PHYSICS UNIT 3 TOPIC 2 - Electromagnetism
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What is Coulomb's Law?
Coulomb's Law states that the electrostatic force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
What does Coulomb's Law describe?
Coulomb's Law describes the magnitude of the electrostatic force between two charged objects.
How does charge affect the electrostatic force between two objects?
The electrostatic force increases as the magnitude of either charge increases.
F ∝ q1q2
How does distance affect the electrostatic force between two charges?
The electrostatic force decreases as the distance between the charges increases.
F ∝ 1 / r2²
What does it mean that Coulomb's Law follows an inverse-square relationship?
If the distance between two charges doubles, the electrostatic force becomes one-quarter as large.
How do charges interact according to Coulomb's Law?
Like charges repel.
Opposite charges attract.
Describe Coulomb's Law.
Coulomb's Law states that the electrostatic force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
How is Coulomb's Law similar to Newton's Law of Universal Gravitation?
Both are inverse-square laws, meaning the force decreases with the square of the distance between the objects.
What happens to the electrostatic force if one charge doubles?
The electrostatic force doubles.
F ∝ q
What happens to the electrostatic force if the distance is halved?
The electrostatic force becomes four times greater.
F ∝ 1 / r²
What equation is used to calculate the electrostatic force between two charges?
where:
F = electrostatic force (N)
Q, q = charges (C)
r = separation distance (m)
k = 8.99 ×10^9 Nm² C^-2 (aka. Coloumbs constant)
When should Coulomb's Law be used?
When calculating the force between two charged particles or objects.
How does increasing one of the charges affect the electrostatic force?
The force increases proportionally.
F ∝ Qq
If one charge doubles, the force doubles.
In what direction does an electric field act?
The electric field direction is the direction a positive test charge would move if placed in the field.
How do electric field lines behave around positive and negative charges?
Electric field lines point away from positive charges.
Electric field lines point towards negative charges.
What is electric field strength?
Electric field strength is the force experienced per unit positive charge at a point in an electric field.
E = F / q
What does a large electric field strength indicate?
A large electric field strength means a charge placed in the field experiences a large electric force.
What are the units of electric field strength?
NC^-1 (newtons per coloumb)
What is electrical potential energy?
Electrical potential energy is the energy a charged object has because of its position in an electric field.
When does a charged particle gain or lose electrical potential energy?
Electrical potential energy changes when a charge moves within an electric field.
What happens to the electrical potential energy of a positive charge moving in the direction of an electric field?
Its electrical potential energy decreases as energy is transferred from potential energy to kinetic energy.
What equation defines electric field strength?
E = F / Q
where:
E = electric field strength (N C^-1)
F = electric force (N)
Q = charge experiencing the force (C)
What equation relates potential difference, charge and change in electrical potential energy?
V = ΔU / q
where:
V = potential difference (V)
ΔU = change in electrical potential energy (J)
q = charge (C)
What does a potential difference of 1 volt mean?
A potential difference of 1 volt means 1 joule of energy is transferred for every coulomb of charge.
1 V = 1J C^−1
How is work done related to a change in electrical potential energy?
The work done moving a charge in an electric field is equal to the change in electrical potential energy.
W = ΔU
What is a magnetic field?
A magnetic field is a region of space where a magnetic force acts on magnets, magnetic materials, or moving charges.
What produces a magnetic field?
Magnetic fields are produced by:
Magnets
Moving electric charges (electric currents)
How are magnetic fields represented?
Magnetic fields are represented using magnetic field lines that show the direction and strength of the field.
What is the direction of a magnetic field?
The direction of a magnetic field is the direction a north pole would move if placed in the field.
How do magnetic field lines behave around a bar magnet?
Magnetic field lines:
Leave the north pole.
Enter the south pole.
Form continuous closed loops.
How can the strength of a magnetic field be shown on a field-line diagram?
The closer the field lines are together, the stronger the magnetic field.
What is the shape of the magnetic field around a current-carrying wire?
The magnetic field forms concentric circular field lines around the wire.
What can experience a force in a magnetic field?
A magnetic field can exert forces on:
Magnets
Magnetic materials
Moving charged particles
Current-carrying conductors
How is a magnetic field different from an electric field?
An electric field acts on charges whether they are moving or stationary, while a magnetic field only acts on moving charges and magnetic materials.
What is the shape of the magnetic field around a moving electric charge?
A moving electric charge produces circular magnetic field lines around its path of motion.
What is the magnetic field pattern around a straight current-carrying wire?
The magnetic field consists of concentric circles centred on the wire.
How can the direction of the magnetic field around a current-carrying wire be determined?
Use the right-hand grip rule:
Thumb points in the direction of current.
Curled fingers show the direction of the magnetic field.
What is the magnetic field pattern around a bar magnet?
Magnetic field lines emerge from the north pole and enter the south pole, forming closed loops.
In which direction do magnetic field lines point around a magnet?
Outside the magnet, magnetic field lines point from the north pole to the south pole.
How is magnetic field strength shown in a field-line diagram?
Closer field lines indicate a stronger magnetic field.
How is a magnetic field generated from a moving electric charge?
A moving electric charge produces a magnetic field around it. The magnetic field forms concentric circles around the direction of motion of the charge.
Why does a moving charge produce a magnetic field but a stationary charge does not?
A stationary charge produces only an electric field, while a moving charge produces both an electric field and a magnetic field.
What is the shape of the magnetic field around a moving charge?
The magnetic field forms circular field lines around the path of the moving charge.
Why does an electric current produce a magnetic field?
An electric current is a flow of moving charges, and moving charges generate magnetic fields.
What equation is used to calculate the magnetic field strength around a straight current-carrying wire?
where:
B = magnetic field strength (T)
μ0 = 4π × 10^−7 Tm/A
I = current (A)
r = distance from the wire (m)
What equation is used to calculate the magnetic field strength inside a solenoid?
where:
B = magnetic field strength (T)
μ0 = permeability of free space
n = turns per metre
I = current (A)
How can the direction of the magnetic field around a straight wire be determined?
Use the right-hand grip rule:
Thumb → direction of current.
Curled fingers → direction of the magnetic field.
What is the shape of the magnetic field around a straight current-carrying wire?
The field consists of concentric circles centred on the wire.
How does distance from a straight wire affect magnetic field strength?
B ∝ 1 / r
As distance increases, magnetic field strength decreases.
How does current affect the magnetic field around a wire?
B ∝ I
Doubling the current doubles the magnetic field strength.
How does the current affect the magnetic field inside a solenoid?
B ∝ I
A larger current produces a stronger magnetic field.
How does the number of turns per metre affect the magnetic field inside a solenoid?
B ∝ n
More turns per metre produce a stronger magnetic field.
What happens when a current-carrying conductor is placed in a magnetic field?
The conductor experiences a magnetic force that acts perpendicular to both the current and the magnetic field.
What happens when a moving electric charge is placed in a magnetic field?
The moving charge experiences a magnetic force that acts perpendicular to both the direction of motion and the magnetic field.
When does a charged particle experience a magnetic force?
A charged particle experiences a magnetic force only when it is moving relative to the magnetic field.
Does a stationary charge experience a magnetic force?
No. A stationary charge does not experience a magnetic force.
What is the direction of the magnetic force on a moving charge or current-carrying conductor?
The force is perpendicular to both the magnetic field and the direction of motion (or current).
How does a magnetic force affect the motion of a charged particle?
The magnetic force changes the direction of motion of the particle but does not change its speed.
Why can a magnetic field cause a charged particle to move in a circular path?
The magnetic force acts as a centripetal force, continuously changing the direction of the particle's velocity.
What is the motor effect?
The motor effect is the force experienced by a current-carrying conductor placed in a magnetic field.
What equation is used to calculate the magnetic force on a current-carrying wire?
F = BILsinθ
where:
F = magnetic force (N)
B = magnetic field strength (T)
I = current (A)
LL = length of wire in the field (m)
θ = angle between the wire and the magnetic field
What equation is used to calculate the magnetic force on a moving charged particle?
F = qvBsinθ
where:
F = magnetic force (N)
q = charge (C)
v = velocity (m/s)
B = magnetic field strength (T)
θ = angle between the velocity and the magnetic field
When is the magnetic force on a wire or moving charge zero?
When:
θ = 0° or 180°
because: sin0° = 0
What relationship is shown if a graph of force vs current is a straight line through the origin?
The magnetic force is directly proportional to the current.
F ∝ I
What relationship is shown if a graph of force vs magnetic field strength is a straight line through the origin?
The magnetic force is directly proportional to the magnetic field strength.
F ∝ B
What relationship is shown if a graph of force vs conductor length is a straight line through the origin?
The magnetic force is directly proportional to the length of conductor in the field.
F ∝ L
When is the force on a conductor in a magnetic field greatest?
When the conductor is perpendicular to the magnetic field.
θ = 90°
How does magnetic field strength change with distance from a magnet?
Magnetic field strength decreases as the distance from the magnet increases.
What conclusion can be drawn if measurements show magnetic field strength decreasing as distance increases?
The magnet's field becomes weaker further from the magnet.
A graph shows magnetic field strength rapidly decreasing with distance from a magnet. What does this indicate?
The magnetic field is strongest close to the magnet and weakens with increasing distance.
How do magnetic field lines help explain why magnetic field strength decreases with distance?
Magnetic field lines are closer together near the magnet and further apart at greater distances, indicating a stronger field close to the magnet and a weaker field further away.
Where is the magnetic field stronger: 1 cm or 5 cm from a magnet?
The magnetic field is stronger at 1 cm because it is closer to the magnet.
What pattern would you expect in a table of magnetic field strength measured at increasing distances from a magnet?
The magnetic field strength values would decrease as the distance increases.
What is magnetic flux?
Magnetic flux (Φ) is the total magnetic field passing through a given area.
What is magnetic flux density?
Magnetic flux density (B) is a measure of the strength of a magnetic field at a point.
Units:
Tesla (T)
What is electromagnetic induction?
Electromagnetic induction is the generation of an EMF (voltage) in a conductor due to a changing magnetic flux.
What is electromotive force (EMF)?
EMF is the voltage induced in a conductor when the magnetic flux through it changes.
Units:
Volt (V)
What does Faraday's Law state?
Faraday's Law states that an EMF is induced when the magnetic flux through a conductor changes, and the magnitude of the EMF depends on the rate of change of magnetic flux.
According to Faraday's Law, how can a larger induced EMF be produced?
By increasing the rate at which the magnetic flux changes.
Examples:
Move the magnet faster.
Move the coil faster.
Use a stronger magnet.
What does Lenz's Law state?
Lenz's Law states that the induced current flows in a direction that opposes the change in magnetic flux that produced it.
Why is Lenz's Law important?
It determines the direction of the induced current and ensures the conservation of energy.
It determines the direction of the induced current and ensures the conservation of energy.
The magnetic flux through the coil changes, inducing an EMF and current in the coil.
How does Lenz's Law apply when a magnet is moved towards a coil?
The induced current creates its own magnetic field that opposes the approaching magnet.
What equation is used to calculate magnetic flux through a loop?
Φ=BAcosθ
where:
Φ = magnetic flux (Wb)
B = magnetic flux density (T)
A = area of the loop (m²)
θ = angle between the magnetic field and the normal to the loop
When should the equation Φ=BAcosθ be used?
When calculating the magnetic flux passing through a current-carrying loop or coil.
What does B represent in Φ=BAcosθ
Magnetic flux density, measured in teslas (T).
What are the units of magnetic flux?
weber (Wb)
When is magnetic flux maximum?
When:
θ = 0°
because: cos0° = 1
and therefore:
Φ=BA
When is magnetic flux zero?
When:
θ=90°
because:
cos90° = 0
and therefore:
Φ = 0
How does increasing the area of the loop affect magnetic flux?
Magnetic flux increases.
Φ ∝ A
How does increasing magnetic field strength affect magnetic flux?
Magnetic flux increases.
Φ ∝ B
How is an EMF induced across a moving conductor in a magnetic field?
When a conductor moves through a magnetic field, it cuts magnetic field lines. This causes charges within the conductor to separate, producing a potential difference (EMF) across the conductor.
Why does a moving conductor in a magnetic field produce an EMF?
The magnetic field exerts forces on the moving charges within the conductor, causing positive and negative charges to accumulate at opposite ends.
What happens to the charges in a conductor moving through a magnetic field?
The charges experience a magnetic force and separate, creating a voltage (EMF) across the conductor.
Why must the conductor move relative to the magnetic field for an EMF to be induced?
Relative motion changes the magnetic flux experienced by the conductor, which is required for electromagnetic induction.
Is an EMF induced if the conductor and magnetic field are stationary relative to each other?
No. No EMF is induced unless there is a change in magnetic flux.
How is the induction of EMF in a moving conductor used in generators?
Generators rotate conductors through magnetic fields, continually changing magnetic flux and inducing an EMF.
How does Faraday's Law relate to a moving conductor?
The moving conductor changes the magnetic flux, and Faraday's Law states that this changing flux induces an EMF.
How does Lenz's Law apply to a moving conductor?
The induced current produces a magnetic field that opposes the change in magnetic flux that caused the induction.
Describe the process of inducing an EMF across a moving conductor in a magnetic field.
As a conductor moves through a magnetic field, it cuts magnetic field lines. The magnetic field exerts forces on charges in the conductor, causing charge separation and creating a potential difference (EMF) across the conductor.