Physics - mod 6: electromagnetism

Properties of an electric field

• never intersect

• perpendicular to the surface of the charge

• magnitude of charge and the no. of field lines are proportional.

Uniform electric field formula

E = V.d.

Force in an electric field

Stationary

• F = Eq

• W = Eq*d

Coming in from the side

• KE = W = 1/2mv²

• 1/2mv² = qEd

Charges in mag. field - concept

Any moving charge has a magnetic field associated with it. A moving stream of electrons will experience a force inside a magnetic field.

Why?

= The stream of charged particles has its own magnetic field which interacts with the external magnetic field.

External magnetic field formula

F=bvqsintheta

Charges in magnetic field - pathways

The force on a charged particle in a magnetic field always at right angles to the velocity of the charged particles. This creates circular motion

R=mv/qb

Electric vs. magnetic field

Similarities and differences

Motor effect

A current carrying conductor in the presence of a magnetic field will experience a force

F = BILsintheta

Motor effect - variable changes the size of the force

Voltage

Variable —> changes direction of force of wire in magnetic field

Polarity of magnetic wire + direction of flow of positive charge relative to B.

Force between wires

Ampere + Newton’s third law of force between 2 wires

Ampere = the constant current which if maintained in two straight parallel conductors would produce between those conductor a force equal to 2×10-7Nm-1 length

Newton’s third law = if one wire applies a force to a second wire, the second wire will apply a force that is equal in magnitude and opposite in direction on the first wire.

DC Motor - definition + formulas

A device that transforms electrical potential energy into rotational KE through the application of a DC current

T = F.d

F = B.I.L

T = nIABsintheta

Components of a DC motor

1. Split ring commutator = reverses the direction of current every 180 ensuring continuous turning occurs.

2. Magnet = provides an external magnetic field in which the coil rotates

3. Axle = provides a point of rotation for the armature. Transfers the rotational energy to another appliance

4. Coil = carries current to produce mechanical energy

5. Brush = acts as a conductive connection between SRC, armature, & power supply

6. Battery = provides current

1. Two ways the efficiency of the motor can be improved:

2. Why electromagnets

1. Reduce friction + add more coils

2. To turn motor off + control its strength

Magnetic flux definition and formula

The measure of total magnetic field lines in a set area.

Magnetic flux density definition

Is a measure of how densely packed the flux lines are. (T)

Faraday’s law definition + formula

Faraday determined that the size of the EMF that is generated is proportional to the rate at which flux is cut. For EMF to be generated there must be relative movement between conductor and magnetic field.

Lenz’s law

Emf generated by cutting flux will generate a current flow if a complete circuit is provided. When current is generated it will create a magnetic field that opposes the change.

Faraday’s law says that “cutting flux leads to voltage.” Electrical law says “but voltage only leads

Explain how a voltage is produced when the magnet is dropped into the coil?

When magnetic flux lines are cut by a conductor, the charges in the wire conductor experiences a force.

(Internal interacting with external magnetic field).

Steps in Lenz’s law

1. EMF produced by cutting flux

2. EMF drives current

3. The current flowing creates its own magnetic field. This magnet will oppose the magnetic field.

Torque

Turning effect of a force

Back EMF

A motor has an applied EMF which drives current through coil. As the motor starts to spin the armature starts to cut flux. This means that the motor unintentionally acts as a generator. The EMF generated always opposes the applied EMF, due to Lenz’s law,

Lenz’s law and conservation of energy

Ensure that the induced current’s energy comes from the work done against the opposing force.

Eddy current’s role in magnetic breaking

• Generate their own opposing magnetic field, resisting motion

• Kinetic energy convert to heat, slowing the object.

Magnet falling through copper wire

1. Eddy current: as magnet falls, its changing field induces eddy current in pipes.

2. Opposing force: These magnet create a mag. field that opposes the magnet’s motion

   1. Energy loss: kinetic energy is dissipated as heat, reducing net acceleration S

AC induction motor

An AC induction motor is an electric motor where torque is generated by current induced in the rotor via electromagnetic interaction with the stator’s rotating magnetic field.

• The stator’s AC-powered windings produce a rotating magnetic field.

• This changing field induces a current in the rotor conductors (Faraday’s law).

• The rotor’s current creates its own magnetic field, which opposes the stator’s field (Lenz’s law), producing torque.

Stator =

provides the external mag. field in which the rotor rotates

Rotor

comprises a series of conductors (metal bars) and the rotor rotates about the axis of the motor’s shaft.

• Allows a current to flow from one side to the other side of the cage.

Steps in AC induction motor

1. AC is supplied to the stator - the surrounding electromagnet

2. produces an oscillating mag. field

3. induces a current in the rotor

4. turns the rotor into an electromagnet

5. tries to oppose the stator field being generated (Lenz’s law)

6. stator and rotor push against each other (mag. field)

7. cause the rotor to return

Transformers

Simple devices that allow us to use an input voltage and make it into output voltage that is larger or smaller depending on the need. Ste

Step-up transformer

When the no. of coils in the secondary coil is larger

Step-down transformer

When the no. of turns in secondary is less than primary A

Primary coil - transformer

Receives an input voltage and induces a changing mag. field

Secondary coil - transformer

Produces a voltage based on the turn’s ratio, enabling efficient voltage transformation for electricity distribution

Soft core - transformer

Reduce eddy current loss and provide a low reluctance path for flux current.

Two applications in tansformer

Microwave ovens: step-up transformers increase the voltage to power the magnetron

Doorbell systems: step-down transformers reduce voltage for safe operation of the doorbell circuit.

Why is AC current required for the function of a transformer

AC— constant changing current

• Creates a changing mag. field to induce a voltage in the secondary coil

Linkage between transformers + law of conservation of energy

Transformers uphold the law of conservation of energy by transferring electrical energy from one circuit to another with minimal loss. I

Ideal transformers

Would have no energy loss, meaning the energy output would exactly equal the energy input. However, real-world transformers always have some losses, making them less than 100% efficient.

Eddy current

Are spiral shaped currents that form within electrical conductors when they are placed inside a changing magnetic field.

Limitations of transformers