Current, potential difference and resistanceIB MYP Sciences: Revision notes

Electric charge is measured in coulombs (C). In a metal wire, the charge is carried by electrons. An electric current is the rate of flow of charge. It is measured in amperes (A) and found using:

I = Q ÷ t

where I is current (A), Q is charge (C) and t is time (s).

Worked example: a current of 2.0 A flows for 30 s. Q = I × t = 2.0 × 30 = 60 C.

Potential difference (p.d.), measured in volts (V), tells you how much energy is transferred to or from each coulomb of charge as it moves between two points. A 6.0 V battery gives 6.0 J of energy to every coulomb that passes through it. When that charge passes through a lamp or resistor, the energy is transferred to the surroundings as light and heat.

A greater p.d. across a circuit pushes a greater current round it.

Resistance is how much a component opposes the flow of current. It is measured in ohms (Ω). For a given p.d., a bigger resistance gives a smaller current. The three quantities are linked by:

V = I × R

so R = V ÷ I and I = V ÷ R.

Worked example: a resistor has 12 V across it and a current of 0.40 A. R = V ÷ I = 12 ÷ 0.40 = 30 Ω.

The resistance of a wire is bigger when it is longer, thinner or made of a poorer conductor, and it increases as the wire heats up.

An ammeter measures current. It is connected in series, in the same loop as the component, so that the current passes through it. A voltmeter measures p.d. It is connected in parallel, directly across the component.

An ideal ammeter has a very low resistance and a voltmeter a very high one, so that neither changes the circuit being measured.

A fixed resistor has constant resistance. A variable resistor (rheostat) has a resistance you can change by sliding a contact. It is used to control current, for example in a dimmer switch.

A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases. It is used in automatic street lights. A thermistor is a resistor whose resistance changes with temperature. For the usual (NTC) type, resistance decreases as temperature increases. It is used in thermostats and fire alarms.

A good investigation names three types of variable. For how wire length affects resistance, the independent variable is length, the dependent variable is resistance (calculated from V ÷ I) and the control variables are the wire material, thickness and temperature.

Use a low p.d. so the wire stays cool, take several lengths, repeat and average. Then process the data in a table or graph. If resistance is directly proportional to length, you get a straight line through the origin. Evaluate by commenting on anomalies, whether the wire heated up, and how the method could be improved. A fair conclusion refers back to the hypothesis.