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Half Life

The radioactivity of a sample decreases over time. Half life is a measurement of this decrease.

What is Half life?

The radioactivity of a sample decreases over time. Half life is a measurement in this decrease.The half–life of a radioactive substance is a constant for each radioactive material. It measures the time it takes for a known amount of the substance to become reduced by half due to radioactive decay, and therefore, the emission of radiation.

Half life demo

In this tutorial you will learn how to calculate the half life of a radioactive material.

Chilled practice question

The activity of a radio-isotope is 1536 cpm. What is its activity after five half lives?

Frozen practice question

A radio-isotopes activity falls from 1880 cpm to 235 cpm in 72 minutes. Calculate it’s half life.

Science in context

The radioactivity of a sample decreases over time. Half life is a measurement of this decrease

Millie’s Master Methods

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Gravitational Potential Energy

Lifting an object in a gravitational field transfers energy into the objects gravitational energy store. Gravitational potential energy is the energy an object has due to its height above Earth.

What is Gravitational potential energy?

Lifting an object in a gravitational field transfers energy into the objects gravitational energy store. Gravitational potential energy is the energy an object has due to its height above Earth. The equation for gravitational potential energy is GPE = mgh, where m is the mass in kilograms, g is the gravitational field strength (9.8 N/Kg on Earth), and h is the height above the ground in meters.

Gravitational potential energy equation

To calculate Gravitational potential energy we use this equation.

$E_p = mgh$

Gravitational potential energy demo

In this tutorial you will learn how to calculate the energy stored in an elevated object.

Chilled practice question

Copy out the question and attempt to calculate the answer before watching the solution. Write down the equation and show all of your working, remember to add the units to your answer, this routine will guarantee you maximum marks in an exam. Mark your solution and correct if needed.

A barrel is lifted onto a shelf 3.5 m from the ground. The barrel has a mass of 22 Kg. Calculate the energy in its G.P.E store.

Frozen practice question

Copy out the question and attempt to calculate the answer before watching the solution. Write down the equation and show all of your working, remember to add the units to your answer, this routine will guarantee you maximum marks in an exam. Mark your solution and correct if needed.

A ski lift transfers 11 KJ of energy into a mans G.P.E energy store. The man has a mass of 55 Kg, calculate the height he was elevated.

Science in context

Lifting an object in a gravitational field transfers energy into the objects gravitational energy store.

Millie’s Master Methods

The Fridge Physics Store

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 the total input energy that is converted to useful energy.

Millie’s Master Methods

The Fridge Physics Store

Energy Transformed

The potential difference between two points is the energy transferred per unit charge. An electrical circuit is an energy transformation device.

What is Energy transformed?

The potential difference between two points is the energy transferred per unit charge. An electrical circuit is an energy transformation device. Energy is provided to the circuit by an electrochemical cell, battery, generator or another electrical energy source. Energy is delivered by the circuit. The rate at which this energy transformation takes place has a great relevance to the design of an electrical circuit for useful functions.

Energy transformed equation

To calculate Energy transformed we use this equation.

Energy = power x time

$ {\mathit E \, \text = \mathit P \mathit t}$

Energy = charge flow x potential difference

$ {\mathit E \, \text = \mathit Q \mathit V}$

Energy transformed demo

In this tutorial you will learn how to calculate the energy transformed through an electrical circuit.

Chilled practice question

Calculate the energy transformed if a circuit is supplied with 6 V and 32 C of charge flows.

Frozen practice question

Calculate the energy transformed when a current of 4 A flows with a p.d of 230 V for 18 s.

Science in context

The potential difference between two points is the energy transferred per unit charge

Millie’s Master Methods

The Fridge Physics Store

Forces and Work Done

The unit for work done is the joule (J), or Newton meter (N-m). One joule is equal to the amount of work that is done when 1 N of force moves an object over a distance of 1 m.

Demo

In this tutorial you will learn how to calculate the work done to move an object through a known distance.

Note-Some mobile devices may require you to tap full screen during playback to view video content.

The formula for this equation is written like this:

w = f x d

Chilled practice question

Copy out the question and attempt to calculate the answer before watching the solution. Write down the equation and show all of your working, remember to add the units to your answer, this routine will guarantee you maximum marks in an exam. Mark your solution and correct if needed.

How much work has been done if a car is pushed along the road 550 cm with a force of 400 N ?

Frozen practice question

Copy out the question and attempt to calculate the answer before watching the solution. Write down the equation and show all of your working, remember to add the units to your answer, this routine will guarantee you maximum marks in an exam. Mark your solution and correct if needed.

75 J of work is done pulling a trolley 150 cm. Calculate the force applied.

Science in context

Work can be calculated with the equation: Work = Force × Distance. The unit for work is the joule (J), or Newton • meter (N • m). One joule is equal to the amount of work that is done when 1 N of force moves an object over a distance of 1 m.

Millie’s Master Methods

The Fridge Physics Store

Momentum

All moving objects have momentum. Forces can cause changes in momentum. The total momentum in a collision or explosion is conserved and stays the same. Car safety features absorb energy involved in a crash they slow down the collision thus reducing the force of impact.

Demo

In this tutorial you will learn how to calculate the momentum in a moving object.

The equation is written like this:

$P = { \text m\; \text x \; \text v}$

Chilled practice question

Calculate the momentum of a 1200 Kg vehicle travelling at 5 m/s.

Frozen practice question

A girl has a mass of 60 Kg. How fast is she moving if her momentum is 240 Kgm/s ?

Science in context

All moving objects have momentum. Forces can cause changes in momentum. The total momentum in a collision or explosion is conserved and stays the same. Car safety features absorb energy involved in a crash they slow down the collision thus reducing the force of impact.

Millie’s Master Methods

The Fridge Physics Store

Change in Momentum

All moving objects have momentum. Forces can cause changes in momentum. The total momentum in a collision or explosion is conserved and stays the same. Car safety features absorb energy involved in a crash they slow down the collision thus reducing the force of impact.

Demo

In this tutorial you will learn how to calculate the change in momentum and the affect it has on impact forces.

The equation is written like this:

$F = { mv \unicode{x2013} mu \over \text{t}}$

Chilled practice question

Calculate the force acting if there is a change in momentum from zero to 168 Kgm/s for 7 s.

Frozen practice question

A 50 kg gymnast is thrown in the air and travels at 5 m/s. The contact time to throw the gymnast is 0.5 seconds. Calculate the force acting on the gymnast.

Science in context

.All moving objects have momentum. Forces can cause changes in momentum. The total momentum in a collision or explosion is conserved and stays the same. Car safety features absorb energy involved in a crash they slow down the collision thus reducing the force of impact.

Millie’s Master Methods

The Fridge Physics Store

Wave Period

The period of a wave is the time it takes to complete one cycle. The unit for a wave period is seconds, and it is inversely proportional to the frequency of a wave, which is the number of cycles of a wave that occur in one second.

Demo

In this tutorial you will learn how to calculate the wave period.

The equation is written like this:

$T = { \text 1 \; \text / \; \text f}$

Chilled practice question

Calculate the wave period for a wave with a frequency of 2 KHz.

Frozen practice question

Calculate the frequency for a wave with a time period of 20 seconds.

Science in context

The period of a wave is the time it takes to complete one cycle. The unit for a wave period is seconds, and it is inversely proportional to the frequency of a wave, which is the number of cycles of a wave that occur in one second.

Millie’s Master Methods

The Fridge Physics Store

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 magnetizes its core which is made from iron which induces a voltage in the secondary coil.

Millie’s Master Methods

The Fridge Physics Store

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.

Millie’s Master Methods

The Fridge Physics Store

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