skoool.co.za

Potential Difference in Circuits

Measuring Current | Circuit Diagrams and Symbols | Ohm’s Law | Series and Parallel Circuits

Measuring Current

As electrons pass through a cell or power supply, they gain energy.

The difference in energy between electrons entering and leaving the cell is called the potential difference (p.d.) or voltage.

There is also a potential difference across every component in a circuit, because energy is transferred to them as the current flows through.

The bigger the potential difference across a component, the bigger the current that flows through it.

Components resist a current flowing through them. The bigger their resistance, the smaller the current produced by a particular voltage.

Resistance is measured in ohms (Ω).

The p.d. across a component is measured in volts (V) using a voltmeter connected in parallel with the component.

The current flowing through a component is measured in amperes (A) using an ammeter connected in series with the component.

Circuit Diagrams and Symbols

Circuit diagrams are drawn using the following electrical symbols.

The diagram shows how the current flowing through a lamp and the p.d. across it can be measured. If the voltage of the power supply is altered, the current through the filament lamp and voltage across it would alter as shown in the current-voltage graph below.

As more current flows, the filament gets hotter, so its resistance increases and current is no longer proportional to voltage.

Ohm's Law

For a resistor at constant temperature, current is directly proportional to voltage. This is known as Ohm's Law.

	potential difference =	current	x	resistance
or	V =	I		R
	(volt, V)	(ampere, A)		(ohm,Ω )

A diode only allows current to flow through in one direction, because it has a high resistance in the opposite direction.

The resistance of a light dependent resistor decreases as the light intensity increases.

The resistance of a thermistor decreases as the temperature increases.

Series and Parallel Circuits

Series circuit	Parallel circuit
See diagram below	See diagram below
• The same current flows through each component	• The total current through the whole circuit is the sum of the currents through each component
• The total p.d. of the supply is shared between the components, so each bulb glows dimly. As more bulbs are connected, the dimmer they glow	• There is the same p.d. across each component, so each bulb shines brightly, no matter how many are connected
• If one bulb blows, they all go off	• If one bulb blows, the others stay on
• The total resistance of the components is the sum of their separate resistances

When cells are connected in series, their total p.d. is the sum of them all.