AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Required practical 10: Investigate how the force on a wire varies with…
Full specification wording
Magnetic flux density (A-level only): Required practical 10: Investigate how the force on a wire varies with flux density, current and length of wire using a top pan balance.
This practical exploits Newton's third law.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical exploits Newton's third law. A rigid wire segment sits horizontally between the poles of a magnet assembly resting on a top pan balance. When current flows, the field exerts a force on the wire; the wire exerts an equal and opposite force on the magnet, so the balance reading changes by F = BIl. Switching the current reverses the sign of the change, so readings are taken with current on and off, and the difference gives the force. Vary one quantity at a time: current (ammeter and variable resistor), flux density (different magnet assemblies or different numbers of magnets), and length (different rigid wire loops or a jockey tapping along a wire). Plot F against each independent variable; a straight line through the origin supports direct proportionality, and the gradient yields B, l or I as appropriate.
What earns the marks
- Sets up the magnet assembly on the top pan balance with the rigid wire segment horizontal and perpendicular to the magnetic field between the poles.
- Uses the balance reading difference between current on and current off to obtain the magnetic force, recognising the third-law pairing between force on wire and force on magnet.
- Varies one independent variable at a time: current with an ammeter and variable resistor, flux density with different magnet assemblies, length with different wire segments or a tapping jockey.
- Plots a graph of force against the independent variable and interprets a straight line through the origin as direct proportionality, using the gradient to find B, I or l.
- Controls relevant variables, such as keeping the wire perpendicular to the field, keeping the wire in the same region of field, and avoiding heating effects by limiting current.
- Repeats readings and averages to reduce random uncertainty, and considers systematic effects such as the balance zero and the mass of connecting leads.
Common mistakes and how to put them right
Assuming the balance reading itself equals the magnetic force.
Put it right: Use the difference between the current-on and current-off readings.
Leaving the wire at an angle to the field.
Put it right: Align the wire perpendicular to the field so that the full F = BIl relationship applies.
Using large currents for long periods.
Put it right: Keep currents modest and switch off between readings to avoid heating that changes resistance and field conditions.
Exam tips
- State clearly that the balance measures the force on the magnet, which is equal and opposite to the force on the wire, and explain how the difference in readings gives F.
- When describing how to vary flux density, refer to changing the magnet assembly or the number of magnets rather than vaguely 'using a stronger magnet'.
- Link each graph shape to the equation F = BIl: force proportional to current and to length at constant B, and proportional to flux density at constant I and l.
Try the lesson
The start of the Magnetic flux density (A-level only): Required practical 10: Investigate how the force on a wire varies with flux density, current and length of wire using a top pan balance. lesson, made from checked teaching notes for this specification point.
Have a go first
In the standard setup for Required practical 10, a magnet assembly rests on a top pan balance and a rigid wire is clamped horizontally between its poles. When a current flows through the wire, the balance reading changes. What does this change in reading allow a student to determine directly?
Listen
This practical exploits Newton's third law. A rigid wire segment sits horizontally between the poles of a magnet assembly resting on a top pan balance. When current flows, the field exerts a force on the wire; the wire exerts an equal and opposite force on the magnet, so the balance reading changes by F = BIl. Switching the current reverses the sign of the change, so readings are taken with current on and off, and the difference gives the force. Vary one quantity at a time: current (ammeter and variable resistor), flux density (different magnet assemblies or different numbers of magnets), and length (different rigid wire loops or a jockey tapping along a wire). Plot F against each independent variable; a straight line through the origin supports direct proportionality, and the gradient yields B, l or I as appropriate. A strong answer earns marks for these points. Sets up the magnet assembly on the top pan balance with the rigid wire segment horizontal and perpendicular to the magnetic field between the poles. Uses the balance reading difference between current on and current off to obtain the magnetic force, recognising the third-law pairing between force on wire and force on magnet. Varies one independent variable at a time: current with an ammeter and variable resistor, flux density with different magnet assemblies, length with different wire segments or a tapping jockey. Plots a graph of force against the independent variable and interprets a straight line through the origin as direct proportionality, using the gradient to find B, I or l. A common slip: Assuming the balance reading itself equals the magnetic force. Instead, use the difference between the current-on and current-off readings. Exam tip: State clearly that the balance measures the force on the magnet, which is equal and opposite to the force on the wire, and explain how the difference in readings gives F.
Key words
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Newton's third law applied to magnetic forces
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Direct proportionality and graphical analysis
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Control of variables in a practical investigation
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Measurement uncertainty and experimental design
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See every definition
- Newton's third law applied to magnetic forces
- The field exerts a force on the wire, and the wire exerts an equal and opposite force on the magnet, so the balance reading changes.
- Direct proportionality and graphical analysis
- A straight line through the origin on a graph of force against the independent variable supports direct proportionality, and the gradient yields B, l or I as appropriate.
- Control of variables in a practical investigation
- One quantity is varied at a time, keeping the wire perpendicular to the field, in the same region of field, and avoiding heating effects by limiting current.
- Measurement uncertainty and experimental design
- Readings are repeated and averaged to reduce random uncertainty, and systematic effects such as the balance zero and the mass of connecting leads are considered.
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