# Current of Electricity

### Questions Worth Pondering

1. How is electric current different from electric charge?
2. How does an ammeter work?
3. Can we connect the ammeter in parallel with the circuit?
4. Are e.m.f and p.d the same?
5. Can we connect the voltmeter in series with the circuit?
6. Should a heating element have high or low resistance? why?
7. How does a variable resistor (a.k.a rheostat) work?
8. Does resistivity of a material change with temperature?

### Exam Tips

1. Although the ratio of V/I for non-ohmic conductor is not constant, we still can use this V/I to find the resistance of the conductor at a particular current/voltage. You simply can’t generalise. That’s all.
2. You need to know when to use e.m.f = W/Q or V= W/Q. When the question talks about moving charge around the circuit, you will use e.m.f. If the question talks about moving the charge through/across the component then you should use V= W/Q.
3. For questions involved finding the new resistance of a new dimension of the same material, it is easier and faster to use the proportionality concept. R ∝ l and R ∝ 1/A.

### Higher Tier Content

• Why conductors tend to have their resistance increase with an increase in temperature?

In metals, metal ions and free electrons are held together by strong metallic bonds.

Resistivity is a measure of resisting power of a specific material to the flow of an electric current.

At higher temperature, the metal ions vibrate more vigorously and collide more frequently with the free electrons making it more difficult for current to flow. This means resistivity of the material increases and hence resistance increases.

• Why insulators tend to have their resistance decrease with an increase in temperature?

Similar argument except there are no free electrons in insulator. Thus, the increase in temperature must have caused the “captive electrons” to become free and thus lowering the resistivity.