Electromagnetic effects
Exam style questions from www.s-cool.co.uk

World strongest magnet from youtube.com

4.6.1 Electromagnetic induction
• Show understanding that a conductor moving across a magnetic field or a changing magnetic field linking with a conductor can induce an e.m.f. in the conductor
• Describe an experiment to demonstrate electromagnetic induction
Connect a solenoid with an ammeter. Move a magnet in and out of the solenoid. Observe the ammeter reading when the magnet moving in and out of the solenoid. A current flowing in both directions can be observed. This means that there is an electromotive force [e.m.f] induced. When you move the magnet in one direction, the ammeter presents a positive value. It also shows a negative value when you move the magnet in the reverse direction. There is no voltage induced when the magnet remains still. To have an induced e.m.f, a coil must cut through the magnetic field lines or a magnet must cut through the magnetic field lines of a coil.

E.M.F [Electromotive force]:
It provides electrical force to charges to drive the circuit. It is measured in V[volts]. A voltage generated by battery or by a magnetic force.

Dynamo Effect:
The induced e.m.f as a result of a conductor moving in and out of a magnetic field. Voltage is generated whenever a wire is moved in or out of a magnetic field.

• State the factors affecting the magnitude of an induced e.m.f.
The magnitude of the induced e.m.f depending on
  • The rate/speed in which the magnet moves
  • The strength of the magnet
  • The number of coils of the solenoid

Supplement
• Show understanding that the direction of an induced e.m.f. opposes the change causing it
Lenz's law.jpg
Image from www.physchem.co.za

Faraday's Electromagnetic Lab** from Phet
external image faraday-screenshot.png
  • Predict the direction of the magnetic field for different locations around a bar magnet and an electromagnet.
  • Compare and contrast bar magnets and electromagnets.
  • Identify the characteristics of electromagnets that are variable and what effects each variable has on the magnetic field's strength and direction.
  • Relate magnetic field strength to distance quantitatively and qualitatively.
  • Compare and contrast how both a light bulb and voltmeter can be used to show characteristics of the induced current.
1. Investigate Faraday's Electromagnet Lab paying attention to what you can change and what tools you can use to make measurements. We will using the Bar Magnet and Electromagnet tabs for this activity and the other tabs later.

2. Read the first four learning goals and design experiments using the simulation that would help you learn thee specific things. You do not have to write the procedures, but be prepared to explain to the teacher or another student your designs. Write a document that gives evidence that you can meet the learning goals. (Include illustrations drawn by hand.)

Challenge
A. Pretend you and your lab partner are designers for the PhET simulations and want to make a simulation for students to investigate gravity fields. Think about what you know about gravity and what kinds of experiments a student might want to do to learn about gravity. You may have to refresh your memory by using the text. Draw a design, by hand, for a gravity simulation. Explain why you included each component and explain at least one experiment that a student could do.

B. Compare and contrast the fields of gravity and magnets qualitatively.
=> To check your writing, each person will meet with a person from a different group. Read each others paragraphs and talk about your reasoning. Write one new paragraph that you both agree is well done.

Supplement
• State and use the relative directions of force, field and induced current
The direction of current in the coil generated in a direction which opposes the movement of the approaching magnetic field.
Direction of induced emf.jpg
Direction of the induced e.m.f from faculty.www.edu
German physicist Heinrich Lenz observed that the direction of the induced current in a conductor is always such as to oppose the motion which produced it. This is called Lenz's Law.

4.6.2 a.c. generator
• Distinguish between direct current (d.c.) and alternating current (a.c.)
D.C.(Direct current):
An electric current flowing in one direction only. / A continuous electric current that flows in one direction only, without substantial variation in magnitude.

A.C.(Alternating current):
An electric current that reverses direction in a circuit at regular intervals. / A continuous electric current that periodically reverses direction, usually sinusoidally.

AC and DC

Supplement
• Describe and explain a rotating-coil generator and the use of slip rings
Sketch a graph of voltage output against time for a simple a.c. generator
• Relate the position of the generator coil to the peaks and zeros of the voltage output

A rotating coil generator.jpg
Image of a rotating-coil a.c. generator from micro.magnet.fsu.edu
While the coil rotating in the magnetic field, the slip rings and brushes allow the coil to rotate freely.
Generators from schoolphysics.co.uk

Questions
1. One design of a byclcle dynamo does not produce a big enough voltage. How could it be increased?
2. Explain why generators need slip rings and brushes? We use slip rings in a.c. generators to have contact between external circuit and a rotating coil to allow the coil rotate freely.
3. What makes the generators in a power station turn around?

Electromagnetic induction worksheet from Mr.Lin

4.6.3 Transformer
• Describe the construction of a basic transformer with a soft iron-core, as used for voltage transformations
• Recall and use the equation (Vp /Vs) = (Np /Ns)
• Understand the terms step-up and step-down

Transformer.jpg
Image from www.polytechnichub.com
(Vp / Vs) = (Np / Ns) where,
Vp: Primary voltage,
Vs: Secondary voltage,
Np: Number of turns in primary coil,
Ns: Number of turns in secondary coil

• Describe the use of the transformer in high-voltage transmission of electricity
• Give the advantages of high-voltage transmission
Transmitting electricity over long distances without wasting energy is difficult. The advantage of transferring electricity at high voltages is to minimize the heat energy loss during the transmission.
To transfer electricity over long distances at a given power, you either need a high voltage and low current or a low voltage and high current, since power is given by the equation: P=VI
The problem with transferring electricity at a high current is that you need a very wide cable to carry the huge amount of current to reduce the resistance of the wire. This makes it more expensive that using a transformer to step-up the voltage, which in turn reduces the current flow. With a low current, the wires can be thinner, making it more cost-beneficial. When the electricity reaches to our household, transformers are used to step-down the high voltage of it.

Supplement
• Describe the principle of operation of a transformer
An A.C. current in the primary coil produces an alternating magnetic field. The change of magnetic field within the core of wire will induced the voltage in the secondary coil.

• Recall and use the equation Vp Ip = Vs Is (for 100% efficiency)
POWERinput = POWERoutput
Primary power supplied is equal to Secondary power supplied

Voltage(V)primary * Current(I)primary = Voltage(V)secondary * Current(I)secondary

Assuming that there is no energy loss to the surroundings and the circuit is 100% efficient.

• Explain why power losses in cables are lower when the voltage is high
The major energy loss happen when current is high.
Due to the flow of electrons in the wire, there is heat energy loss while transferring electrical energy in the circuit . The larger the current flowing through the wire, the bigger the energy loss as heat energy wasted to its surroundings.

4.6.4 The magnetic effect of a current
• Describe the pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids

Mageticfield around a current carrying conductor.jpg
Image from everythingscience
• Describe applications of the magnetic effect of current, including the action of a relay
Supplement
• State that the direction of a magnetic field line at a point is the direction of the force on the N pole of a magnet at that point
A magnetic field pattern of a solenoid resembles that of a bar magnet.
Magneti field around a flat coil.jpg
Magnetic filed around a solenoid.jpg
The magnetic field strength in a solenoid can be increased by:
1) increasing the current,
2) increasing the number of turns per unit length of the solenoid, or
3) placing a soft iron core within the solenoid. The soft iron core concentrates the magnetic field lines, thereby increasing the magnetic field strength.
Electromagnetism from schoolphysics.co.uk
Relay
A device to control the switch of another circuit without any direct electrical contact between them.
Relay.jpg
Supplement
• State the qualitative variation of the strength of the magnetic field over salient parts of the pattern
• Describe the effect on the magnetic field of changing the magnitude and direction of the current
1. Magnetic fields that are in the same direction make the combined fields stronger.
2. Magnetic fields that are in opposite direction make the combined fields weaker.
Force on a current carrying wire in magnetic fields
Force on a current carrying wire in MF.jpg

4.6.5 Force on a current-carrying conductor
• Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing:
(i) the current
(ii) the direction of the field

MIT Physics Demo -- Forces on a Current-Carrying Wire mittechtv Uploaded on 8 Aug 2008

Force on current-carrying conductors experiment

MIT Physics Demo -- Jumping Wire mittechtv Uploaded on 8 Aug 2008

Force on current carrying conductors experiment.jpg

Supplement
• Describe an experiment to show the corresponding force on beams of charged particles
A change in the direction of the magnetic field will cause a change in the deflection of the charged particles, with the direction of the particles staying constant.
Force on charged particles
Force on a charged particles.jpg

• State and use the relative directions of force, field and current
Fleming's left hand rule from penguinphysics
Flemings left hand rule.jpg

4.6.6 d.c. motor
• State that a current-carrying coil in a magnetic field experiences a turning effect and that the effect is increased by:
– increasing the number of turns on the coil
– increasing the current
– increasing the strength of the magnetic field

To increase the turning effect on the wire coil, we can:
1) Increase the number of turns on the wire coil.
2) Increase the current in the coil.
3) Insert a soft-iron cylinder at the centre of the coil of wires and increase the strength of the magnetic field.

• Relate this turning effect to the action of an electric motor including the action of a split-ring commutator
A direct current (DC) motor is a fairly simple electric motor that uses electricity and a magnetic field to produce torque, which turns the motor.
DC Motor
DC Motor.jpg
Mortors and Generators from UNSW

Electromagnetism revision questions from GCSESCIENCE.COM

Electromagnetism summary worksheet from TES
Electromagnetic induction worksheet from Mr.Lin

Magnet quiz

Go visit what2learn.com and play the magnets and electromagnets game.
The motor effect true or false questions.

Are these statements about the motor effect true or false?
T / F
1.
Reversing the magnetic field has no effect on the direction of the force.

2.
A current-carrying wire held parallel to a magnetic field will experience a force.

3.
Reversing the current reverses the direction of the force.

4.
Increasing the strength of the magnet or the current will increase the size of the force.

5.
The length of wire in a magnetic field has no effect on the size of force.

6.
You only need to know the direction of the current to predict the direction of the force.


Melt metal with magnets from youtube.com

Electromagnetic Induction: GCSE Revision Published on 4 Apr 2011

Revision questions from www.gradegorilla.com Electricity, Magnetism , Moter effect