OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how to show that a current can create a magnetic effect and…
Full specification wording
Magnets and magnetic fields: describe how to show that a current can create a magnetic effect and describe the directions of the magnetic field around a conducting wire
A current-carrying wire is surrounded by a magnetic field, and this can be shown with a plotting compass or iron filings.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 5 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A current-carrying wire is surrounded by a magnetic field, and this can be shown with a plotting compass or iron filings. Place a long straight wire through a card, connect it to a cell and switch, and pass a current. A small plotting compass placed near the wire deflects; reverse the current with the switch and the needle deflects the opposite way, showing the field is produced by the current. Iron filings sprinkled on the card form concentric circles centred on the wire. Around a straight wire the field lines are circles in a plane at right angles to the wire. Looking along the wire with the conventional current flowing away from you, the field lines run clockwise; with current towards you they run anticlockwise. Reversing the current reverses the field direction.
What earns the marks
- States that a plotting compass near a current-carrying wire deflects, showing a magnetic effect.
- Describes sprinkling iron filings around a straight wire to reveal concentric circular field lines.
- Identifies that the field lines around a straight wire form circles in a plane perpendicular to the wire.
- Applies the right-hand grip rule to give the field direction for a stated current direction.
- Explains that reversing the current reverses the direction of the magnetic field.
Common mistakes and how to put them right
Draws the field lines as straight lines parallel to the wire.
Put it right: Draw them as concentric circles around the wire.
Claims the compass needle points along the wire.
Put it right: Note that the needle aligns with the circular field, so it turns around the wire.
States that reversing the current leaves the field direction unchanged.
Put it right: State that reversing the current reverses the field direction.
Exam tips
- Name the apparatus explicitly, such as a plotting compass or iron filings, rather than saying only that a field is detected.
- State the plane of the circles: perpendicular to the wire, not along it.
- Use the right-hand grip rule and state the current direction before giving the field direction.
Try the lesson
The start of the Magnets and magnetic fields: describe how to show that a current can create a magnetic effect and describe the directions of the magnetic field around a conducting wire lesson, made from checked teaching notes for this specification point.
Have a go first
A student wants to show that a current-carrying wire produces a magnetic effect. Which observation would demonstrate this?
Listen
A current-carrying wire is surrounded by a magnetic field, and this can be shown with a plotting compass or iron filings. Place a long straight wire through a card, connect it to a cell and switch, and pass a current. A small plotting compass placed near the wire deflects; reverse the current with the switch and the needle deflects the opposite way, showing the field is produced by the current. Iron filings sprinkled on the card form concentric circles centred on the wire. Around a straight wire the field lines are circles in a plane at right angles to the wire. Looking along the wire with the conventional current flowing away from you, the field lines run clockwise; with current towards you they run anticlockwise. Reversing the current reverses the field direction. A strong answer earns marks for these points. States that a plotting compass near a current-carrying wire deflects, showing a magnetic effect. Describes sprinkling iron filings around a straight wire to reveal concentric circular field lines. Identifies that the field lines around a straight wire form circles in a plane perpendicular to the wire. Applies the right-hand grip rule to give the field direction for a stated current direction. Exam tip: Name the apparatus explicitly, such as a plotting compass or iron filings, rather than saying only that a field is detected.
Key words
Say what each word means before you turn the card.
Magnetic effect of a current
Try answering aloud or in your head before turning the card.
Plotting compass and iron filings as detection methods
Try answering aloud or in your head before turning the card.
Circular field pattern around a straight wire
Try answering aloud or in your head before turning the card.
Right-hand grip rule and effect of reversing current
Try answering aloud or in your head before turning the card.
See every definition
- Magnetic effect of a current
- A current-carrying wire is surrounded by a magnetic field, shown by a plotting compass deflecting or iron filings forming concentric circles.
- Plotting compass and iron filings as detection methods
- A plotting compass placed near the wire deflects, and iron filings sprinkled on the card form concentric circles centred on the wire.
- Circular field pattern around a straight wire
- Around a straight wire the field lines are circles in a plane at right angles to the wire.
- Right-hand grip rule and effect of reversing current
- Looking along the wire with the conventional current flowing away from you, the field lines run clockwise; reversing the current reverses the field direction.
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