Physics Y9 Electricity

• When certain insulating materials are rubbed against each other, they become electrically charged. 
• Electrical charges can move easily through electrical conductors, such as metals.

  Electrical charges cannot move easily through electrical insulators, such as plastics and rubbers. 
• Electrical wires and cables have a core made of an electrical conductor so electric charge can flow through it easily, and a casing made of an electrical insulator to stop you from getting an electric shock by touching it.
• Static charge is an electric charge which cannot move. 
• Static charge is often (but not always) found on electrical insulators where charge cannot flow freely.
• Static charge can be positive (+ve) or negative (–ve)
• Static charge can build up on an insulator if isolated.
• When certain insulating materials are rubbed together, the friction causes negatively charged electrons to be rubbed from one to the other. This leaves a positive static charge on the one that loses electrons and a negative static charge on the one that gains electrons. The amount of electrons transferred equals the charge number.
• Positive static charges never move, only negative electrostatic charges.
• With the polythene rod, electrons move from the duster to the rod. The rod becomes negatively charged, and the cloth has an equal positive charge. 
• With the acetate rod, electrons move from the rod to the duster. The rod becomes positively charged, and the cloth has an equal negative charge.
• As electric charge builds up on an isolated object, the potential difference between it and the Earth (at 0V) increases. The p.d. causes an electric field between the object and the earth/earthed conductor. This electric field causes electrons in the air particles to be removed (aka ionisation). Air is normally an insulator, but when ionised it is much more conductive. Therefore, if the p.d. gets high enough, electrons can jump across the gap between the object and the earth– a spark. Electrons can also jump between a charged object and any earthed conductor nearby.
• When 2 electrically charged objects are brought close together they exert a force on one another. These forces will cause the objects to move if they can. This is known as electrostatic attraction/repulsion and is a non-contact force. These forces get weaker the further apart the two objects are. Opposite electric charges are attracted to each other, but the same electric charges will repel each other.
• An electric field is created around any electrically charged object. The closer to the charged object you get, the stronger the field is. 
• You can show an electric field using field lines. 

  • Point away from positive charge
  • Point towards negative charge
  • Always at right angles to the charged object’s surface 
  • Closer lines equal a stronger field
  • Lines can be unevenly spread at points between the objects
• When a charged object is placed in the electric field of another object, it experiences a force. The force between the objects decreases as the distance between them increases. This force is what causes the attraction or repulsion in electrostatic attraction/repulsion.
• A switch is used to turn a circuit on (closed) and off (open).
• A lamp has an electrical current that heats the filament in a bulb so that it gives out light.
• A resistor restricts or limits the flow of electrical current. A fixed resistor has a resistance that does not change.
• A resistor restricts or limits the flow of electrical current. Moving the position of the slider on a variable resistor changes the resistance. A variable resistor is used in some dimmer switches and volume controls.
• The resistance of a thermistor depends on its temperature. At low temperatures, the thermistor has a high resistance. As the temperature increases, the resistance decreases. A thermistor can be used in thermostats or heat-activated fire alarms.
• The resistance of an LDR depends on light intensity. At low light levels, the LDR has a high resistance. As the light intensity increases, the resistance decreases. An LDR can be used as a sensor in cameras or automatic lights that come on when it gets dark.
• A diode allows current to flow in one direction only. Current will not flow in the other direction. Diodes are used to convert an alternating current into a direct current.
• Current- the (rate of) flow of charge
• Cells/batteries always have a positive terminal (the longer line) and a negative terminal (the shorter line)– current flows from positive to negative around a circuit. 
• The size of the current equals the rate of flow of charge, measured in amperes (A) using an ammeter. An ammeter is always connected ‘in line’ or ‘in series’ with a component.
• When current flows past a point in a circuit for a length of time then the charge that has passed is given by this formula:

  Q = It

  Charge flow (C) = current (A) x time (s)
• A bigger current means there is more charge passing around the circuit.
• The current through a component depends on both the resistance of the component and the potential difference across it. 
• Potential difference- the driving force that pushes the charge around. Also a measure of energy, per unit of charge, transferred between two points in a circuit.
• Electrical charge will only flow around a complete circuit if there is a potential difference, so a current can only flow if there’s a source of potential difference, aka a cell/battery.
• Potential difference is measured in Volts (V) using a voltmeter. A voltmeter is always connected ‘in parallel’ with a component.
• To find potential difference use this formula:

  V = E/Q

  Potential Difference (V) = energy transferred/charge flow (C)
• Resistance- opposition to the flow of current
• The greater the resistance of a component, the smaller the current (for a given potential difference across the component). Measured in Ohms (Ω).
• Resistance is linked to p.d. and current in this formula:

  V = IR

  Potential Difference (V) = current (A) x resistance (Ω)
• In a series circuit, current is the same through each component, and has the same value at any point in the circuit.

  Formula- Itotal = I1 = I2 = …etc

  To calculate- I= V/R
• In a series circuit, there is a bigger potential difference when more cells are connected in series (if all cells are connected the same way). It is shared between the various components (equally if the components are identical).

  Formula- Vtotal = V1 + V2 + …etc

  To calculate- V = IR and V = E/Q
• In a series circuit, resistors have to share the total potential difference. The potential difference across each resistor is lower, therefore so is the current. So, if a resistor is added, the current decreases and the total resistance of the circuit increases.

  Formula- Rtotal = R1 + R2 + …etc

  To calculate- R = V/I 
• In a parallel circuit, current splits between each branch. 

  The current through each component in one branch is equal (like a series circuit). The same amount of current that entered the branch must leave the branch when it rejoins the rest of the circuit. 

  If 2 identical components are connected, the same current will flow through each component.

  Total current flowing = total of all the currents flowing through the separate branches.

  Formula- Itotal = I1 + I2 + …etc