Define a magnetic field .

A magnetic field (or B-field ) is a field of force created by moving electric charge or by permanent magnets.

True or False? A stationary electric charge produces a magnetic field.

False. Only moving charge (or permanent magnets) produce a magnetic field; a stationary charge does not.

How can magnetic fields be observed, given that they are invisible?

By the force they exert on magnetic materials, for example iron filings or the movement of a needle in a plotting compass.

What happens when two like magnetic poles are brought together? What about two opposite poles?

Two like poles (N-N or S-S) repel each other. Two opposite poles (N-S) attract each other.

Magnetic field lines on a bar magnet always point from the .......... pole to the south pole.

Magnetic field lines on a bar magnet always point from the north pole to the south pole.

State three key rules for drawing magnetic field lines correctly.

Field lines: come out of the north pole and into the south pole; are stronger where lines are closer together; never cross ; are continuous .

Define a uniform magnetic field .

A uniform magnetic field is one where the magnetic field strength is the same at all points, represented by equally spaced, parallel field lines.

Describe the shape and direction of the magnetic field lines around a straight current-carrying wire.

They form concentric circles centred on the wire, strongest near the wire and weaker further away. Reversing the current reverses the direction of the field.

Define Maxwell's right-hand screw rule .

Point the right-hand thumb in the direction of the conventional current in the wire; the curled fingers show the direction of the magnetic field around the wire.

How is the right-hand grip rule used to find the direction of a solenoid's magnetic field?

Grip the coil so the fingers point in the direction of conventional current flow; the thumb then points in the direction of the field lines through the coil, from north to south.

State two ways to increase the strength of the magnetic field produced by a solenoid.

Add a ferrous (iron) core , or add more turns to the coil.

The magnetic field lines around a solenoid emerge from the .......... pole and return to the south pole.

The magnetic field lines around a solenoid emerge from the north pole and return to the south pole.

True or False? The magnetic field pattern of a flat circular coil is completely different from that of a solenoid.

False. A flat circular coil produces field lines similar to a single turn of a solenoid; field lines emerge from one face (north pole) and return to the other (south pole).

Define Fleming's left-hand rule .

A rule used to find the direction of the magnetic force on a current (or moving charge) in a magnetic field: the First finger represents the field , the se Cond finger represents the (conventional) current , and the thu Mb represents the force — all mutually perpendicular.

What do a dot and a cross represent when showing a magnetic field's direction in a diagram?

A dot represents the field directed out of the page. A cross represents the field directed into the page.

When using Fleming's left-hand rule for a beam of electrons, in which direction should the second finger point?

In the opposite direction to the electrons' motion, since current is defined as the direction of flow of positive charge.

If the directions of any two of the force, field and current on a conductor are known, what can Fleming's left-hand rule be used to find?

The direction of the third quantity — for example, the current direction can be found if the force and field directions are already known.

When using Fleming's left-hand rule to relate force, field and velocity, the second finger represents the direction of velocity of a .......... charge.

When using Fleming's left-hand rule to relate force, field and velocity, the second finger represents the direction of velocity of a positive charge.

True or False? The magnetic force on a moving charged particle is always parallel to its velocity.

False. The force is always perpendicular to the particle's velocity, which is why it acts as a centripetal force , producing circular motion.

Define magnetic flux density , B .

The force acting per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field.

One tesla is the flux density of a magnetic field such that a wire carrying a current of 1 A , placed normal to the field, experiences a force per unit length of 1 N m -1 .

Approximately how does the Earth's magnetic flux density compare with that of an ordinary fridge magnet?

The Earth's field is about 0.032 mT , much weaker than a typical fridge magnet at around 5 mT .

A current-carrying conductor experiences .......... force when it is orientated parallel to the magnetic field.

A current-carrying conductor experiences zero force when it is orientated parallel to the magnetic field.

True or False? The direction of conventional current used in these calculations is the same as the direction of electron flow.

False. Conventional current is the direction of flow of positive charge, which is opposite to the direction of electron flow.

In the experiment to determine magnetic flux density, what are the independent and dependent variables?

Independent variable = current , I . Dependent variable = mass , m , recorded on the top-pan balance.

Why must the top-pan balance be zeroed before taking readings, and what type of error does this avoid?

It is zeroed with no current flowing, to avoid a zero error — a systematic error.

Why should high currents be avoided when carrying out this experiment?

High currents cause the wire to heat up , increasing its resistance, and pose a safety risk of burns.

How can random error be reduced in this experiment, besides repeating and averaging readings?

Repeat the experiment with the magnet and wire turned through 90° .

In this experiment, the length of the wire in the magnetic field, L , is treated as a .......... variable.

In this experiment, the length of the wire in the magnetic field, L , is treated as a control variable.

True or False? In this experiment, the magnetic force on the wire is measured directly using a newton-meter.

False. The force is found indirectly: it equals the weight ( mg ) shown as a change in mass on the top-pan balance, using Newton's third law .

What is the magnetic force on a charged particle travelling parallel to the magnetic field lines?

Zero — there is no magnetic force when the particle moves parallel to the field.

Explain why the direction of "current" for a beam of electrons is opposite to their direction of motion.

Current is defined as the rate of flow of positive charge, so for a flow of negative charge (electrons), the equivalent conventional current direction is opposite to the electrons' motion.

True or False? The magnetic force on a moving charged particle can change its speed.

False. The force is always perpendicular to the particle's velocity (a centripetal force), so it changes the particle's direction but not its speed.

Define centripetal force .

Centripetal force is the resultant force acting on an object moving in a circular path; it is always directed towards the centre of the circle.

What path does a charged particle follow when travelling perpendicular to a uniform magnetic field?

A circular path, since the magnetic force is always perpendicular to the particle's velocity and directed towards the centre of the path.

Particles with a greater mass travelling in a magnetic field move in .......... circular paths.

Particles with a greater mass travelling in a magnetic field move in larger circular paths.

True or False? The magnetic force acting on a charged particle in a magnetic field does work on it, increasing its speed.

False. The magnetic force is always perpendicular to the velocity, so it does no work on the particle — its speed stays constant and only its direction of travel changes.

Define velocity selector .

A device consisting of perpendicular electric and magnetic fields, used to filter charged particles so that only those travelling at one specific velocity pass through undeflected.

What is a velocity selector used for in a device such as a mass spectrometer?

To produce a beam of charged particles that are all travelling at the same velocity .

In a velocity selector, the electric force on a charged particle .......... on its speed, whereas the magnetic force does.

In a velocity selector, the electric force on a charged particle does not depend on its speed, whereas the magnetic force does.

What happens to particles travelling faster or slower than the selected speed v in a velocity selector?

The electric and magnetic forces on them are no longer balanced, so they are deflected and collide with one of the charged plates , removing them from the beam.

True or False? Gravity is the dominant force acting on charged particles as they pass through a velocity selector.

False. The gravitational force on the charged particles is negligible compared with the electric and magnetic forces, so it can be ignored in calculations.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsMagnetic Fields

Magnetic Fields (OCR A Level Physics): Flashcards

Exam code: H556

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Define a magnetic field.

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Cards in this collection (54)

Define a magnetic field.
A magnetic field (or B-field) is a field of force created by moving electric charge or by permanent magnets.
True or False?
A stationary electric charge produces a magnetic field.
False.
Only moving charge (or permanent magnets) produce a magnetic field; a stationary charge does not.
How can magnetic fields be observed, given that they are invisible?
By the force they exert on magnetic materials, for example iron filings or the movement of a needle in a plotting compass.
What happens when two like magnetic poles are brought together? What about two opposite poles?
Two like poles (N-N or S-S) repel each other. Two opposite poles (N-S) attract each other.
Magnetic field lines on a bar magnet always point from the .......... pole to the south pole.
Magnetic field lines on a bar magnet always point from the north pole to the south pole.
State three key rules for drawing magnetic field lines correctly.
Field lines: come out of the north pole and into the south pole; are stronger where lines are closer together; never cross; are continuous.
Define a uniform magnetic field.
A uniform magnetic field is one where the magnetic field strength is the same at all points, represented by equally spaced, parallel field lines.
Describe the shape and direction of the magnetic field lines around a straight current-carrying wire.
They form concentric circles centred on the wire, strongest near the wire and weaker further away. Reversing the current reverses the direction of the field.
Define Maxwell's right-hand screw rule.
Point the right-hand thumb in the direction of the conventional current in the wire; the curled fingers show the direction of the magnetic field around the wire.
How is the right-hand grip rule used to find the direction of a solenoid's magnetic field?
Grip the coil so the fingers point in the direction of conventional current flow; the thumb then points in the direction of the field lines through the coil, from north to south.
State two ways to increase the strength of the magnetic field produced by a solenoid.
Add a ferrous (iron) core, or add more turns to the coil.
The magnetic field lines around a solenoid emerge from the .......... pole and return to the south pole.
The magnetic field lines around a solenoid emerge from the north pole and return to the south pole.
True or False?
The magnetic field pattern of a flat circular coil is completely different from that of a solenoid.
False.
A flat circular coil produces field lines similar to a single turn of a solenoid; field lines emerge from one face (north pole) and return to the other (south pole).
Define Fleming's left-hand rule.
A rule used to find the direction of the magnetic force on a current (or moving charge) in a magnetic field: the First finger represents the field, the seCond finger represents the (conventional) current, and the thuMb represents the force — all mutually perpendicular.
What do a dot and a cross represent when showing a magnetic field's direction in a diagram?
A dot represents the field directed out of the page. A cross represents the field directed into the page.
When using Fleming's left-hand rule for a beam of electrons, in which direction should the second finger point?
In the opposite direction to the electrons' motion, since current is defined as the direction of flow of positive charge.
If the directions of any two of the force, field and current on a conductor are known, what can Fleming's left-hand rule be used to find?
The direction of the third quantity — for example, the current direction can be found if the force and field directions are already known.
When using Fleming's left-hand rule to relate force, field and velocity, the second finger represents the direction of velocity of a .......... charge.
When using Fleming's left-hand rule to relate force, field and velocity, the second finger represents the direction of velocity of a positive charge.
True or False?
The magnetic force on a moving charged particle is always parallel to its velocity.
False.
The force is always perpendicular to the particle's velocity, which is why it acts as a centripetal force, producing circular motion.
Define magnetic flux density, B.
The force acting per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field.
Define the tesla.
One tesla is the flux density of a magnetic field such that a wire carrying a current of 1 A, placed normal to the field, experiences a force per unit length of 1 N m^-1.
Approximately how does the Earth's magnetic flux density compare with that of an ordinary fridge magnet?
The Earth's field is about 0.032 mT, much weaker than a typical fridge magnet at around 5 mT.
State the equation for the magnetic force F on a current-carrying conductor, in terms of B, I, L and the angle θ between the conductor and the field.
$F = B I L \sin θ$
For what angle between a current-carrying conductor and a magnetic field is the force on the conductor a maximum, and what does the equation become?
Maximum when θ = 90° (conductor perpendicular to the field), giving $F = B I L$
A current-carrying conductor experiences .......... force when it is orientated parallel to the magnetic field.
A current-carrying conductor experiences zero force when it is orientated parallel to the magnetic field.
True or False?
The direction of conventional current used in these calculations is the same as the direction of electron flow.
False.
Conventional current is the direction of flow of positive charge, which is opposite to the direction of electron flow.
In the experiment to determine magnetic flux density, what are the independent and dependent variables?
Independent variable = current, I. Dependent variable = mass, m, recorded on the top-pan balance.
Why must the top-pan balance be zeroed before taking readings, and what type of error does this avoid?
It is zeroed with no current flowing, to avoid a zero error — a systematic error.
What quantities are plotted on each axis to determine B, and how is B calculated from the graph?
Plot mass m (y-axis) against current I (x-axis) and find the gradient of the line of best fit, then $B = \frac{g r a d i e n t \times g}{L}$
Why should high currents be avoided when carrying out this experiment?
High currents cause the wire to heat up, increasing its resistance, and pose a safety risk of burns.
How can random error be reduced in this experiment, besides repeating and averaging readings?
Repeat the experiment with the magnet and wire turned through 90°.
In this experiment, the length of the wire in the magnetic field, L, is treated as a .......... variable.
In this experiment, the length of the wire in the magnetic field, L, is treated as a control variable.
True or False?
In this experiment, the magnetic force on the wire is measured directly using a newton-meter.
False.
The force is found indirectly: it equals the weight (mg) shown as a change in mass on the top-pan balance, using Newton's third law.
State the equation for the magnetic force on a moving charged particle.
$F = B Q v \sin θ$ where F = magnetic force, B = magnetic flux density, Q = charge, v = speed, and θ = angle between the velocity and the field.
At what angle between a charged particle's velocity and the magnetic field is the magnetic force a maximum, and what does the equation simplify to?
Maximum at θ = 90° (particle moving perpendicular to the field), giving $F = B Q v$
What is the magnetic force on a charged particle travelling parallel to the magnetic field lines?
Zero — there is no magnetic force when the particle moves parallel to the field.
Explain why the direction of "current" for a beam of electrons is opposite to their direction of motion.
Current is defined as the rate of flow of positive charge, so for a flow of negative charge (electrons), the equivalent conventional current direction is opposite to the electrons' motion.
The equation $F = B I L \sin θ$ applies to a current-carrying conductor, whereas $F = B Q v \sin θ$ applies to an isolated .......... charged particle.
The equation $F = B I L \sin θ$ applies to a current-carrying conductor, whereas $F = B Q v \sin θ$ applies to an isolated moving charged particle.
True or False?
The magnetic force on a moving charged particle can change its speed.
False.
The force is always perpendicular to the particle's velocity (a centripetal force), so it changes the particle's direction but not its speed.
Define centripetal force.
Centripetal force is the resultant force acting on an object moving in a circular path; it is always directed towards the centre of the circle.
What path does a charged particle follow when travelling perpendicular to a uniform magnetic field?
A circular path, since the magnetic force is always perpendicular to the particle's velocity and directed towards the centre of the path.
Equating the magnetic force to the centripetal force gives $B q v = \frac{m v^{2}}{r}$ . Rearrange this to find the equation for the radius r of a charged particle's circular path in a magnetic field.
$r = \frac{m v}{B q}$
where m = mass, v = speed, B = magnetic flux density, q = charge.
Particles with a greater mass travelling in a magnetic field move in .......... circular paths.
Particles with a greater mass travelling in a magnetic field move in larger circular paths.
According to $r = \frac{m v}{B q}$ , how does increasing a particle's charge affect the radius of its circular path in a magnetic field?
The radius decreases, since r ∝ 1 / q: particles with greater charge move in smaller circles.
According to $r = \frac{m v}{B q}$ , how does increasing the magnetic flux density B affect the radius of a charged particle's circular path?
The radius decreases, since r ∝ 1 / B: a stronger magnetic field produces a smaller circle.
True or False?
The magnetic force acting on a charged particle in a magnetic field does work on it, increasing its speed.
False.
The magnetic force is always perpendicular to the velocity, so it does no work on the particle — its speed stays constant and only its direction of travel changes.
Which law relates the centripetal force and acceleration of a charged particle moving in a circular path in a magnetic field?
Newton's second law: $F = m a$ , with the acceleration directed towards the centre of the circular path.
Define velocity selector.
A device consisting of perpendicular electric and magnetic fields, used to filter charged particles so that only those travelling at one specific velocity pass through undeflected.
What is a velocity selector used for in a device such as a mass spectrometer?
To produce a beam of charged particles that are all travelling at the same velocity.
What condition must be satisfied for a charged particle to pass through a velocity selector undeflected?
The electric force and magnetic force on the particle must be equal in magnitude but opposite in direction: $F_{E} = F_{B}$
Starting from $F_{E} = F_{B}$ , derive the equation for the selected velocity v of a velocity selector.
$E Q = B Q v$
The charge Q cancels, giving:
$v = \frac{E}{B}$
In a velocity selector, the electric force on a charged particle .......... on its speed, whereas the magnetic force does.
In a velocity selector, the electric force on a charged particle does not depend on its speed, whereas the magnetic force does.
What happens to particles travelling faster or slower than the selected speed v in a velocity selector?
The electric and magnetic forces on them are no longer balanced, so they are deflected and collide with one of the charged plates, removing them from the beam.
True or False?
Gravity is the dominant force acting on charged particles as they pass through a velocity selector.
False.
The gravitational force on the charged particles is negligible compared with the electric and magnetic forces, so it can be ignored in calculations.

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