Resistance
- As electrons move through the metal wire of a circuit (or any other component), they transfer some of their electrical potential energy to the positive ions of the metal
Free electrons collide with metal ions which resist their flow
- This energy results in an increase in the kinetic energy of the lattice
- Which means a higher internal energy of the metal
- The macroscopic result of this transfer is the heating up of the wire
- Some metals heat up more than others
- The higher the heating, the higher the resistance
- Wires are often made from copper because copper has a low electrical resistance
- The resistance R of a component is defined as:
The ratio of the potential difference across the component to the current flowing through it
- It is calculated as follows:
- Where:
- V = potential difference in volts (V)
- I = electric current in amperes (A)
- R = resistance in ohms (Ω)
- This means that the higher the resistance of a component, the lower the current flowing through it and vice versa
- In terms of SI base units: 1 Ω = 1 kg m2 s–3 A–2
Worked example
A charge of 5.0 C passes through a resistor at a constant rate in 30 s. The potential difference across the resistor is 2.0 V.
Calculate the resistance R of the resistor.
Step 1: Write down the known quantities
- Charge, Δq = 5.0 C
- Time, Δt = 30 s
- Potential difference, V = 2.0 V
Step 2: Write down the equation for the resistance R
Step 3: Calculate the current I from the charge and time
Step 4: Substitute the numbers into the above equation
I = 0.17 A
Step 5: Substitute this value of the current into the equation for the resistance given in Step 2
R = 12 Ω