9. (a) Which of these could be the output for a dynamo? **(1)** **B** (b) (i) Figure 17 shows the output from a battery - Edexcel - GCSE Physics - Question 9 - 2018 - Paper 1

To calculate the potential difference across the secondary coil, we use the transformer equation:

$\frac{V_s}{V_p} = \frac{N_s}{N_p}$

Where:

• $V_s$ = potential difference across the secondary coil
• $V_p$ = potential difference across the primary coil (25 V)
• $N_s$ = number of turns in the secondary coil (150)
• $N_p$ = number of turns in the primary coil (30)

Substituting the values:

$\frac{V_s}{25} = \frac{150}{30}$

Rearranging gives:

$V_s = 25 \times \frac{150}{30}$

Calculating:

$V_s = 25 \times 5 = 125 V$

Thus, the potential difference across the secondary coil is 125 V.

Using high voltage transmission cables and transformers enhances the efficiency of electrical power distribution in several key ways:

1. Reducing Power Losses: High voltage transmission reduces current for a given power level according to the formula $P = V × I$. Since power loss due to resistance in the wires can be described by $P_{loss} = I^2 × R$, decreasing the current minimizes losses over long distances.

2. Transformers Step-Up Voltage: Transformers are used to step up voltage before transmission, allowing electricity to travel more efficiently across long distances. At the receiving end, another transformer steps down the voltage to a safer level for consumer usage.

3. Lowering Current for Safety: As the transmission wires operate at high voltages, the resultant low current reduces the risk of overheating and allows for the use of thinner wires, which are cheaper and easier to install.

4. Increasing Transmission Capacity: Higher voltages allow for more power to be transmitted without increasing current, thus enhancing the overall capacity of the electrical grid.

In summary, high voltage transmission cables and transformers work together to minimize energy losses, enhance safety, and improve the efficiency of power distribution across the national grid.