Electricity & Magnetism

Question 1

What is charge?

Answer 1

charge – the flow of electrons.

Question 2

What is static electricity?

Answer 2

a concentration/excess of positive or negative charge which is stationery.

Question 3

What is current electricity?

Answer 3

The flow of electrical charges along a conducting pathway (circuit).

Question 4

What are conductors?

Answer 4

Electrical conductors are substances that allow charges to flow through easily. Metals are good conductors as valence electrons are loosely held and freely move from atom to atom

Question 5

What are insulators?

Answer 5

Electrical insulators are substances that do not allow charge to pass through. e.g: plastic.

Question 6

What is charging by friction?

Answer 6

charging by friction is when a rubbing motion creates an OPPOSITE charge. When two materials are rubbed together, electrons can get transferred, causing both objects to become oppositely charged.

Question 7

What is charging by contact?

Answer 7

If a neutral object is touched by a charged object, charging of the neutral object will take place.

e.g: If an an object is negatively charged and touches a neutral object, electrons will pass on to the neutral object (unlike charges attract). Result is that the neutral object acquires a negative charge and the initially charged object becomes less negative.

Question 8

What is charging by induction?

Answer 8

When a charged object is near a neutral object, it causes the object’s electrons to move, making some positive and negative charge separate in the neutral object. The object is still neutral overall.

Question 9

What is electrical earthing?

Answer 9

Grounding/Earthingis the process ofremovingthe excess charge [static electricity] on an object by the transfer of electrons between it and another object.

Question 10

What is an electrical discharge through the air?

Answer 10

When large amounts of positive and negative charge have been separated (causing very high voltages to develop) then a sudden discharge through the air (an electrical spark) can occur.

Question 11

Explain the differences between charging an insulator and charging a conductor.

Answer 11

When insulators become charged, the excess/shortage of electrons stays in the area where the charging occurred.

When conductors become charged the excess/shortage of electrons is distributed all over the outside surface of the conductor. If conductor has any sharp points/edges, then the greatest concentration of charge will occur there.

Question 12

Describe the interaction of objects with like charges and objects with unlike charges.

Answer 12

When two charged objects are placed close together, they exert a force on each other.
Direction of force is given by: like charges repel, unlike charges attract

Question 13

Describe a Van de Graaf generator and some experiments you can do with it.

Answer 13

If somebody touches the dome of the van de graaf generator while standing on an insulator (such as a polystyrene slab) electrons from the person will be transferred from the person to the dome. The person’s hair will become positively charged.

• Electric motor rotates bottom roller
• Causes rubber belt to rub past the metal comb that is connected to Earth
• Friction between rubber belt and comb brushes electrons from belt comb
• Leaves belt positively charged
• At the top of the generator a metal comb connected to the dome brushes against the belt
• Electrons pass from the dome of the generator onto the positively charged belt and are carried away
• Dome becomes positively charged

Question 14

Describe an electroscope and some experiments you can do with it.

Answer 14

Used to detect the type and quantity of charge on an object.

Consists of insulated metal rod and thin leaf of metal foil placed inside glass jar or glass fronted container.

How it works:

If the rod is negatively charged…

1. Rod touches cap of neutral electroscope
2. Electrons travel from the rod stem to thin leaf of foil
3. Foil and stem now both negatively charged, so they will repel each other, Foil rises.
4. The height to which the leaf rises indicates the quantity of charge transferred from the rod

If the rod is positively charged…

1. Rod touches cap of neutral electroscope
2. Attracts electrons from the electroscope, leaving stem and foil positively charged

Question 15

What is a magnetic field?

Answer 15

Magnetic field is a region in space where magnetic forces can be detected.

The churning motion of molten iron in the Earth’s core generates an Electromagnetic field.
Substances containing iron, cobalt or nickel can exhibit magnetic properties, as they are the only known elements to generate a magnetic field.

Strong field between unlike poles
Between like poles net magnetic force = 0 (shown
by no lines between the poles)

The magnetic field strength can be shown by
the proximity of lines of force.

• lines are close together – strong magnetic field

• lines far apart – weak magnetic field
Field is strongest near the poles and weaker towards the centre of the bar magnet.

Question 16

Describe the magnetic field pattern around permanent magnets.

Answer 16

Any magnet has 2 poles = north and south. The fields go from North to South.

Question 17

Describe the magnetic field pattern around the Earth.

Answer 17

Compasses – which are magnetic North poles – are attracted to magnetic South poles.
The North (seeking) pole of the Earth is actually a (magnetic) South pole. The South (seeking) pole of the Earth is actually a (magnetic) North pole.  If two magnetic fields are of equal strength where a compass is located there will be equal forces on a compass needle. The forces are in opposite directions so the net force on the needle is zero; needle points northward due to Earth’s magnetic field.

Question 18

Describe the magnetic field pattern around coils.

Answer 18

• Solenoid is a coil of wire
• Coiling a length of wire has the effect of concentrating the magnetic field so a strong magnetic field exists around the individual coils
• Magnetic field around a solenoid is similar to the field around a bar magnet
  Direction of the magnetic field around a solenoid can be determined by using the right hand solenoid rule

Question 19

State the symbol for magnetic field strength.

Answer 19

Teslas (T).

Question 20

What is a solenoid?

Answer 20

A solenoid is an electromagnet. An electromagnet is something that uses electricity to create a moving electric charge (magnetic field). When electric current flows through wire, magentic field is created around the wire. By winding the wire into a coil and around an iron core, we can strengthen the magentic field. ELECTROMAGNETS are made from coils like this. Magnetic field is same as bar magent, however can be reversed by changing the direction of the current (by turning the battery around), magnetism can be turned on and off by turning the current on and off.

To increase the strength of an electromagnet:
• Use more coils of wires
• Increase voltage/current supplied to the coil
• Use a soft iron core (soft iron does not retain
its magnetism once the circuit is broken).

Question 21

Describe uses for an electromagnet (solenoid)

Answer 21

1. The Reed Switch
   - When a current flows in the coil, the reeds (thin strips of metal) are magnetised and attract each other, closing the switch in the process
2. The Electric Bell
   - When the button is pushed, the electromagnet magnetises the soft iron cores, which attracts the armature.
   - The armature is connected to the hammer which rings the bell.
   - When the bell is rung the circuit is broken and the electromagnet loses its magnetism.
   - The armature returns to its original position, the spring presses against the contact screw and the circuit is closed again
   This process repeats for as long as the button is pushed

Question 22

Solve problems using the equation B = kI/d

Answer 22

The strength of the magnetic field produced by a
current-carrying straight conductor can be
calculated by:

The relationship: B = kI/d

where B = magenetic field strength [measured in teslas (T)], k = constant (in T/mA), I = current (A), d = distance from wire (m)

Question 23

What is electric current?

Answer 23

• the amount of electrons passing a point per second.
• coulombs per second, AMPS
• measured in Amperes/Amps (A)

Question 24

What is voltage?

Answer 24

• also called potential difference
• measured in Volts (V)
• voltage is the energy released per coulomb
• joules per coulomb
• to make a current flow through a conductor, there must be a voltage across it!!

Question 25

What is resistance?

Answer 25

• the hindering of charge flow, depends on length (double the length of object double resistance), surface area of object cross section (thin wire = higher restistance than thick wire), temperature and others.
• measured in Ohms
• is volts per amp

Question 26

What is power?

Answer 26

• the rate at which energy is transformed from one form to another
• joules per second
• measured in Watts (W)

Question 27

What is the direction of conventional current?

Answer 27

Conventional Current: flows from negative to positive. It flows from the negative (flat) end of the battery to the positive end, (part with the bump).

Question 28

State the equations for electric circuits.

Answer 28

V = IR, P = IV, P = E/t, R = R1 + R2 + … (series), 1/R = 1/R1 + 1/R2 (parallel)

Question 29

Advantage of series and parallel circuits

Answer 29

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Question 30

Lightning Conductors

Answer 30

Tall buildings use lightning conductors to protect the building from lightning damage. It is a metal rod attached to the highest point on a building, connected to the ground by a thick metal strip. When the lightning strikes, the current flows through the metal strip – the easiest and shortest path - to the ground rather than through the building, protecting the building.