Electricity (M4)

Question 1

Potential divider equation

Answer 1

V out = R2/(R1+R2) X V in

Question 2

emf equation

Answer 2

E = V + Ir
E = I(R+r)

Question 3

Power equations

Answer 3

P=VI
P=IIR
P=VV/R

Question 4

Potential difference

Answer 4

The energy transferred by charge carriers from electrical energy to other forms (eg heat or light) per unit charge

Question 5

Volt

Answer 5

Unit of potential difference and emf.
The energy transferred per unit charge, whether energy Is being transferred to or from the charge
V=JC^-1

Question 6

emf

Answer 6

When energy is transferred from chemical energy (or other forms) to electrical energy per unit charge.

Question 7

Internal resistance

Answer 7

The resistance of a source of emf which causes a loss in energy/voltage as charge passes through the source.
Symbol: r
Unit: ohm

Question 8

Terminal potential difference

Answer 8

The p.d. across a power source. When there is no current, this is equal to the emf, but if there is a current in the source, it is equal to the emf minus the lost volts.

Question 9

Lost volts

Answer 9

When the terminal p.d. is less than the actual emf due to some energy being ‘lost’ (transferred to heat) when there is a current in the power source and not all the energy transferred tonthe charge is available to the circuit.

Question 10

Kilowatt-hour

Answer 10

A derived unit of energy, most often associated with paying for electrical energy.
Calculated by multiplying the power in kW by time in hours.
Symbol:kWh
1kWh = 3.6MJ

Question 11

Kirchhoff’s second law

Answer 11

In any circuit, the sum of electromotive forces is equal to the sum of the p.d.s around a closed loop.

Question 12

Resistors in series

Answer 12

R = R1 + R2 + …

Question 13

Resistors in parallel

Answer 13

1/R = 1/R1 + 1/R2 + …

Question 14

Ohm’s law

Answer 14

For a conductor at a constant temperature, the current is directly proportional to the potential difference across it.

Question 15

Electrical resistance

Answer 15

A property calculated by dividing the potential difference across it by the current in it.
Symbol: R
Unit: ohm
V = IR

Question 16

Resistivity

Answer 16

A property of a material defined as the product of the resistance of a component made of the material and its cross sectional-area divided by its length.
Symbol: rho
Unit: ohm meter

Question 17

Electric current

Answer 17

The rate of flow of charge, measured in amperes (A). Normally a flow of electrons in metals or a flow of ions in electrolytes.
Symbol: I
Q = It

Question 18

Coulomb

Answer 18

Unit of electrical charge. 1 C passes through a point in 1 second when there is an electric current of 1 A.
C = As

Question 19

Ampère

Answer 19

The base SI unit of electric current. It is defined as the current flowing in 2 parallel wires in a vacuum 1m apart such that there is an attractive force of 2x10^-17 N per metre length of wire between them.

Question 20

Charge of an electron

Answer 20

Negative elementary charge
-1.6x10^-19

Question 21

Kirchhoff’s first law

Answer 21

At any point in an electrical circuit, the sum of currents into that point is equal to the sum of electrical currents out of that point.
Electrical charge is conserved.

Question 22

Drift velocity equation

Answer 22

I = Anev

Question 23

Conductor

Answer 23

Material with very high number density
Order of 10^28 per m^3

Eg metals
Number density = number of free electrons per cubic metre

Question 24

Semiconductor

Answer 24

Material with number density around 10^17.
Eg silicon and germanium.

Question 25

Insulator

Answer 25

Material with very low number density.
Eg wax and rubber