Voltage in Electric Currents

Overview

• Voltage is the energy transferred or work done per unit charge; it can be imagined as the push of a battery or power supply in a circuit.
• Potential difference (p.d.) is the work done by a unit charge passing through an electrical component; another name for the voltage between two points.
  • The p.d. is also the difference in electrical potential between two points:
    • the point where the current enters a component
    • the point where the current leaves a component
• A voltmeter is a meter for measuring the p.d. (voltage) between two points.
  • Voltmeters are always connected in parallel with a component, that is, across a component.
  • Voltmeters can either have an analogue display (with needle) or a digital display (with numerical value).
• The electromotive force (e.m.f) is the electrical work done by a source (cell, battery, etc.) in moving a unit (coulomb) charge around a circuit; the voltage across the terminals of a source.

Combining e.m.f.s

• Many battery-operated electrical appliances need more than one cell to make them work.
  • For example, a radio may need four 1.5V cells. Each has an e.m.f. of 1.5V and, when connected together in series, they give a combined e.m.f. of 6V. When cells are connected in series, their e.m.f.s add up.

What is a volt?

Electromotive force

• The reason why high voltages are used for our mains supply is that a supply with a high e.m.f does a lot of work on the charge that it pushes around the circuit.
  • For example, a 230V mains supply does 230J of work on each coulomb of charge that travels round the circuit.
  • This helps us figure out what is meant by a volt.
• A supply with an e.m.f of 1V does 1J of work on each coulomb of charge it pushes round a circuit. Therefore, $1V=1J$.
• The chemical energy supplied by the cell is what pushes electrons around a circuit. The e.m.f tells you how much work is done on each coulomb of charge as it passes through the cell. This is described by the formula: $$\text{e.m.f.}=\frac{\text{work done on the charge (J)}}{\text{charge (C)}}$$ $$E=\frac{W}{Q}$$
• The bigger the e.m.f of a cell, the more strongly electrons are pushed around the circuit.

Potential difference

• The potential difference across a device (e.g., a lamp) is a measure of how much electrical work is done by each coulomb as it passes through a component.
• This can be described as the formula: $$\text{p.d. (V)}=\frac{\text{work done by the charge (J)}}{\text{charge (C)}}$$ $$V=\frac{W}{Q}$$
• The voltage/p.d. is the work done (or energy transferred) per unit charge.
• As electric current flows through a circuit, the chemical energy of the cell is transferred to the components as internal energy (in a resistor) or kinetic energy (in a motor) or transferred by light (in a lamp), etc.
• Remember that work done = energy transferred.