QMU–July 2012

The fourth lecture was about a topic my year 13 students get to know and love: Fields and Potential. Given By Robert Birke.

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Detecting the electric force between two charged plates.

When you are doing KS3 and KS4 physics you learn there are different kinds of force divided up into two groups: Contact (friction, drag, upthrust, compression and stretch, etc.) and Non-contact (gravitational, electric and magnetic). In reality the contact forces don’t exist. Gravitational forces are felt by objects having mass; electric forces are felt by charged objects and magnetic forces are felt by magnetic materials. Non-contact forces are “action at a distance” forces. This means they act away from the source through empty space. A field is a region of space where something experiences a force. A charged particle experiences a force in an electric field; a mass experiences a force in a gravitational field and a magnetic pole experiences a force in a magnetic field.

There is a problem with “action at a distance”. It has been difficult to come up with the correct physics.

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This doesn’t explain how objects not in contact affect each other.

If you connect two parallel plates to an EHT power supply you can prove that the electric force between them is constant. This involves putting a small piece of charged aluminum foil on a polythene rod and placing the rod between the plates. The foil is deflected but the deflection doesn’t change if you move the rod about between the plates.

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Identical field in all directions.

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Away from the edges the force is constant. The electric field is the force per unit charge. The limit of this is when the charge tends to zero. The field is a vector as it has size and direction. The field lines are parallel and equally spaced and show the path that a lone positive charge would take if free to move.

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This is a practical that I show my year 13 students to show up the electric field lines.

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I use this experiment to do two things. For year 12 I use it to illustrate the link between charge and current and for year 13 I use it to illustrate the force on charged objects in an electric field. If a pico-ammeter is included in the circuit a current can be seen due to the movement of charges. The ball is usually a ping pong ball painted in aquadag. If the ball is positive it is first repelled by the positive plate and attracted to the negative plate. When it touches the negative it gains lots of negative charge so is repelled by the negative plate and attracted to the positive plate and the cycle gets repeated.

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Equipotentials and field lines are always perpendicular to each other.

Work done on moving the charge = force x distance. Energy gained is proportional to the distance if the force is constant. Moving a positive charge towards the positive plate means doing work against the force. This potential energy change is the same despite the path.

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Electrical uniform field E = F/q = Force per unit test “charge”

Gravitational uniform field g = F/m = Force per unit test “mass”

Force = – gradient of potential energy. Bound states have negative values.

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The minus sign shows the force is attractive.

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Force gets smaller as the distance increases so equal steps means less and less energy.

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It is possible to calculate escape velocity’s GMe = 4xE14. The negative sign is because it shows a bound state.

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There is no negative sign here as charges can be positive or negative.

Surprisingly the electric force is greater than the gravitational force. This can easily be demonstrated by using a charged polythene rod to pick up a piece of paper against the force of gravity. Gravity appears to be bigger as it can act over a greater distance. This may be due to the fact that gravity is only positive but charges can be either and this leads them to cancel each other out.

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