2. Electricity

Question 0

What are some uses of electrostatic charges?

E.g. Photocopiers and inkjet printers

Answer 0

There are some events in which having two objects of opposite charge is very useful.

An example of this is in photocopiers and inkjet printers where the ink is given a charge, and the parts of the paper where its wanted is given the opposite charge, so that the ink is automatically attracted to the right parts of the paper.

Question 1

Describe an experiment to measure the speed of sound in air

Answer 1

Measure the distance between two places, have a sound made in one place, as soon as you see the sound has been made start a stop watch, as soon as you hear the sound made stop the stopwatch.

Distance= speed/ time

Question 2

What are the potential dangers of electrostatic charges?

E.g. In fuelling aircrafts and tankers

Answer 2

When a large electrostatic charge builds up it can create a spark. When refuelling vehicles the fuel rubbing along the pipe can cause an electrostatic charge, if this sparks if could ignite the fuel causing a fire or explosion. (This can be avoided if the charge is brought to earth by a wire attached to the plain or tanker)

Question 3

Explain electrostatic phenomena in terms of the movement of electrons

Answer 3

Electrostatic phenomena is an event where static electricity has a specific effect: for example a static shock. Electrons move from one material to another, the material with a negative charge will then look for some way to earth its charge: like clouds through lightening or a car through your hand and body.

Question 4

Explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

Answer 4

If two materials are rubbed along each other one will gain electrons from the other.

The one that has gained electrons has a negative charge. The one that has lost electrons will have a positive charge. The charges are electrostatic because they are not flowing.

Question 5

Describe experiments to investigate how insulating materials can be charged by friction

Answer 5

Get a polyethene rod and rip up some small pieces of paper; the rod will have no effect on the paper.
Rub the polyethene rod with a cloth, now the rod will attract the pieces of paper, this is because it now has a charge they are attracted to.

Question 6

Identify common materials which are electrical conductors or insulators, including metals and plastics

Answer 6

Electrical conductors are materials that allow a current to pass through them.

To do this they need to have ‘free’ electrons, because current is a flow of electrons. Metals have free electrons because of the way they are bonded (atoms and electrons within a lattice) this means they are good electrical conductors.

Plastics are polymers which are bonded in a way that means electrons aren’t free and so can’t move. No flow of electrons means no electric current so they are insulators.

Conductors:

• copper
• aluminium
• water (dirty)

Insulators:

• oil
• plastic
• air
• water(pure)

Question 7

What is voltage?

What is a ‘volt’?

Answer 7

Voltage is the energy transferred per unit of charge passed.

The unit ‘volt’ is a joule per coulomb.

Question 8

What is electric current?

Answer 8

Electric current is a flow of electrons.

Question 9

What equation links charge, current, and time?

Charge=

Answer 9

Charge = current x time

Question 10

What is ‘current’?

Answer 10

Current is the rate at which charge is flowing through a circuit.

Question 11

What equation links voltage, current, and resistance?

Voltage=

Answer 11

Voltage = current x resistance

Question 12

How can lamps and LEDs be used to indicate the presence of a current in a circuit?

Answer 12

For an LED to light up there must be a current in a circuit. If a LED is in a circuit but not emitting light then there must be no current. If an LED is illuminated then it will have a current flowing through it.
By this we know that if the LED in our circuit is shining then there is a current, if it isn’t then we don’t.

Question 13

What is an LDR?

Describe the qualitative variation of resistance of LDRs with illumination and of Thermistors with temperature

Answer 13

An LDR is a light dependent resistor. Its resistance changes with the intensity of light: the brighter it is the less resistance; the less light the more resistance.

Thermistors are temperature dependent resistors. In hot conditions there will be less resistance where as in the cold the resistance is high.

Question 14

Describe the qualitative effect of changing resistance on the current in a circuit

Answer 14

Increasing the resistance will decrease the current. This can be achieved by adding more components or ones with higher resistance.

Decreasing the resistance will increase the current. This can happen if components are removed or replaced by those with lower resistance.

Question 15

describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

Answer 15

If you increase the resistance, the current will decrease. Resistors, metal filament lamps and diodes all create resistance in a circuit and so will decrease the current.
This can be investigated using an ammeter and measuring the current with and without these components, or with different voltage levels (measured by voltmeter.)

Question 16

understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

Answer 16

The current in a series circuit is the same through out all parts of the circuit. It is worked out using the equation I= V/R. ( current = voltage / resistance). So its the total of the voltages received by the components divided by the total of all the components resistances.

Question 17

explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Answer 17

In a series circuit everything is connected on one line. This means that the voltage is shared out between every component: this makes it useful for supplying low power things like fairy lights.

In a parallel circuit different components are connected separately to the supply. This means that of one component breaks the others can continue being powered as the whole circuit is still functioning, this makes it practical to use. It is also good for charging higher power things as the potential difference is equal all over a parallel circuit so each component receives the full voltage.

Question 18

understand the difference between mains electricity being alternating current (a.c) and direct current (d.c.) supplied by a cell or battery.

Answer 18

Direct current: flows in one direction only. It is supplied by cells and batteries. It comes out as a straight line on an oscilloscope.

Alternating current: changes from one direction to another rapidly. Mains electricity is alternating.

Question 19

What equation links energy transferred, current, voltage, and time?

Answer 19

Energy transferred = current x voltage x time

This is the same thing as saying ‘power x time’

Question 20

What equation links power, current, and voltage?

Power=

Answer 20

Power = current x voltage

Question 21

What does a current in a resistor result in?

(Energy transfer, temperature)

How can this be used in a variety of domestic contexts?

Answer 21

As a resistor slows down the movement of electrons, the kinetic energy that was moving them is converted into heat energy. This can be used, for example, in hair dryers or heaters.

Question 22

In domestic appliances, what are the uses of…

Insulation?
Double insulation?
Earthing? (Earth wire)
Fuses?
Circuit breakers?

Answer 22

Insulation is covering a live wire with a material that won’t conduct the electricity.

Double insulation is a precaution that makes sure the live wire cannot touch the casing (so no shock can be conducted) usually by putting extra insulation round that wire. Double insulation can also mean that the casing of an object is plastic so even if the wire touches it, it wont conduct.

An earth wire is touching the case so that if a current is in the case, it will be directed through the earth wire, this will then take the current to the earth. Additionally the surge of electricity in the wire may break the fuse.

Fuses are sections of wire in the circuit that melt if too high a current goes through them. They come with different maximum currents.

Circuit breakers have an electromagnet that is activated if the current goes above a certain limit. the electromagnet pulls an iron switch towards it, this opens the switch and breaks the circuit.

Question 23

What are the hazards of…

Frayed cables?
Long cables?
Damaged plugs?
Water around sockets ?
Pushing metal objects into sockets?

Answer 23

frayed cables: the insulation has worn down exposing live wires, electricity can be conducted from these.

Longer cables: a higher risk of being damaged and there is more resistance with longer wires making them more at risk of over heating.

Damaged plugs: create a risk that some of the safety features may be broken.

Water conducts electricity and can cause energy from the circuit to flow trough it creating a fire and electrocution risk. Metal objects in sockets have the same dangers.