4.2 Energy, power and resistance

4.2.1 Circuit symbols

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) circuit symbols

(b) circuit diagrams using these symbols.

4.2.2 E.m.f. and p.d

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) potential difference (p.d.); the unit volt

(b) electromotive force (e.m.f.) of a source such as a cell or a power supply

(c) distinction between e.m.f. and p.d. in terms of energy transfer

(d) energy transfer; W =VQ

(e) energy transfer   for electrons and other charged particles.

There are two v's in this equation, which is obviously confusing.  The first one is the voltage, so eV is a charge times a voltage and therefore, from the equation above, an energy.  The second v is obviously velocity, so the KE of the electron is equal to the amount of electrical energy gained by the electron accelerating through the potential difference.

4.2.3 Resistance

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) resistance; V=IR ; the unit ohm

b) Ohm’s law

(c) (i) I–V characteristics of resistor, filament lamp, thermistor, diode and light-emitting diode (LED)

(ii) techniques and procedures used to investigate the electrical characteristics for a range of ohmic and non-ohmic components.

(d) light-dependent resistor (LDR); variation of resistance with light intensity.

4.2.4 Resistivity

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) (i) resistivity of a material; the equation

(ii) techniques and procedures used to determine the resistivity of a metal.

(b) the variation of resistivity of metals and semiconductors with temperature

You don't need to know very much about this.  You should recall that the resistance of a metal increases with temperature as the ions vibrate more and make it harder for the electrons to get through.  The resistance of semiconductors decreases with temperature because more charge carriers are available as the temperature rises.  If the resistance increases, then the resistivity much also increase.

(c) negative temperature coefficient (NTC) thermistor; variation of resistance with temperature.

4.2.5  Power

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) the equations P = VI, P = I R2 and P = V2/R

(b) energy transfer; W = VI t

This is just a combination of P = E/t, which is just the definition of power, with P=IV.

(c) the kilowatt-hour (kW h) as a unit of energy; calculating the cost of energy.