Current, Voltage and Resistance

The flow of electricity through an object, such as a wire, is known as the current (I). It is measured in amps (A); if the current is very small then it is described in milli-amps (mA), 1000 mA = 1A. The driving force (electrical pressure) behind the flow of a current is known as the voltage and is measured in volts (V) (Voltage may also be referred to as the potential difference, or electromotive force). The property of a material that limits current flow is known as its resistance (R), the unit of resistance is the ohm (Ω). Resistance to alternating current is more properly called impedance but, in this application, resistance and impedance can be considered to be equivalent.

The relationship between current, voltage and resistance is expressed by Ohm’s Law. This states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit, provided the temperature remains constant.

Ohm’s Law:        Current (I) = Voltage (V) / Resistance (R)

To increase the current flowing in a circuit, the voltage must be increased, or the resistance decreased.

A simple electrical circuit is depicted in Figure 1a. The flow of electricity through this circuit is further illustrated by analogy to the pressurized water system in Figure 1b.

In the electrical circuit the power supply generates electrical pressure (voltage), equivalent to the pump creating water pressure in the pipe; the current is equivalent to the rate of flow of water; and the light bulb provides the resistance in the same way as the restriction in the water system. The ammeter is equivalent to the flow meter and the voltmeter measures the difference in electrical pressure each side of the restriction in the water system. There will be a drop in voltage due to the energy used up in driving the current through the light bulb, which has a higher resistance than the wire in the circuit. Similarly, the water pressure at (A) will be less than at (B).

Figure 1a Simple Electric Circuit	Figure 1b Pressurised Water System

The overall resistance of an object depends on a number of properties including its length, cross-sectional area and the type of material. The longer a conductor, the greater its resistance; for example, a two metre wire has twice the resistance of a one metre wire of similar properties. The larger the cross-section of a conductor, then the lower its resistance: overhead power cables have a much lower resistance than a lamp flex of the same length. Different materials also have different abilities to conduct electricity. Metals conduct very well but materials such as ceramics or glass do not usually conduct electricity at all and are known as insulators.

Animals contain a high proportion of liquid that will conduct electricity well; however skin, fat, bone and hair are poor conductors. Electrical current will take the path of least resistance through animal tissue, with the result that only a small proportion of the measured current will penetrate the brain. Animals with heavy fleeces, thick skin, fat layers or thick skulls will have a high electrical resistance. Table 1 shows how the relationship between current, voltage and resistance differs when stunning sheep of different physical condition. In this example, the minimum current required for an effective stun is one amp.

Table 1 Examples of the application of Ohm's Law when stunning sheep

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