P4.5.4P4. Electricity and Magnetism
Force on a current-carrying conductor — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 15 of 19 1 quiz
Lesson summary
P4.5.4 Force on a current-carrying conductor — COMBINED SCIENCE notes: Explain the motor effect, find the direction of the force with Fleming's left-hand rule and describe the deflection of charged particles in a magnetic field.Motor effect = a current-carrying conductor in an ex…
- Why the wire moves
- Reversing the force
- Perpendicular fingers
- Larger current
- Stronger magnetic field
- Longer length of wire in the field
Section 1 of 9The motor effect
Motor effect
Motor effect a current-carrying conductor in an external magnetic field experiences a force
- The conductor produces its own magnetic field, which interacts with the external field.
- Examplea copper rod in a uniform field moves when current is passed through it.
Condition
- Condition the force is greatest when the current is perpendicular to the field and zero when the current is parallel to the field
Why the wire moves
- Why the wire moves the two fields add on one side of the wire and oppose on the other side
- The wire is pushed from the stronger region towards the weaker region.
- The movement is at right angles to both the current and the field.
Section 2 of 9Direction of the force
Reversing the force
- Reversing the force reverse either the current or the magnetic field
- Reversing both gives a force in the original direction.
Fleming's left-hand rule
- Fleming's left-hand rule gives the direction of the force (thrust)
- The first finger points in the direction of the magnetic field, from north to south.
- The second finger points in the direction of the conventional current, from positive to negative.
- The thumb then points in the direction of the force, or thrust, on the conductor.
Perpendicular fingers
- Perpendicular fingers all three directions are at right angles to each other
- The force can therefore point into or out of the page.
Section 3 of 9Size of the force
Size of the force
Larger current
- Larger current increases the force
Stronger magnetic field
- Stronger magnetic field increases the force
Longer length of wire in the field
- Longer length of wire in the field increases the force
Section 4 of 9Demonstrating the motor effect
1
Place a stiff wire, or a loose rod on two rails, between the poles of a U-shaped magnet at right angles to the field.
2
Connect the wire to a d.c. supply through a switch and a variable resistor.
3
Close the switch; the wire moves at right angles to both the field and the current.
4
Reverse the current, then reverse the magnet; each time the wire moves the other way.
5
Increase the current with the variable resistor; the force on the wire is larger.
Section 5 of 9Charged particles in a magnetic field
Charged particles in a magnetic field
Moving charge
Moving charge a current, so the field exerts a force on each charged particle, for example each electron in a wire
Force direction
- Force direction the force is at 90° to both the direction of travel and the field lines
- Fleming's left-hand rule gives its direction.
- The path of the particle is deflected into a curve.
Electrons
- Electrons their negative charge means the second finger points opposite to the direction of motion
- Conventional current flows opposite to the electron flow.
A positive beam moving the same way is deflected in the opposite direction to an electron beam.
Angle to the field
- Travelling perpendicular to the field lines gives the maximum force.
- Travelling parallel to the field lines gives no force.
- Travelling at an angle to the field lines gives a smaller force.
Section 6 of 9Uses of the motor effect
D.c. motors
- D.c. motors a current-carrying coil in a field turns
Loudspeakers
- Loudspeakers a coil in a magnetic field moves a cone
Moving-coil meters
- Moving-coil meters a coil in a field turns against a spring
Section 7 of 9More from the specification
1
Electromagnetic induction
- A voltage is induced in a conductor when it cuts magnetic field lines or when the flux changes, such as moving a magnet into a solenoid.
2
Size of induced voltage
- Increases with faster movement, stronger magnetic field strength and more turns;
- a sensitive voltmeter detects it.
3
Transformer
- Two coils on an iron core;
- an alternating current in the primary creates a changing field that induces a voltage in the secondary.
4
Transformer equation
- Vp/Vs = Np/Ns, and for 100% efficiency the power is equal in both circuits, VpIp = VsIs.
5
Force on a conductor
- A current-carrying wire in a field experiences a force, found with Fleming's left-hand rule.
6
Electromagnetic induction
- A voltage is induced in a conductor when it cuts magnetic field lines or when the flux changes, such as moving a magnet into a solenoid.
7
Size of induced voltage
- Increases with faster movement, stronger magnetic field strength and more turns;
- a sensitive voltmeter detects it.
8
Transformer
- Two coils on an iron core;
- an alternating current in the primary creates a changing field that induces a voltage in the secondary.
9
Transformer equation
- Vp/Vs = Np/Ns, and for 100% efficiency the power is equal in both circuits, VpIp = VsIs.
10
Force on a conductor
- A current-carrying wire in a field experiences a force, found with Fleming's left-hand rule.
11
Generator
- A device that converts movement into an electrical output by electromagnetic induction.
12
Induced magnet
- A magnetic material that becomes a magnet only while in a magnetic field.
13
Potential difference
- The work done by a unit charge passing through a component.
14
Step-up transformer
- A transformer with more turns on the secondary coil than the primary, raising the voltage.
15
Step-down transformer
- A transformer with fewer turns on the secondary coil, lowering the voltage.
16
Electromagnetic induction
- The production of an e.m.f. in a conductor when there is relative movement between the conductor and a magnetic field.
17
Permanent magnet
- A magnet whose field is always present.
18
Transformer
- A device with a primary coil and secondary coil on an iron core that changes an alternating voltage.
19
Solenoid
- A coil of wire that produces a magnetic field like a bar magnet when a current flows.
20
Voltage
- A measure of the electrical energy supplied or changed in a circuit.
21
Voltmeter
- Meter that measures voltage, connected in parallel.
22
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Motor effect | The force on a current-carrying conductor placed in a magnetic field. |
| Fleming's left-hand rule | First finger = field, second finger = conventional current, thumb = force (thrust). |
| Thrust | The force on the current-carrying conductor in a magnetic field. |
| Conventional current | Current taken as flowing from positive to negative, opposite to electron flow. |
| Magnetic field | The region around a magnet or a current in which a magnetic force is felt. |
| Deflection | The change in direction of the path of a charged particle in a magnetic field. |
Section 9 of 9Exam technique
1
Using the right hand for the force — the force on a current uses the left hand (the right-hand dynamo rule is for induction)
2
Pointing the second finger along the electron motion — it must point along the conventional current, so opposite to electron motion
3
Saying the force acts along the wire or along the field — it is at right angles to both
4
Thinking a current parallel to the field feels a force — it feels no force
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