A-level Physics (Advancing Physics)/Voltage

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Charge moves through a circuit, losing potential energy as it goes. This means that the charge travels as an electric current. Voltage is defined as the difference in potential energy per. unit charge, i.e.

V = \frac{E}{Q}

where V is voltage (in V), E is the difference in potential energy (in joules) and Q is charge (in coulombs).

There are two electrical properties which are both measured in volts (commonly abbreviated V), and so both are known under the somewhat vague title of 'voltage'. Both are so called because they change the potential energy of the charge.

Electromotive Force (EMF)[edit]

Keep in mind, that EMF as the name suggests is not an electrical force, it is basically the potential difference across the terminals when the key is open i.e. when no current is drawn from the cell. EMF is named so by the scientists who performed faulty experiments and named it so, hence, just a tribute to their contribution to physics it is still called EMF but the definition has changed with time.

Potential Difference[edit]

As charge travels around a circuit, each coulomb of charge has less potential energy, so the voltage (relative to the power source) decreases. The difference between the voltage at two points in a circuit is known as potential difference, and can be measured with a voltmeter.

Series Circuits[edit]

In a series circuit, the total voltage (EMF) is divided across the components, as each component causes the voltage to decrease, so each one has a potential difference. The sum of the potential differences across all the components is equal to the potential difference (but batteries have their own 'internal resistances', which complicates things slightly, as we will see).

Parallel Circuits[edit]

In a parallel circuit, the potential difference across each branch of the circuit is equal to the EMF, as the same 'force' is pushing along each path of the circuit. The number of charge carriers (current) differs, but the 'force' pushing them (voltage) does not.

Questions[edit]

1. A battery has an EMF of 5V. What is the total potential difference across all the components in the circuit?

2. The voltages (relative to the voltage of the battery) on either side of a resistor are -6V and -5V. What is the potential difference across the resistor?

3. At a given point in a circuit, 5C of charge have 10 kJ of potential energy. What is the voltage at this point?

4. Why do the electrons move to a point 1cm further along the wire?

Worked Solutions