Section 6: Electricity

Question 1

What is Ohm’s Law?

Answer 1

Provided the physical conditions, such as temperature, remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.

Question 2

Define Current.

Answer 2

The rate of flow of charge.

Question 3

What is meant by the electromotive force (e.m.f.), ε

Answer 3

The amount of electrical energy the battery produces and transfers to each coulomb of charge.

Question 4

What is a potentiometer and why is it useful?

Answer 4

A potentiometer has a variable resistor replacing R₁ and R₂ of the potential divider. You move a slider or turn a knob to adjust the relative sizes of R₁ and R₂.

This is useful for when you want to vary voltage continuously, like volume of a speaker.

Question 5

What does the resistance-temperature graph look like for a thermistor?

Answer 5

Question 6

What are the three main types of semiconductors?

Answer 6

Thermistors, LEDs, and LDRs.

Question 7

What is potential difference between two points?

Answer 7

The work done in moving a unit charge between two points.

Question 8

What is an ideal voltmeter and ammeter ?

Answer 8

Voltmeters are assumed to have an infinite resistance (so no current flows through them) and ammeters are assume to have no resistance (and so will have no potential difference across them)

Question 9

Describe how to work out current, voltage and resistance in parallel circuits.

Answer 9

1) Current splits at each junction so I = I₁ + I₂ … + I_n

2) There is the same p.d. across all components as the p.d. across each branch is equal to the e.m.f of the circuit

3) 1/R_t = 1/R₁ + 1/R₂ … + 1/R_n

Question 10

How does an increase in temperature affect the resistance of metal conductors?

Answer 10

Heating a metal makes it’s ions vibrate more, this means that they are more likely to obstruct the path of moving electrons, and hinder the flow of charge. This means that the hotter a metal is, the higher its resistance.

Question 11

What is the I-V characteristic for an ohmic conductor?

Answer 11

Straight line graph, through the origin.

Question 12

What is a superconductor?

Answer 12

When electricity flows through a material, it heats up, and some of the electricity energy is wasted as heat. You can lower the resistivity of materials like metals by cooling them down. If you cool some materials down to below a critical temperature called the ‘transitional temperature’ their resistivity disappears entirely and they become a superconductor. A material with no resistance.

Question 13

What is the I-V characteristic for a Filament Lamp?

Answer 13

A curve that starts steep but gets shallower as the voltage rises.

Question 14

What are potential dividers used for?

Answer 14

Supplying a varying p.d supply, or just one that is at a lower p.d. then the power supply.

Question 15

What are the uses of superconductors?

Answer 15

1) Power cables that transmit electricity without any loss of power.

2) Really strong electromagnets that have lots of applications, - Maglev trains.

3) Electronic circuits that work really fast with minimal energy loss, because there’s no resistance to slow the current down.

Question 16

What is the I-V characteristic for a semiconductor diode?

Answer 16

A curve that requires a voltage of around 0.6V in the forward bias/direction before they will conduct - called the threshold voltage. In reverse bias, the resistance of the diode is very high and almost no current flows.

Question 17

How is charge conserved in a dc circuit?

Answer 17

As charge flows through a circuit, it doesn’t get used up or lost. This means that whatever charge flows into a junction will flow out again. Current = rate of flow of charge , it follows that current entering a junction = current leaving the junction. (K1L)

Question 18

What is a thermistor and how does it work?

Answer 18

The resistance of the component decreases as the temperature increases. Warming the thermistor gives more electrons enough energy to escape from their atoms. Therefore more charge carriers are available, resistance is lowered. This property makes them good temperature sensors.

Question 19

Why and How are variable resistors, thermistors and LDRs used in a potential divider?

Answer 19

Replacing one of the fixed resistors with a variable resistor would allow you to vary V_out.

A LDR or thermistor can be used to supply an output voltage that varies with the light level or the temperature.

Question 20

What is meant by the terminal p.d.?

Answer 20

The potential difference across the load resistance. It is the energy transferred when one coloumb of charge flows through the external circuit.

Question 21

What is a semiconductor?

Answer 21

A material that isn’t as good at conducting electricity as metals, because they have far fewer charge carriers available. However, if energy is supplied to them, more charge carriers can be released and the resistivity of the material decreases. This makes them good at detecting changes in their environment.

Question 22

Describe how to work out voltage , current and resistance in a series circuit.

Answer 22

1) There will be the same current at all points of the circuit. 2) The e.m.f is split between the components by K2L e = V1 + V2 … + Vn. 3) The voltage splits proportionally to the resistance as V=IR 4) Rt = R1 + R2 … + Rn

Question 23

How is energy conserved in a dc circuit?

Answer 23

Energy is always conserved. In electric circuits, energy is transferred around the circuit. Energy transferred to a charge is e.m.f, and energy transferred from a charge is p.d. In a closed loop, these two quantities must be equal to conserve energy (which must always happen). (K2L)

Question 24

What is a potential divider?

Answer 24

A circuit with a voltage source and a couple of resistors is series. The potential difference across the voltage source is split across the resistors in the ratio of the resistances.