Easy Electrical Stuff *updated* with new images

If it were possible to connect a voltmetervoltm across the ends of a battery,without any charge passing through either the voltmeter or the battery (it isn’t but nearly), the voltmeter would measure the EMF (electromotive force) of the battery. The voltmeter shown is an analogue voltmeter, where the data measured is continuous, unlike a digital one which samples 0’s and 1’s.

Connecting the battery to a circuit pushes charges round the circuit. Their flow rate in coulombs per second is the current in amps (1A = 1C/s). The ammeter is a charge flowmeter and is connected in series with the components.

The resistance in ohms of a component is the p.d in volts across it divided by the current in amps through it. Resistances add together round a series loop.

It takes energy to drive charge through a component like a light bulb. Easy to see because the light bulb is giving out energy in the form of heat and light which it gets from the battery. The difference in energy between two points in the circuit when 1C of charge flows between them is the potential difference between the two points, measured in volts. So 1V = 1J/C . The voltmeter is an energy comparer so it is connected across a component, in parallel.

If 1A=1C/s and 1V = 1J/C, then volts x amps is measured in J/s or Watts, the rate of energy use of the component. So, a 40W light bulb transfers 40J of electrical energy every second as heat and light.

Imagine a long, thin pipe filled with water. It’s hard to drive water down the pipe. Making the pipe shorter reduces its resistance to flow. It’s like this with current in a wire. If a wire is short, it will have a lower resistance than a long wire. Twice the resistance for twice the length. If the wire is fatter its resistance decreases. Doubling the cross-sectional area, the ‘fatness’, halves the resistance.

When two identical resistors are connected in parallel, the available current is split in half.

res

The one single resistor we could use to replace the pair is HALF the value of either.

This is a picture of a big, old wire wound variable resistor. The longer the wire, the greater the resistance . The red slider is moved  along to include longer or shorter wire in the circuit – more or less resistance.

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