Topic P3- Electricity

Question 1

What is static electricity?

Answer 1

• Static electricity is the result of an imbalance between negative and positive charges in an object.
• These charges can build up on the surface of an object until they find a way to be released or discharged.

Question 2

How can you build-up static electricity?

Answer 2

The rubbing of certain materials against one another can transfer negative charges, or electrons.

Question 3

What happens when two conducting materials are rubbed together? Do they produce static electricity?

Answer 3

• Static charge won’t build up on conductors unless they are isolated
• because as soon as you put too many electrons in one place,
• they repel each other and spread out, reducing or eliminating the effect.
• Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.

Question 4

What happens when two insulating materials are rubbed together? Do they produce static electricity?

Answer 4

• When insulating materials rub against each other, they may become electrically charged .
• Electrons , which are negatively charged, are able to flow
• and may be ‘rubbed off’ one material
• and on to the other.
• The material that gains electrons has a negative static charge.
• The material that loses electrons is left with a positive static charge.

Question 5

Why can’t protons move or flow during the production of static electricity?

Answer 5

Protons do not move because they are tightly bound in the nuclei of atoms.

Question 6

What happens when static charge is discharged?

Answer 6

• When a static charge on an object is discharged, an electric current flows through the air.
• This can cause sparks.
• Lightning is an example of a large amount of static charge being discharged.

Question 7

Explain what causes charged objects to attracted, repelled and what kind of force is used between the objects:

Answer 7

• Electrically charged objects exert a force on one another
• These forces get weaker as the distance increases between the objects
• Objects with opposite electric charges will attract
• Objects with the same electric charges will repel.
• The force between the two oppositely charged objects is called electrostatic attraction.
• The force between the two same charged objects is called electrostatic repulsion.
• It is a non-contact force, as the objects don’t need to be interacting to apply this force.

Question 8

How can you test whether an object is charged?

Answer 8

• Electrically charged objects attract small neutral objects placed near them.
• E.g. if you hold a charged rod above some neutral scraps of paper, the paper will jump towards the rod
• This happens because the rod induces an electric charge in the paper
• If the rod is positively charged, it attracts the electrons in the paper towards it
• If it is negatively charged it repels the electrons.
• (Negative charges in paper repelled from negatively charged rod, which means the parts of the paper near to the rod have a positive charge.)
• This gives the surface of the paper an opposite charge to the rod, so the rod and the paper are attracted together.
• You can also test if the rod is charged by holding it near a stream of water
• the rod will induce a charge in the water, so the stream will be attracted to the rod and. bend towards it.

Question 9

How can you test for a charge present in an object using a gold leaf electroscope?

Answer 9

• If a negatively charged insulator touches the zinc plate, some of its charge is transferred to the electroscope,
• and conducted down to the metal stem and gold leaf.
• This negatively charges both the stem and the gold leaf, which repel each other.
• This makes the gold leaf rise
• If you touch the plate with a positively charged insulator, electrons flow into it from the plate, stem and leaf.
• Again, the stem and leaf will have the same charge and the leaf will rise.

Question 10

What do electric field lines show?

Answer 10

• Electric field lines go from positive to negative.
• They are always at a right angle to the surface of the object
• at the point where they touch the surface.
• The closer the electric field lines are, the stronger the field
• and the stronger the force a charged object in the field experiences.
• For charged spheres (diagrams- look at pg 43), field lines get further apart the further from the sphere you are,
• so the force another charged object feels due to an electric field decreases with distance.

Question 11

When two charged objects interact, how do the electric field lines represent attraction and repulsion?
What force is produced?

Answer 11

• When electric fields around two charged objects interact, a force is produced.
• If the field lines between the charged objects point in the same direction, the field lines ‘join up’ and the objects are attracted to each other
• This happens when the two charges are free to move, the field lines will straighten and shorten as the charges move together.
• When the field lines between the charged objects point in opposite directions, the field lines ‘push against’ each other and the objects repel each other.
• This happens when two charges are the same type.
• *Draw the field lines for:
• repulsion of positively charged spheres
• repulsion of negatively charged spheres
• attraction of a negative and positive charge spheres.

Question 12

What is current, in a circuit?

Answer 12

• Current is the rate of flow of electric charge (electrons) around a circuit
• It will only flow if there is a potential difference across that component
• and if the circuit is complete (closed circuit)
• Current is measured in amperes, A.

Question 13

What is potential difference, in a circuit?

Answer 13

• Potential difference is the driving force that pushes the current round.
• It is the energy transferred (E) per coulomb of charge (Q) that passes between two points in an electrical circuit.
• May be referred to as voltage (same thing)
• It is measured in volts, V.

Question 14

What is resistance, in a circuit?

Answer 14

• Resistance is a measure of how easily charge can flow.

- It is measured in ohms, Ω

Question 15

What is the equation for calculating charge at a given point?

Answer 15

charge (A) = current (C, coulombs) x time (seconds)

Question 16

What is the equation for calculating the energy transferred from a given p.d.?

Answer 16

energy transferred (J) = charge (C) x potential diff. (V)

Question 17

What does an ammeter do?

Answer 17

• The ammeter measures the current (in amps)
• flowing thought the component
• Must be placed in series (in line with) the component
• can be put anywhere in series in the main circuit, but never in parallel

Question 18

What does a voltmeter do?

Answer 18

• The voltmeter measures the potential difference across the component
• Must be placed in parallel with the component under test, NOT the resistor variable or battery
• so it can compare the energy the charge has before and after passing though the component.

Question 19

What is the equation for calculating resistance?

Answer 19

potential difference (V) = current (A) x resistance (Ω)

**you can rearrange this equation to calculate resistance, but then again, always rearrange every physics equation to find the wanted variable!!

Question 20

Explain why increasing the temperature increases the resistance in a circuit:

Answer 20

1) When an electron flows through a resistor, some of its energy is transferred to it thermal energy store of the resistor
- heating it up.
- The thermal energy store of a substance is the kinetic energy store of its particles.
- As the resistor heats up, its particles start to vibrate more.
- With the particles vibrating more and more, it is difficult for the charge-carrying electrons to get through the resistor
- so the current cannot flow as easily and the resistance increases.
- More current means an increases in temperature,
- which means an increase in resistance,
- which means the current decreases again.

-this is why the graph for filament lamps level off at high currents.

Question 21

What are diodes, and what are some of their uses?

Answer 21

• A diode is a device made from a semi-conductor material like silicon
• It lets current flow freely though it in one direction, but not in the opposite direction
• There is high resistance in the reverse direction
• They are useful in electronic circuits like radio receivers
• and can also be used to get direct current from an alternating supply.

Question 22

What is an LDR, and what are some of its useful applications?

Answer 22

• An LDR is a resistor that is dependent on the intensity of light.
• In darkness, the resistance is highest
• as light levels increases, the resistance falls
• so the current through the LDR increases
• They can be used in automatic night-lights, outdoor lighting and burglar detectors.

Question 23

What is a thermistor, and what are some of its uses?

Answer 23

• Thermistor is a temperature dependent resistor.
• in hot conditions the resistance drops
• In cool conditions, the resistance increases.
• In constant conditions, the I-V graphs are curved as they current increases, so the thermistor warms up and the resistance decreases.
• They are used in temperature detecting devices such as thermostats, irons and car engines.

Question 24

What is a series circuit?

Answer 24

• The different components are connected in a line, end to end
• between the +ve and -ve terminals of the power supply
• current has to flow through all of the components to get round the circuit
• so if you remove one component, it can have a big effect on the others.

Question 25

What is a parallel circuit?

Answer 25

• In parallel circuits, each component in separately connected to the +ve and -ve terminals of the supply.
• This means if you remove or disconnect one of them, it will hardly affect the other at all.
• often most sensible way of connecting things, used in cars and household electrics, where you have to be able to switch everything on and off separately.

Question 26

If you want to add more cells to a circuit to create a larger p.d., should you connect them in series or parallel, and why?

Answer 26

• Connect them in series
• connecting several cells in series, all the same way ( + to - ) gives a bigger total p.d.- because each charge in the circuit passes through each cell and gets a ‘push’ from each one.

Question 27

In a series circuit, how is potential difference distributed?

Answer 27

• The total potential difference of the supply is shared between the various components.
• So the p.d.s round a series circuit always adds up to equal the p.d. across the power supply:

V = V1 + V2

-This is because the total energy transferred to the charges in the circuit by the power supply equals the total energy transferred from the charges to the components.

Question 28

In a series circuit, how is current distributed?

Answer 28

• Current is the same everywhere
• The same current flows though all parts of the circuit:
• i1 = i2 = i3
• The size of the current is determined by the total p.d. of the power supply and the total resistance of the circuit, i.e. I = V/R

Question 29

In a series circuit, how is resistance behave and how is it distributed?

Answer 29

• Resistance adds up
• The total resistance is just the sum of the individual resistances:

R = R1 + R2 + R3

• You can treat multiple resistors connected in series as a single resistor with equivalent resistance R
• The resistance of two (or more) resistors in series is bigger than the resistance of just one of the resistors on its own
• because the battery has to push each charge through all of them.
• the bigger the resistance of a component, the bigger its share of the total p.d. because more energy is transferred
• from the charge when moving through a large resistance than a small one.
• If the resistance of one component changes (variable resistor, light-dependent resistor, thermistor) then the potential difference across all the components will change too.

Question 30

In a parallel circuit, how is the potential difference distributed?

Answer 30

• Potential difference is the same across all branches.
• All branches get the full source of p.d. so the p.d. is the same across all branches:

V1 = V2 = V3

-This is because each charge can only pass down one branch of the circuit so it must transfer all the energy supplied to it by the source p.d. to whatever’s on that branch.

Question 31

In a parallel circuit, how is current distributed?

Answer 31

• Current is shared between branches.
• The total current flowing round the circuit equals the total of all the currents through the separate branches.

i = i1 + i2

• You can find the current in a branch using I = V/R
• There are junctions where the current either splits or rejoins.
• The total current going into a junction has to equal the total current leaving.

Question 32

In a parallel circuit, how is resistance distributed?

Answer 32

• The total resistance of a parallel circuit is hard to get a definite answer to,
• but it is always less than that of the branch with the smallest resistance
• The resistance is lower because the charge has more than one branch to take
• only some of the charge will flow along each branch
• a circuit with two resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves
• which means the parallel circuit will have a higher total current.

Question 33

What does the power of a component tell you?

Answer 33

-The power of a component tells you how much energy it transfers per second.

Question 34

What is the formula for calculating energy transferred?

Answer 34

energy transferred (J) = power (W) x time (seconds)

Question 35

What is the formula for calculating power?

Answer 35

power (W) = p.d. (V) x current (A)

Question 36

How can you calculate power with a given resistance?

Answer 36

power (W) = current^2 (A) x resistance (Ω)