P3.2 - Simple Circuits

• For a current to flow, potential difference is required.
• In a battery or cell, there are substances that react to separate the charges. This makes one side of the battery positively charged and the other negatively charged.
• The positively charged terminal of a battery has a higher electrical potential than the negative terminal.
• The potential difference and the current relies on the construction of the cell, not its size.
• Potential difference is measured in volts (V).
• When a potential difference is applied on a wire, an electrical field is set up inside it. This allows for the charged electrons to move straight away.
• Potential difference is measured with a voltmeter.
• Electrical working is work done transferred by the charges in the component.
• Energy transferred (J) = potential difference (V) * charge (C)
• A voltmeter is needed at both sides of a component as it measures the "rise in potential" across a cell or battery, and a "drop in potential" across a component. There is little "drop" in wires, so the voltmeter would read zero by itself.
• In parallel circuits, the current in one loop adds up to match the current in the loop nearest to the battery.
• Resistance is measured in Ohms.
• Current relies on resistance and potential difference.
• Current is the dependent variable in the formula:

  Current (A) = potential difference (V) / resistance
• Resistance is produced by the delocalised electrons colliding with the positive ion lattice.
• A variable resistor is used to alter the resistance of a wire. It is used to make dimmer switches.
• A graph of current against potential difference is called a characteristic graph. The data comes from taking measurements from ammeters for different potential differences, before reversing the battery and doing the same process again.
• Fixed resistor graph:

  • The wire is a linear circuit element - its resistance does not change as the potential difference does.

  
• If the wire gets hot, the resistance varies and the graph is not a straight line, this is Ohm's Law.

  • For a non-linear circuit, the resistance is not constant.
  • The electrons in a wire collide with the ions. The ions vibrate more as the wire gets hotter. This causes more collisions and the wire heats up more. The current increases as the potential difference increases, but at a slower rate.

  
• A diode is a component that only allows current to flow one way. Some diodes emit light (LEDs).

  • As a potential difference is added, very little current flows until it suddenly increases. If the potential difference is reversed, no current flows.

  
• A thermistor is a resistor that relies on temperature. It is not like a piece of wire, it is made of a semi-conducting material (e.g: silicon). Electrons in a semi-conductor do not need much energy to escape from the atoms to form a current.

  
• As temperatures increase, resistance decreases in a thermistor so current increases.

  
• If there are more electrons moving per second for a particular potential difference, current increases. This means resistance gets smaller.
• A Light-Depended Resistor (LDR) changes with light intensity.

  
• As light intensity increases, more electrons leave the atoms in the semi-conductor and increases current while decreasing resistance.

  
• By changing the components or its order, the current can change, subsequently changing net resistance.
• Adding a lamp in series:

  • Current decreases
  • Net resistance increases
• Adding a lamp in parallel:

  • Total current increases
  • Net resistance decreases.
• The potential differences across the components add up to equal the potential difference across a battery.
• Thermistors can be used in sensing circuits.
• The potential difference tells you the energy transferred by each charge.
• Potential difference (V) = energy transferred / charge
• Power = potential difference * current
• Energy transferred (J, kWh) = power (W) * time (s)
• Power = current^2 * resistance