotting a graph to improve reliability.
- Include the equation B = F / (IL) and show how the gradient of an F–I graph gives LB.
Try the lesson
The start of the Magnetic fields: (ii) techniques and procedures used to determine the uniform magnetic flux density between the poles of a magnet using a current-carrying wire and digital balance lesson, made from checked teaching notes for this specification point.
Listen
To measure the flux density between the poles of a magnet, mount a rigid straight wire horizontally between the poles so it is perpendicular to the field, and support the magnet on a digital balance. With no current, record the balance reading. Pass a known current I through the wire and record the new reading; the change in reading gives the force F on the wire (using the balance's force reading, F = mg where needed). Measure the length L of wire between the poles. Since the wire is perpendicular to the field, F = ILB, so B = F / (IL). Repeat for several currents and plot F against I; the gradient is LB, so B = gradient / L. This averages random errors and improves reliability. A strong answer earns marks for these points. Describe the apparatus: a straight wire clamped perpendicular to the field between the poles of a magnet, with the magnet resting on a digital balance. Explain that the change in balance reading when current flows gives the magnetic force on the wire. Measure the length L of wire within the field and the current I, then use B = F / (IL) because the wire is perpendicular to the field. Repeat for several values of current and plot a graph of force against current; the gradient equals LB, so B = gradient / L, reducing random error. A common slip: Using the total length of the wire instead of the length between the magnet poles. Instead, only the length inside the uniform field contributes to the force. Exam tip: State clearly that the balance reading changes by the magnetic force, and explain how you isolate that change.
See it
Key words
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Experimental technique
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Magnetic flux density measurement
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Graphical analysis
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Error reduction
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See every definition
- Experimental technique
- Mounting a rigid straight wire horizontally between the poles so it is perpendicular to the field, with the magnet supported on a digital balance.
- Magnetic flux density measurement
- Using the change in balance reading when current flows to give the force F on the wire, then B = F / (IL) because the wire is perpendicular to the field.
- Graphical analysis
- Repeating for several currents and plotting F against I, where the gradient is LB, so B = gradient / L.
- Error reduction
- Repeating for several currents and plotting a graph, which averages random errors and improves reliability.
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