Current, potential difference and resistance
Jonny Nelson explains resistance with a GCSE Physics practical experiment
The Moving electric charges, eg electrons moving through a metal wire. in a circuit depends on the The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them. and the The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms.. It is easy to think of each of these in the following way:
- the current is the flow of electric Property of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge.
- the potential difference provides the 'push'
- the resistance restricts the flow of charges
Resistance
Electric wires are made of metal, which have electrical resistance. The atoms in a solid metal are arranged in a regular A regular grid-like arrangement of atoms in a material. structure. The outer Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. from each atom are free to move through the structure, forming a current. However, they may collide with atoms or meet defects in the lattice. This reduces the number of electrons flowing, which reduces the current.
An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. have a low resistance and Material that is a poor conductor of electricity or heat. have a high resistance.
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Calculating potential difference
When a charge moves through a potential difference, Energy transferred by a force. Work done = force 脳 distance moved in the direction of the force. is done and energy is transferred. Potential difference can be calculated using the equation:
potential difference = current 脳 resistance
This is when:
- potential difference is measured in volts (V)
- current is measured in amps (A)
- resistance is measured in ohms (\(\Omega\))
Example
A current of 2.0 A flows through a component with a resistance of 40 \(\Omega\). Calculate the potential difference.
potential difference = current 脳 resistance
= 2.0 脳 40
= 80 V
Question
A current of 2.0 A flows through a component with a potential difference of 12 V. Calculate the resistance of the component.
Rearrange the equation to find resistance and then substitute in the known values.
potential difference = current 脳 resistance
\(resistance = \frac{potential~difference}{current}\)
\(resistance = \frac{12}{2.0}\)
\(= 6 \: \Omega \)
Resistance in series and parallel circuits
Resistors in series
In a A circuit where one component follows directly from another, eg three bulbs in a row with no junctions are said to be connected in series. circuit the total resistance across all of the components (the 'net resistance') increases as more components are added.
The two resistors have the same current flowing through them. The potential difference across them will be different if they have different resistances. The total resistance for this circuit is: R1 + R2.
Resistors in parallel
In a parallel circuit the net resistance decreases as more components are added, because there are more paths for the current to flow through.
The two resistors have the same potential difference across them. The current flowing through them will be different if they have different resistances.
The total current in the circuit is the sum of the currents through each branch. The total resistance for this circuit is calculated by dividing the potential difference of the cell by the total current for the circuit.