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Magnetic Fields AS Unit 4 Drawing Field Diagrams As with gravitational and electric fields, we often draw field diagrams around magnets to show the magnetic field they exert. Electric fields show the direction that a positive charge would travel in that field. In a similar way, magnetic fields demonstrate demonstra how a North magnetic monopole would behave. Although monopoles are (theoretically) impossible to isolate, this is a useful way of representing the field.

Magnetic Fields around (a) a solenoid, and (b) a bar magnet. Notice that the field lines point from N to S. In other words, they show the direction in which a theoretical North monopole would feel a force.

Magnetic Field around a Wire When a current flows through a wire (in (i other words, when charge is moving), it produces a magnetic field around aro that wire. This is orientated in a plane perpendicular to the wire.

Right Hand Grip Rule

To find out which direction the field is in (i.e., ( where the arrows are), we can use the Right Hand Han Grip Rule.

3D Diagrams on 2D Paper: Arrowhead Notation We often need to represent 3D magnetic fields on 2D paper. To overcome this we can use arrowhead notation. A point is like an arrow coming towards you, and a cross is like the flights of the arrow moving away from you.

Magnetic field around a solenoid. Here, the current is shown using arrowhead notation, enabling the magnetic field to be drawn in the same plane as the paper.

Using your right hand, if the current is going in the direction your thumb is pointing, then the magnetic field will be pointing in the direction your fingers are.

A very similar method is used to show the direction of a linear magnetic field.

Magnetic Fields AS Unit 4 Current in a Magnetic Field When a current flows through a wire wire in a magnetic field, that wire experiences a force. This can be explained by drawing the field diagram: the cancelling of field lines results in a catapult field,, which forces the wire in a certain direction.

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Fleming’s Left Hand Rule The directionss of current flow, field, and motion, are all perpendicular to one another. They form what is called an orthogonal basis. If we know the direction of two of these, we can work out the other using Fleming’s Left Hand Rule.

where F = Force on wire (N) B = magnetic field strength (Teslas, T) I = current through wire (A) l = length of wire (m)

To find out in which direction this force acts, we can use Fleming’s Left Hand Rule. This direction will always be perpendicular to both the direction of current and the direction of the magnetic field.

Moving Charge in a Magnetic Field When a charge moves in a magnetic field, it may experience a force. The magnitude of this force can be found by

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Your First irst Finger represents the field (F), seCond finger the current (C), and your thuMb b the motion (M). Draw the direction of the force in each of these situations:

where F = Force on wire (N) B = magnetic field strength (Teslas, T) q = charge (C) v = velocity of charge (ms-1)

The direction of the force can be found by Fleming’s Left Hand Rule. Note that with that we are interested in current flow, i.e. the movement of positive charge. If a negative charge is moving in a magnetic field, then the current flow is in the direction opposite to its velocity. Because the force is always perpendicular to the direction of motion, this causes the charge to follow a circular path.

Magnetic Fields AS Unit 4 Compare this with the motion of a charge in an electric field: this always feels a force in the direction of the field itself, resulting in a parabola.

Catapult Field When a wire carries a current through a magnetic field, the mechanism that causes the force it feels is the catapult field. The magnetic field around the wire is shown here, as is the field between the poles of the magnet. On one side of each A charge in an electric wire the field lines are in the field follows a parabolic opposite direction, and cancel path, unlike a charge in a each other out. On the other magnetic field. side, though, the field lines reinforce one another. This results in a kind of “low pressure” on one side, which forces the wire out of the field.

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