- Some insulating materials can become electrically charged when they are rubbed against each other.
- The electrical charge then stays on the material, i.e it does not move (the charge is 'static').
- Charge builds up when electrons (which have a negative charge) are 'rubbed off' one material onto another.The material receiving the electrons becomes negatively charged and the one giving up electrons becomes positively charged.
- If a Perspex rod is rubbed with a cloth, the rod loses electrons to become positively charged. The cloth gains electrons to become negatively charged.
- If an ebonite rod is rubbed with fur, the ebonite gains electrons to become negatively charged. The fur loses electrons to become positively charged.
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Repulsion and Attraction
- When two charged materials are brought together, they exert a force on each other so they are attracted or repelled.
- Two materials with the same type of charge repel each other; two materials with different charges attract each other.
- If a positively charged Perspex rod is moved near to another positively charged Perspex rod suspended on a string, the suspended rod will be repelled. We would get the same result with two negatively charged ebonite rods.
- If a negatively charged ebonite rod is moved near to a positively charged suspended Perspex rod, the suspended Perspex rod will be attracted. We would get the same result if the rods were the other way round.
- Example - When cars are spray painted, a panel of the car is positively charged and the paint is negatively charged. The paint particles repel each other, but are attracted to the positively charged panel. This causes the paint to be applied evenly.
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- An electric current is a flow of charge. It is measured in amperes (amps).
- In an electric circuit the components and wires are full of charges that are free to move.
- When a circuit is made, the battery causes these charges to move in a continuous loop.
- The charges are not used up.
- In metal conductors there are lots of charges free to move.
- Insulators have few charges that are free to move.
- Metals contain free electrons in their structure. The movement of these electrons creates the flow of charge (electric current).
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- The amount of current flowing in a circuit depends on the resistance of the components in the circuit and the potential difference (pd) across them.
- Potential difference tells us the energy given to the charge and is another name for voltage. It is a measure of the 'push' of the battery on the charges in the circuit.
- The greater the potential difference (or voltage) across a component, the greater the current that flows through the component.
- Two cells together provide a bigger potential difference across a lamp than one cell. This makes a bigger current flow.
- Current is measured using an ammeter and voltage (or potential difference) is measured using a voltmeter.
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Resistance and Current
- Components such as resistors, lamps and motors resist the flow of charge through them, i.e. they have resistance.
- Work is done by the power supply and energy is transferred to the component.
- The greater the resistance of a component or components, smaller the current that flows for a particular voltage, or the greater the voltage needed to maintain a particular current.
- Even the connecting wires in the circuit have some resistance, but it is such a small amount that it is usually ignored.
- Two lamps together in a circuit with one cell have a certain resistance. Including another cell in the circuit provides a greater potential difference, so a larger current flows.
- Adding resistors in series increases the resistance because the battery has to push charges through all of the resistors. Resistance is a measure of how hard it is to get a current through a component at a particular potential difference or voltage.
- Adding resistors in parallel reduces the total resistance and increases the total current because this provides more paths for the charges to flow along.
- When an electric current flows through a component it causes the component to heat up. In a filament lamp this heating effect is large enough to make the filament in the lamp glow.
- As the current flows, the moving charges collide with the vibrating ions in the wire giving them energy; this increase in energy causes the component to become hot.
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- As long as the temperature of a resistor stays constant, the current through the resistor is directly proportional to the voltage across the resistor, regardless of which direction the current is flowing, i.e. if one doubles, the other also doubles.
- This means that a graph showing current through the component and voltage across the component will be a straight line through 0. (If there is no voltage, then there isn't anything to push the current around.)
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Thermistors and LDRs
- LDRs - Light Dependent Resistors
- The resistance of some materials depends on environmental conditions.
- The resistance of a thermistor depends on its temperature. Its resistance decreases as the temperature increases; this allows more current to flow.
- The resistance of a light dependent resistor (LDR) depends on light intensity. Its resistance decreases as the amount of light falling on it increases; this allows more current to flow.
- As the resistance of an LDR and a thermistor can change, this will result in a change in the potential difference for all the other components in the circuit.
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Potential Difference and Current
- The potential difference across a component in a circuit is measured in volts (V) using a voltmeter connected in parallel across the component.
- The current flowing through a component in a circuit is measured in amperes (A), using an ammeter connected in series.
- When batteries are added in series, the total potential difference is the sum of all the individual potential differences.
- When batteries are added in parallel, the total potential difference and current through the circuit remains the same, but each battery supplies less current.
- This sharing of the load makes the batteries last longer.
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- In a series circuit, all components are connected one after another in one loop going from one terminal of the battery to the other.
- The current flowing through each component is the same.
- Potential difference across components adds up to the potential difference across the battery.
- The total energy transferred to each unit charge by the battery must equal the total amount of energy transferred from the charge by the component because energy cannot be created or destroyed.
- If another battery was added to the circuit, it would have the effect of increasing the voltage and current.
- The potential difference is largest across components with the greatest resistance.
- More energy is transferred from the charge flowing through a greater resistance because it takes more energy to push the current through the resistor.
- If a circuit has several identical components, they will have the same potential difference across them. If the components are different, the one with the greattest resistance will have the greatest potential difference across it, and vice versa.
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- Components connected in parallel are connected separately in their own loop going from one terminal of the battery to the other.
- The amount of current that passes through each component depends on the resistance of each component. The greater the resistance, the smaller the current. In the circuit above, bulb Q has half the resistance of bulb P, so twice as much current flows through it.
- The total current from the battery is equal to the sum of the current through each of the parallel components.
- The current is smallest through the component with the greatest resistance.
- The same voltage causes more current to flow through a smaller resistance than a bigger one.
- The potential difference across each component is equal to the potential difference of the battery.
- The current through each component is the same as if it was the only component present. For example, a circuit with a battery and a bulb has a 1 amp current. If a second identical component is added in parallel, the current through each would be 1 amp. The total current through the battery will increase.
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- In electromagnetic induction, movement produces voltage. If a wire or a coil of wire cuts through the lines of force of a magnetic field (or vice versa), then a voltage is induced (produced) between the ends of the wire. If the weire is part of a complete circuit, a current will be induced.
- Moving the magnet into the coil induces a current in one direction. A current can be induced in the opposite direction by moving the magnet out of the coil or moving the other pole of the magnet into the coil.
- Both of these involve a magnetic field being cut by a coil of wire, creating an induced voltage.
- If there is no movement of the magnet or coil there is no induced current, because the magnetic field is not being cut.
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- Mains electricity is produced by generators. Generators use the principle of electromagnetic induction to generate electricity by rotating a magnet inside a coil.
- The size of the induced voltage follows the same rules as with the magnet and coil. The voltage will be increased by:
- increasing the speed of rotation of the magnet
- increasing the strength of the magnetic field
- increasing the number of turns on the coil
- placing an iron core inside the coil
- As the magnet rotates, the voltage induced in the coil changes direction and size.
- The induced current reverses its direction of flow every half turn of the magnet and is, therefore, alternating current.
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Power and Transformers
- When charge flows through a component, energy is transferred to the component. Power, measured in watts (W), is a measure of how much energy is transferred every second, i.e. the rate of energy transfer.
- Transformers are used to change the voltage of an alternating current. They consist of two coils, called the primary and secondary coils, wrapped around a soft iron core.
- When two coils of wire are close to each other, a changing magnetic field in one coil can induce a voltage in the other.
- Alternating current flowing through the primary coil creates an alternating magnetic field.
- This changing field then induces an alternating voltage across the secondary coil, which causes an alternating current through the secondary coil.
- The amount by which a transformer changes the voltage depends on the number of turns on the primary and secondary coils.
- If the number of turns on each coil is the same, the voltage does not change.
- If there are more turns on the secondary coil, the voltage goes up; if there are fewer turns, the voltage goes down.
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How an a.c. Generator Works
- In simple terms, an a.c. generator works by placing a coil of wire within a magnetic field and rotating it.
- As it rotates and cuts through the magnetic field, a voltage is induced across the coil and a current is induced in the coil.
- One side of the coil moves up during one half turn, and then down during the next half. This means that the current reverses direction, or alternates, every half turn.
- Each end of the coil is attached to a separate metal ring, called a slip ring, which rotates with it.
- The generator can be connected to an external circuit (e.g. to power a light) using brush contacts. These brushes are spring-loaded so that they continuously push against the slip rings and the circuit remains complete. They need o be replaced regularly because they gradually wear away.
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Types of Current
- A direct current (d.c.) always flows in the same direction. Cells and batteries supply direct current.
- An alternating current (a.c.) changes the direction of flow back and forth continuously. The number of complete cycles per second is called the frequency, and for UK mains electricity this is 50 cycles per second (hertz, Hz).
- In the UK, the mains supply has a voltage of about 230 volts which, if it is not used safely, can kill.
- We use a.c. instead of d.c. for our electrical supply for different reasons: a.c. is easier to generate and distribute over large distances within the National Grid. When d.c. is transmitted over large distances, it loses a lot of electrical energy within the wires due to heat etc. This would mean that the further you live from the power station, the less energy you would get.
- a.c. can be increased and decreased using transformers that reduce energy loss due to heat. Therefore a.c. can be transmitted with hardly any power loss. d.c. cannot be used without transformers. At the power station, transformers step up the voltage to between 150,000V and 400,000V. Before electricity is consumed by the domestic user, transformers step down the voltage of the electricity to a level that is safe to use e.g. 230V.
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Principles of the Motor Effect
- In the motor effect, current produces movement.
- When a conductor (wire) carrying an electric current is placed in a magnetic field, the magnetic field formed around the wire interacts with the permanent magnetic field causing the wire to experience a force, which makes it move.
- This force acts at right angles to both the current and magnetic field.
The size of the force on the wire can be increased by:
- increasing the size of the current (e.g. having more cells).
- increasing the strength of the magnetic field (e.g. having stronger magnets).
The direction of the force on the wire can be reversed by:
- reversing the direction of flow of the current (e.g. turning the cell around).
- reversing the direction of the magnetic field (e.g. swapping the magnets around).
The wire will not experience a force if it is parallel to the magnetic field.
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The Direct Current Motor
- Electric motors rely on the principle of the motor effect. They form the basis of a vast range of electrical devices both inside and outside the home.
- As a current flows through the coil, a magnetic field is formed around the coil, creating an electromagnet. This magnetic field interacts with the permanent magnetic field that exists between the two poles, North and South. A force acts on both sides of the coil, which rotates the coil to give us a very simple motor.
- To make the motion continuous, a commutator is used. The commutator makes the direction of the current reverse every half turn, so the motor keeps turning in the same direction.
- A motor can therefore make something move. Motors are used to move the drum in a washing machine, spin the disc in a DVD player and a hard disk drive, and turn the wheels of an electric car. Tiny electric motors drive the wheels in toy cars, and huge motors drive electric trains.
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