Resistance in a circuit

The two main ways of increasing the current in an electrical circuit are by increasing the voltage or by decreasing the resistance.

Resistance in a circuit

Changing the voltage

If you increase the voltage across a component, there will be more current in the component. Too high a voltage and the bulb will blow.

Three circuits. Number 1 has a cell at the top, an ammeter reading 0.1A on the right and a lamp giving off a little light at the bottom. Number 2 has a battery at the top, an ammeter reading 0.2A on the right and a lamp giving off a lot of light at the bottom.  Number 3 has two batteries at the top, an ammeter reading 0.3A at the right and a blown bulb at the bottom.

Changing the resistance

If you increase the number of lamps in a series circuit, there will be less current. The lamps resist the current, so if you put more lamps into the circuit, there is more resistance.

Two circuits. Number 1 has a cell and closed switch on the top, an ammeter reading 0.4A at the right and a brightly lit bulb at the bottom.  Number 2 also has a cell and closed switch at the top, an ammeter reading 0.2A at the right and two dimly lit bulbs at the bottom.

You could do the same with a variable resistor.

Three circuits. Number 1 has a cell at the top, an ammeter reading 0.2A on the right and a lamp giving off a little light at the bottom right and a variable resistor bottom left. Number 2 has a cell at the top, an ammeter reading 0.4A on the right and a lamp giving off a medium amount of light at the bottom right and a variable resistor at bottom left.  Number 3 has a cell at the top, an ammeter reading 0.5A on the right and a lamp giving off a lot of light at the bottom right and a variable resistor at bottom left.

The quantities voltage, current and resistance are linked by the relationship:

{\mathop{\rm Re}\nolimits} sistance = \frac{{voltage}}{{current}}

R = \frac{V}{I}

  • The symbol for resistance is R, it is measured in ohms (\Omega)
  • The symbol for voltage is V, it is measured in volts (V)
  • The symbol for current is I, it is measured in amperes (A)

If there is more than one voltage or current, you use the voltage across the resistor and the current through it, not just any values that you see in the question.

Question

A torch lamp takes a current of 0.3 amperes from a 3 volt battery. Calculate its resistance.

To calculate the resistance you need to take the following steps:

{\mathop{\rm Re}\nolimits} sistance = \frac{{voltage}}{{current}}

R = \frac{V}{I} = \frac{3}{{0.3}} = 10\,ohms\,(10\Omega )

Click to listen to the Naked Scientists' explanation of electrical resistance.

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