# OCR Combined Science A – P6/P12

### Wave Speed

Wave speed is given in meters per second (the number of waves that pass per second). Wavelength is measured in meters and frequency is measured in hertz (Hz), or number of waves per second. Demo In this tutorial you will learn how to calculate the speed of a wave. The equation for this calculation is written like this: $v = { \text f \; \text x \; \lambda }$ Chilled practice question Calculate the velocity of a wave with a wavelength of 6 m and a frequency of 50 Hz Frozen practice question Find the velocity of a wave which has a time period of 10 s and a wavelength of 24 m, you will need to calculate the frequency from the wave period equation first. Science in context Wave speed is given in meters per second (the number of waves that pass per second). Wave Speed = Frequency x Wave length. Wavelength is measured in meters and frequency is measured in hertz (Hz), or number of waves per second.

### Efficiency

The efficiency of a device is the proportion of input energy that is converted to useful energy. What is Efficiency? The efficiency of a device is the proportion of input energy that is converted to useful energy. Efficiency is a measure of how much work or energy is conserved in an energy transfer, work or energy can be lost, for example as wasted heat energy. The efficiency is the useful energy output, divided by the total energy input, and can be given as a decimal always less than 1 or a percentage. No machine is 100% efficient. Efficiency equation To calculate Efficiency we use this equation. $efficiency = {\text{useful energy output} \over\text{total energy input}}$ Efficiency demo In this tutorial you will learn how to calculate how efficient a device is at transferring energy from one form to another. Chilled practice question Calculate the efficiency of a light bulb with a total input energy of 500 J. The bulb emits 200 J of light energy and 300 J of heat energy. Frozen practice question A tumble drier is 80% efficient. Its useful energy is 45 KJ what is the total input energy in Joules. Science in context The efficiency of a device is the proportion of…

### Transformers

A transformer is an piece of electrical apparatus which will increase or decrease the voltage in an alternating current. It can be designed to “step up” or “step down” voltages and is based on the magnetic induction principle. Demo In this tutorial you will learn how to calculate voltages in step up and step down transformers. The equation is written like this: ${ \text V_{1} \; \text / \; \text V_{2} \; \text = \; \text N_{1} \; \text / \; \text N_{2} \;}$ Chilled practice question A transformer has 300 turns on its primary coil and 100 turns on it secondary coil. What voltage is induced in the secondary coil if the primary voltage is 96 V ? Frozen practice question A transformer has 320 turns on the primary coil and 800 turns on the secondary coil. Find the voltage across the primary coil when the voltage across the secondary coil is 1320 V. Science in context A transformer is an piece of electrical apparatus which will increase or decrease the voltage in an alternating current. It can be designed to “step up” or “step down” voltages and is based on the magnetic induction principle.When a voltage is introduced to one coil, called the primary coil it…

### The Motor Effect

A current-carrying wire or coil can exert a force on a permanent magnet. The force increases if the strength of the magnetic field and/or current increases. This is called the motor effect. Demo In this tutorial you will learn how to calculate the force on a wire carrying a current in a magnetic field. The equation is written like this: $F = { \text B \; \text x \; \text I \; \text x \; \text L}$ Chilled practice question Calculate the force on a wire carrying a current of 3 A. 0.25 m of the wire is in the magnetic field. The magnetic flux density is 15 T. Frozen practice question The force on a wire in a magnetic field is 30 N. The magnetic flux density is 3 T and the length of wire in the magnetic field is 50 cm. Calculate the current. Science in context A current-carrying wire or coil can exert a force on a permanent magnet.The force increases if the strength of the magnetic field and/or current increases. This is called the motor effect.
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