Magnetic Fields

magnetic field

A region around a magnet or electric current in which magnetic forces can be observed. It is represented by field lines that show the direction and strength of the magnetic force.

magnetic fields around a wire

are generated by electric current flowing through the wire, creating concentric circles of magnetic influence.

solenoid

A cyclindrical coil of wire designed to create a magnetic field (acting as a magnet) when an electric current passes through it. Solenoids are used in various applications, including electromagnets and switches.

resultant force on a current carrying wire in a magnetic field

the combined effect of all the forces acting on the wire, determined by the current direction and strength, as well as the magnetic field direction.

what does the size of the resultant force acting on a wire in a mag. field depend on?

It depends on the strength of the magnetic field, the amount of electric current flowing through the wire, and the angle between the wire and the magnetic field. the size of the force depends on the component of the magnetic field that is perpendicular to the current - the direction of the force is always perpendicular to current and magnetic field

magnetic flux density

A measure of the strength of a magnetic field in a given area, often denoted by the symbol B. It is defined as the amount of magnetic flux through a unit area perpendicular to the field.

the force on 1m of wire carring a current of 1A at right angles to a magnetic field

at what angle to the magnetic field is the force on a current carrying wire greatest?

90 degrees (perpendicular)

describe what occurs when a moving charge passes through a magnetic field

A moving charge experiences a force perpendicular to both its velocity and the magnetic field, leading to circular or helical motion as it acts as a centripetal force.

relationship between force experienced by mass and the mass

force experienced by mass is independent of the mass

relationship between mass and centripetal acceleration of mass

centripetal acceleration experienced depends on mass - Newton’s 2nd Law

what happens to the radius of curvature when the mass or velocity of the particle increases

The radius of curvature increases as mass or velocity increases if the force remains constant, due to a greater momentum.

what happens to the radius of curvature when the strength of the magnetic field or charge of the particle increases

decreases

is the frequency of rotation of a charged mass moving in a magnetic fielddependent / independent of its velocity

independent of its velocity

equation for r for a moving charge in a mag. field

r = mv/BQ

equation for f of rotation of a charged particle in a mag. field

f = BQ/(2πm)

describe how a cyclotron works

1. cyclotron made of 2 hollow semi-circular electrodes (Dees) with a uniform mag. field between them applied perpendicular to the plane of the electrodes

2. alternating p.d. applied between the electrodes

3. charged particles are produced and fired into one of the electrodes, where m-field causes them to follow circular path and leave electrode

4. applied p.d. then accelerates particles across gap until enter next electrode

5. due to higher v, radius of circular motion within dee will increase

6. p.d. reversed again so accelerated to other dee

7. process repeats as the particle spirals outwards, increasing in speed before finally exiting

magnetic flux

measures the strength of a magnetic field passing through a given area

magnetic flux linkage

the product of the number of turns in a coil and the magnetic flux through one turn. It represents the total magnetic field strength through a coil of wire.

the rate of change in flux linkage tells you…

the induced electromotive force (emf) in the coil.

a change in flux linkage of 1 weber per second will induce an emf of 1 volt in a loop of wire

electromagnetic induction

is the process of generating an electromotive force through a change in magnetic flux.

when a conducting rod moves relative to a mag. field, the electrons in the rod will experience a force (as they are charged particles) and build up on one side of the rod, causing an emf to be induced in the rod - this is known as electromagnetic induction.

if the coil forms a complete circuit, a current is also induced

Faraday’s Law

the magnitude of induced emg is equal to the rate of change of flux linkage

Lenz’s Law

states that the direction of induced current opposes the change in magnetic flux that produced it.

describe how Lenz’s law can be demonstrated

1. as the magnet approaches the coil, there is a change of flux through the coil so an emf and current is induced

2. due to Lenz’s law, the direction of induced current is such as to oppose the motion of the magnet so the same pole as the pole of tha magent approaching the coil will be induced in order to repel the magnet. this causes the magnet to slow down, due to electrostatic forces of repulsion

3. as the magnet passes through the centre of the coil, there is no change in flux so no emf is induced

4. as the magnet begins to leave the coil, there is a change in flux, so a current is induced that opposes the motion of the magnet. therefore, an opposite pole is induced by the magnet causing it to slow down once again, due to electrostatic forces of attraction

what does the gradient of the graph of flux linkage against time show

It represents the induced emf in the circuit.

what does the area under an emf-t graph show?

flux linkage

induced emf in a rotating coil

when a coil rotates uniformly in a mag. field, the coil cuts the flux and an alternating emf is induced

in a rotating coil, how will increasing the speed of rotation affect the frequency and max emf induced?

Increasing the speed of rotation will increase the frequency of the induced emf, leading to a higher maximum emf induced in the coil.

in a rotating coil, how does increasing the mag. flux density affect the max emf and frequency?

Increasing the magnetic flux density will increase the maximum emf induced in the coil, while the frequency of the induced emf remains unchanged.

generator / dynamo

A device that converts mechanical energy into electrical energy using electromagnetic induction, typically through the rotation of a coil in a magnetic field. the generator has slip rings and brushes to connect the coil to an external circuit. the output and current change direction with every half rotation of the coil, producing an alternating current

what will an oscilloscope with the time base on/off show for a direct current?

on: straight horizontal line parallel to x-axis at the height of the output voltage

off: dot at the height of the output voltage

what will an oscilloscope with the time base on/off show for an alternating current?

on: trace will show a repeating sinusoidal waveform which shows the variation of output voltage with time

off: a straight vertical line will appear on screen, showing all possible voltages

peak voltage

the maximum voltage reached by an alternating current during a cycle, indicating the highest point of the waveform. (distance from equilibrium to the highest/lowest point)

peak-to-peak voltage

the difference in voltage between the maximum positive and maximum negative peaks in an alternating current waveform. (distance from minimum to a maximum point)

rms voltage

the effective value of an alternating current voltage, calculated as the square root of the average of the squares of instantaneous voltage values over a cycle, representing the equivalent direct current value.

mains electricity

UK: 230V (rms value) so peak-to-peak voltage is 650V

transformers

Devices that transfer electrical energy between circuits through electromagnetic induction, often used to increase or decrease voltage levels. They change the size of the voltage for an alternating current.

what kind of material is needed for the core of a transformer and why?

The core of a transformer needs to be made of ferromagnetic (magnetically soft) material, such as iron, because it enhances the magnetic field, allowing for efficient electromagnetic induction and minimizing energy losses.

describe how a transformer works

A transformer works by using electromagnetic induction, where an alternating current in the primary coil generates a changing magnetic field in the core, inducing a voltage in the secondary coil. The voltage transformation ratio depends on the number of turns of wire in each coil.

step down transformers

Transformers designed to reduce voltage from a higher level to a lower level while increasing current. less turns in the secondary coil

step up transformers

Transformers designed to increase voltage from a lower level to a higher level while decreasing current, having more turns in the secondary coil.

sources of inefficiency in a transformer core

are often due to hysteresis losses, eddy current losses, and copper losses, which result in energy being converted to heat rather than efficiently transformed.

eddy currents

looping currents induced by the changing flux, which induces an emf in the core. they create a magnetic field that acts against the field that induced them, reducing the field strength they also dissipate energy by generating heat
due to lenz’s law, they oppose the field that produced them, reducing the field’s flux density and they generate heat, which causes energy to be lost

how can the effect of eddy currents be reduced

• laminated cores, which increase resistance to the flow of these currents

• use a metal with a higher resistivity that minimizes their formation.

how can a transformer be made more efficient?

• magnetically soft core - easily magnetises and demagnetises

• low resistivity copper wires in coils - thick and larger diameter

• coils as close together

why are transformers important in the national grid?

Transformers are essential in the national grid because they efficiently step up (increase) or step down (decrease) voltage levels, allowing for the transmission of electricity over long distances with minimal energy loss and ensuring that power is delivered at safe and usable voltage levels to consumers.