SP11-SP13: Static Electricity, Magnetism and the Motor Effect & Electromagnetic Induction

What can happen if certain insulating materials are rubbed together?

They can become statically electrically charged.

Why can insulators become electrically charged when rubbed together?

Electrons transfer from one material onto the other. The material gaining electrons becomes negatively charged, and the material losing electrons becomes equally positively charged.

What happens when two identically charged objects are brought close together?

They exert a repulsive non-contact force on each other and repel.

What happens when two oppositely charged objects are brought close together?

They exert an attractive, non-contact force in each other and attract.

How does a balloon stick to the wall?

Rubbing a balloon on your hair or clothes transfers electrons to the balloon and it becomes negatively charged. When attached to the wall, it repels the negative charges on the surface of the wall leaving a positive charge on the surface. This is called attraction by induction.

Why do we feel an electric shock when too much static builds up?

When an object builds up an electrostatic charge it needs to be earthed via a conductor. Electrons flow to/from the earth to balance charge. This can also occur if you touch a charged object whilst you are earthed. If earthing occurs through a person/animal it causes an electric shock

Why does lightning occur?

Clouds can build up an electrostatic charge (static electricity) due to friction When this charge becomes large enough the clouds discharge via the air to the earth.

How is static electricity used in electrostatic sprayers?

The spray gun is charged, which charges up the small drops of paint. Each paint drop repels all the others, since they got the same charge, so you get a fine, even spray. The object to be painted is given an opposite charge to the gun, which attracts the fine spray of paint.

How can static electricity be produced when refuelling cars?

As fuel flows out the pipe into an aircraft or tanker, static charge can build up due to friction. When the charge builds up, a spark may be discharged. The spark could start a fire if it reacts with the flammable fuel.

How can static electricity be produced on airplanes?

As planes fly through the air, friction between the air and the plane causes the plane to become charged. This build up of static charge can interfere with communication equipment.

How does earthing prevent the build up of static electricity?

It provides a path with a low resistance for the electrons to flow. This means no charge can build up to give a shock or make a spark. The electrons flow down the conductor to the ground if the charge is negative and flow up the conductor from the ground if the charge is positive.

What is an electric field?

A region in which a charged object will experience a non-contact electrical force.

What does an object that becomes statically charged generate?

An electrical field

How are sparks caused?

When there is a high enough potential difference between a charged object and the earth (or earthed object). A high potential difference causes a strong electrical field between the charged object and the earthed object. The strong electric field causes electrons in the air particles to be removed (ionisation). The air is now much more conductive so a current can flow through it, which is the spark.

Describe the appearance of electric field lines in a parallel field.

Straight lines, which are parallel to each other and point from the positive plate to the negative plate. At the edges, the lines are partially curved.

At which part of a magnet are the magnetic forces strongest?

The poles of the magnet (North pole and South pole)

In what direction does a magnetic field point?

In the direction that a north pole would experience a force if placed in the field. From the north seeking pole to the south seeking pole of a magnet

What is a permanent magnet?

A magnet that produces its own magnetic field

What is an induced magnet?

A material that becomes a magnet when it is placed in a magnetic field

What is the difference between a permanent magnet and an induced magnet?

A permanent magnet produces its own magnetic field. An induced magnetic becomes magnetic when placed in a magnetic field.

Why is the force between a magnet and a magnetic material always attractive?

The south pole of the magnet induces a north pole in the magnetic material, and vice versa.

Give four examples of magnetic materials.

Iron, steel, nickel and cobalt

What are the uses of magnetic materials?

Fridge doors have a permanent magnetic strip to keep it closed. Cranes use induced electromagnets to attract and move magnetic materials e.g. scrap metal in scrap yards. Doorbells use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise. Magnetic separators are used in recycling plants to sort metal items. Maglev trains use magnetic repulsion to make trains float slightly above the track to reduce losses from friction and to propel them along. MRI machines use magnetic fields to create images of the inside of bodies without having to use ionising radiation. Speakers and microphones also use magnets.

What is produced when a current flows around a conducting wire?

A magnetic field. The field is made up of concentric circles perpendicular to the wire, with the wire in the centre.

How does changing the direction of the current affect the direction of the magnetic field?

A change in direction of current leads to a change in direction of the magnetic field.

What determines the strength of magnetic field around a current-carrying wire?

The magnitude of the current through the wire.

What is the motor effect?

When a force is exerted between a magnetic field and a current-carrying wire placed in that field.

What is Fleming's left hand rule?

Thumb - Force (movement). First finger - Magnetic Field. Second finger - current

What factors affect the size of the force on a current-currying wire in a magnetic field?

The magnitude of the current flowing through the wire and the strength of the magnetic field that the wire is placed in.

How does the motor effect work?

When you place a loop of wire at 90° angles in a magnetic field, the wire will experience the full force on both the left and right hand sides. The force always acts in the same direction relative to the magnetic field and the direction of the current in the wire. Therefore, changing the direction of either the magnetic field or current will change the direction of the force.

What equation is used to size of the Force, linking magnetic flux density, current and length?

Force (N) \= Magnetic Flux Density (T) x Current (A) x Length (M)

What is the magnetic flux density?

A measure of how many field (flux) lines there are in a region - shows the strength of the magnetic field.

If the current and the magnetic field are parallel to each other, which direction would the force act in?

No force will act.

How does an electric motor work?

A coil of wire, carrying a current, is placed in a magnetic field. The forces on the two sides perpendicular to the field act in opposite directions. This causes a rotational effect

What is the use of the split-ring commutator

It will swap the contacts every half turn in order to keep the motor turning in the same direction

How can the direction of the motor be reversed?

By swapping the polarity of the d.c supply (reversing the current) or swapping the magnetic poles over (reversing the field).

What is a solenoid?

A coil of current-carrying wire that produces a strong magnetic field

What is a solenoid core made up of?

An added iron core, which increases the strength of the magnetic field. The iron core becomes an induced magnet.

Describe the magnetic field found inside a solenoid

Strong and uniform. Inside the solenoid, there are lots of field lines pointing in the same direction.

Why is the field outside of a solenoid weak?

The fields from each coil overlap and cancel out, making the outside field weak apart from at the ends of the solenoid.

What is an electromagnet?

A magnet with a magnetic field that can be turned on and off using an electrical current.

What is electromagnetic induction?

The induction (creation) of a potential difference (and current if there is a complete circuit) in a wire which is experiencing a change in magnetic field

How can electromagnetic induction be produced?

If an electrical conductor (e.g. a coil of wire) and a magnetic field move relative to each other. It can also be produced if the magnetic field through an electrical conductor changes.

How can you produce an electric current using an electrical conductor and a magnet?

Moving/rotating a magnet in a coil of wire, or moving/rotating a coil of wire in a magnetic field ('cutting' the magnetic field lines). Once a potential difference is induced, if there is a complete circuit, a current will also be induced.

What happens if you keep moving a magnet (or coil) backwards and forwards in a magnetic field?

An alternating current is produced.

What happens when the magnetic field through an electrical conductor changes?

A potential difference is induced.

What factors affect the size of an induced potential difference?

The number of turns on the coil of wire. The strength of the magnetic field. The speed of movement/change of the magnetic field.

What can be said about the direction of the magnetic field produced by an induced current?

The direction of the field is such that it opposes the change that induced the current. This change is either the movement of the conductor or a change to the field.

How does a transformer work?

They have 2 coils of wires, the primary and secondary, joined with an iron core. When an alternating potential difference is applied across the primary coil, it produces an alternating magnetic field. The iron in the core is a magnetic material that is easily magnetised and demagnetised. Because the coil is producing an alternating magnetic field, the magnetisation in the core also alternates. The secondary coil cuts through the changing magnetic field, inducing an alternating potential difference across the ends of the coil

What do transformers use?

They use induction to change the size of the potential difference of an alternating current.

What are step-up transformers?

They 'step up' (increase) the potential difference (more number of turns on the secondary coil).

What are step-down transformers?

They 'step down' (decrease) the potential difference (less number of turns on the secondary coil).

What can be said about the efficiency of a transformer?

It is almost 100% effective

What is the equation for the power of a transformer?

P.D. across Primary Coil (Vp) x Current through Primary Coil (Ip) \= P.D. across Secondary Coil (Vs) x Current through Secondary Coil (Is)

What are two ways a generator produces different types of current?

An alternator produces alternating-current, a dynamo produces direct current.

How does a dynamo generate direct current?

Generators apply a force to rotate a coil in a magnetic field (or a magnet in a coil). As the coil spins, a current is induced in the coil. This current changes direction every half turn. Dynamos have a split-ring commutator which swaps the connection every half turn to keep the current flowing in the same direction.

How does an alternator generate alternating current?

Generators apply a force to rotate a coil in a magnetic field (or a magnet in a coil). As the coil spins, a current is induced in the coil. Alternators have slip-rings and brushes so the contacts don't swap every half turn, which will cause the current to change direction while rotating. This means an alternator produces an alternating potential difference and therefore an alternating current if the coil is part of a complete circuit.

How does a microphone use electromagnetic induction to generate an electrical signal?

It uses the generator affect by converting pressure variations into alternating current. In the device, sound waves produce pressure variations that hit a flexible diaphragm that is attached to a coil of wire. The coil of wire surrounds one pole of a permanent magnet and is surrounded by the other pole. This means that as the diaphragm vibrates (and so the coil moves), a current is generated in the coil.

How does a loudspeaker use electromagnetic induction to generate sound waves?

It uses the motor effect to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound. A cone wrapped in wire is connected to an a.c. power supply and is placed in a permanent magnetic field. The coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil which moves the cone and produces sound. When the current is reversed, the force acts in the opposite direction.

How is the pitch of the sound from a loudspeaker changed?

The frequency of the a.c. current is altered, which creates a different frequency of vibration in the cone.

How does a power station use a turbine to turn a generator?

Fuel is used to heat water to convert it into steam which turns a turbine. The turbine s connected to a powerful magnet inside a generator - a huge cylinder wound with coils of copper wire. As the turbine spins, the magnet spins with it, inducing a large potential difference and alternating current in the coils. The coils are joined together in parallel to produce a single output from the generator.

How does the national grid use transformers to produce a high potential difference and a low current?

The national grid needs to transfer huge amount of energy each second. They can do this by either increasing the voltage or increasing the current. However, increasing the current makes wires heat up, so loads of energy is wasted to thermal energy stores. To reduce these losses, transformers are used to make the national grid more efficient, high-voltage and low-resistant. Step up transformers at power stations boost the potential difference up really high (400,000V) and keep the current low. Step down transformers then bring it back down to safe, usable levels at the consumers' end.