OCR GCSE Physics A (J249) — Magnetism and magnetic fields
Have you ever noticed a compass needle twitch when you turn on a heavy electrical appliance nearby? A current-carrying conductor generates a magnetic field around it. This field is a region where a magnetic material experiences a force. Whenever you describe this effect, always assume the flow of conventional current, which travels from the positive terminal to the negative terminal of the power source.
To prove that moving charge produces a magnetic field, you can set up a simple laboratory demonstration. The goal is to observe how a magnetic field interacts with a plotting compass, which contains a tiny bar magnet.
First, arrange a straight conducting wire vertically so it passes through a horizontal piece of card. Next, place a plotting compass on the card near the wire and note the needle's starting position, which will usually point North. Then, close the switch in the circuit to allow current to flow through the wire. Finally, observe the needle deflect (move away) from its original position, aligning itself perpendicular to the wire.
You can also change the variables in this demonstration. If you reverse the direction of the current by swapping the terminal connections, the compass needle will deflect in the opposite direction. Furthermore, a larger current will result in a greater deflection, indicating a stronger magnetic field.
The magnetic field around a straight wire forms a distinct pattern of concentric circles. These circular field lines exist in a plane that is exactly perpendicular to the wire.
To demonstrate the shape of this field, you can sprinkle iron filings onto the horizontal card around the wire. When you tap the card gently, the filings will align themselves into the concentric circle pattern. Alternatively, to find the direction of the field, place multiple plotting compasses around the wire in a circle. You can mark dots at the tips of the compass needles and join them together to map the complete field line.
The strength of the magnetic field () is directly proportional to the size of the current (). If the current is zero, the magnetic field disappears completely.
The field is strongest closest to the wire and gets weaker as the distance () from the wire increases. This mathematical relationship is written as:
When drawing these field lines, the circles must never touch or cross each other. To represent the decreasing field strength, the circles are not evenly spaced. Instead, they must be drawn close together near the wire and increasingly further apart as you move away from the centre.
The Right-Hand Grip Rule is a reliable method to determine the exact direction of the magnetic field lines around a wire.
Point your right thumb in the direction of the conventional current. When you curl your fingers, the direction they wrap around the wire indicates the direction of the circular magnetic field lines.
In 2D diagrams, standard symbols are used to show current moving in three dimensions. An 'X' represents current flowing into the page, resembling the feathers of an arrow moving away from you. A dot ('.') represents current flowing out of the page, like the tip of an approaching arrow.
A straight wire passes vertically through a piece of card. A student views the wire from above, and the current is represented by an 'X' symbol. Describe how to use the Right-Hand Grip Rule to determine the direction of the magnetic field lines.
Step 1: Identify the meaning of the 2D symbol.
Step 2: Apply the Right-Hand Grip Rule.
Step 3: Determine the direction of the curled fingers.
Step 4: State the final answer clearly.
The grip rule can also be adapted for a solenoid, which is a long coil of wire. When current passes through a solenoid, it produces a magnetic field similar to a bar magnet. If you curl your right fingers in the direction of the current flowing through the coils, your extended right thumb will point towards the North Pole of the solenoid's magnetic field.
Students often confuse the Right-Hand Grip Rule (used to find the magnetic field direction around a wire) with Fleming's Left-Hand Rule (used to find the force on a wire). Ensure you use your right hand for field patterns!
In drawing questions, examiners specifically look for the spacing of your concentric circles. You must draw them increasingly further apart as you move away from the wire to show the magnetic field getting weaker.
Always use the specific command word 'deflect' when describing how a compass needle responds to a nearby current-carrying wire in exam answers.
Remember that the dot and cross symbols for current are visual representations of an arrow: a dot is the tip coming towards you (out of the page), and an 'X' is the feathers moving away from you (into the page).
Current-carrying conductor
A material, such as a wire, that is allowing an electrical current to flow through it and consequently generates a magnetic field.
Magnetic field
A region in space where a magnetic material, such as a compass needle or iron filing, experiences a magnetic force.
Conventional current
The theoretical flow of electrical charge in a circuit, moving from the positive terminal to the negative terminal of a power source.
Plotting compass
A small device containing a tiny bar magnet, used to identify the direction and shape of a magnetic field.
Deflect
The visible movement of a compass needle away from its original resting position when influenced by an external magnetic field.
Concentric circles
A pattern of circular lines that all share the exact same centre point, such as the field lines around a straight conducting wire.
Right-Hand Grip Rule
A hand-gesture mnemonic used to determine the direction of magnetic field lines around a current-carrying wire by pointing the thumb in the direction of the current.
Solenoid
A long coil of wire that generates a strong, uniform magnetic field internally (similar to a bar magnet) when a current is passed through it.
Put your knowledge into practice — try past paper questions for Physics A
Current-carrying conductor
A material, such as a wire, that is allowing an electrical current to flow through it and consequently generates a magnetic field.
Magnetic field
A region in space where a magnetic material, such as a compass needle or iron filing, experiences a magnetic force.
Conventional current
The theoretical flow of electrical charge in a circuit, moving from the positive terminal to the negative terminal of a power source.
Plotting compass
A small device containing a tiny bar magnet, used to identify the direction and shape of a magnetic field.
Deflect
The visible movement of a compass needle away from its original resting position when influenced by an external magnetic field.
Concentric circles
A pattern of circular lines that all share the exact same centre point, such as the field lines around a straight conducting wire.
Right-Hand Grip Rule
A hand-gesture mnemonic used to determine the direction of magnetic field lines around a current-carrying wire by pointing the thumb in the direction of the current.
Solenoid
A long coil of wire that generates a strong, uniform magnetic field internally (similar to a bar magnet) when a current is passed through it.
