6. Electricity

Question 1

What is a coulomb?

Answer 1

A coulomb is a unit of charge, and it is the amount of charge that passes in one second when the current is one amp.

Question 2

What is potential difference?

Answer 2

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

Question 3

Which equation links voltage, charge and work done?

Answer 3

V = W / Q

Question 4

What is Ohm’s law?

Answer 4

Current is directly proportional to potential difference.

Question 5

What are the characteristics of ideal voltmeters and ammeters?

Answer 5

An ideal voltmeter has infinite resistance so that no current flows through it and an ideal ammeter has no resistance so that no potential difference flows through it.

Question 6

What are the characteristics and graphs of ohmic conductors, filament lamps and diodes?

Answer 6

Ohmic conductors: I = kV
Filament lamps: Like a ln(x) graph in the top right and bottom left quadrants of the graph, due to heat energy heating the wire causing elections to vibrate more and increasing the resistance.
Diodes: Like an x^2 graph but only in the top right quadrant, although a small amount of current can flow backwards so there is a line with a shallow gradient just under the x axis.

Question 7

What does the resistivity of a wire depend on?

Answer 7

• Resistance.
• Cross sectional area.
• Length.

Question 8

What is the definition of resistivity?

Answer 8

The resistance of a 1m length with a 1m^2 cross sectional area.

Question 9

What is a volt?

Answer 9

The amount of electrical energy transferred per unit charge.

Question 10

What is an amp?

Answer 10

The rate of flow of electric charge. 1 amp = 1 coulomb of charge per second.

Question 11

What are semiconductors?

Answer 11

Semiconductors are materials that have few charge carriers, but if energy is supplied to the, more charge carriers are released and their resistivity decreases.
An example is a thermistor.

Question 12

If temperature increases, what happens to an NTC thermistor (negative temperature coefficient)?

Answer 12

Resistance decreases.

Question 13

What is a superconductor?

Answer 13

When a material is lowered to a critical temperature, its resistivity is 0, becoming a superconductor. This is often difficult to do as the critical temperatures for these things are so low.

Question 14

What can superconductors be used for?

Answer 14

• Power cables.
• Electromagnets for e.g. Maglev trains.
• Really fast electronic circuits.

Question 15

What are the equations for energy that you get from substituting into power equations (3)?

Answer 15

E = IVt
E = V^2t / R
E = I^2Rt

Question 16

What is internal resistance?

Answer 16

The resistance caused by electrons colliding with atoms and losing energy.

Question 17

What is load/external resistance

Answer 17

The resistance of all the components in the external circuit.

Question 18

What is terminal p.d.?

Answer 18

The potential difference measured at the terminals of the cell.

Question 19

What is e.m.f?

Answer 19

E.m.f (electro-motive force) is the internal voltage of the power supply.

Question 20

What is the difference between terminal p.d. and e.m.f.?

Answer 20

E.m.f. Is internal whereas some of the voltage is lost due to internal resistance so the output p.d. Is measured from the terminals of the cell.

Question 21

What are lost volts?

Answer 21

The volts lost due to internal resistance.
E.m.f. - terminal p.d.

Question 22

What is little r in the equation for e.m.f?

Answer 22

Internal resistance.

Question 23

What is Kirchhoff’s first law?

Answer 23

The total current entering a junction = the total current leaving it.

Question 24

What is Kirchhoff’s second law?

Answer 24

The total e.m.f. Around a series circuit = the sum of the p.d.s across each component.

ε = ∑IR

Question 25

What is the formula for total e.m.f. in a circuit?

Answer 25

ε(total) = ε(1) + ε(2) + ε(3) etc..

Question 26

What is the formula for potential dividers?

Answer 26

V(out) = (R(2) / (R(1) + R(2))) V(source) Where V(out) is the potential difference across R(2).

Question 27

What is a potentiometer?

Answer 27

When in a potential divider, there aren’t two resistors, instead there is one variable resistor. This can be useful in e.g. car stereos.

Question 29

What is the formula linking I, Q and t?

Answer 29

I = ∆Q / ∆t Q = It

Question 30

How could the resistivity of a wire be determined experimentally?

Answer 30

By setting up a circuit with a variable resistor, an ammeter, and a voltmeter in parallel around the circuit, recording values for an IV graph, where the y-intercept is the e.m.f.